Compensating transport trends in the Drake Passage frontal regions yield no acceleration in net transport

Domain: earth_science
Published: 2023-11-28
Editorial outcome: accept

Abstract
Although the westerly winds that drive the Antarctic Circumpolar Current (ACC) have increased over the past several decades, the ACC response remains an open question. Here we use a 15-year time series of concurrent upper-ocean temperature, salinity, and ocean velocity with high spatial resolution across Drake Passage to analyze whether the net Drake Passage transport has accelerated in the last 15 years. We find that, although the net Drake Passage transport relative to 760 m shows insignificant acceleration, the net transport trend comprises compensating trends across the ACC frontal regions. Our results show an increase in the mesoscale eddy activity between the fronts consistent with buoyancy changes in the fronts and with an eddy saturation state. Furthermore, the increased eddy activity may play a role in redistributing momentum across the ACC frontal regions. The increase in eddy activity is expected to intensify the eddy-driven upwelling of deep warm waters around Antarctica, which has significant implications for ice-melting, sea level rise, and global climate.

Introduction
The Antarctic Circumpolar Current (ACC) is one of the critical components of the Southern Ocean (SO) and the global ocean climate system. The ACC flows eastward unbounded around Antarctica, connecting the three major ocean basins, promoting exchange between them, and allowing the establishment of a global-scale overturning circulation1. The ACC and the overturning circulation transport heat, salt, carbon dioxide, and other tracers around the globe and strongly influence the Earth’s climate. Therefore, studying the three-dimensional structure of the ACC and its properties is pivotal to assessing how the SO is responding to climate change.

An intensification of the SO westerly winds that drive the ACC has been documented over the past several decades, reflecting an intensification of the Southern Annular Mode (SAM; see refs. 2,3), the leading mode of atmospheric variability in the SO. Modeling studies show that the positive trend in the winds is a response to both depletion of the atmospheric ozone and increments in greenhouse gas emission over the last decades4–6. The intensification in eastward wind stress, and therefore northward Ekman transport1, increases the horizontal density gradients that characterize the fronts that comprise the ACC. In coarse-resolution global climate models, the steepened gradients accelerate the flow in the ACC fronts, resulting in increased ACC transport7,8. In contrast, eddy-permitting global climate models show that the enhanced available potential energy resulting from the steepened isopycnals is released through baroclinic instabilities, which feed into the vigorous eddy field within the ACC, reaching an eddy saturation state, i.e., eddy activity increases as the wind stress increases with no net acceleration of the ACC9–15.

Overall the SO is a meagerly sampled section of the global oceans as it represents a challenge to observe due to inhospitable conditions and remoteness. An exception is Drake Passage, which has historically provided an ideal location for monitoring the ACC transport16–20, as it represents the narrowest constriction (800 km) through which the ACC navigates on its circumpolar path (Fig. 1a). As such, Drake Passage transport and its long-term variability serve as valuable metrics for validating ocean and global climate models. However, to date, none of the observational studies have found a clear trend in the net Drake Passage transport10,17–20. Moreover, these studies were unable to explore trends within the ACC frontal regions owing to the lack of direct current velocity measurements at a sufficient spatial resolution to resolve the frontal regions and mesoscale activity. The lack of long-term in situ observations (velocity, temperature, salinity) with high horizontal resolution in the SO hampers the exploration of trends in the ACC transport and properties across the different frontal regions.

This study uses a unique observational time series of year-round near-repeat upper-ocean temperature, salinity, and velocity across the Drake Passage (Fig. 1a) to explore whether the Drake Passage transport in the upper 760 m shows significant acceleration in the last 15 years. The repeated upper-ocean velocity transects provide the most efficient way to measure the upper-ocean transport21. The high along-track spatial resolution of the transects allows us to observe patterns on the order of the first baroclinic Rossby radius (20–10 km)22. Surprisingly we found that, although the net Drake Passage transport relative to 760 m shows insignificant acceleration, there are compensating trends across the ACC frontal regions. Our results show an increase in the mesoscale eddy activity between the fronts due to buoyancy changes in the fronts consistent with an eddy saturation state.

Results
Climatology of cross-passage velocity, temperature, and salinity
Climatological mean sections of cross-transect velocity utot, potential temperature θ (temperature hereinafter), and salinity S across Drake Passage using all available velocity (237) and temperature/salinity (114) transects (see Methods) are shown in Fig. 1b–d. The cross-transect velocity climatology reveals the jets associated with the main ACC fronts (Fig. 1b). The narrow Subantarctic Front (70–150 km, Fig. 1b) is the strongest jet with mean velocities reaching 60 cm s−1 near the surface (Fig. 1b). Located south of the Subantarctic Front, the jet associated with the Polar Front extends for almost 200 km across Drake Passage and has a weaker mean velocity than the Subantarctic Front (300–500 km; Fig. 1b). Further south (750–800 km) the weakest (u < 15 cm s−1) of the three jets is the Southern ACC Front (Fig. 1b). Relatively warm water masses (θ > 2 °C) are located northward of the Polar Front (<400 km; Fig. 1c). The Subantarctic Front marks the region with the strongest temperature stratification and lateral gradients (Fig. 1c). The northernmost subsurface tongue of cold, fresh Antarctic Winter Water (AWW; θ < 1 °C, S ~ 34) marks the location of the Polar Front, the boundary with the warm subtropical waters (Fig. 1c, d). Below the AWW (<−200 m), the dense Upper Circumpolar Deep Water (UCDW; θ ~ 2 °C, S > 34.40) is very homogeneous in temperature (Fig. 1c, d).

Trends in Drake Passage transport
To explore long-term trends, we estimated the time series of Drake Passage transport in the upper 760 m. We defined Drake Passage total transport Utot as the integral of the cross-track velocity utot from 0 to 760 m depth and from 0 to ~850 km across Drake Passage. When temperature and salinity observations were available, the geostrophic velocity ugeo and therefore the geostrophic transport (density-driven, depth-dependent relative to 760 m; Ugeo) could also be estimated. For the subset of 60 coincident velocity and temperature/salinity transects occupied from October 2005 to April 2019, the total Drake Passage transport based on direct velocity observations was decomposed into a geostrophic component plus a residual (Uref, depth-independent; see “Methods”). Hereinafter, we interpret the acceleration and deceleration of the eastward transport and flow as positive and negative trends, respectively. Trends are statistically significant when the probability p < 0.05 after the modified Mann–Kendall test (see “Methods”).

None of the Drake Passage transport time series show statistically significant acceleration over the last two decades (p > 0.05; Fig. 2). Trends were statistically insignificant regardless of the period used or number of transects used (Supplementary Fig. S1). Our results are consistent with previous Drake Passage observational studies using coarser resolution in situ observations17,18 and altimetry-derived geostrophic currents20 which found no significant acceleration of the Drake Passage transport. In the following subsection, we explore an alternative hypothesis to explain the lack of trends in the net Drake Passage transport.

