Topographically driven vigorous vertical mixing supports mesoscale biological production in the Tsugaru Gyre

Domain: earth_science
Published: 2025-04-22
Editorial outcome: major_revision

Abstract
The Tsugaru Strait is a critical conduit linking the Sea of Japan and the Pacific Ocean and is characterized by a vigorous eastward current. Here, we use high-resolution surface current data acquired from a high-frequency radar system in conjunction with direct turbulence measurements to show that the large increase in chlorophyll-a within a hundred kilometre-scale anticyclonic gyre is driven by the intensification of turbulence, extraordinarily high diffusive nitrate fluxes and divergent flow fields limited to a very narrow area. Enhanced mixing and nutrient fluxes are caused by the presence of a distinct topographic feature—an underwater seamount—within the path of the Tsugaru Strait water flow. Our long-term datasets reveal a positive correlation between recurring submesoscale divergence related to the topographic features and the downstream distribution of chlorophyll-a. This finding reinforces the significant role of the underwater seamount in enhancing the productivity of the downstream anticyclonic gyre. The results of this study emphasize the importance of re-evaluating nutrient supply processes with respect to submesoscale interactions with underwater seamounts for large-scale enhanced biological productivity.

Introduction
Vertical mixing is key to supplying nutrients from deeper ocean regions to plankton communities1,2. Among the different nutrient supply mechanisms to the euphotic zone, tidal-driven flows over seamounts are noteworthy because of their enhanced vertical mixing3,4. Moreover, strong currents, such as the Kuroshio Current, also induce disturbances as they traverse topography, thus positively affecting nutrient supplies from greater depths to the surface5,6. However, the effects of topographic mixing on large-scale biological productivity remain poorly understood due to the sparsity of observations and the difficulty in linking small-scale mixing measurements with larger-scale productivity.

In this study, we introduce a case study to address the above issue by investigating the interaction between a strong current and local topography, revealing that this interplay fosters a vigorous nutrient supply within a confined area at the outlet of the Tsugaru Strait in the North Pacific, Japan (Fig. 1a). The strait features an eastward throughflow, commonly known as the Tsugaru Warm Current, which can reach speeds of 1 ms−1 or more at the fastest points7. The flow path of the current exhibits notable seasonal variations8–11 (Fig. 1b, c). In winter, the Tsugaru Warm Current flows southwards along the coast of the Shimokita Peninsula as the “coastal mode” (Fig. 1a, b), whereas in summer and autumn, the eastward Tsugaru Warm Current contributes to the formation of a clockwise mesoscale gyre known as the Tsugaru Gyre on the Pacific side8,9 (Fig. 1c–e). The seasonal variations in the horizontal flow path are accompanied by significant changes in the vertical outflow characteristics (Supplementary Fig. 1) over the topographic feature known as Shiriya Spur, which is near 41.4°N–41.7°N, 141.5°E–141.6°E, and extends northwards from Cape Shiriya (Fig. 1e). According to an ocean data assimilation model (called JCOPE-TDA; Downscaled and Assimilated version of the Japan Coastal Ocean Predictability Experiment, Tide enabled12), subsurface upwelling and surface divergence on the eastern side of the topography (~41.65°N, 141.60°E–141.65°E) can be markedly enhanced during the Tsugaru Gyre development season (Supplementary Fig. 1d and b, respectively). This localized upwelling and expected vertical nutrient supply can sustain surface primary production and the ecosystem of the downstream gyre.

To quickly test this hypothesis, we normalized the long-term (2014–2022) seasonal (August and September) mean surface chlorophyll-a concentration (mg m−3) by its pixelwise standard deviation (mg m−3) and denoted the normalized data as Ichl (unitless). This index (Ichl) revealed elevated values (>2) within the gyre compared to those outside the gyre (Fig. 1e). Assuming that the chlorophyll-a concentration at each grid is normally distributed during the season, the elevated value of Ichl within the gyre supports the notion of enhanced chlorophyll-a levels with small temporal variation during that season (note that in general, cautions should be exercised when applying this model to the distribution of chlorophyll-a, particularly in the case of short-term blooms, although we have assumed a normal distribution model in this study, e.g., Fig. 1f). The Ichl exhibited a horizontal maximum in the region corresponding to the area immediately east of the spur (approximately 41.7°N, 141.6°E–141.7°E; Fig. 1e). This finding suggests the existence of a nutrient source around the spur that supports stable chlorophyll-a levels, as shown by Ichl.

A study by Tanaka et al.13. reported vertical turbulent nutrient flux over a sill located at approximately 140.35°E (off the Cape Tappi region) in the western part of the strait (Fig. 1e). The reported flux of 1 mmol N m−2 day−1 (below the subsurface chlorophyll-a maximum layer) is significantly greater than the values observed in the open ocean. While this nutrient supply off Cape Tappi is substantial, the sustained productivity in the Tsugaru Gyre suggests the presence of an additional nutrient source, given the distance from Cape Tappi and the spatial distribution of maximum Ichl in the Shiriya Spur (Fig. 1e).

The Tsugaru Warm Current, which passes over the Shiriya Spur, may induce disturbances that more directly impact biological production within the Tsugaru Gyre. However, the nutrient supply capacity at this topographic feature, the Shiriya Spur, has not been assessed, nor has there been a study linking turbulence intensity with large spatiotemporal variations in biological production (over several hours to days). We suggest that this steady supply of localized nutrients at the outlet support sustained phytoplankton growth in the downstream environment.

To better understand the drivers of sustained phytoplankton productivity downstream of the Tsugaru Warm Current, we conducted drift turbulence measurements on 26th August 2021 following an advected water column in the eastern part of the strait, crossing the Shiriya Spur. In this area, continuous surface current velocity data from a high-frequency radar (HFR) system have been collected since 2014. Here we suggest that nutrient supply at the submesoscale (O(1–10) km) around the Shiriya Spur sustains biological productivity downstream at the mesoscale (O(100) km) gyre, through a combination of turbulence observations, HFR monitoring, and ocean data assimilation model (JCOPE-TDA) outputs.

Results
First, we briefly present the seasonal variability characteristics of the Tsugaru Warm Current and Tsugaru Gyre, and the associated topographically driven upwelling and surface velocity divergent field, on the east side of the Tsugaru Strait, using outputs from the JCOPE-TDA (Supplementary Fig. 1, with the 2019 output as an example10; see the Methods section). This analysis was motivated by the challenges in continuously and repeatedly acquiring the spatial structure, including the vertical velocity of the ocean interior. We then introduce results based on field observations using HFR and turbulence measurements. Finally, we discuss the implications for productivity in the Tsugaru Gyre using long-term chlorophyll-a data and empirical estimates of primary productivity.

Seasonal variability characteristics of the topographically relevant upwelling and surface velocity divergence of the Tsugaru Warm Current – implications of numerical modelling

The outputs from the JCOPE-TDA model reasonably reproduce the seasonal variations in the current and the formation of the Tsugaru Gyre8–10 (Supplementary Fig. 1a and b). For the vertical east–west section of each season at 41.65°N (Supplementary Fig. 1c and d), in the region where the eastward flow overtakes the Shiriya Spur near 141.5°E, remarkable upwelling (downwelling) is observed on the western side of (above) the summit at 141.4°E–141.5°E (141.5°E–141.6°E) for the January–June and December time averages (Supplementary Fig. 1c), when density stratification is weak. In July–November, when stratification develops, upwelling occurs at depths shallower than 100 m at approximately 141.60°E–141.65°E (Supplementary Fig. 1d), and divergence fields of surface velocities are observed at 41.6°N–41.7°N and 141.5°E–141.7°E (Supplementary Fig. 1b).

Therefore, the surface divergence field that occurs in the stratified season on the east side of the Shiriya Spur summit (approximately 41.6°N–41.7°N, 141.5°E–141.7°E) serves as an indicator of vertical upward flow and nutrient supply from the lower layers. During this season, vertical transport from the lower layers, including turbulent mixing, significantly affects the nutrient supply for phytoplankton in the surface layer, as surface nutrients have been depleted by previous spring blooms14.

Turbulence over the spur and associated diffusive nitrate flux

Then, we focused on the region where the Tsugaru Warm Current intersects the Shiriya Spur. The reason for this is that active mixing is suspected to have occurred at this location, and this mixing may have been accompanied by a vertical supply of nutrients that bolsters surface biological production within the Tsugaru Gyre (Fig. 1e). Drift turbulence measurements were conducted near the axis of the Tsugaru Warm Current at a latitude of approximately 41.65°N, allowing us to collect data on changes in nutrient distribution and turbulence intensity as the water column was advected downstream from west to east (Fig. 1e). The vessel’s travel distance of ~14.8 km in approximately 3 h, which was almost entirely due east during the drift observations (Supplementary Table 1), corresponds to an advection velocity of ~1.37 ms−1, which is in good agreement with the actual characteristics of the Tsugaru Warm Current.

The drift turbulence observations from the Shiriya Spur (Fig. 1e) revealed highly elevated turbulent energy dissipation rates, which exceeded 10−6 W kg−1 near the surface and bottom layers from the western region to the top of the spur (west of 141.55°E) (Fig. 2a). Near the spur’s top (approximately 141.55°E), the isopycnal surfaces, such as 24.0 σ θ, rapidly deepened (Fig. 2b), and turbulence intensities exceeding 10−8 W kg−1 were observed throughout the entire depth range, reaching 10−5 W kg−1 at certain locations near the bottom (Fig. 2a). The isopycnal surfaces, such as 24.0 σ θ, undulated from the top to the dorsal surface on the east side of the summit. The layer with water temperatures between 12°C and 14°C (approximately 25.5–26.0 σ θ) at approximately 141.6°E was thicker than that on the western side of the spur, with a rising internal surface area of 25.5 σ θ, associated with dissipation rates of O(10−5) W kg−1 (Fig. 2a). The decrease and increase in the depth of the isopycnal surface from west to east of the summit (approximately 141.55°E) are analogous to those observed off Cape Tappi13 (Fig. 1e for the location). Although a similar phenomenon was observed for Cape Tappi, notably, the magnitude of mixing was significantly greater in the present study: the vertical diffusivity east of the summit (~141.60°E) was O(10−1) m2 s−1 (Fig. 2b), which is an order of magnitude greater than that reported for Cape Tappi13. This magnitude of vertical diffusivity was approximately 10,000 times or more significant than the background level in the open ocean15–20.

The spatial distribution of nitrate exhibited significant variation between the western (i.e., upstream) and eastern (i.e., downstream) regions of the topography. Nitrate concentrations were low in the surface layer west of the summit of the Shiriya Spur (<141.55°E), with a nitricline observed at approximately 50 m (Fig. 2c). From the summit to the eastern flank of the sill (141.55°E–141.60°E), nitrate concentrations varied along the rippled isopycnal surface. East of 141.60°E, a more concentrated nitrate (~2 mmol N m−3) distribution extended to the near-surface layer (<20 m). This implies a significant nitrate supply to the surface from greater depths during the flow of water through the sill. The highest levels of vertical nitrate flux occurred immediately east of the summit (141.55°E–141.60°E), where the flux reached O(10–102) mmol N m−2 day−1 (Fig. 2c). This value was much greater than the values reported in other open ocean areas2,20–22. The magnitude was also approximately 100 times greater than that estimated for another characteristic topography near Cape Tappi in the western part of the strait (Fig. 1e shows the location; i.e., 1 mmol N m−2 day−1)13. This value is also comparable to those reported in areas close to land, such as the western boundary region, where strong currents and topography interact5,6,23–31. Fluxes at the nitricline (Fig. 2d) showed large variations but increased from west of the summit (141.55°E), ~0.75 mmol N m−2 day−1, to ~10 mmol N m−2 day−1 east of the summit as the mean. This result suggested that the nutrient content at the surface on the east side of the spur is greatly influenced by vertical nitrate fluxes. The east–west cross-sectional distribution of nitrate concentrations on isopycnal surfaces revealed a tendency for nitrate concentrations to increase within the 24.0–25.5 σ θ range at approximately 141.55°E, near the topographic summit (Fig. 2e). A further increase in nitrate concentration was also observed at 141.60°E within a density range of less than 24.5 σ θ. The vertical convergence of the fluxes at approximately 24.5–25.0 σ θ at 141.58°E–141.60°E (Supplementary Fig. 2) suggests nitrate addition through turbulent vertical mixing on the east side of the summit. This outcome aligns with the process of nitrate addition from lower layers facilitated by turbulent vertical fluxes during the passage of the Tsugaru Warm Current over the topography.

The zonal (east–west) velocities observed along the drift line demonstrated a robust eastward current surpassing 1 ms−1 from 141.50°E to the region just above the sill (Fig. 2f). By employing the method outlined in a previous study13, we calculated the layer Froude number, Frl. The obtained value for Frl was greater than 1 at both 141.56°E and 141.58°E, where intense eastward flow and undulating isopycnal surface were observed (Fig. 2f). This suggests the presence of an internal hydraulic jump. Specifically, when the background flow velocity approximates the propagation speed of the internal gravity wave, stagnation and the superposition of internal gravity waves near the critical point (Frl = 1) are expected. This condition leads to significant isopycnal displacement, resulting in heightened turbulence due to instability. The enhancement of turbulence by such hydraulic jumps has been investigated in various locations, including the Strait of Gibraltar32. The zonal flow divergence demonstrated a convergent field near the surface from just above the sill to just east (141.55°E–141.60°E), matching the structure of the rippling and deepening density surface (Supplementary Fig. 3). In contrast, the surface layer east of 141.65°E exhibited a divergent field (Supplementary Fig. 3).