Trends in transport across Drake Passage
We investigated whether there are opposing regional trends in the ACC frontal jets consistent with no trend in Drake Passage transport. We estimated trends in the cross-transect total utot and geostrophic ugeo velocity components for the common sampling period October 2005–April 2019.

Significant opposing trends in cross-track velocity are found within Drake Passage. All significant trends (p < 0.05) show no change in sign with depth (Fig. 3). Since the jets of the ACC are equivalent barotropic (velocity is self-similar with depth; see ref. 23), the sign of the trends likely holds over the entire water column, although the magnitude may decay. The largest significant acceleration of the eastward flow is found in the region between the Subantarctic Front and Polar Front (150–300 km, Fig. 3a, c). A weak, significant acceleration is also found in the Southern ACC Front (>750 km, Fig. 3a, c). In contrast, a significant deceleration of the total flow component is found within the Polar Front (400–500 km, Fig. 3a) and to the south (550–600 km, Fig. 3a). A broader and weaker deceleration is found in the geostrophic component, but mainly within the Polar Front (350–500 km, Fig. 3c). A similar distribution in space and sign is found (although with slightly larger amplitudes) when total velocity trends are calculated with the subset of the total velocity transects that coincide with the temperature/salinity transects (Fig. 3b).

We estimated the time series of cross-transect total utot, geostrophic ugeo, and reference uref transport per distance bin. The net total, geostrophic, and reference Drake Passage transports are 75.54 ± 0.64 Sv, 15.85 ± 0.20 Sv, and 51.08 ± 0.85 Sv, respectively (Fig. 4b, e, h). The sum of the geostrophic, reference and Ekman (8.09 ± 0.22 Sv; see “Methods”) transports yields 75.02 ± 0.90 Sv, which agrees with the net total transport calculated using the concurrent transects (77.87 ± 1.04 Sv; green line in Fig. 4b). Extending the total transport calculation to the upper 970 m yields a 15-year mean of 89.41 ± 4.69 Sv, in close agreement with ref. 24’s mean of 95.00 ± 2.10 Sv based on the first five years of data (only using transects along the most repeated line), and the 40-year mean modeled transport of 92.7 Sv in the upper 1000 m calculated in ref. 21 from a data-constrained eddy-permitting ocean model. The 75.54 Sv net total transport in the upper 760 m is 56% of the full-depth total canonical transport of 134 Sv estimated in ref. 17 and 48% of the 40-year mean full-depth model transport of 157 Sv estimated in ref. 21.

Transport anomalies for the three components are the largest in the northern half of Drake Passage, especially in the region between the Subantarctic Front and Polar Front (0–500 km, Fig. 4a, d, g). This region has the largest mesoscale eddy activity (meandering of the fronts and eddy formation) within Drake Passage25–28. South of this region the transport anomalies are the weakest (>500 km, Fig. 4a, d, g). While the time series of transport anomalies do not readily show discernible trends, the estimated trends (Fig. 4c, f, i) show clear significant trends consistent with the velocity trends (Fig. 3). All transport components show a clear acceleration of the region between the Subantarctic Front and Polar Front (150–300 km; Fig. 4c, f, i) and deceleration of the southern flank of the Polar Front (350–550 km; Fig. 4c, f, i). Similar patterns are found when total transport trends are calculated with the total velocity transects that coincide with the temperature/salinity transects (Fig. 4c), or using the transects falling along the most repeated crossing (not shown). The geostrophic and reference transport trends (Fig. 4f, i) summed across Drake Passage yield −0.10 Sv year−1 and 0.04 Sv year−1, respectively, and are in close agreement with the insignificant trends calculated from the time series of net Drake Passage transport (Supplementary Fig. S1). The sum of the total transport trends (Fig. 4c) yields 0.08 Sv year−1, which is of the opposite sign (but insignificant) compared to that estimated from the net transport time series (Supplementary Fig. S1).

The results based on binning the transport by distance indicate that the different ACC frontal regions in Drake Passage have tended to compensate each other over the last 15 years such that the net Drake Passage transport shows no significant acceleration.

Nonetheless, the acceleration of the region between the fronts could be an effect of an increase in the meandering of the ACC frontal jets and eddy formation, an aspect not captured when using the fixed cross-passage coordinate system. To explore this hypothesis we constructed time series of transport anomalies and trends analogous to Fig. 4, but where the binning was done per pair of streamlines (Fig. 5), using synoptic sea surface height (SSH) derived from a combination of daily altimetric sea-level anomalies and mean geostrophic streamlines (Fig. 1a; see “Methods”). Contours of SSH provide a natural coordinate system (streamlines) for geostrophic flows. The streamlines allowed us to track the position of the fronts that comprise the ACC, which can vary (both in position and distance between each pair of streamlines) daily due to strong meandering of the fronts and eddy formation25,28.

As with the time series of transport anomalies in passage coordinates (Fig. 4a, d, g), the transport anomalies in streamwise coordinates (Fig. 5a, d, g) are largest in northern Drake Passage, but are concentrated north of the Polar Front. Similar to the time series in passage coordinates (Fig. 4a, d, g), trends in the streamwise time series are difficult to discern (Fig. 5a, d, g). The magnitude and pattern of the estimated trends for the total transport in streamwise coordinates (Fig. 5c) are much reduced relative to passage coordinates (Fig. 4c), suggesting that at least some of the apparent acceleration in the zone between the Polar and Subantarctic Fronts (Figs. 3 and 4c) was due to fronts meandering and eddies. When binned by streamline, the transport anomalies within meanders and eddies contribute to the anomalies of the fronts rather than the interfrontal zone. The main significant trend in the streamwise coordinate system is that the Subantarctic Front has significantly accelerated over the last 15 years (Fig. 5c). This result is only evident when using the streamwise coordinate system, which is able to capture the effect of the steering of the fronts due to meandering and eddy detachment, an aspect that is missed when using the passage coordinates (Fig. 4).

We will explore the increasing mesoscale activity hypothesis in “Trends in eddy activity” by estimating trends in eddy kinetic energy and momentum flux. In the following subsection, we explore buoyancy changes across Drake Passage that may contribute to increased eddy formation.

Buoyancy changes across Drake Passage
To understand buoyancy changes across the ACC frontal regions in Drake Passage, we estimated trends in the temperature and salinity in the upper 760 m and decomposed them into along neutral density layers (spice), vertical movement of density layers (heave), and total (depth) trends29,30 (see “Methods”).

Significant warming and salinification due to spice is found in the Subantarctic Front (<200 km; Fig. 6a, d). Increased heat uptake from the atmosphere to the ocean due to increasing greenhouse gas emissions over the last decades could be responsible for the warming in the Subantarctic Front31. In contrast, wind-driven cold and fresh water is upwelled along isopycnals32 in the southern (cold) flank of the Polar Front (400–500 km; Fig. 6a, d); this upwelling is likely due to the SO westerly wind stress intensification3. The heave component shows cooling and salinification across the Polar Front in the upper 300 m, and it is maximum in the front’s south flank (400–500 km; Fig. 6b, e), which makes up most of the signal at depth in the temperature and salinity trends (Fig. 6c, f). The heave-driven cooling potentially stems from a more negative wind stress curl (Ekman suction or upwelling) over the Drake Passage area during the last 15 years (Supplementary Fig. S2), consistent with previous trends estimated for a shorter period of time33. A similar cooling trend in sea surface temperature in Drake Passage was found from 1993 to 201734. Our study finds that the cooling trends between 2005 and 2019 are a factor of two-three larger than those in ref. 35 potentially owing to the coverage over a different period and the average area used in ref. 35. South of the Polar Front (below 150-m depth), strong heave-driven warming is associated with the upwelling of warm-salty UCDW (>500 km; Fig. 6b, e). The upwelled UCDW below the mixed layer could minimize AWW’s formation, which could produce a warmer water column.

Trends in eddy activity
As discussed in “Trends in transport across Drake Passage”, the acceleration of the transport in the region between the Subantarctic and Polar Fronts (Figs. 3 and 4) potentially arise from an increase in the steering and meandering of the ACC fronts due to enhanced mesoscale eddy activity. To examine this hypothesis, we constructed time series of depth-averaged eddy kinetic energy and eddy momentum fluxes per pair of streamlines (<EKE> and <EMF>, respectively; see “Methods”).

Time series of streamwise <EKE> show that eddy energy increases during the last quarter of the sampling period (Fig. 7a) in the zone between the Subantarctic Front and Polar Front (−0.60 m > SSH > −1.00 m). This zone exhibits the largest time-mean (denoted by overbar) depth-averaged eddy kinetic energy (<EKE>), consistent with previous studies26–28; <EKE> decreases significantly away from this region (Fig. 7b). Trends show a significant increase in <EKE> in the south flank of the Subantarctic Front over the last 15 years (Fig. 7c), indicating an increase in the meandering of the front’s streamlines and eddy formation. Surprisingly, trends show a significant decrease of <EKE> in the Polar Front (Fig. 7c). Our means are unaltered when they are calculated using only those (60) velocity transects that have a coincident temperature/salinity transect (Fig. 7b). Trends using the coincident transects show a positive peak centered in the zone between the Subantarctic and Polar Fronts; this peak lies poleward of the significant positive trend estimated using all transects, although trends using the coincident transects are insignificant (Fig. 7c). Our results are consistent with an eddy saturation state, i.e., the net ACC transport remains invariant to the increased wind-forcing whereas the eddy activity increases9,15.

Time series of streamwise <EMF> show that the largest negative values occur in the region between the Subantarctic and Polar Fronts, whereas relatively smaller positive values dominate in the Subantarctic Front (Fig. 7d). The time-mean depth-averaged eddy momentum flux <EMF> reflects the pattern seen in the time series; it is positive in the Subantarctic Front (SSH > −0.65 m) and negative in the region between the fronts (−0.65 m > SSH > −1.00 m) (Fig. 7e). Within and south of the Polar Front (SSH < −1.00 m), <EMF> is again positive although its value is close to zero (Fig. 7e). Positive values imply a northward flux of eastward momentum and negative values imply a southward flux of eastward momentum26. This interpretation suggests that, on average, the eddies in the interfrontal zone redistribute zonal momentum towards the Subantarctic Front and the Polar Front. The positive trend of <EMF> in the southern flank of the Subantarctic Front (Fig. 7f) suggests that eddies have significantly increased the northward flux of zonal momentum in the south flank of the Subantarctic Front over the last 15 years. Similar results are obtained when means and trends are calculated using only those transects that have a coincident temperature/salinity transect (Fig. 7e, f).

Methods
The Drake Passage datasets
Underway upper-ocean velocity, temperature, and salinity were collected aboard the Antarctic Research and Supply Vessel (ARSV) Laurence M. Gould (LMG) that transits between South America and the Antarctic Peninsula (Fig. 1). Since February 2005, a 38 kHz phased array acoustic Doppler current profiler (ADCP; OS38) has sampled velocity in the upper 1000 m at 24-m vertical resolution with the first depth bin at 46 m. Transects were discarded if one of the following criteria were met: (a) transects were not completed in less than 4 days, (b) along-track/vertical gaps were too large to fill, or (c) if transects with gaps were outside the area enclosed by the gridded objectively mapped mean. Following these criteria allows us to estimate the mean and trends in both the net total transport and transport per distance bin with the same number of degrees of freedom (transects). From February 2005 to December 2019, 248 (of 286 total crossings) ADCP transects met these criteria, with 147 transects along the most commonly repeated line (Supplementary Fig. S1). From October 2005 to April 2019, the common sampling period between the ADCP and the temperature/salinity transects (details below), 237 transects were available with 140 transects falling along the most repeated line.

Velocity data were processed using the Common Ocean Data Access System (CODAS) software43. Following refs. 24,44, returned ping data were averaged over 5-min (100 pings) ensembles and screened using amplitude, error velocity, and percent good criteria. Velocities were transformed from ship-relative to absolute ocean currents using GPS position and attitude measurements. The absolute current velocities were further averaged to 15-min resolution, i.e., ~4.50 km along-track resolution (assuming a ship’s speed of 5 m s−1). Barotropic tidal currents were removed from the absolute velocity by subtracting the tidal prediction of the TPXO 7.2 tide model45. A slab layer was assumed for the upper 46 m where the OS38 ADCP did not sample. The single ping velocity uncertainty is 23 cm s−1; thus the uncertainty of a 300 ping average is 1.3 cm s−1. Combined with the error in ship speed from GPS (0.8 cm s−1) the error in absolute velocity in a 15-min average is 1.5 cm s−1. Averaging over 83 min (i.e., ~25 km along-track assuming an average ship speed of 5 m s−1) reduces the error σtot to 0.65 cm s−1.

Since February 1997, on 6–7 LMG transects per year ~70 eXpendable Bathythermograph (XBT) probes are deployed that measure temperature in the upper 900 m. The spatial resolution of the XBT sampling is 6–10 km across the Subantarctic Front and Polar Front, and 10–15 km elsewhere46. All data were quality-controlled and corrected as in ref. 46. The error associated with the systematic fall rate error is ~1% of the depth due to random XBT probe differences. The thermistor calibration error is 0.05 °C. The majority of crossings also deployed twelve expendable conductivity–temperature–depth (XCTD) probes that directly measure temperature and salinity in the upper 1000 m with a sample spacing of 25–50 km. The XCTD probe salinity accuracy of ~0.05 accounts for the errors in conductivity, temperature, and pressure47. To obtain cross-transect salinity, a look-up temperature-salinity-depth-position relationship was first constructed from historical hydrography. For each XBT temperature profile, a corresponding salinity was obtained. The salinity anomaly was then determined from the XCTD measurements with respect to the historical data, and the salinity anomaly was objectively mapped along the transect and added to the corresponding XBT-derived salinities. Overall, there were 114 XBT/XCTD (temperature/salinity) transects from February 1997 to December 2019 that were used to calculate the time-mean temperature and salinity fields; 53 of these transects fall along the most commonly sampled line (Supplementary Fig. S1). From October 2005 to December 2019, 69 of 114 transects were available of which 60 transects coincide with the OS38 ADCP transects.