Internal hydraulic jumps, upwelling and surface divergence as suggested by JCOPE-TDA data

To demonstrate the close relationship between instability in the ocean interior and the divergence of surface current velocities, we also used JCOPE-TDA outputs in August and September 2021 (Supplementary Fig. 4). We focused on the distributions of vertical flow, the layer Froude number, and the inverse Richardson number (Ri−1, an indicator of shear instability). In August and September 2021, the long-term mean flow axis location of the Tsugaru Warm Current, defined as the latitude where the eastward flow is maximal at each longitude, was distributed at approximately 41.60°N–41.65°N in the region 141.1°E–141.6°E (Supplementary Fig. 4a). The flow velocities showed eastward flow maxima exceeding 1 ms−1 at depths shallower than 50 m west of the Shiriya Spur crest, approximately 141.40°E–141.55°E (Supplementary Fig. 4b). The layer Froude number exceeded 1 at approximately 141.5°E (Supplementary Fig. 4c). Furthermore, the temporal mean of Ri−1 was greater than 1 and closer to its threshold of 4 in the region from 50–100 m and the bottom layer from approximately 141.5°E–141.6°E than in other regions, such as <141.40°E. This increase in Ri−1 indicates a potential field for shear instability there (Supplementary Fig. 4c). The upward flow developed at approximately 50–100 m at 141.6°E, indicating that the area shallower than 50 m has a diverging field of horizontal velocities at approximately 141.6°E (Supplementary Fig. 4d).

A significant positive correlation was observed between the daily averaged Frl in the region 141.45°E–141.60°E, corresponding to the Shiriya Spur, and the daily average Ri−1 in the region 141.55°E–141.65°E east of the summit (averaged at depths greater than 20 m to exclude surface stratification effects) (Supplementary Fig. 4e). A significant positive correlation was also shown between this mean Frl and the divergence field of surface flow velocities averaged over a day in the region 141.60°E–141.65°E (Supplementary Fig. 4f). These modelling results suggest that the enhanced surface divergence field east of the spur is related to the vertical flow and vertical mixing associated with internal hydraulic jumps. Therefore, we investigated the two-dimensional divergence field of the surface velocities obtained by the HFR system.

Intra-seasonal spatial and temporal changes in the surface velocity field around the Shiriya Spur

The seasonal variations in surface divergence observed by the HFR system (Fig. 3a, b) were consistent with the JCOPE-TDA outputs (Supplementary Fig. 1): during July–November, a remarkably divergent region of surface velocity appeared at approximately 41.65°N–41.70°N, 141.60°E–141.70°E (Fig. 3b). The locations of the divergence observed during drift observations at approximately 19:00 on 26th August 2021 (41.65°N–41.70°N, 141.60°E–141.70°E; Supplementary Fig. 5d) were generally consistent with those represented by the July–November observational means (Fig. 3b), although the subdaily variability in velocity was also significant (Supplementary Fig. 5) due to tides around the spur33. Therefore, the directly measured nutrient supply due to turbulent mixing in this study corresponds to the divergence of the surface layer observed from the HFR system. Given the limited opportunities for observing turbulence and the temporal resolution of chlorophyll-a data (1 day or more), we focused on another timescale to further support this idea on the basis of the observed data.

Owing to the unique characteristics of the Tsugaru Strait, this current–divergence mechanism can change drastically on timescales ranging from a few days to approximately 10 days (i.e., subinertial frequency range10; Fig. 3c) as well as on a seasonal timescale. This is due to the half-open channel on the northern side (41.8°N, 141.20°E–141.50°E; e.g., Fig. 1e), the protruding spur being in the middle of the strait (~41.7°N at 200 m; e.g., Fig. 1e), and the disturbing excitation in the subinertial frequency range by wind and other factors10. In the 2021 example, a decrease in the surface velocity divergence east of the Shiriya Spur summit (41.65°N–41.70°N, 141.60°E–141.65°E; hereafter divES) was noticeable during July–November, coinciding with the northward shift of the current axis at 141.63°E (Fig. 3c). The wavelet coherence of the spatially averaged divES and the axis latitude at 141.63°E revealed that there was an increase in the signal in the subinertial frequency range around August and September (Fig. 3d). Readers can also grasp the correspondence between the north–south shift of the current axis and the intensity changes in the layer Froude number, subsurface upwelling, and surface divergence on the east face of the Shiriya Spur reproduced by the JCOPE-TDA output (Supplementary Fig. 6a–f).

On the basis of these findings, we contend that the divergence of the surface flow field on the east side to the sill observed by the HFR system might act as a proxy of the vertical nutrient supply to the surface layer via significant turbulent vertical mixing and the associated nutrient supply occurring east of the sill to a certain extent. We used a daily average of the surface velocity divergence field for the subsequent analysis, accounting for the shortest timescale in the GlobColour chlorophyll-a data34,35. Focusing on the daily mean divES for July–November 2021, we calculated the distribution of the ratio of the 1-day lag daily surface chlorophyll-a concentration when divES was positive to negative. The daily lag was assumed here to be the advection time, as discussed below. Consequently, the distribution of ratios greater than one was consistent with the anticyclonic Tsugaru Gyre distribution (Fig. 4). These results suggest that divES is closely linked to the chlorophyll-a concentration in the broader area. Then, we computed and mapped the correlation between the broader chlorophyll-a distribution east of the sill and the surface divergence east of the summit of the spur during August–September utilizing data from 2014–2020 (excluding 2015 due to the data defects for the HFR system). The results are presented in the next section.

Divergence east of the spur and chlorophyll-a variation in the summertime Tsugaru Gyre

The HFR system data revealed a significant positive 1-day lag correlation on the basis of the Pearson linear correlation coefficient between the daily and spatial averages of divES and the surface chlorophyll-a concentrations in the downstream area between August and September of 2014 and 2022 (except for the 2015 data; Fig. 5a), which overlaps well with the distribution area of the Tsugaru Gyre (indicated by the contours and vectors in Fig. 5a).

To confirm the above lag-1-day correlation with respect to the timescale of advection, we performed offline particle tracking using surface velocities from JCOPE-TDA in late August 2021 (Fig. 6). This tracking experiment effectively reproduced a pronounced clockwise circulation at approximately 41°N–42°N, 141.5°E–143.0°E (Fig. 6). It was also suggested that the particles released from the eastern side of the Shiriya Spur generally travel approximately half a gyre in approximately one day. This indicates that intense surface flows, which are not detectable in long-term time averages (e.g., Fig. 1e), may be responsible for active horizontal transport. Therefore, the broad-scale positive correlation observed on lag-1 day in Fig. 5a is not incomprehensible or widely improbable, given the advection timescale. The 2021 case study also revealed that significant wavelet coherence between divES and chlorophyll-a dominated in the Area 1 and Area 2 regions, as shown in Fig. 6, at subinertial frequencies with small phase differences (Fig. 7a and b, respectively). This suggests that variations in divES intensity associated with the north–south shift of the flow axis impact the surface chlorophyll-a concentration in these areas (see also Fig. 4).

The particle tracking example shows that surface circulation of the Tsugaru Gyre (41°N–42°N, 141°E–143°E) can complete a full cycle in approximately two or three days (Fig. 6). The fact that the positive correlation region becomes even larger at a lag of 3 days (Supplementary Fig. 7b) than at 0 lag (Supplementary Fig. 7a) and at a lag of 1 day (Fig. 5a) suggests the effect of this recirculation. At a lag of 5 days, the area of regions with significant positive correlations within the Tsugaru Gyre decreased (Supplementary Fig. 7c).

In the 7-day lag case, a notable (p < 0.05) positive correlation zone was identified at approximately 41.0°N–41.25°N and 142.5°E–143.0°E, which lies distant from the region where the divergence was calculated (Fig. 5b). This is partly an effect of the second round of recirculation. In addition, there may be a converging effect of phytoplankton advected by another larger circulation (39.5°N–41.5°N, 142°E–144°E), as expressed in Fig. 6. This larger anticyclonic gyre is likely caused by the influence of a mesoscale eddy, which also appeared in the long-term average field in August (Supplementary Fig. 8). The wavelet coherence analysis shown in Fig. 7 with divES and chlorophyll-a revealed subinertial frequency variability along this large circulation, with a tendency for the phase shift to increase along the path from Area 2 to Area 5, as shown in Fig. 6 (Fig. 7b–e). In the particle tracking example, the timescale for the advection of particles along this large circulation into the positively correlated area (41.0°N–41.25°N and 142.5°E–143.0°E, around Area 6) was approximately 7 days. Chlorophyll-a anomalies, excited by subinertial frequency fluctuations and advected along this large circulation, likely contributed to the strengthening of lag correlations of approximately 7 days in Area 6 (Fig. 7f; e.g., September).

In contrast, the divergence west (i.e., upstream) of the summit (41.65°N–41.70°N, 141.50°E–141.55°E) did not exhibit a significant positive correlation within the region where the gyre was expected (Fig. 5c and d). The results support the idea that the areas where mixing and nutrient additions influencing broad-scale production occur are concentrated in the eastern part of the Shiriya Spur. We also conducted a correlation analysis using high-passed time series data using a 3-day moving average to eliminate the recirculation resonance effect (Fig. 6). As the area deviated from approximately 41.60°N–41.70°N in latitude and moved away from the Shiriya Spur, the positive correlation area to the east tended to diminish (Supplementary Fig. 9). These findings suggest that, primarily, the nutrient supply area is geographically quite confined and can be characterized as submesoscale, likely dictated by the width of the Tsugaru Warm Current. The results of this long-term correlation analysis (2014–2022) also suggest that physical processes that significantly control phytoplankton production within the broad Tsugaru Gyre, such as the nutrient supply associated with upwelling (e.g., Supplementary Figs. 1d and 4d) and vertical mixing (Fig. 2a–d), are restricted to the eastward (leeward) side of the spur.

Methods
Shipboard observations were conducted over the Shiriya Spur on 26th August 2021 with the research vessel (R/V) Wakataka-Maru, which is owned by the Japan Fisheries Research and Education Agency. The microstructure measurements were repeated 24 times via a free-fall turbulence ocean microstructure acquisition profiler (TurboMAP; JFE Advantech Co., Ltd.; Nishinomiya, Hyogo, Japan) that was loosely tethered during drift observation and equipped with two shear probes (Supplementary Table 1). The calculation method for the estimation of the dissipation rate, ε, was consistent with that used in a previous study conducted on the western Tsugaru Strait13, where shear probe data were sampled at 512 Hz and processed via standard procedures47. For each 8-metre segment of each profile, the turbulent kinetic energy dissipation rate, ε, was estimated by integrating the shear spectrum from 1 cycle per metre to the Kolmogorov wavenumber, employing the Nasmyth universal spectrum48. Segments with spectral shape deviations from the Nasmyth universal spectrum due to various factors, such as rapid changes in the fall rate, were excluded from the ε evaluation. The turbulent vertical eddy diffusivity, Kρ, was calculated as Kρ=Γ×ε/N2, assuming that the mixing coefficient of Γ was constant at 0.249. The buoyancy frequency, N, was derived from the temperature and conductivity sensors attached to the TurboMAP.

Simultaneously, with the turbulence observations, nitrate profiling was carried out via Deep SUNA v2 (Sea-Bird Scientific, Inc.) attached to TurboMAP. During the drift observations, surface water samples were taken at approximately every other point of TurboMAP deployment (Supplementary Table 1) for correction of offset values at the surface nutrient concentrations by the nitrate sensor. The profile offset was adjusted with reference to the surface water sample data. Unfortunately, SUNA’s batteries ran out during the drift observations, so data for deployments east of ~141.65°E could not be obtained (Supplementary Table 1). To estimate the vertical turbulent nitrate fluxes, we adopted an approach from a previous study13,50:

FNO3 = −Kρ ∂NO3/∂z = Γε ∂NO3/∂ρ / g, (1)

where g is the gravitational acceleration and where ρ is the potential density. In this study, we estimated the nitrate gradient on the density surface for each profile via a fitting function. To obtain the vertical distribution of the background nutrient distribution, eliminate contamination due to spike errors in SUNA and exclude fluctuations due to short-term disturbances, we utilized a multiple regression model with density as an explanatory variable, ranging from the first to the tenth order, and log-transformed nitrate concentration. The model with the smallest Akaike information criterion was chosen. From this fitting function for density and nitrate concentration, we estimated the nitrate gradient in the density plane (Supplementary Fig. 11). We also processed each vertical nitrate flux profile to address the near-surface extrapolation effect resulting from the aforementioned fitting process. Specifically, we ensured that the flux was zero at depths where the nitrate concentration was determined to be zero. The nitrate profiles for this terminal processing underwent offset correction through surface samples during the drift observations following the application of a 30-metre moving average due to spike errors in SUNA. Even with the moving average scale of 10 metres, there was no notable disparity observed in the flux magnitude estimates depicted in Fig. 2d. In the present study, the depth of the nitricline was defined as the depth at which the vertical gradient was greatest for each 30-metre filtered nitrate vertical profile. This depth corresponded relatively well with the 2 mmol N m−3 nitrate concentration, except for 141.58°E–141.60°E immediately east of the summit of the Shiriya Spur (141.55°E) (Fig. 2c). Fluxes were averaged over a range of ±5 m at this depth and were defined as fluxes in the nitricline (Fig. 2d). Even at the 10-metre scale of the moving average for the nitrate profiles, there was no noticeable difference in the estimates of the magnitude of the fluxes shown in Fig. 2d.