Objective mapping temperature and salinity transects
Each temperature and salinity transect was objectively mapped to a latitude-depth grid with a horizontal and vertical resolution of 1/10° and 10 m starting from the surface to 760 m, respectively, a depth range commonly reached by most XBT and XCTD probes. Gaps in the XBT/XCTD data below 760 m were relatively large, therefore, making the normalized mapping error larger than the noise-to-signal ratio. For the temperature horizontal grid, two Gaussian functions were used: one with a large-scale decorrelation scale of 100 km and another with a scale of 30 km. Likewise, two scales were employed for the XCTD salinity: a 100-km scale and a 50-km scale. The latter was employed since the spacing between XCTD profiles is larger than that for the XBT profiles.

Constructing time series of total velocity
The velocity vector was rotated to an along/cross-transect coordinate system. For this study, only the cross-transect component utot was kept (positive flow is eastward) and then bin-averaged onto a 25-km along-track grid24. For transects with continuous gaps ≤150 km, the gaps were filled using an objective mapping as follows. Velocity anomalies were estimated by subtracting the time-mean cross-transect velocity component from the utot. The time-mean velocity is obtained by calculating objectively mapped mean velocity vectors as in ref. 28 (and references therein). In calculating the time-mean velocity, velocity transects from all 286 crossings were employed. For each transect, the nearest time-mean velocity (in a 25 km × 25 km grid-box) to the utot velocity profile was employed and rotated to the along/cross-transect coordinates to estimate the time-mean cross-transect velocity component and subtract it from utot. Next, the total velocity anomalies were objectively mapped into an along-track/depth grid using a Gaussian covariance function with horizontal and vertical decorrelation scales of 50 km and 300 m, respectively. Subsequently, the gaps were filled with the mapped anomalies plus the time-mean cross-transect component. Finally, the OS38 time series were corrected following ref. 24 due to a systematic offset between northbound and southbound Drake Passage total transport. The transport bias arises from a bias in the cross-transect velocity and is consistent with a rotation calibration error due to uncertainty in the measured transducer alignment. This angle error results in a component of the ship’s speed erroneously projected in the cross-track component of velocity. Supplementary information provides details about the method employed to minimize transport bias.

Total, geostrophic, and reference transport
The upper-ocean net total Drake Passage transport was calculated as Utot = ∫L ∫0z0 utot dz dx, where L is the length of the transect, dx is the horizontal spacing (25 km), and z0 = −760 m is the common maximum depth between the OS38 and XBT/XCTD profiles. The error in absolute transport in the upper 760 m per 25-km along-track bin (dx) is estimated from the absolute velocity error (σtot) as σtot × dx × H / √n, where H = 760 m and n is the number of observations (transects), yielding 8 × 10−3 Sv for 237 transects for the October 2005–April 2019 period. Similarly, the standard error in absolute net Drake Passage total transport, estimated as σtot × L × H / √n, is 0.28 Sv.

The geostrophic cross-transect velocity component ugeo and Drake Passage transport Ugeo were estimated from the objectively mapped temperature and salinity transects. The transects were employed to estimate the geopotential anomaly Φ, which is the geostrophic stream function, given as:

Φ = ∫p0p δ(x,p) dp, (1)

where p0 = 760 db is the reference pressure, p is pressure, and δ is the specific volume anomaly estimated from the temperature and salinity transects. Cross-transect geostrophic velocity ugeo was then calculated as the first difference of Φ with respect to the along-track distance:

ugeo = − 1/f ∂Φ/∂x, (2)

where f is the local Coriolis frequency, and x is the along-track distance. The geostrophic Drake Passage transport was calculated as Ugeo = ∫L ∫0z0 ugeo dz dx (dx ~ 12 km).

The total cross-transect velocity utot, defined as that of the ADCP velocities, can be decomposed into geostrophic ugeo, reference uref and Ekman uEkm components (i.e., utot = ugeo + uref + uEkm) when XBT/XCTD transects are available. We estimated the Ekman component by subtracting the geostrophic component profile from the total component profile [i.e., uEkm = utot − ugeo; ref. 48] within the Ekman layer (z ≤ zEkm). The Ekman layer was defined where the total shear showed an exponentially decaying profile, after first averaging per transect and then averaging for the entire time series48. For our observations, the base of the Ekman layer was zEkm = −90 m since the mean shear velocity reduces to a constant value below 90 m and is in good agreement with the geostrophic shear. Here, we interpolated the total velocity to the geostrophic velocity along-track and depth grid for each transect. Thus the Ekman transport was calculated as UEkm = ∫L ∫0zEkm uEkm dz dx. Consequently, the reference component uref was defined as the layer-averaged residual velocity between the base of the Ekman layer and the deepest bin z0:

uref = 1/(zEkm − z0) ∫z0zEkm (utot − ugeo) dz, (3)

Therefore, the reference Drake Passage transport is Uref = ∫L ∫z0zEkm uref dz dx.

Isopycnal heaving and spiciness
To understand the changes in potential temperature θ and salinity S with depth and position across Drake Passage, temperature and salinity trends were decomposed into two main contributions29: modification of temperature and salinity along isopycnals (neutral density layers γ) called “spiciness”, and vertical displacement “heave” of neutral density layers49. As discussed in ref. 30, the analysis when done along pressure-density surfaces allows a direct calculation of the component of temperature and salinity change due to vertical heave of neutral density surfaces, and that due to spice. The decomposition of the total potential temperature and salinity changes ∂θ/∂tp and ∂S/∂tp on a pressure surface is:

∂θ/∂tp = ∂θ/∂tγ + ∂θ/∂γ ∂γ/∂tp + Res, (4)

∂S/∂tp = ∂S/∂tγ + ∂S/∂γ ∂γ/∂tp + Res, (5)

The first terms on the right-hand side of Eqs. (4) and (5) represent the changes on neutral density levels, whereas ∂θ/∂γ and ∂S/∂γ are the local gradient of mean θ and S in γ, and ∂γ/∂tp represents the neutral density change on a pressure surface. Res represents other terms from the Taylor expansion that are considered as a residual.

An important point is that the spiciness changes represent a shift in the θ/S profiles at a constant γ. Consequently, a change in salinity involves a change in temperature and conversely, temperature change along γ involves a change in salinity. The heave component reflects adiabatic processes such as wind-driven Ekman pumping and low-frequency Rossby waves.