Other vertical water sampling independent of the turbulence observations was also performed via a carousel water sampler (SBE32; Sea-Bird Scientific, Inc.; Bellevue, Washington, USA) at two stations, i.e., at the start of the drift observation (41.60°N, 141.41°E) and at the end of the drift observation (41.69°N, 141.72°E). The samples were collected at 10, 20, 30, 50, 60, 80, 100, 125, 150, 200, and 300 dbar in addition to the surface. However, the starting point of the drift observation was located slightly further from the initial deployment point of water sampling (41.60°N, 141.41°E) because the latitude at which the drift observation started was fine-tuned to account for the position of the flow axis of the Tsugaru Warm Current (approximately 41.68°E). The end point (41.69°N, 141.72°E) was also far from the correction station because the SUNA battery ran out during the drift observations. Therefore, the nitrate distribution from this water sampler was used to visually confirm the consistency of the vertical structure of nitrate and was not used to correct the nitrate profile during drift (Supplementary Fig. 12).

We utilized velocity data obtained from an acoustic Doppler current profiler mounted on the R/V Wakataka-Maru (SADCP, Ocean Surveyor, Teledyne RD instruments, 38 kHz, Poway, California) in our study. The data were corrected for misalignment angle error due to the SADCP installation51, and data with a data flag value generally called “percent good” (indicating the fraction of available pings for a given ensemble) of 80% or lower were excluded. We used short-period average data (STA) at 1-minute intervals (including 6 pings per 1 ensemble) obtained while crossing the Shiriya Spur. The data were spatially averaged in the longitudinal direction to match the horizontal resolution of the HFR of 3 km and the grid. In the vertical direction, the first layer is 25 m long with a vertical resolution of 8 m and a maximum of 60 layers (497 m at the maximum depth).

The HFR monitoring system employed in this study was a SeaSonde (13.9 MHz, CODAR, Mountain View, USA). The HFR data provided a spatial resolution of ~3 km covering an area of ~3 to 60 km from each antenna (Fig. 1e)9–11. Approximately every 30 min, the system calculates the distribution of surface currents, which are uploaded to the Mutsu Institute for Oceanography (MIO) Ocean Radar data site for the eastern Tsugaru Strait (MORSETS; http://www.godac.jamstec.go.jp/morsets/e/top/). This HFR system has been operational in the eastern Tsugaru Strait since 2014. The basic processing procedure followed that of prior studies52. A raw Doppler spectrum was acquired at 2 Hz for 512 sweeps, approximately every 4.5 min. Four raw Doppler spectra were averaged (i.e., a 15-minute average) to estimate the Doppler spectrum for determining the radial velocity via the multiple signal classification (MUSIC) algorithm53, which was output every 10 min. Additionally, seven 15-minute averages were also utilized to estimate the final radial velocity. Therefore, each 30-minute output contained approximately 75 min of time information. We excluded grids with data acquisition rates below 90% in 2021 for Supplementary Fig. 5 and 95% for 2014 and 2016–2022 for the correlation analysis with reference to the previous method10. We excluded data for 2015 from the correlation analysis because of an observed data malfunction. Additionally, we verified the consistency of the HFR velocity with that obtained from the SADCP data along the drift trajectory (Supplementary Fig. 13).

We analysed the velocity fields for a region larger than the coverage of the HFR using surface velocity data from the Ocean Surface Current Analysis Real-time (OSCAR) dataset, which provides a third-degree resolution of ocean surface currents (Ver. 1. Physical Oceanography Distributed Active Archive Center, National Aeronautics and Space Administration, California, USA.). The data covering the period from 2014 to 2021 were accessed on 9th June 2022 at https://doi.org/10.5067/OSCAR-03D0154. To delineate the extent of the Tsugaru Gyre, the relative vorticity was calculated via the following definition: ∂V/∂x−∂U/∂y, where U and V represent the east–west and north–south currents of the OSCAR, respectively, and where x and y denote the east–west and north–south directions, respectively.

The surface distributions of chlorophyll-a were analysed via data from GlobColour (http://globcolour.info), which was developed, validated, and distributed by ACRI-ST, France55,56. We selected data products with 4-km resolution and 1-day resolution via the level-three Garver-Siegel-Maritoren (GSM01) merged model55,56 for correlation analysis (Fig. 5, Supplementary Figs. 7–9). The merged data encompassed information acquired from three satellite sensors: SeaWiFS (Sea-Viewing Wide Field-of-View Sensor) on GeoEye’s Orbview-2 mission, MODIS (Moderate Resolution Imaging Spectroradiometer) on NASA’s (National Aeronautics and Space Administration) Aqua mission, and MERIS (Medium Resolution Imaging Spectrometer) on ESA’s ENVISAT (Environmental Satellite) mission. The period of study extended from 2014 to 2022.

We used the output of the ocean data assimilation model (JCOPE-TDA12) to describe the physical processes linking the observed turbulence and the surface divergence field. The system provides hourly outputs of variables, such as temperature, salinity, and ocean current velocities, with a horizontal resolution of 1/36°. It incorporates tidal fluxes and sea level anomalies for 11 constituents at the lateral boundaries and surface pressure gradients because of the 25 potential tidal constituents. The atmospheric forcing data were distributed from the Global Forecast System of the National Centers for Environmental Prediction. Using multiscale three-dimensional variational data assimilation, the JCOPE-TDA system assimilates observational data, including satellite sea surface height anomalies, satellite sea surface temperatures, and in situ temperature–salinity profiles. Further details on the JCOPE-TDA configuration are provided in a previous study12. The correspondence between the model and observations of seasonal variations around the Tsugaru Strait was documented in our previous study10. The data selection year includes outputs from 2019, which are part of the results from the previous study, and 2021, when turbulence observations were made. The vertical layers were output every 5 m from 0 m to 350 m.

To obtain information on the spatiotemporal scale of advection of potentially eutrophicated surface water near the Shiriya Spur, we performed an offline particle tracking experiment using surface velocity data from JCOPE-TDA. Using hourly velocity data, we investigated the particle transport pathway from 00:00 on 30th August 2021 for 7 days as an example (Fig. 6). The discharge locations were three north–south grids centred at 41.65°N and 141.65°E. For each time step, data from the nearest grid to the expected transport location latitude and longitude were used. The effect of horizontal dispersion was added via random walk steps57, and 10 sequences were performed per emission grid. Here, the effect of horizontal diffusion is expressed by the following equation:

Δx = 2D H Δt R x, Δy = 2D H Δt R y, (2)

where R is a random number ranging from −1 to 1. The subscripts x and y represent the longitude and latitude, respectively. Here, Δt was set to 3600 sec. In the present study, a horizontal diffusivity coefficient, DH, of 102 m2 s−1 was employed.

We computed the divergence field of surface velocity within the latitudinal band of 41.65°N–41.70°N, encompassing the latitude of the drift turbulence observations, within the 141.60°E–141.65°E area. This calculation utilized the 1-day average of the divergence field of surface current velocities (〈∂u/∂x+∂v/∂y〉t, where u and v represent the east–west and north–south currents observed by the HFR, respectively, and the subscript t indicates the time average). We defined this divergence east of the summit of the Shiriya Spur as divES. Subsequently, employing 1-day resolution data of surface chlorophyll-a in the Tsugaru Gyre, we conducted a lag correlation analysis between the divergence and chlorophyll-a, utilizing data from August and September spanning the years 2014 to 2022, excluding 2015, due to an observed data malfunction. The Pearson linear correlation coefficients were examined.

Following the methodology presented in a previous study in the western Tsugaru Strait13, we determined the layer Froude number via the following formula:

Frl = Ul / (Nl hl), (3)

where Ul, Nl, and hl represent the zonal velocity, vertically averaged buoyancy frequency, and thickness of the layer between the 24.0 σ θ isopycnal surface and the bottom layer to which the free-fall measurement instrument (TurboMAP) descended (for N calculation), respectively58. We also calculate the layer Froude number via the JOCPE-TDA outputs.

Furthermore, we determine the gradient Richardson number (Ri) via JCOPE-TDA outputs via the following definition to examine the potential shear instability field:

Ri = N2 / (∂u/∂z)2, (4)

where ∂u/∂z is the vertical shear of the zonal velocity. Here, we assumed predominance of the eastward flow over the meridional velocity. We used the inverse of the Richardson number, Ri−1, as an indicator of instability. In this case, the criterion suggesting shear instability is Ri−1 = 4. However, we primarily used the time average of the logarithm, which understates the impact of instantaneous instability. We also calculated the divergence from the horizontal velocities of the JCOPE-TDA outputs.

To estimate primary production within the Tsugaru Gyre, we employed two models that use surface chlorophyll-a concentrations. First, we utilized the CBEM previously suggested37,38, as mentioned in the main text. The CBEM estimation equation is expressed as log10(PP)=a+blog10(Chl). In this equation, PP and Chl represent the primary production and chlorophyll-a concentration at the sea surface, respectively, whereas the empirically given constants a and b have fixed values of 2.793 and 0.559, respectively37,38. We also used VGPM39 to estimate primary production, accounting for the effects of the local sea surface water temperature and light environment on primary production. Here, vertically integrated primary production over the euphotic layer was calculated via the following equation:

PP = PBopt Chl 0.66125E0irr / (E0irr + 4.1) Zeu [mg C m−2 day−1], (5)

where PBopt is the maximum carbon fixation rate within the water column and is calculated via the sea surface temperature, T, as follows:

PBopt = −3.27×10−8×T7+3.4132×10−6×T6−1.348×10−4×T5 +2.462×10−3×T4−0.0205×T3+0.0617×T2+0.2749×T +1.2956 (6)

The variables E0irr, Zeu, and D are the sea surface daily photosynthetically available radiation (PAR), euphotic layer depth, and photoperiod, respectively. In the present study, the monthly average (August and September of 2014 to 2022) sea surface temperature and PAR were employed; the former was calculated from the ocean data assimilation reanalysis products (JCOPE2M59,60), and the latter was the result of Aqua-MODIS (Moderate-Resolution Imaging Spectroradiometer; level 3, 4 km) distributed from the NASA Ocean Biology Distributed Active Archive Center (OB.DAAC). The photoperiod was subsequently calculated from the monthly mean sunshine duration (defined as the period during which direct solar irradiance exceeds 120 W m−2) for the onshore observation points surrounding the Tsugaru Gyre (Fig. 1e and Supplementary Table 2) provided by the Japan Meteorological Agency for 1991–2020. The depth of the euphotic layer was estimated to be 60 m from the observed PAR at 41.60°N, 141.41°E (the first station for the carousel water sampler, Sea-Bird Scientific), and information from a previous study36.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "GENERAL COMMENT\nIn this study, Kaneko et al. describe concomitant measurements of turbulence and nitrate along a transect across a sill in the\nTsugaru strait, and show elevated levels of turbulence and very large nutrient fluxes (by ocean standards) associated with\ndivergent flow around the topographic feature. They propose that the nutrients fluxed into the surface layer are advected\noffshore fueling intense primary production in the Tsugaru gyre. They test their hypothesis by assessing the correlation\nbetween downstream chlorophyll and flow divergence at the sill, as measured by a surface radar. This is in my view the\nmost interesting achievement of this paper, since it is normally difficult to track the effect of turbulent mixing on primary\nproductivity, which is probably enabled by the very intense fluxes. As such, this is a very interesting study and deserves\npublication. However, I have a series of major concerns about the research/manuscript that would require some effort before\nI can accept publication. I outline these in detail below, together with some other minor comments and suggested edits but in\ngeneral my major points revolve around these issues:\n-while the correlation between flow divergence and chlorophyll downstream is very promising, I feel the analysis is still quite\nshallow and leaves many questions open (some of which I detail below). It would be interesting to understand better the\nspatio-temporal dynamics of the flow divergence before going further. From Fig. 3 it appears that divergence is highly\nvariable in space and in very short time scales. It left me wondering how the biological response couples with such high\nfrequency fluctuations and whether there are lower frequencies as well that are dominating the divergence variability (I think\nI am calling for a frequency analysis here). Another key issue, is that no lagged correlations were investigated, and it\nappears logic to me that there must be some lag between the nutrient supply at one point, and the biological response\ndownstream. Is the flow that fast that this is not relevant? Maybe this is what the authors intend to convey in their last\nparagraph but it wasn’t entirely clear.\n-I feel the conclusion in Lines 122-124 that the diapycnal nutrient flux is visible in the nitrate distribution is not entirely\nconvincing as it is presented. I am not saying is not happening, it probably is, but the data could be presented in a more\ncompelling way.\n- I find the text a bit confusing in many placing and needs substantial improvement for clarity. I have made a number of\nsuggestions on that regard below.\n- There is a very nice discussion on how the enhanced turbulence impacts the productivity in the Tsugaru gyre, but I feel that,\nin order to publish the manuscript in Nature Communications, further efforts are done to highlight the broader relevance\nof the paper. There are a couple of very general sentences in the abstract and conclusions, but I feel the message could be\nmade stronger with a more focussed dissertation: highlight more regions that could experience similar dynamics and discuss\ntheir relevance for the large scale production of subtropical gyres, e.g. highlight the disproportionate contribution of the\nTsugaru gyre to carbon export in the subtropical Pacific with respect to surface area. And also, in the abstract you mention\nsustainability and marine conservation. This sounds a bit too broad, unless you mention long term changes in the current\nacross the strait and the importance of monitoring it or something like that.\n- Some more details would be beneficial about some parts of the methodology\nDETAILED COMMENTS\nLine 21-24: “Nutrients transported vertically in specific hotspot areas that reach the surface can be conveyed downstream by\nbackground currents, thereby augmenting surface biological production over a wider expanse, including robust currents\ntraversing seamounts.”, sentence a bit convoluted/confusing. Suggest rewritting\nLine 32. Suggestion: “A surface divergence field” -> “a divergent surface flow field”\nLine 34. Suggestion: “Formation area” -> “area”\nLine 36. Last sentence is a bit too general and lacks real substance. It would be OK if you explored further the implication for\necosystem management in the manuscript, but not really justified at the moment\nIntroduction. The first paragraph sets up the general relevance but it entirely lacks of references. Some reference would\nincrease the credibility\nLine 52. “[...] the Tsugaro gyre” could be added at the end of the sentence for clarity\nLine 61. “higher distribution” -> “higher values/concentration”?\nLine 68. In the paper you report nitrate flux in mmol/m2/s, but here is per day. It would be good to make the units more\nuniform to make comparisons easier\nLine 95. “the similar localized increase”, unclear phrase\nLine 103 and after. “the isotherm”, which isotherm? All isotherms? In general lines 100-112 seem a bit confusing to me, they\ncould benefit from some polishing\nLine 116. It is confusing that you have focused on the temperature description above, and here you emphasize comparing\nnutrients with density instead of temperature. It would be simpler to pick one, since, I guess, they must be closely related\nLine 122 – 124. It is hard to verify this conclusion by looking at the nitrate plot here, since nitrate is not shown together with\ndensity, so it is difficult to tell whether there was a significant impact of diapycnal flux on nitrate concentration. Further,\n“similar depths displaying different nitrate concentrations does not necessarily imply a diapycnal flux, if isopycnals are not\nflat. You could modify the plots slightly to get the message across more effectively. Perhaps adding a subplot with\nnitrate+density could help. I also suggest (see below), maybe putting the last panel in an independent figure. Your\nconclusions may be reinforced by performing some along stream nitrate budget / advection diffusion balance: knowing the\nflow speeds and the nitrate evolution along the stream, you may infer a diapycnal flux that you could compare with your\nmeasured flux. I am thinking of something along these lines: https://doi.org/10.1029/2007GL032785. I am not sure if this is\ngoing to work (possibly not, due to unsteadiness), but may be worth having a try.\nLine 128-129. This sentence is somewhat repetitive\nLine 131. The reporting of the flux values is confusing. Try expanding the sentence.\nLine 134-139. You could maybe explain to the readership of NC what are the implications of the existence of a hydraulic\njump. It may not be obvious for some of your biologically-oriented readership.\nLine 142. I think you should say that the HFR helps you support the assumption that the zonal divergence is representative\nof the three-dimensional structure of the float.\nHFR divergence analysis. Fig 3 depicts very substantial temporal variability of the flow divergence. I am not very familiar\nwith the HFR, it would be good to know how confident you’re with this patterns and what is the instrumental noise level for\ntheir detection. If they are robust, you could comment a bit on the significance of the short term variability. Before proceeding\nto the correlation with chlorophyll it would be desirable to better understand the spatio-temporal dynamics of the flow\nconvergence/divergence: how flow convergence varies in time and space and how consistent and persistent are the\npatterns described during the cruise. Maybe some EOF or time-series analysis could be presented, in the main text or as SI.\nIt would be useful also, some discussion about what drives those changes.\nLines 158-161. Sentence not clear. The link between radar flow and nutrients is not obvious. Also, what do you mean by the\n\"transport of nutrient supply\"? Do you mean the transport of recently upwelled nutrients or something on this line?\nCorrelation analysis. This is a very interesting promising thing, but I think it should be explored in more depth. First thing,\nhave you tried lagged correlations? I am surprised of this direct correlation with no lag, when your results imply that there is\nan advective time-scale for this response (the time it takes for the waters with the upwelled nutrients to reach a certain point).\nIn general, I feel that there is a lack of understanding of the underlying spatio temporal dynamics of the physico-chemical\ncoupling. Would something like a wavelet coherence analysis help? Maybe it is too complex. I would encourage the authors\nto provide some more insights with the approach they consider most useful. This analysis would strengthen conclusions like\nthose in sentences 178-182. Also it is unclear why the convergence zone shows no correlation, I would assume that the\nconvergence-divergence patterns are linked to each other. Could you expand a bit on that? If panel b is not helping you, you\nmay as well dropped, but now that you showed it I am intrigued by it.\nLine 183. Confusing sentence. Turbulence does not “uptake” nutrients, it supplies nutrients which are then taken up by\nplankton.\nLines 185-190. I understand what you mean here, but I think you could use a more direct language to make the point clearer.\nSomething along these lines: “the throughflow advects water that is poorer in nutrient compared with the generally dominant\nsubarctic waters”\nLine 192. “uptake” -> transfer or supply\nLine 198. Not sure if “carbon flux” is an accurate term here, I would rather say it this the new production of organic carbon\nthat could be sustained with the influx of nutrients (and will potentially become a downward flux of organic carbon)\nLine 202-203. you may as well make a, b values explicit there, but it would be important also to say where this model comes\nfrom and whether is known to apply locally\nLine 205. “about daily advection” is unclear, maybe something like “though horizontal advection over the course of one day”\nLine 296. From “reproduction” does not make sense to me. Do you mean regenerated production?\nConcluding remark and relevance. The mention to a single other place in the globe feels a bit insufficient here for a\ngeneralist journal. Would it be able to expand a bit on the broader relevance? E.g. showcase other locations with potentially\nsimilar structures?\n-Figure 4. You show potential vorticity contours there, but I can recall seeing a reference in the text as to why those are there.\nI guess is to highlight the gyre location. Make sure that this is explicit somewhere.\nMethods\n- You mention that you followed a drift sampling strategy, but without a more detailed description it is not straightforward to\nguess what it exactly means and what are the advantages of this approach for your study. You could expand on the\nsampling strategy a bit\n-Line 219. What do you mean by modifications in latitude\n-Line 221-223. unclear, did you take surface water samples to assess the drift in the SUNA sensor? How did the sensor\nperformed? It would be good to have some more information. Also, could you use these samples to reinforce your message\nof nutrient upwelling on the lee side of the sill?\n-Lines 124 onwards. Some more details would be welcome. How many profiles? How many shear sensors have you got?\n-Nitrate density – fits. Do you do the fitting for each profile, or all profiles combined? In the main text you suggest significant\nnitrate changes along isopycnals, how does that impact this estimation? (probably not a lot, but just checking).\n-SADCP. Also some more details could be useful: What was the configuration, in terms of bin size, segment averaging etc?", "reviewer_2": "This study makes excellent use of a dataset comprising of 1) long-term high frequency radar measurements of surface\ncurrents and divergence and satellite measurements of chlorophyll a together with, 2) shipboard measurements of turbulent\nenergy dissipation to reveal that the wide expanse of chlorophyll a within the Tsugaru Gyre, is tied to the extremely high\nvalues of vertical diffusive nitrate fluxes caused by the presence of a Shiriya Spur seamount in the Tsugaru Strait. On the\nbasis of statistical analysis the authors were able to show that there was a strong correlation between long-term high-\nfrequency radar measurements of surface divergence downstream of the sea mount and long-term satellite measurements of\nchlorophyll a. Although the measurements are high-quality, and the findings interesting, I couldn’t find anything significantly\nnovel in this manuscript that would merit its publication in Nature Communications. There have been several previous\nreports on the role of seamounts from elsewhere as contributing to areas of elevated biological productivity. From within the\nTsugaru Strait, some of the authors of this manuscript as well as others have shown that presence of sea mounts are\nresponsible for excessive increases in turbulent energy dissipation rates along the path of the Tsugaru current and\nconsequently nutrient injection into the euphotic column and the elevated chlorophyll concentrations downstream in the\nTsugaru Gyre. Perhaps the only novelty in this study is the strong correlation between surface divergence immediately\ndownstream of the seamount and surface chlorophyll a concentration. Despite this interesting observation, I found the\ndiscussion of the results very weak, in particular, the few lines addressing the lack of correlation between surface divergence\nupstream of the Shriya Spur seamount and sea surface chlorophyll in the Tsugaru Gyre seemed cursory at best. Although it\nseems clear that stratification downstream is essential to phytoplankton growth. I also note that the discussion lacked any\nmention of the potential role of the anti-cyclonic Tsugaru Gyre in accumulation surface chlorophyll. Finally the section\ndescribing the extrapolation of shipboard nitrate fluxes over the region of intense divergence to estimates of carbon fluxes,\nand the estimates of satellite measurements using surface chlorophyll a measurements to be over-simplistic. For a journal of\nthis repute, I would expect a more rigorous evaluation of both carbon fluxes and net primary productivity especially using an\napproach that views this hot spot of chlorophyll a as a continuous algal culture system. which is receiving pulses of nutrients\ncontinuously that sustains the anti-cyclonic Tsugaru Gyre’s elevated chlorophyll a. If the authors intend resubmitting, they\nshould restate as to what makes their results different from those reported for other seamounts, what makes the findings in\nthis manuscript different from those reported earlier for the Tsugaru Current, the role of the Tsugaru gyre and stratification in\nthe build up of chlorophyll a", "reviewer_3": "Review of the manuscript \"Vigorous vertical mixing supports biological production in the mesoscale Tsugaru Gyre\"\nThe paper presents a case in which observational data were used to suggest that during the summer, when the Tsugaru\nStrait throughflow intensifies, turbulence over the Shiriya Spur and the associated nutrient supply rate significantly increase.\nThe supplied nutrients are then transported downstream resulting in enhanced primary production across a broader region\nwithin the Tsugaru Gyre. The study highlights the critical role played by the divergence field in nutrient supplies and the\ntransport mechanisms within the Tsugaru Gyre. Since this understanding holds great importance for sustaining fisheries and\nmarine ecosystem conservation, I believe the broader scientific community would benefit from this report including\noceanographers, fishery scientists, etc. My comments are as follows.\n1. The presentation of results requires substantial revision to enhance clarity and consistency. One prominent issue is the\ninconsistency in the use of terminology. The terms \"rear of the sill\", \"back of the sill\", \"behind the sill\", and others, were\nemployed without clear specification of whether they refer to the same concept or different concepts, and this causes\nconfusion. To make the research more accessible to a broader audience, I would recommend using simpler and more easily\nunderstandable terms, such as \"east of the sill.