Tracking the position of the ACC streamlines
To determine the trends across Drake Passage as a function of the ACC frontal regions, we employed a synoptic streamwise coordinate system (sea surface height; SSH) in a similar way as used in ref. 28 for eddy heat flux. We used a combination of the mean dynamic topography50 and the SSALTO/DUACS daily maps of sea-level anomalies from AVISO to track the position of the ACC streamlines. The mean dynamic topography is directly tied to the large-scale surface geostrophic circulation constrained by 20 years of in situ observations such as drifters corrected due to wind/Ekman currents, altimetry, Argo temperature/salinity profiles, and conductivity–temperature–depth (CTD) casts50. The daily maps were obtained from multiple satellite altimeters and objectively mapped to a 0.25° × 0.25° Cartesian grid51. The sea-level anomalies are relative to a 20-year mean of the sea surface height field. Adding the mean dynamic topography to the sea-level anomalies produces maps of dynamical SSH that enable tracking of the shifting and meandering of the ACC streamlines and mesoscale eddies. We used the daily maps from February 1999 to December 2019, which covers our period of interest, and we subtracted a 0.6 m constant from the daily maps following ref. 28 (and references therein), which does not affect the position of the fronts and their horizontal gradients.

The daily SSH are smoothed with a 20-day box-car window; we tested if the binning, and thus the trends, are sensitive to the smoothing window, however, no significant changes in the trends were observed. The tracking method provides an algorithm to attribute and bin data on occasions when a transect crosses the same front more than once. For example, transects often cross the Subantarctic Front twice, as it turns steeply from eastward to northward following the South American shelf break, forming meanders that can detach as eddies with closed contours. Consequently, more profiles fall within closed contours when the fronts meander or during eddy formation and detachment (see Appendix in ref. 28). Hence, we expect that an increase in meandering and eddy formation augments the number of transects that cross a meander or eddy. This aspect of the synoptic SSH method could impact our interpretation of the acceleration of the jets on the ACC fronts. Velocity profiles falling within each unique pair of SSH streamlines are integrated vertically with depth and distance to format transport estimate per pair of streamlines per transect. utot is re-interpolated to a ~12 km along-track distance (similar to that of ugeo and uref) so that the criterion of one point per pair of streamlines per transect is met.

Estimating eddy kinetic energy and eddy momentum flux
To quantify trends in the eddy activity in Drake Passage and the eddy momentum transfer with the ACC jets, we estimated time series of depth-averaged eddy kinetic energy and eddy momentum flux from the 5-min (100 pings; 1.5 km along-track resolution) OS38 ADCP records corrected for transducer misalignment angle (see supplementary information Eq. (4)). We focus on the period of October 2005–April 2019 (237 velocity transects), which is the period employed to estimate trends in cross-transect velocity and streamwise transport. Following 28, eddy velocity terms per transect per depth bin were estimated as u′ = u − u, where u and u are the 5-min velocity vector and the time-mean velocity vector. Subsequently, depth-averaged (0–760 m) eddy kinetic energy (EKE) and eddy momentum flux (EMF) per transect were calculated as

EKE = 1/H ∫z00 0.5(u′2 + v′2) dz, (6)

EMF = 1/H ∫z00 u′v′ dz, (7)

Next, each EKE and EMF transect was bin-averaged at a ~12 km along-track distance and then streamwise averaged using the synoptic SSH to construct time series of <EKE> and <EMF> per transect per pair of streamlines, where the angle brackets <⋅> denote the bin and streamwise averaging. The ~12 km resolution allows us to meet the criterion of at least one point per pair of streamlines per transect.

Statistical analysis: trends and significance
Trends in Drake Passage transports (Utot, Ugeo, and Uref), cross-transect velocities (utot, ugeo, and uref), salinity S, potential temperature θ, <EKE> and <EMF> were calculated after removing the seasonal cycle (sum of the annual and semiannual harmonics). The trends were estimated by least-squares fitting ^y = a0 + a1t to each time series; the second term a1 on the right-hand side of the equation is the trend. Uncertainty in the calculated least-square trends is reported as the 95% confidence intervals52 using

CIa1 = ± tα/2 × RMSE × 1 / √∑(t − t)2, (8)

RMSE = √∑n (^yi − yi)2 / (n − μ), (9)

where tα/2 is the Student’s t statistic for 95% confidence (i.e., α = 0.05), RMSE is the root mean square error, ^y is the predicted variable, yi is the observed variable, n − μ is the number of observations minus the number of parameters fitted μ = 2, and t is the average time in years.

Statistical significance of the trends employed a modified Mann–Kendall test53, a commonly used nonparametric trend test. The modified test takes into account autocorrelations within the time series. The variance of a variable for autocorrelated data can be calculated as

V* = σ2 × n/n* = n(n−1)(2n+5) / 18 × n/n*, (10)

n/n* = 1 + 2/n(n−1)(n−2) ∑i=1n−1 (n−i)(n−i−1)(n−i−2)C(i), (11)