\" The headings in the results section as well should offer clear and concise\ndescriptions of the content being introduced (please refer to the details).\n2. Furthermore, the discussion is notably lacking. While the research introduces several critical figures and concepts, it falls\nshort of providing an in-depth discussion beyond that of correlation maps, which are insufficient to effectively elucidate the\nproposed mechanisms. The advection of nutrients is controlled not only by surface currents, but also by factors such as\nphytoplankton assimilation rate, etc. The concept of the Froude number and its association with the idea of an \"internal\nhydraulic jump\" was also introduced. However, no adequate discussion of what these concepts mean in the context of this\nstudy was given. A more in-depth discussion of these critical elements is necessary.\n3. The methods section lacks essential details. For instance, it is unclear which type of correlation analysis was conducted\nto examine the relationship between chlorophyll-a and the surface velocity divergence. Moreover, the treatment of\nchlorophyll-a data remains unspecified. Satellite-derived chlorophyll-a is prone to missing pixels, often due to cloud cover,\nespecially at the temporal scales used in the correlation, but it was not addressed how this was handled in the correlation\nanalysis.\n4. The quality of the observations was also not thoroughly discussed. For instance, while calibration data for nitrogen\nmeasurements was collected, the paper omits its presentation. Lines 238-239 mention the adjustment of nitrate profile offset\nwith reference to bottle data, but it lacks clarity regarding why this adjustment was necessary and how it was carried out.\nProbably this section could benefit from a clear separation between data and methods.\nDetails:\n21-24: the sentence needs to be improved to simplify the flow.\n95: It does not follow from the above statement that the high chlorophyll-a was caused by turbulent mixing.\n110: \"...gradually shoaled,...\"?\n111-115: The sentence needs revision to improve the flow and clarity. I think the point being made is that, “while a similar\nphenomenon has been observed off Cape Tappi, it is noteworthy that the magnitude of this phenomenon was significantly\ngreater in the case of Shiriya Spur”?\n112: It would be more precise to use terms like \"west of the summit\" or \"east of the summit\" instead of \"behind\" in this\ncontext, as the latter term is unclear and ambiguous.\n119: Near the topography?\n125: levels of vertical nitrate flux were immediately behind the sill?\n132: In a geographic context, using cardinal directions namely north, south, east, and west is indeed more intuitive and\nspecific compared to the term \"rear,\" which can be vague and open to interpretation. So, to provide clear and unambiguous\ngeographical references I’d advise to stick to the above cardinal direction.\n137: How significant was this difference from 1? Notably, this number exhibits a decrease in an eastward direction,\ncoinciding with the areas where intense mixing was observed. It would be beneficial to provide a brief explanation of how it\nis derived and offer additional context, as it plays a crucial role in supporting the claims made in this paper. Later in the\nmethods section, the paper describes its calculation in more detail, but a concise overview and context would enhance the\nreader's understanding of its importance.\n139: What does “internal hydraulic jump” mean in this context?\n147, 166: Should be revised to provide a clear and concise description of these paragraphs.\n183: What is meant by the “concept of nutrient uptake” in this context? Typically, it describes the process in which\nmicroorganisms, such as phytoplankton, absorb nutrients from their surroundings to facilitate growth and metabolic activities.\nHowever, it appears that this definition may not apply here.\n195: I would suggest removing the term “back of the sill”. The given longitude and latitude ranges provide more precise\ncontext than the above term, which is ambiguous and can lead to potential confusion.\n198: It's important to make a distinction since the carbon flux referred to here is potential. The estimates are based on the\nrate of nitrogen supply and not the actual nitrogen uptake rate by phytoplankton.\n199: What is the relevance of the speed or mass advection in the estimate of primary production? It appears only chlorophyll-\na concentration is important. It would be more intuitive to clarify what the coefficients “a” and “b” are and how they are\nobtained.\n203: Providing context of background concentration\n216: Was water sampling performed only “at the start and at the end”, or was it conducted “from the start to the end”?\n222: Surface water samples collected at approximately every alternate location were primarily used to calibrate the nitrate\nsensor? What other parameters were measured during these water samplings, and at what frequency were they collected?\n227: “The calculation method...” for what?\n234: Like the other parameters in the equation, Γ should also be accompanied by a brief description.\n245: Please provide a context as to why this approach was adopted. Adding the reason help readers understand the\nmotivation behind the methodology.\n249-251: The sentence contains a lot of information and is difficult to follow. First, “flux” here refers to nitrate flux? There is no\nmention of nitrate correction until “corrected nitrate.” What correction is this referring to? Second, was the 30-m running mean\nfiltering applied along the entire transect within the surface region? To improve clarity, please break down the sentence.\n253: Oh, so the filtering was along the nitrate vertical profile!\n254: What does “downstream of the topography” refer to or mean in this context?\n259: Briefly explain what the “correction for misalignment” does. Additionally, it is not clear how the “percent” was estimated\nin this case.\n280: 8-day resolution data was used in Fig. 1c. What was the purpose of 1-day resolution data?\n292: What would be considered the \"observed\" bottom layer in this context? Would this be the bottom layer to which a\nmeasurement instrument descended to or simply the water depth.\n294: The manuscript referred to appears to be about the “light-saturated photosynthetic rates” and not primary production.\nPlease verify. Also, probably the coefficients were derived in a specific site. Are they applicable to the region of this study?\n519: What “reference numbers” are those?\n520: indicate the nutrient flux in ... regions?\n527: What do the vectors indicate?\n532: The criteria for defining the spatial average of the surface divergence is not clear. The area average boxes have\ndifferent sizes. How were they defined? Probably a Hovmöller plot of divergence and chlorophyll-a would provide additional\ninsights."}, "author_response": "R01-G01:\n-while the correlation between flow divergence and chlorophyll downstream is very\npromising, I feel the analysis is still quite shallow and leaves many questions open\n(some of which I detail below). It would be interesting to understand better the\nspatiotemporal dynamics of the flow divergence before going further. From Fig. 3 it\nappears that divergence is highly variable in space and in very short time scales. It left\nme wondering how the biological response couples with such high frequency\nfluctuations and whether there are lower frequencies as well that are dominating the\ndivergence variability (I think I am calling for a frequency analysis here). Another key\nissue, is that no lagged correlations were investigated, and it appears logic to me that\nthere must be some lag between the nutrient supply at one point, and the biological\nresponse downstream. Is the flow that fast that this is not relevant? Maybe this is what\nthe authors intend to convey in their last paragraph but it wasn’t entirely clear.\nResponse:\nThank you sincerely for your thorough and constructive feedback. In light of your\ncomments, we have incorporated additional analyses to address your inquiries. We have\nincluded details on the spatiotemporal distribution of the flow field (refer to newly\nadded Fig. 3) and conducted a wavelet analysis of the surface current divergence in the\narea of interest (see newly added Fig. 4). Additionally, we have performed a lag\ncorrelation analysis of surface divergence and chlorophyll-a (Figs. 6 and 7). We have\nalso included wavelet relationships between this divergence and the area-averaged\nchlorophyll-a concentrations in 18 subregions within the Tsugaru Warm Current (Fig.\n8).\nThe updated manuscript now features an illustration of the mean surface current\nvelocity distribution during the summer period (August to September) from 2014 to\n2022. It confirms that the background current (Tsugaru Warm Current) flowing through\nthe Tsugaru Straits consistently exceeds the seafloor topography just offshore of Cape\nShiriya as an average state over time. We used this background as the basis for selecting\nthe area in which to calculate the divergence field of surface current velocities\n(41.65°N–41.70°N, 141.60°E–141.65°E).\nThe wavelet analysis (as depicted in the newly added Fig. 4) clearly illustrates\ndiurnal and semidiurnal variations in the divergence of surface current velocities, which\nwe believe correspond to tides. Furthermore, the findings suggest that subinertial\nfrequency variations (ranging from a few days to 10 days) tend to intensify during the\nsummer and fall seasons. This phenomenon may align with the north–south variation in\nthe transit latitude of the Tsugaru Warm Current, as demonstrated in our previous study\n(Kaneko et al., 2022; https://doi.org/10.1186/s40645-022-00509-z). Specifically, the\nTsugaru Warm Current typically flows over the terrain on average but exhibits\navoidance behavior on the mentioned time scales with periodic north–south shifts.\nFig. 9 in Kaneko et al. (2022). Example of shorter-term variation in “the axis”\n(indicated as lines) of the Tsugaru Warm Current estimated from the high-frequency\nradar during summer.\nUsing high-pass filtered data, we further explored the correlation between surface\nvelocity divergence and chlorophyll-a distribution. We observed that, in this case, the\ncorrelation was notably significant only in a narrow region adjacent to the topography\n(refer to Supplementary Figure 6). Building upon these results, we theorized that the\nsubinertial periodic variation of the divergence field, synchronized with the advection\ntime scale (approximately one week for a half-circumnavigation), could be linked to the\npositive correlations observed over a broad area in the Tsugaru Gyre (as shown in Fig.\n6a). The detection of remarkable wavelet coherence in subinertial frequencies within the\nTsugaru Gyre during August and September (Fig. 8) would be construed as evidence\nsupporting this notion.\n\nR01-G02:\n-I feel the conclusion in Lines 122-124 that the diapycnal nutrient flux is visible in the\nnitrate distribution is not entirely convincing as it is presented. I am not saying is not\nhappening, it probably is, but the data could be presented in a more compelling way.\nResponse:\nThank you very much for your suggestion. We have incorporated the information\nregarding the distribution of nitrate on the isopycnal surface as shown in Fig. 2e.\n\nR01-G03:\n- I find the text a bit confusing in many placing and needs substantial improvement for\nclarity. I have made a number of suggestions on that regard below.\nResponse:\nThank you for bringing this to our attention. We have addressed and corrected\nindividual revisions based on the comments provided. Additionally, since the authors\nare not native English speakers, we have utilized an English proofreading service (AJE,\nhttps://www.aje.com/) to ensure that the wording is more suitable. However, we would\ngreatly appreciate it if you could reiterate any points that may still be difficult to\nunderstand. Thank you once again for taking the time to review and provide feedback.\n\nR01-G04a:\n- There is a very nice discussion on how the enhanced turbulence impacts the\nproductivity in the Tsugaru gyre, but I feel that, in order to publish the manuscript in\nNature Communications, further efforts are done to highlight the broader relevance\nof the paper. There are a couple of very general sentences in the abstract and conclusions,\nbut I feel the message could be made stronger with a more focussed dissertation:\nhighlight more regions that could experience similar dynamics and discuss their\nrelevance for the large scale production of subtropical gyres, e.g. highlight the\ndisproportionate contribution of the Tsugaru gyre to carbon export in the subtropical\nPacific with respect to surface area.\nResponse:\nThank you for your valuable comments. In response, we have included an\nestimation of production within the Tsugaru Gyre in the second paragraph of Results\nsection. As previously mentioned in our correspondence with the editor, we have\nconducted two estimations: one using an empirical equation based solely on surface\nchlorophyll-a concentrations, and the vertical generalized production model (VGPM,\nBehrenfeld & Falkowski, 1997; https://doi.org/10.4319/lo.1997.42.1.0001) that\nconsiders additional factors such as local light environment and sea-surface water\ntemperature. We believe that both estimates are comparable in magnitude and closely\naligned. We have also compared these estimations with values from previous studies\nconducted in other ocean regions, such as the western North Pacific subtropics by\nShiozaki et al. (2009) (https://doi.org/10.3354/meps07837) and the subarctic by Imai et\nal. (2002) (https://doi.org/10.1016/S0967-0645(02)00198-4). These comparisons\nindicate that primary production within the Tsugaru Gyre is relatively high.\nFurthermore, we contend that production within the Tsugaru Gyre can be characterized\nas stable, exhibiting minor fluctuations from year to year (refer to the newly added Fig.\n1d). Therefore, we are confident that we have effectively addressed your feedback.\n\nR01-G04b:\nAnd also, in the abstract you mention sustainability and marine conservation. This\nsounds a bit too broad, unless you mention long term changes in the current across the\nstrait and the importance of monitoring it or something like that.\nResponse:\nThank you for bringing this to our attention. We have made significant revisions to\nthe abstract to accommodate the word limit, which was imposed because the manuscript\nyou previously reviewed was transferred from another Nature journal. As a result of the\nword limit in Nature Communications, we were unable to address the issue of marine\nconservation directly in the present abstract. Instead, we have briefly summarized it\nwithin the context of the impact of submesoscale nutrient supply.\nWhile it is possible to speculate that variations in the intensity and changes in the\npath of the background flow (i.e., Tsugaru Warm Current) might also play a role in the\ndivergence east of the spur, the precise nature of the different timescale variations of the\nTsugaru Warm Current remains uncertain. We are currently exploring other studies,\nsuch as investigating the impact of decadal-scale variations on the Tsugaru Warm\nCurrent. Looking ahead, we aim to investigate whether the diverse timescale variations\nmight affect production within the Tsugaru Gyre through the key area of nutrient input.\n\nR01-G05:\n- Some more details would be beneficial about some parts of the methodology\nResponse:\nThank you for your comments. We will provide detailed explanations of the\nmodifications in separate comments below. We have included the number of turbulence\nsensors in TurboMAP, sampling details for the correction of the SUNA nitrate sensor\n(additional information has also been added in Supplemental Table 1), and details of the\nSADCP data acquisition settings.\n\nDETAILED COMMENTS\nAbstract\n\nR01-D01:\nLine 21-24: “Nutrients transported vertically in specific hotspot areas that reach the\nsurface can be conveyed downstream by background currents, thereby augmenting\nsurface biological production over a wider expanse, including robust currents traversing\nseamounts.”, sentence a bit convoluted/confusing. Suggest rewriting\nResponse:\nThank you for your feedback. As noted in the general comments section above, we\nhave significantly revised the abstract to align with the new word limit for abstracts,\nfollowing our transition from another Nature Journal. We appreciate your\nunderstanding.\n\nR01-D02:\nLine 32. Suggestion: “A surface divergence field” -> “a divergent surface flow field”\nResponse:\nWe adjusted the wording as per your instructions initially, but during the subsequent\nreorganization, some of the phrasing had to be altered due to the character limit\nmentioned above. We appreciate your understanding, again.\n\nR01-D03:\nLine 34. Suggestion: “Formation area” -> “area”\nResponse:\nDue to the aforementioned word limit, we have omitted this sentence in our\norganization. We trust you comprehend our decision.