where n/n* represents a correction related to autocorrelation in the time series, and C(i) denotes the autocorrelation between the ranks of observations. This study assumes that trends were statistically significant when the modified Mann–Kendall test resulted in a probability p < 0.05.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "In “compensafing transport trends in the Drake Passage frontal regions yield no accelerafion in net transport”, Gufierrez-Villanueva et al. presented the Drake Passage transport calculated from 15 years of direct measurements (2005-2019) for the top 760 m, based on shipboard ADCP surveys along with XBT/XCTD measurements (that covered from 1997 to 2019). The key finding is that while the net transport measured exhibits no accelerafion, the key front regions do exhibit stafisfically significant trends that have opposite signs. They further suggested that the accelerafion of the frontal jets results from an increase in the mesoscale eddy acfivity due to buoyancy changes in the fronts, consistent with an eddy saturafion state. I believe this observafional work is of great value to the community and, to some degree, I think the authors can emphasize a bit more on the strength and/or the importance of this unique observafion, i.e., high-resolufion direct measurement of the current/transport across the full passage (for the upper ocean). However, there are some places I think the point is a bit unclear or potenfially misleading, hence some clarificafions are needed.\n1. The compensafing trends in frontal regions “explain” (L20) or “are likely responsible for” (L69) the lack of trend in the net transport. Probably a minor wording issue but I am not sure if this causafive implicafion is appropriate. The net transport is made of the transports of the front regions, so it is somewhat strange to call one explains/is responsible for the other. Simply stafing the results, i.e., no trends in the net transport, but compensafing trends in the frontal regions, is more appropriate to me.\n2. I do not think it is accurate to state “SAF and PF jets have accelerated while the currents located between the front have decelerated …” (L17-18). If one reads Fig. 3 correctly, the strongest increase is located between SAF and PF, the southern half of the PF and the region between PF and SACCF show decreasing velocity (L130-134; L185-186).\n3. Not sure if “accelerafion of frontal jets results from an increase of mesoscale eddy acfivity” is consistent with eddy saturafion state (L22-23). If an increase of mesoscale eddy acfivity leads to accelerafion of the frontal jets, it should ulfimately lead to a stronger net ACC transport, right? And what contributes the decreasing trend in the southern part of PF and north of SACCF?\n4. I wonder if it helps to remove the mean from Fig 5, because it is difficult to see trend (which might be the reason to have Fig. 6). More importantly, I think it is valuable to have a similar fime series plot from the geophysical coordinate perspecfive, showing over fime the change of verfically integrated velocity that corresponds to the trend in Fig. 3 (This plot can even include those secfions that did not cross 1000 m isobath, if those secfions have been processed, just not included in transport calculafion).\n5. The ADCP based transport is calculated for the top 760 m, not to 1000 m as in the previous work covering 4.5 years of the observafions (Firing et al., 2011). I assume this is because many secfions do not extend to 1000 m, otherwise, one may argue it is valuable to have a separate fime series that cover the top 1000 m to be consistent with the previous work (separafing between geostrophic and reference transport at 760 m seems of less importance to me, although that is arguable).\nOther details:\nL39. Ref 4 may be placed at the end of this sentence as it is about both ozone and greenhouse gas emissions (It should be menfioned that it is a modeling work). Not sure why ref 5 (also a modeling work) is included here as it is mostly about ocean warming, not so much about wind or southern annular mode.\nL58, “… fime series were not yet long enough” and L111. “… using short sampling periods” are not accurate for refs. 16 and 19. The geostrophic transports based on hydrographic surveys (ref 16) cover a prefty long period (although less frequent). Ref 19 rely much on satellite SSH which is not direct measurement of current. I think the strength or value of this ‘unique’ observafion needs to be highlighted a bit more, i.e., high-resolufion, direct measurements of the current across the full Drake passage over a long period of fime (One limitafion is that it covers only upper part of the water column).\nL113. Refs. 21, 22 find no accelerafion in the Drake passage but their main result is that the warming is leading to accelerafion in zonal flow in much of the Southern Ocean, and I do not think low-resolufion climate model is the best examples to show no accelerafion in ACC transport (because without eddies, a stronger wind tends to drive a stronger ACC transport).\nL114-L120. Ref. 23 found that “baroclinic and barotropic transports are not correlated; thus, monitoring either baroclinic or barotropic transport alone may be insufficient to assess the temporal variability of the total ACC transport”. They did not imply that the lack of trend in their modeled transport is due to two components compensafing each other. Furthermore, the separafion of geostrophic and reference velocity at 760 m has different meaning from the baroclinic - barotropic separafion for full water column. On a different perspecfive, I think it is noteworthy that ref 23 carefully evaluated the modeled ACC transport structure using various observafions, including the result of the first 4.5 years of the ADCP measurement that are presented in this paper. The agreement between model and the ADCP measurement for the top 1000 m was one key reason that gives the confidence in their modeled transport. Thus, the observafions like this are also of great value to modeling community as well. I think this point could be noted somewhere in the paper.\nL176. “[32] found a similar …” you may rephrase this as “a similar cooling … [32]”.\nFig.1 I assume panels b-d is along the common secfion? If yes, it should be menfioned in the Figure capfion and noted that the locafion of this secfion is indicated in S1, or add a one line in panel a.", "reviewer_2": "This paper uses a 15-year fime series of subsurface temperature, salinity, and velocity in the Drake passage to invesfigate the structure of the transport trends of the Antarcfic Circumpolar Current. The paper presents the trends in the overall net transport across the Drake Passage, cross-passage velocity and streamwise transport, and link them with the buoyancy changes across the passage.\nThese results hold significant importance as they offer a detailed descripfion of the transport trends in the Drake Passage, parficularly in relafion to the ACC's response to strengthening winds, which remains an acfive research quesfion. It is worth nofing that although the Drake Passage is the most regularly sampled secfion of the Southern Ocean, long-term fime series of temperature, salinity, and velocity across the enfire Southern Ocean are scarce and highly valuable.\nThe paper is well wriften and only a few minor issues need to be addressed or clarified in my opinion.\nComments / Quesfions:\nThe paper frequently refers to the impact of wind on buoyancy and velocity change. Have the authors aftempted to compute trends in winds or wind stress curl along the transect? If so, how consistent are these trends with Figure 4 and the changes described in Secfion 4?\nL144: “is located”?\nFigure 3 and Secfion 3 may confuse readers. While Figure 3 sets the methodology for separafing the total, reference, and geostrophic components, and shows in a simple way that the compensafion of the ACC frontal regions, it presents results that, at first glance, seem to contradict the final conclusions. Specifically, it shows a decelerafion of velocifies in the SAF and PF zones, as well as an accelerafion in the region between them, whereas the abstract menfions that the subantarcfic and polar jets have accelerated while the currents between them have decelerated. While this might be confusing at a first read, this confusion could be partly avoided by adding a few similar words as the opening of secfion 5. I leave this decision up to the authors.\nL173-175: The difference in magnitude between Fig. 4 and Fig. S2 is surprising. Does that quesfion the robustness of the trends?\nL283: “to rise”\nL299: Is there any specific criterion used do discard the transects that significantly deviated from the main course?\nL338-340: How many of the transects between October 2005 and December 2019 fall into the most sampled line? I am just wondering how the zonal spreading of the transects could impact the cross-transect velocity, temperature and salinity trends. More specifically, can that explain part of the differences in the trends from the longer and shorter fime periods (Fig. 3a vs Fig Fig.3b; Fig.4 vs Fig S2).\nL425-428: What is the mofivafion behind the use of two different datasets sources for the Mean Dynamic Topography and Sea Level Anomaly? This could introduce discrepancies between the datasets that could be avoided with the use of the full SSH signal from SSALTO/DUACS.\nL447-449: Does that mean that profiles falling into closed contours are discarded?\nL459: You might mean a_0 instead of a_1\nFigure 3. capfion: “Trends in (a) and (b) are esfimated using all available (237) velocity transects and only those (63) velocity transects that have a coincident temperature/salinity transect used to compute the geostrophic ugeo component in (c), respecfively.”\nPlease split this sentence into two parts: Trends in (a)… while trends in (b)…", "reviewer_3": "This paper analyses a 15-year fime series of Drake Passage made up of underway ADCP measurements and XBT/XCTD deployments. The data has been collected during crossings of ARSV Laurence M. Gould during transits from South America to the Antarcfic Peninsula. The measurements cover the top 1000 m, or so, of the water column with analysis restricted to between depths of 20-40 m and 760 m, due to instrument constraints and quality control processing. The resulfing transects clearly show the 3 main jets of the Antarcfic Circumpolar Current (ACC) and the steeply inclined isopycnals that accompany them. Analysis of least square trends shows that there are no meaningful trends in the Drake Passage transport, consistent with previous publicafions. However, there are stafisfically significant trends in the cross-transect velocity, which compensate when the area-integrated cross-transect transect is calculated. The locafions of these trends indicates a stafisfically significant accelerafion of the Polar Front and Subantarcfic Front, with compensafing decelerafion between the fronts. This is interpreted as being a result of increased wind stress leading to locally steepened isopycnals and increased Eddy Kinefic Energy (EKE), with the more vigorous eddy field then sharpening and accelerafing the jets due to their momentum transport.\nMy experfise is not in analysing and processing observafional data. This makes it difficult for me to comment on these aspects of the paper. The methods secfion appears thorough and well thought out. It is supported by citafions to previous applicafions of similar techniques with similar correcfions, etc, being found necessary in the current paper. The stafisfical tests are consistently applied and systemafically reported. With my inexpert eye, this all seems quite reasonable.\nThe paper makes a valuable contribufion to understanding how the Southern Ocean and ACC has and/or will respond to climate change. Their results are consistent with previous ones showing no increase in ACC transport. However, they are able to add the important detail that this may be because of compensafion between different regions increasing/decreasing their velocity. The authors link this to eddy-driving of the jets via eddy momentum fluxes and provide quanfificafion of the same.\nThere are some points missing from the discussion that I think should be included. The observafions only cover the top 760 m of the water column, amounfing to 61% of the baroclinic transport or 47% of the transport including the near-boftom flow (lines 202-204). However, there is no discussion of whether the results are likely to hold over the whole water column. Is it likely that the same paftern of spafially localised accelerafions and decelerafions hold? The second thing that might warrant some discussion is whether the conclusions would hold upstream/downstream of Drake Passage. A recent paper, Shi et al. (2021), indicates that there might be accelerafion of the northern edge of the ACC elsewhere in the Southern Ocean. Can the authors results shed any light on whether this might affect the circumpolar transport of the ACC? Or is this too speculafive, given limited Southern Ocean observafions?\nMinor Comments\nThere were a few minor things I noficed whilst reading the paper that the authors may wish to consider.\nLines 73-76 : This sentence seems incomplete, did the authors mean to point towards Figure 1?\nFigure 5b : I found it hard to tell the lines apart, because there colour is the same, even with one having prominent markers. Different colours might be befter.\nLines 148-151 & Figure 3b : I was puzzled as to the difference between Figure 3a and 3b unfil I got to this point. It might be befter to put this informafion in the figure capfion or move it earlier in the paragraph, since readers may look at the figure before reading the full paragraph (as I did)."}, "author_response": "Response to Reviewers’ Comments\nNCOMMS-23-17942-T\nManuel O. Gutierrez-Villanuevaa, Teresa K. Chereskin, and Janet Sprintall\namog002@ucsd.edu\nSeptember 26, 2023\nThankyouforreviewingourmanuscript. Below,wehaveprovideddetailedpoint-by-pointreplies. Wehave\nusedboldanditalicfontfortheReviewers’commentswhereasourresponsesarewritteninnormalfont. In\nourresponses,weusedtextinbluetoindicatethattexthasbeenincorporatedintotherevisedmanuscript.\nTextinredintheaccompaniedmanuscriptwithtrackchangesindicatesthattexthasbeenremovedfromthe\noriginalmanuscript.\nMajor changes\nReply: We wish to mention major changes in the manuscript in this revision in response to the reviewer\ncomments.\n1. Revised Figure 2 and supplementary Fig. S1\nPanelb: Wefound6additionalXBT/XCTDtransectsforthegeostrohpictransporttimeseriesandtrendcal-\nculationsshowninFig. 2b. Althoughthetrendisslightlyaltered,theconclusion(nosignificanttrendinthe\nDrakePassagegeostrophictransport)didnotchange.\nPanelc: Wefoundamistakeincalculatingthereferencetransporttimeseriesthatbiaseditlow. Wecorrected\nthe mistake and recalculated the reference transport and trend (also shown in supplementary Fig. S1).