\n\nR01-D04:\nLine 36. Last sentence is a bit too general and lacks real substance. It would be OK if\nyou explored further the implication for ecosystem management in the manuscript, but\nnot really justified at the moment\nResponse:\nThank you for your feedback. As mentioned in the general comments section above,\nwe have substantially revised the abstract to streamline the text in accordance with the\nupdated word limit for abstract. We appreciate your understanding.\nIn the revised abstract, we have refined our assertion to propose that the\nsubmesoscale nutrient injection identified in this study could instigate a reevaluation of\nnutrient supply routes in similar locations. Investigating such injection at the\nsubmesoscale level could prove valuable as a monitoring point for potentially bolstering\nmesoscale primary production.\n\nIntroduction\n\nR01-D05:\nIntroduction. The first paragraph sets up the general relevance but it entirely lacks of\nreferences. Some reference would increase the credibility\nResponse:\nThank you for your highly valuable and significant remarks. We have incorporated\nthe pertinent references and made revisions to the first paragraph of the Introduction\nsection.\n\nR01-D06:\nLine 52. “[...] the Tsugaro gyre” could be added at the end of the sentence for clarity\nResponse: We have made the correction as you suggested.\nPlease refer to the 2nd paragraph of the Introduction.\n\nR01-D07:\nLine 61. “higher distribution” -> “higher values/concentration”?\nResponse: We have addressed the issue as you indicated.\nPlease refer to the 2nd paragraph of the Introduction.\n\nR01-D08:\nLine 68. In the paper you report nitrate flux in mmol/m2/s, but here is per day. It would\nbe good to make the units more uniform to make comparisons easier\nResponse:\nThank you for bringing this to our attention. We have updated the description to\nensure clarity in both units whenever feasible. However, we feel it is important to\nacknowledge that the observed values in our study exhibit significant variability, and\ntranslating them into average values over a day is an assumption. We have articulated\nthis notion in the \"Discussion\" section, contrasting it with primary production as daily\nfluxes.\nPlease also refer to the 2nd paragraph in the “Turbulence over the spur and associated\ndiffusive nitrate flux”, and the 1st paragraph of the Discussion.\n\nResults\n\nR01-D09:\nLine 95. “the similar localized increase”, unclear phrase\nResponse:\nThank you for bringing this to my attention. I have clarified the area and removed\nthe word, \"SIMILAR\".\nPlease refer to the final sentence in the 1st paragraph of the “General situation at the\ntime of the survey”\n\nR01-D10:\nLine 103 and after. “the isotherm”, which isotherm? All isotherms? In general lines\nResponse:\nThe expression “the isotherm” has been revised to emphasize isopycnal surfaces\nwith specific values for their description.\nPlease refer to the “Turbulence over the spur and associated diffusive nitrate\nflux“ Section\n\nR01-D11:\nLine 116. It is confusing that you have focused on the temperature description above,\nand here you emphasize comparing nutrients with density instead of temperature. It\nwould be simpler to pick one, since, I guess, they must be closely related\nResponse:\nAs mentioned earlier (R01-D10), the description has been adjusted to concentrate on\nindividual isopycnal surfaces, while also incorporating specific values from the\nisotherms.\nPlease refer to the \"Turbulence over the spur and associated diffusive nitrate flux\"\nsection.\n\nR01-D12:\nLine 122 – 124. It is hard to verify this conclusion by looking at the nitrate plot here,\nsince nitrate is not shown together with density, so it is difficult to tell whether there\nwas a significant impact of diapycnal flux on nitrate concentration. Further, “similar\ndepths displaying different nitrate concentrations does not necessarily imply a diapycnal\nflux, if isopycnals are not flat. You could modify the plots slightly to get the message\nacross more effectively. Perhaps adding a subplot with nitrate+density could help.\nI also suggest (see below), maybe putting the last panel in an independent figure. Your\nconclusions may be reinforced by performing some along stream nitrate budget /\nadvection diffusion balance: knowing the flow speeds and the nitrate evolution along\nthe stream, you may infer a diapycnal flux that you could compare with your\nmeasured flux. I am thinking of something along these lines:\nhttps://doi.org/10.1029/2007GL032785. I am not sure if this is going to work (possibly\nnot, due to unsteadiness), but may be worth having a try.\nResponse:\nThank you for your valuable feedback. In light of your comments, we have initially\nincluded a figure panel as Fig. 2e depicting the distribution of nitrate on the isopycnal\nsurface along the east–west observation line. We anticipate that this figure will enhance\nour comprehension of elevated nitrate concentrations on isopycnal surfaces to the east\nof the topography.\nWe have also taken into account the comments and the previous study you\nrecommended, investigating the balance between advection and diffusion in the east–\nwest direction, specifically uN/x and /z(K N/z). Regrettably, we did not achieve\nz\nconclusive results indicating closure in these terms alone. As previously mentioned by\nyou, the imbalance primarily will stem from issues related to unsteadiness and the\nchallenge of assessing advection effects in the north–south direction using the outcomes\nof this observation. Additionally, the Sharples method is tailored to extract nutrient\ngradients closer to the background field [i.e., ideally, /z(K N/z)], hence it cannot\nz t\nfully assess the impacts of short-term changes in nutrient gradients [i.e.,\n/z(K N’/z)].\nz\nTo more accurately evaluate such a balance, conducting more targeted observations\n(such as numerous repetitive measurements of turbulence intensity on parallel sidelines)\nis crucial. However, resolving phenomena with significant temporal variability (like\nturbulence and tides) in a rapidly flowing area poses considerable challenges. We hope\nyou appreciate this aspect.\nInstead, we utilized the average nitrate turbulent vertical flux on the isopycnal\nsurface F and the average depth of the isopycnal surface z to assess the trend of\n \nvertical convergence of the fluxes (F /z ), and to compare the distribution pattern\n \nof the east–west gradient of nitrate on the isopycnal surface (N /x). This comparison\n\nhas been illustrated in Supplementary Figure 2.\nWhile we did not consider the advection velocity of the background flow, we\nobserve a trend towards a positive nitrate gradient (indicating higher concentration in\nthe eastern, downstream area) just east of the summit of the topography (24.5–25.0,\n\n141.59°E–141.62°E, particularly in the flux convergence region (Supplementary Figure\n2).\nThe previous Fig. 2f has been reassigned as Supplementary Figure 3 due to the\nnumber of panels.\n\nR01-D13:\nLine 128-129. This sentence is somewhat repetitive\nResponse: Thank you for your comment. We have revised this sentence.\nPlease refer to the 2nd paragraph in the \"Turbulence over the spur and associated\ndiffusive nitrate flux\" section.\n\nR01-D14:\nLine 131. The reporting of the flux values is confusing. Try expanding the sentence.\nResponse:\nThank you for your comment. We have also addressed the issue as you indicated.\nPlease refer to the 2nd paragraph in the \"Turbulence over the spur and associated\ndiffusive nitrate flux\" section.\n\nR01-D15:\nLine 134-139. You could maybe explain to the readership of NC what are the\nimplications of the existence of a hydraulic jump. It may not be obvious for some of\nyour biologically-oriented readership.\nResponse:\nThank you for your significant comment. We have included explanations of the\nhydraulic jumps.\nPlease refer to the 3rd paragraph in the \"Turbulence over the spur and associated\ndiffusive nitrate flux\" section.\n\nR01-D16:\nLine 142. I think you should say that the HFR helps you support the assumption that the\nzonal divergence is representative of the three-dimensional structure of the float.\nHFR divergence analysis. Fig 3 depicts very substantial temporal variability of the flow\ndivergence. I am not very familiar with the HFR, it would be good to know how\nconfident you’re with this patterns and what is the instrumental noise level for\ntheir detection. If they are robust, you could comment a bit on the significance of the short\nterm variability. Before proceeding to the correlation with chlorophyll it would be\ndesirable to better understand the spatiotemporal dynamics of the flow\nconvergence/divergence: how flow convergence varies in time and space and how\nconsistent and persistent are the patterns described during the cruise. Maybe some EOF\nor time-series analysis could be presented, in the main text or as SI. It would be useful\nalso, some discussion about what drives those changes.\nResponse:\nThank you for your valuable feedback.\nFirstly, we noted the prevailing dominance of east–west flow velocities. However,\nwe have also included the following statements: “In reality, the north–south velocity\ndistribution may also affect the divergence field and the vertical structure of the density\nsurface. Therefore, we investigated the two-dimensional divergence field of the surface\nvelocities obtained by the HFR system”. (Please refer to the final three sentences in the\n\"Turbulence over the spur and associated diffusive nitrate flux\" section).\nA Hovmöller spatiotemporal analysis (Fig. 3) was employed to characterize the\neast–west flow and the divergence of the surface flow within the area of interest. \nAdditionally, a wavelet analysis (Fig. 4) was utilized to provide further insight into the\nspatiotemporal variability in the divergence.\nWe believe that these additional results provide the reader with more compelling\ninsights into why we focused on the divergence of surface velocities at this location.\nFurthermore, it appears evident that the diurnal and semi-diurnal variations due to tides\nare significant. In other words, the substantial variation in divergence observed during\nthe drift period could be attributed to these tides rather than noise from the HFR. The\nmean signal-to-noise ratio of the HFR system at the observation site during the study\nwas greater than 5, surpassing the threshold proposed by previous studies (Kirinchich et\nal., 2012, https://doi.org/10.1175/JTECH-D-11-00160.1; Hains et al., 2017,\nhttps://doi.org/10.1175/JTECH-D-16-0203.1).\nPlease note that the comparison between the field exhibiting short-term variations\nand the internal structure of the ocean is aimed at illustrating a systematic relationship\npattern between turbulence intensification and divergence field of the surface current\nvelocity.\nIn addition to these short-term variations, we contend that the behavior of the\nlonger-period Tsugaru Warm Current and the heightened divergence of surface currents\nat subinertial frequencies, as outlined in the general comment, aligns with our assertion\n(Figs. 4 and 8). Specifically, when the Tsugaru Warm Current intersects the Shiriya\nSpur at this frequency, it boosts surface nutrient supply in the submesoscale region,\nleading to increased divergence of surface currents on the eastern side of the\ntopography, consistent with the relationship (spatial pattern) between the current and the\ntopography during the shipboard expedition. We believe this synchronicity at the\nfrequency further substantiates the validity of our hypothesis, indicating elevated\nnutrient supply in the submesoscale region with divergence of surface current east of the\ntopography.\n\nR01-D17:\nLines 158-161. Sentence not clear. The link between radar flow and nutrients is not\nobvious. Also, what do you mean by the \"transport of nutrient supply\"? Do you mean\nthe transport of recently upwelled nutrients or something on this line?\nCorrelation analysis. This is a very interesting promising thing, but I think it should be\nexplored in more depth. First thing, have you tried lagged correlations? I am surprised\nof this direct correlation with no lag, when your results imply that there is an advective\ntime-scale for this response (the time it takes for the waters with the upwelled nutrients\nto reach a certain point).\nIn general, I feel that there is a lack of understanding of the underlying spatio temporal dynamics of the physico-chemical\ncoupling. Would something like a wavelet coherence analysis help? Maybe it is too complex. I would\nencourage the authors to provide some more insights with the approach they consider\nmost useful. This analysis would strengthen conclusions like those in sentences 178-\n182. Also it is unclear why the convergence zone shows no correlation, I would assume\nthat the convergence-divergence patterns are linked to each other. Could you expand a\nbit on that? If panel b is not helping you, you may as well dropped, but now that you\nshowed it I am intrigued by it.\nResponse:\nThank you very much for your invaluable comments. We clarified the concept that\nthe divergence field of surface current velocities east of the Shiriya Spur, as detected by\nthe HFR system, could serve as an indicator of vertical nutrient supply through vertical\nmixing, supported by the observed correspondence (as mentioned in the final paragraph\nof the \"Spatial and temporal changes in the surface velocity field around the Shiriya\nSpur\" section). Although horizontal transport through advection is not addressed here,\nwe discuss its effects in a later section concerning lag correlation analyses in the revised\nmanuscript.\nThe correlation analysis in the previous manuscript was considerably simplified due\nto space constraints. However, in response to reviewer comments, we have endeavored\nto enhance the analysis by incorporating lag correlations. Initially, we included 7-day\nlag correlation maps as shown in Fig. 6c (and Fig. 6d). Subsequently, we partitioned the\nTsugaru Gyre into 18 subregions and showcased their lag correlations in Fig. 7. This\ndivision into subregions along with spatial averaging, aims to mitigate the impact of\npotential chlorophyll-a data gaps. Furthermore, we investigated the distribution of\ncorrelation coefficients derived from the high-pass filtered data (see Supplementary\nFigure 6). We have also included wavelet relationships between the divergence and the\narea-averaged chlorophyll-a concentrations in the 18 subregions within the Tsugaru\nWarm Current (Fig. 8).\nWith the inclusion of the lag analysis shown in Fig. 7, we observe that correlations\ntend to be high in regions with short lag times (~ 0 day) in the waters east of the Shiriya\nSpur (e.g., R04 and R05 in August). Additionally, peaks or notable correlations with lag\nare noted in R08 and R11 at and 6–7 days in August, and in R08 and R11 at 5–6 days in\nSeptember. Additional high-passed correlations (Supplementary Figure 6) revealed that\nthe region of notable correlation was predominantly centered around the Shiriya Spur,\nwith no widespread correlations evident in the region of 41.0°–41.5°N, 142°E–143°E of\nFig. 6a. These outcomes lead us to infer the lag attributed to advection.\nWe included wavelet analysis for the divergence of surface current velocities in the\narea east of the Shiriya Spur (Fig. 4). Considering that signal enhancements in the\nsubinertial range (spanning from a few to 10 days) typically occur during summer and\nautumn (Fig. 4c), the broad range of correlations at small lag (Fig. 6a) is attributed to\nsynchrony between the timescale of advection (~7 days assuming a 150 km diameter for\nthe Tsugaru gyre and a mean current velocity of 0.4 m s1) and the periodic\nenhancement of divergence in the immediate eastern region of the spur. We believe that\nthis increased divergence at subinertial frequencies is a result of the north–south\nmovement of the Tsugaru Warm Current, as previously noted. The notable wavelet\ncoherence signals between the divergence and chlorophyll-a observed in the Tsugaru\nGyre at subinertial frequencies, as depicted in Fig. 8, will further substantiate this\nconcept.\nWe are unsure about the specific time scale at which the biological response initiates\ndue to the absence of observations on this cruise, but we anticipate it to occur between\none and several days after the nutrient supply takes place.\n\nDiscussion\n\nR01-D18:\nLine 183. Confusing sentence. Turbulence does not “uptake” nutrients, it supplies\nnutrients which are then taken up by plankton.\nResponse:\nThank you very much. You are absolutely right; we have amended it to \"supply\"\nalong with R01-D20.\nPlease refer to the 1st paragraph in the Discussion section.\n\nR01-D19:\nLines 185-190. I understand what you mean here, but I think you could use a more\ndirect language to make the point clearer. Something along these lines: “the\nthroughflow advects water that is poorer in nutrient compared with the generally\ndominant subarctic waters”\nResponse:\nFollowing your comments, this section has been revised and moved to the 1st\nparagraph of the \"Divergence east of the spur and chlorophyll-a variation in the\nsummertime Tsugaru Gyre\" section in order to make the text more concise.\n\nR01-D20:\nLine 192. “uptake” -> transfer or supply\nResponse:\nThank you for your feedback. This has also been rectified, along with R01-D18.\nPlease refer to the 1st paragraph in the Discussion section.\n\nR01-D21:\nLine 198. Not sure if “carbon flux” is an accurate term here, I would rather say it this\nthe new production of organic carbon that could be sustained with the influx of nutrients\n(and will potentially become a downward flux of organic carbon)\nResponse:\nThank you for your feedback. We consider that over an extended period, it could be\nviewed as a sinking flux (carbon sequestration from the surface), but we suspect that it\nis likely advected near the surface within a certain timeframe. However, at the present\nstage, assessing the quantitative aspects of new production, export, and advection seems\nchallenging. In the revised manuscript, our initial approach involves estimating primary\nproduction based on surface chlorophyll-a concentrations using two different models.\nSubsequently, we convert it to nitrogen content using the Redfield ratios and compare\nthis to the vertical turbulent nitrate fluxes estimated through observation.\n\nR01-D22:\nLine 202-203. you may as well make a, b values explicit there, but it would be\nimportant also to say where this model comes from and whether is known to apply\nlocally\nResponse:\nThank you for your invaluable feedback. It is mentioned in the Method section that\nthese coefficients are empirically determined. Additionally, another model (VGPM) was\nemployed to estimate primary production in the Tsugaru Gyre, taking into account local\nsea-surface water temperature and light conditions. We have also rectified the incorrect\nreference cited in the previous manuscript, as it should have been Berger (1989). Our\napologies.\nIn light of your comments, we have introduced these models and the level of\nprimary production within the Tsugaru Gyre in the second subsection in the Results,\ncomparing it to other oceanic regions.\n\nR01-D23:\nLine 205. “about daily advection” is unclear, maybe something like “though horizontal\nadvection over the course of one day”\nResponse:\nThank you very much for your feedback. Concerning the discussion section, we\nfound that the previous manuscript became overly complex due to numerous\nassumptions regarding quantification of the vertical turbulent nitrate fluxes, estimation\nof the area where nutrient inputs to the surface layer occur, and distances due to\nadvection. As a result, the revised manuscript adopts a simpler approach, employing a\ncomparison method based on per unit area values. Consequently, this portion has been\nomitted. Thank you for your understanding.\n\nR01-D24:\nLine 296. From “reproduction” does not make sense to me. Do you mean regenerated\nproduction?\nResponse:\nWe have identified the part you mentioned as L206. While we have made\nsubstantial revisions to the \"Discussion\" section as outlined earlier, we have also\naddressed the language you highlighted, changing it to \"regenerated production\".\nPlease refer to the 2nd paragraph in the Discussion.\n\nR01-D25:\nConcluding remark and relevance. The mention to a single other place in the globe feels\na bit insufficient here for a generalist journal. Would it be able to expand a bit on the\nbroader relevance? E.g. showcase other locations with potentially similar structures?\nResponse:\nIn accordance with your suggestion, we have updated the text by incorporating\nfindings from other oceanic regions, such as the Mediterranean Arboreal Gyre, the Kuril\nIslands eddy, the Kuroshio and Kuroshio Extension currents and their recirculation, and\nthe Antarctic Circumpolar Current.\nPlease refer to the 3rd paragraph in the Discussion.\n\nR01-D26:\n-Figure 4. You show potential vorticity contours there, but I can recall seeing a\nreference in the text as to why those are there.\nI guess is to highlight the gyre location. Make sure that this is explicit somewhere.\nResponse:\nWe have included a description and rationale for relative vorticity in the Methods\nsection. Furthermore, we have incorporated a velocity vector into this figure, which\ncorresponds to Fig. 6 in the revised version.\nPlease also refer to the 1st “Divergence east of the spur and chlorophyll-a variation in\nthe summertime Tsugaru Gyre” section.\n\nMethods\n\nR01-D27:\n- You mention that you followed a drift sampling strategy, but without a more detailed\ndescription it is not straightforward to guess what it exactly means and what are the\nadvantages of this approach for your study. You could expand on the sampling strategy\na bit\nResponse:\nWe have included explanations of the significance of the drift sampling in the text.\nPlease refer to the 4th paragraph in the Introduction and the 3rd paragraph in the\n“Chlorophyll-a distribution and estimated primary production within the Tsugaru\nGyre” section.\n\nR01-D28:\n-Line 219. What do you mean by modifications in latitude\nResponse:\nExplanation has been added in the Method section (3rd paragraph). This entails\nadjusting the starting latitude of the drift to be more northerly than originally planned,\nbased on information obtained from the Tsugaru Warm Current Axis Latitude Survey\n(Fig. 1c will also help your understanding).\n\nR01-D29:\n-Line 221-223. unclear, did you take surface water samples to assess the drift in the\nSUNA sensor? How did the sensor performed? It would be good to have some more\ninformation. Also, could you use these samples to reinforce your message of nutrient\nupwelling on the lee side of the sill?\nResponse:\nAs you pointed out, we utilized it to assess sensor offsets, so we have included that\ninformation. Details such as sampling latitude and longitude have been summarized in\nSupplementary Table 1. Analysis of surface water samples revealed a more pronounced\ntrend of increasing SiO2 concentrations east of the summit, while the distribution of\nNO3 exhibited a consistent trend but with a weak signal at the surface, which is not\nelaborated upon in this study.\nFurthermore, while this manuscript does not delve into it, the root mean square error\nbetween the six CTD water samples and the offset-corrected profiles obtained from\nSUNA near the Shiriya Spur was approximately 1.2 uM during the same voyage.\nIn the preceding manuscript, details concerning the spatial dispersion of nitrate from\nthe carousel water sampling (41.60°N, 141.41°E and 41.69°N, 141.72°E) were\ninadvertently omitted due to the replacement of panels in Fig. 2 during our writing\nprocess. We believe this may have led to some confusion, and we apologize for any\ninconvenience caused. We have included this transect information as Supplementary\nFigure 8.\n\nR01-D30:\n-Lines 124 onwards. Some more details would be welcome. How many profiles? How\nmany shear sensors have you got?\nResponse:\nDetails regarding the number of turbulence deployments during drift have been\nincluded in Supplementary Table 1. The TurboMAP is equipped with two shear sensor\nprobes.\n\nR01-D31:\n-Nitrate density – fits. Do you do the fitting for each profile, or all profiles combined?\nIn the main text you suggest significant nitrate changes along isopycnals, how does that\nimpact this estimation? (probably not a lot, but just checking).\nResponse:\nIn this study, we conducted the fitting for each profile based on the concept of\nestimating the time-averaged nitrate concentration gradient relative to the density field.\nWe believe that this approach helps mitigate the impact of spike contamination and the\ninfluence of short-term disturbances on the nitrate gradient estimation. However, since\nthe distribution of nitrate on the isopycnal surface varies at each observation point along\nthe flow direction of the Tsugaru Warm Current, fitting was performed individually at\neach point.\nUpon examining the fitted cases on the isopycnal surface using all profiles, we\nobserved that the flux estimates tended to be approximately half of the fitted cases for\neach profile.\nHowever, because the fluxes vary spatially due to the intermittent nature of\nturbulence intensity, we believe that enhancing the accuracy of turbulence intensity\nestimation is a more critical issue for achieving more precise flux estimation.\nPlease note that during the drift observation, priority was given to turbulence\nintensity observation, so only surface water sampling was utilized for correction\npurposes. This was done to rectify the offset of SUNA.\n\nR01-D32:\n-SADCP. Also some more details could be useful: What was the configuration, in terms\nof bin size, segment averaging etc?\nResponse: We have included additional information.\nPlease refer to the 5th paragraph in the Method."}, {"round": 2, "reviewer_comments": {"reviewer_1": "The authors have made a very substantial effort to improve the manuscript, including a series of new, insightful analyses, as\nsuggested. I deeply appreciate such an effort. The manuscript is on the right track to become much stronger. However, one\nof the key results of the manuscript (the correlation between localised at the Shiriya spur and productivity in the Tsugaru\ngyre) is now less clear cut than it appeared to be with the previous, much shallower analysis. There was a substantial effort to\nstill make this point, which is now spread across 6 figures (3 in the main text and 3 as supplementary). The most\nprominent feature in those figures seems to be the high correlation at 1 day lag in Figure 6, which is (perhaps deceivingly) at\nodds with the “advective” hypothesis that the authors are putting forward. The answer to this apparent contradiction may be\nthere already but is diluted in the wealth of figures and information. I would recommend taking a step back and thinking\nabout the best way of showing their point more succinctly and convincingly. There are other relevant points I list below. I also\nattach an annotated PDF with minor comments.\nOverall, my perception is still that the manuscript is of high quality and makes an interesting and appealing point. It could\neventually reach Nature Communication standards but getting to that point still requires some work to strengthen the main\nconclusions and the clarity of the arguments.\nIt is not entirely clear/explicit why you can assume a direct correspondence between the elevated mixing rates during the\ndrift experiment and the flow divergence downstream of the spur (end of page 8). You should provide a more straightforward\nmechanistic connection between the two. It may relate to the hydraulic jump’s dynamics, but it would be good to see an\nexplicit argument. This weakness could be solved by adding one or two sentences explaining the dynamics that justify the\nlink.\nA second weakness pertains to the main idea that mixing of nutrients at the Tsugaru Strait supports productivity over the\nadjacent gyre. The authors have made a significant effort to justify this point, but the wealth of information becomes a bit\noverwhelming, and the point is somehow lost. The paper would benefit from a synthesis effort. Right now, you have 3 main\nfigures to make this point, plus several supplementary figures. This feels like too much, and the ideas get diluted.\nIf I understood correctly, the advection time-scale seems to be too long (~ 1 week) to explain the strong correlations at zero-\nlag between localised divergence and large-scale chlorophyll distribution in the wider Tsugaru Gyre area (the authors\nacknowledge that around line 332). If there was an advective connection between divergence and productivity, progressing\nanti-cyclonically along the gyre’s rim, I would expect to see some evidence of that in your correlation maps in Figure 6, 7.\nThere is some of that in Figure 7, but I feel it could be more convincing. Perhaps, I am distracted by the prominent signal\nindicating a wide-spread short-lag coherence in Fig 6a. This would rather suggest a common larger-scale driver at ~daily\ntimescales (tides? - can you comment on what this driver may be?). I understand that then you decide to focus on sub-inertial\ntimescales, so perhaps this is your point. If that is the case, you could focus more on this idea and strengthen your\narguments. Since the short-term coupling appears more prominent in Figure 6, you need to convince the reader that the\ncoupling at subinertial frequencies matters as well or matters more. For example, you could\nestimate how much of the chlorophyll variability is contained at subinertial frequencies\nand how much of it is explained by the covariance with divergence. Other points of\nimprovement are explaining better what point Figure S6 makes and adding some\nsignificance contours to the wavelet analysis in Figure 8 and discuss how this analysis\ncompares with the lag correlations in Figure 7.\nYou could explain up front why you focus on the period August-September. Is it because this is the time of the year when the\ngyre / the localised divergence are active? Is nutrient limitation in the gyre stronger during this time of the year, so you see a\nclearer signal when nutrients are supplied? Maybe expanding on that could help you highlight the importance of localised\nmixing, e.g. if productivity in the gyre is higher than expected for this time of the year. The addition of productivity estimates is\na good added element of the revised manuscript. If you could show that productivity is higher than expected for the region in\ntimes of the year when the divergence is more active, that could make your argument stronger.\nYou could elaborate more in the connection between the sub-inertial variability and your previous study on the Tsugaru\nWarm Current position (line 245-247). Can you demonstrate/illustrate this somehow, with a figures\nLine 90. A brief explanation of what the drift sampling strategy means in practice, and how close it is to Lagrangian, would\nbe useful in the main text.\nLines 119-124. An important feature seen in this figure is that chlA concentrations are maximum east of the spur. You could\nuse this feature to strengthen your argument.\nPage 10-11. You should explain why you picked 1 day and 7 day lags in Figure 6.\nFigure 7. Correlation subplots have different limits for their y-axis. This makes interpretation unnecessarily complicated.\nSupplementary Figures 4,5. I struggle to interpret these 3-dimensional plots. Given that this is is SI and you have plenty of\nspace, I would suggest showing some slices as subplots rather than a 3d figure.\nDiscussion. You compare the areal nutrient supply at the spur with uptake in the gyre and show that the former is higher.\nHowever, enhanced fluxes presumably occur over a small area, while the uptake occurs over a much broader area. When\nscaled over the respective areas, do supply and uptake still match?\nGeneral: although the language has improved, I still feel that some parts of the introduction could be more clearly written\n(see annotated PDF).", "reviewer_2": "General comments: I would like to commend the authors for incorporating many of the suggestions made by all the reviewers\nof the previous version of the manuscript. The changes made, including the addition of new datasets and analysis have\ncertainly helped improve the robustness and validity of the results. The manuscript is still in need of editorial changes for\nimproving its clarity.\nI have tried to help as best as I can by making many editorial suggestions which can be found in the annotated version of the\nmanuscript attached with my review.\nSpecific comments:\n1) In my previous review, I had requested the authors to comment on the novelty of their work, specifically outlining what sets\ntheir work apart from the previous studies by Tanaka et al. 2021 within the same region, and by others who have already\ndescribed the importance of underwater seamounts and physical mixing mechanisms triggered by these mounts and their\nrole in facilitating nutrient fluxes into and the growth of phytoplankton in the euphotic zone. The response that the studies of\nTanaka et al. (2021) were undertaken at a location 100km away from the present study seem to me at best lame. I would\nhave liked more details - for instance whether the mixing, nutrient enhancement, divergence mechanisms differed or were\nnot reported in Tanaka et al. adequately.\n2) Although I had not pointed this out earlier, and for the sake of their readers, I would like the authors to provide more details\nas to whether the area of enhanced chlorophyll downstream of the TWC is a year-round feature? This is especially important\ngiven the importance of this process in sustaining this productive ecosystem. I assume that this mixing is a year round\nfeature, albeit with slight changes in strength given the uni-directional flow of this current through the Tsugaru Strait and\nperhaps slight changes in current velocities over the year. If this is not a year round feature and the direction and strength of\nthe TSW flow does not change much, what is the reason for seasonal variability of Chl-a within the gyre. Is it related to\nweaker divergence, light limitation, grazing pressure, lack of water column stability etc?\nAs I have stated above - I have made several edits in the annotated version of the manuscript. I would like the authors to\nhave a look at edits to the abstract which currently lacks clarity and conciseness. I also made some edits to the introductory\nsection which would benefit from clear statements as to the motivation for this study. In the other sections of the manuscript,\nI have also made some edits and have made some notes about the figures some of which appear to be labeled inadequately", "reviewer_3": "Comments on \"Vigorous vertical mixing supports biological production in the mesoscale Tsugaru Gyre”\nThe authors have shown responsiveness to the comments provided by this and other reviewers. However, I remain\nconcerned about the lack of structure and depth in the discussion section. This weakness persists in the current version.\nAdditionally, the language requires further consideration.\nKey aspects covered by this manuscript include (1) nutrient supply via turbulent mixing, (2) nutrient transport to the\ndownstream Trugaru Gyre, and (3) enhanced productivity resulting from this nutrient supply. However, these elements, aside\nfrom correlations, lack in-depth discussion. Even the correlations themselves require further interpretation to discern their\nsignificance. I recommend enhancing the discussion to address these points more comprehensively, particularly focusing on\nthe implications of the observed positive correlation between nutrient supply and chlorophyll-a levels. Additionally, the\ninsights provided by the lag correlations are noteworthy and deserve further elaboration.\nI'm uncertain whether the newly added wavelet figures add significant value to the main text or if they might be more suitable\nfor supplementary materials. Additionally, there seems to be an abundance of introduced materials without subsequent\ndiscussion. It's crucial that the discussion begins with an exploration of nutrient supply and transport mechanisms, followed\nby an elucidation of their effects on biology. This sequential approach would enhance the coherence and depth of the\nmanuscript.\nDetails\nL25: “…the impacts of localized nutrient supply on…?”\nL27: add comma after Pacific Ocean.\nL27-29: a lot of unnecessary words that can be removed to give space to more critical information in the abstract.\nL34: the significance of the study can be made more impactful.\nL52: on long-term, how? Maybe include references. I thought this study was trying to demonstrate this control over broad-\nscales?\nL88: This should mean surface water mass?\nL92: “A previous study” can be name since it is repeatedly referenced.\nL109: what timescales?\nL179: since VGPM is referred to by name, the other model too can be referenced by name.\nL357: very long sentence and difficult to follow.\nL396: another long sentence.\nL562: Since no significant differences were found, I would suggest to keep it simple in the manuscript and show the absolute\nvalue rather than ratio. Also, the y-axis should have the same scale for all subplots. This makes comparison easy and\nstraightforward.\nL611: area cannot support a concept.\nL626: extrapolated vertical mean?\nL627: range -> flux? This comparison is a bit strange. Is the nitricline in the same depth range as the euphotic layer?"}, "author_response": "Reviewer #1 (Remarks to the Author):\nI want to than the authors for the efforts they devoted to address my comments and craft a very thorough response letter and\na much improved manuscript. The authors have now introduced new analysis, based on modelling efforts, to clarify my\nquestions and I am happy to say that they dissipated my concerns, which regarded the advective time-scales of the nutrient\nsignatures around the gyre. I would like to praise again their effort to introduce new clever analysis. I am now happy with the\nscience, it is very interesting and the arguments are now well supported by different threats of evidence.\nI still have some substantial editorial recommendations to improve the clarity and conciseness of the manuscript. I list the\nmain points here, but I also attach an annotated PDF with more details and additional editorial comments (I am not listing\nthose comments separately here for the sake of returning this review more quickly, I hope the authors don't mind that).\nThe clarity of the manuscript is much improved now, however, I have the feeling that some parts of the manuscript may be a\nbit difficult to understand for a reader that hasn't have access to the previous versions of the article. I am struggling to judge\nthe clarity with full confidence, given that I read already the previous versions already, but I'd have some concrete\nsuggestions:\n- Rewrite the first paragraph of the introduction, with a clear, simpler message, perhaps following this thread: (1) Mixing is\nkey to supplying nutrients to plankton communities; (2) mixing is enhanced at topographic features (cite some examples); (3)\nthe effects of topographic mixing on productivity remain poorly understood due to the sparsity of observations and the\ndifficulty of linking small-scale mixing measurements with larger-scale productivity; (4) here we address this issue for a case\nstudy region.\n- I struggled to follow section \"Spatial and temporal changes in the surface velocity field around the Shiriya Spur\", although it\nis really promising. Try to bring the focus back to your main messages here, and guide the reader at the start, explaining why\nyou look at different scales of variability (in particular the subinertial variability of the jet. This variability is key in subsequent\nanalysis but I don't feel it has been introduced properly) and why this matters for divergence/nutrient supply. I would invite\nthe authors to reconsider whether all the information in Fig. 5 is really needed in the main text. I am happy if they decide to\nleave everything, I am just mindful of conciseness and clarity.\n-Lines 362-371. I also invite the authors to reconsider whether this paragraph adds something new and relevant to the main\ntext, or it can be dropped.\n-For a series of other editorial recommendations, see attached PDF.\n\nReviewer #2 (Remarks to the Author):\nI am very happy that the authors have incorporated many of the changes recommended at every stage of the review process.\nThe result is that they have a very very strong and compelling manuscript \" \"Vigorous vertical mixing supports mesoscale\nbiological production in the Tsugaru Gyre The work holds substantial significance for both the field and related disciplines\"\nthat meets the high standards for publication in Nature Communication. The work holds substantial significance for both the\nfield of physical and biological oceanography and is richly deserves to be published\n\nReviewer #3 (Remarks to the Author):\nComments\nOverall, I feel the text has improved, but I strongly encourage the authors to reconsider the narrative, particularly by reducing\nthe reliance on general model outputs. The core issue, as noted in the previous round of reviews, is that the story is\nbecoming less clear. The combination of observations with general model results is not compelling. If the model outputs are\nincluded, they should be more focused on explaining specific events, such as in the case of Fig. 7. Several figures could be\nstreamlined to enhance focus, for instance, simplifying Fig. 1 and moving several others to supplementary material. A simple\nand clear narrative can be achieved by focusing on parts of Fig 1 and 5, as well as Figs 3, 6, and 7. The remaining figures\nadd too much material without significantly enhancing the manuscript.\nI also notice a tendency to force connections between observations and phenomena without providing clear, logical\nexplanations (e.g., L168-171), which weakens the manuscript. As I mentioned in the previous review, the section titles need to be more informative to help readers understand the content. For instance, \"General situation at the time of the survey\" is\nvague, general situation of what? A more specific title would improve clarity. Furthermore, from the current title, one would\nexpect it detailing the conditions associated with the survey. Instead, it discusses chlorophyll-a enhancement and the\nimplied impact of vertical mixing on nutrient supply near the spur. This makes the section feel misplaced. As such, it would\nbe better positioned after \"Turbulence over the spur and associated diffusive nitrate flux\", which details the mechanisms of\nnutrient supply.\nOverall\n-Text clarity remains an issue: despite revisions, convoluted passages remain and should be streamlined for better flow. Try\nusing shorter sentences to isolate ideas. I am inclined into thinking text clarity is not merely a language problem but more\nabout the narrative focus.\n-Too many figures and study focus remains unclear: the number of figures have increased considerably but they did not\nsignificantly improve the clarity or focus of the manuscript, particularly in balancing observation-based findings with general\nmodel outputs. Fig. 7 from model outputs is useful, but other figures dilute the focus.\n-Section order and titles: The structure needs revision for better flow. The results are mixed with discussion and this adds to\nthe confusion. Section titles should be more specific to guide the reader.\n-I suppose high-resolution current data and turbulence measurements should be the main focus of the manuscript.\nIn summary, I recommend focusing on simplifying the manuscript’s narrative, rather than expanding it with new material.\nDetails\nL25: “…the impacts of localized nutrient supply on…?”\nL27: add comma after Pacific Ocean.\nL27-29: a lot of unnecessary words that can be removed to give space to more critical information in the abstract.\nL34: the significance of the study can be made more impactful.\nL52: on long-term, how? Maybe include references. I thought this study was trying to demonstrate this control over broad-\nscales?\nL88: This should mean surface water mass?\nL92: “A previous study” can be name since it is repeatedly referenced.\nL109: what timescales?\nL179: since VGPM is referred to by name, the other model too can be referenced by name.\nL357: very long sentence and difficult to follow.\nL396: another long sentence.\nL562: Since no significant differences were found, I would suggest to keep it simple in the manuscript and show the absolute\nvalue rather than ratio. Also, the y-axis should have the same scale for all subplots. This makes comparison easy and\nstraightforward.\nL611: area cannot support a concept.\nL626: extrapolated vertical mean?\nL627: range -> flux? This comparison is a bit strange. Is the nitricline in the same depth range as the euphotic layer?"}, {"round": 3, "reviewer_comments": {"reviewer_3": "Comments\nI would like to commend the authors for their responsiveness to reviewer comments and their considerable effort to make the\nmanuscript more accessible. It is now significantly easier and more pleasant to read and grasp the ideas being introduced.\nThe text has clearly improved in clarity and conciseness. While I still find a few sections in need of further revisions, which I\ndetail below, I believe these are minor issues that can be resolved in preparation for publication.\nDue to time constraints, I have not separated my commentary from editorial suggestions; please see specifics below.\nSpecifics\nI might be missing the point, but “vertical mixing sustained” and “vertical mixing supports” convey different nuances. The\nlatter seems to better capture the manuscript’s intent to emphasize that “vertical mixing supports” high biological productivity\nin Tsugaru Gyre.\nL32-35 blends general and specific information resulting in a diluted message that doesn't clearly articulating the\nmanuscript's main point. Please consider the following:\n“Our long-term datasets revealed a positive correlation between recurring submesoscale divergence related to the\ntopographic feature and the downstream distribution of chlorophyll-a. This finding reinforces the significant role of the\nunderwater seamount in enhancing the productivity of the downstream anticyclonic gyre.”\nIntegrate L48 into L49 for a seamless flow\n“In this study we introduce a case study to address the above issue by investigating the interaction between a strong current\nand local topography, revealing that...”\nL54: Fig. 1b and c show seasonal changes in Tsugaru Gyre characteristics, yet the text discusses “the flow path of the\ncurrent...”, in this case, the Tsugaru Warm Current. This reference to Fig. 1b and c may be misleading or confusing to\nreaders. Additionally, is the text implying that the Tsugaru Warm Current (Fig. 1a) and the Tsugaru Gyre (Fig. 1b and c) are\nthe same thing? Please clarify the reference to Fig. 1b and c.\nL68-80: “To quickly test this hypothesis, we normalized the long-term (2014–2022) seasonal (August and September)\nmean surface chlorophyll-a concentration (mg m^−3) by its pixelwise standard deviation (mg m^−3) and denoted the\nnormalized data as Ichl (unitless). This index (Ichl) revealed elevated values (>2) within the gyre compared to those outside the gyre\n(Fig. 1e). Assuming that the chlorophyll-a concentration at each grid is normally distributed during the season, the elevated\nvalue of Ichl within the gyre supports the notion of enhanced chlorophyll-a levels with small temporal variation during that\nseason. The Ichl exhibited a horizontal maximum in the region corresponding to the area immediately east of the spur\n(approximately 41.7°N, 141.6°E–141.7°E; Fig. 1e). This suggests the existence of a nutrient source around the spur that\nsupports stable chlorophyll-a levels, as shown by Ichl.”\nI believe the assumption of normality for chlorophyll-a data can be tested and may not be readily accepted by biological\noceanographers. It would be beneficial to address this assumption more critically in the text.\nL75: “As chlorophyll-a concentrations within the gyre remain above zero during the season” What does this mean? Is there\nany period when chlorophyll-a levels are below zero?\nL86: “...given the distance from Cape Tappi and the spatial distribution of Ichl...”\nL106: I believe that including a paragraph to introduce the results would greatly enhance the reader's experience. Currently,\nthere is a significant jump from the introduction to the results, which may disrupt the flow.\nL105: “Firstly, we briefly present the seasonal variability characteristics of the Tsugaru Warm Current and Tsugaru Gyre, and\nassociated topographically driven upwelling and surface velocity divergent field, on the east side of the Tsugaru Strait, using\noutputs from the JCOPE-T DA (Supplementary Fig. 1, with the 2019 output as an example; see the Methods section). This\nanalysis is motivated by the challenges in continuously and repeatedly acquiring the spatial structure of surface currents,\nincluding the vertical velocity of the ocean interior. We then introduce results based on field observations using HFR and\nturbulence measurements. Finally, we discuss the implications for productivity in the Tsugaru Gyre using long-term\nchlorophyll-a data and empirical estimates of primary productivity.”\nSeasonal characteristics of velocity divergence from the JCOPE-T DA model\n“The outputs from the JCOPE-T DA model reasonably reproduce the seasonal variations in the current and the formation of\nthe Tsugaru Gyre (Supplementary Fig. 1a and b). For the vertical east–west section of each season at 41.65°N...”\nL130: Adding a reference would help support this claim and strengthen the argument.\nL133 could be moved after the results of turbulence and nitrate fluxes.\nL135-137: This information is redundant and can be removed, as it has already been discussed in the introduction.\nL153-156: I am not clear what this statement is trying to convey. It feels too speculative and should be removed from the\nresults. A clear statement of the rationale behind this comparison would enhance clarity.\nAlternatively, it would be more reasonable to focus on the estimates of nutrient input resulting from the interaction between\nthe Tsugaru Warm Current and the spur, assessing its impact on primary production during August and September against\nbackground conditions. This approach would provide a clearer understanding of the contribution.\nL157 feels like the introduction of L169.\nL164: “The R/V’s travel distance”?\nL180: indicating→associated with?\nL200: “...western strait topography...?”\nL205: “This result suggests that the nutrient content at the surface on the east side of the spur is greatly influenced by vertical\nnitrate fluxes.”\nL266: “...from→during...”\nL906: The concept of normalized chlorophyll-a is becoming unnecessarily complex. The notation〈chl(x,y,t)〉 is not clear,\nas the “t” inside the brackets makes it difficult to interpret the average. What exactly does it convey? If we are to calculate the\npixelwise mean at a given time, shouldn’t it simply correspond to the original value?\nL951: “from” July to November 2021, or for July “and” November 2021?"}, "author_response": "I sincerely appreciate all of your essential, valuable, and insightful comments and suggestions. We will address each point individually below. This process has been an incredibly valuable experience for me. Thank you once again."}]

Decision letter