\nHowever,theconclusion(nosignificanttrendintheDrakePassagenettransport)isunaltered.\ncross-transect(a)totalU ,(b)geostrophicU ,and(c)referenceU DrakePassagetransport(1Sv=106\ntot geo ref\nm3 s−1). Theseasonalcycle(annual+semiannualharmonics)wasremovedforeachtimeseries. Standard\ndeviations for each calendar year are shown by shading. Thick solid lines show the linear fits for the time\nseries. Graysolidlinein(b)isthelinearfitfortheperiodofOctober2005-April2019,coincidingwith(c)\nreferencetransportperiod. Least-squarestrendsand95%errorbarsareshownwithtext[Svyear−1]. None\nofthetrendsarestatisticallysignificantafterthemodifiedMann-Kendalltest(p>0.05).\nyear−1]. Triangles indicate that all available transects are employed. Trends estimated for the period of\nOctober2005-April2019aremarkedwithsquares. Redcircleanddiamondshowthetrendsestimatedwith\nthosevelocitytransectsthathaveacoincidenttemperature/salinity(XBT/XCTD)transect(subsampled)and\nalongthemostrepeatedline(redthicklineinsmallinset;seeMethods),respectively. Noneofthetrendsare\nstatisticallysignificantafterthemodifiedMann-Kendalltest(p>0.05).\n3\n2. Revised Figure 3\nPanel d: We removed this panel, as the reference velocity is constant (i.e., depth-independent) and the\ncontent is very similar to the trend in reference transport shown in the new Fig. 4i. Text in section 3\nregarding the trends in cross-passage velocity has been modified to reflect the revised Fig. 3. The revised\nfigureisshownbelow,andtherevisedtextreads:\nLines126-145: WeinvestigatedwhetherthereareopposingregionaltrendsintheACCfrontaljetsconsistent\nwithnotrendinDrakePassagetransport. Weestimatedtrendsinthecross-transecttotalu andgeostrophic\ntot\nu velocitycomponentsforthecommonsamplingperiodOctober2005-April2019.\ngeo\nSignificant opposing trends in cross-track velocity are found within Drake Passage. All significant trends\n(p < 0.05) show no change in sign with depth (Fig. 3). Since the jets of the ACC are equivalent barotropic\n(velocityisself-similarwithdepth;KillworthandHughes,2002),thesignofthetrendslikelyholdsoverthe\nentirewatercolumn,althoughthemagnitudemaydecay. Thelargestsignificantaccelerationoftheeastward\nflow is found in the region between the Subantarctic Front and Polar Front for the total and geostrophic\ncomponents (150-300 km, Fig. 3a,c). A weak, significant acceleration is found also in the Southern ACC\nFront (> 750 km, Fig. 3a,c). In contrast, a significant deceleration of the total flow component is found\nwithin the Polar Front (400-500 km, Fig. 3a) and to the south (550-600 km, Fig. 3a). A broader and\nweaker deceleration is found in the geostrophic component, but mainly within the Polar Front (350-500\nkm, Fig. 3c). A similar distribution in space and sign is found (although with slightly larger amplitudes)\nwhentotalvelocitytrendsarecalculatedwiththesubsetofthetotalvelocitytransectsthatcoincidewiththe\ntemperature/salinitytransects(Fig. 3b).\n4\na SAF PF SACCF\n0\n0\n0\n]-200 1\nm 0 01 2 5\n[ 15 5\n2\nh-400 2\n0 1\nt 02\np 0\n5\ne 0\nD -600 0 Total velocity u\n050 0 50 tot\n211 011\nb SAF PF SACCF\n0\n0\n] m -200 0 11 0 11\n[ 0 052 05\nh-400 2 2 5\nt p 0 2 0\ne 5\nD -600 Total velocity u\n050 5 0 tot 0\n0 211 0 1 1 0 1\nc SAF PF SACCF\n0\n8\n0\n]\nm-200 8 0\n5\n[ 5 0 0\nh-400 0\nt\np 0\ne -600 Geostrophic velocity u\nD 0 geo\n0 200 400 600 800\nDistance from North [km]\nFig.3Trendsincross-transectvelocitycomponents[cms−1 year−1]fromOctober2005-April2019. Trends\nincross-transect(a)-(b)totalu ,and(c)geostrophicu component. Trendsin(a)areestimatedusingall\ntot geo\navailable(237)velocitytransectswhiletrendsin(b)arecalculatedusingonlythose(60)velocitytransects\nthathaveaconcurrenttemperature/salinitytransectusedtocomputethegeostrophicu componentin(c).\ngeo\nBlackthicksolidcontoursshowthezerotrend. Hatchedareasarenon-significanttrendsat95%confidence\nafterthemodifiedMann-Kendalltest(p>0.05). Cyancontoursshowthemeanvelocitycomponent[cms−1].\nThemeanlocationoftheSubantarcticFront(SAF),PolarFront(PF),andSouthernACCFront(SACCF)are\nindicatedbyhorizontalgold,gray,andmagentabarsrespectively.\n5\n3. New Figure 4\nIn responding to reviewer 1, we included a new figure showing the time series of transport anomalies and\nthemeansandtrendsoftransport,binnedbydistanceacrossDrakePassage(Fig. 4)forcomparisonwiththe\nsame calculation made binning by streamlines (revised Fig. 5, details below). The revised figure is shown\nbelow,andtherevisedtextinsection3reads:\nLines 146-174: We estimated time series of cross-transect total u , geostrophic u , and reference u\ntot geo ref\ntransportperdistancebin. Thenettotal,geostrophicandreferenceDrakePassagetransportsare75.54±0.64\nSv, 15.85±0.20 Sv, and 51.08±0.85 Sv respectively (Fig. 4b,e,h). The sum of the geostrophic, reference\nand Ekman (8.09±0.22 Sv; see Methods) transports yields 75.02±0.90 Sv, which agrees with the net total\ntransport calculated using the concurrent transects (77.87±1.04 Sv; green line in Fig. 4b). Extending the\ntotal transport calculation to the upper 970 m yields a 15-year mean of 89.41±4.69 Sv, in close agreement\nwithFiringetal.(2011)’smeanof95.00±2.10Svbasedonthefirstfiveyearsofdata(onlyusingtransects\nalong the most repeated line), and the 40-year mean modeled transport of 92.7 Sv in the upper 1000 m\ncalculated by Xu et al. (2020) from a data-constrained eddy-permitting ocean model. The 75.54 Sv net\ntotaltransportintheupper760mis56%ofthefull-depthtotalcanonicaltransportof134Svestimatedby\n(Cunninghametal.,2003)and48%ofthe40-yearmeanfull-depthmodeltransportof157Svestimatedby\n(Xuetal.,2020).\nTransportanomaliesforthethreecomponentsarethelargestinthenorthernhalfofDrakePassage,especially\nin the region between the Subantarctic Front and Polar Front (0-500 km, Fig. 4a,d,g). This region has the\nlargestmesoscaleeddyactivity(meanderingofthefrontsandeddyformation)withinDrakePassage(Watts\netal.,2016;Lennetal.,2011;Foppertetal.,2017;Gutierrez-Villanuevaetal.,2020). Southofthisregionthe\ntransportanomaliesaretheweakest(>500km,Fig. 4a,d,g). Whilethetimeseriesoftransportanomaliesdo\nnotreadilyshowdiscernibletrends,theestimatedtrends(Fig. 4c,f,i)showclearsignificanttrendsconsistent\nwiththevelocitytrends(Fig. 3). Alltransportcomponentsshowaclearaccelerationoftheregionbetween\ntheSubantarcticFrontandPolarFront(150-300km;Fig. 4c,f,i)anddecelerationofthesouthernflankofthe\nPolarFront(350-550km; Fig. 4c,f,i). Similarpatternsarefoundwhentotaltransporttrendsarecalculated\nwiththetotalvelocitytransectsthatcoincidewiththetemperature/salinitytransects(Fig. 4c),orusingthe\ntransectsfallingalongthemostrepeatedcrossing(notshown).\n6\na b c\n0 16\nd\n]m\nk[ h\ntro N m o rf e cn\na tsiD\n2\n4 6\n8\n0\n0 0\n0\n0\n0 0\ne\nS\nP\nA\nF\nF\n7 7 5 7 . . 5 8\nT\n4 7 ± ±\no\nS\n0 1\nt\n. .\na\nA\n6 0\nl\n4 4\nC\nS S\nC\nv v\nF\nf\nTotal\nA S l u l\nS\nb tr\nS\nA\ns a P a\nA\nC\nn m F s\nF\nC\ne p c l\nF\ne ts d\n-\n-\n-\n1\n1\n1 0 4\n8\n8 4\n2\n6\n2\n]vS\n[ yla\nm o n a tro p sn\na rT\n0 3\ng\n]m\nk[\nh tro\nN m o rf e cn\na tsiD\n2\n4 6\n8\n0\n0 0\n0\n0\n0 0\nh\nGeos\n15\ntr\n.8\no\n5\np\n±\nS\nh\nS\nA\n.\nA\nc\nF\nS\nC\nv\nF\ni\nGeostrophic\nS\nS\nA\nP\nA\nC\nF\nF\nCF -\n-\n-\n0\n1\n2\n3\n2 1\n]vS\n[\nyla\nm o n a tro p sn\na rT\n0\n]m\nk[ h tro\nN m\n2\n4\n0\n0\n0\nRe\nS\nP\nf\nA\ne\nF\nF\nrence Reference\nS\nP\nA\nF\nF\n0\n2 4\n6 ]vS\n[ yla\nm o n\no rf e cn\na tsiD\n6\n8\n0\n0\n0\nS\n0 0 .\nA\n. 8 2 5\nC\nC\nS v v\nF SACCF -\n-\n-\n6\n4 2 a tro p sn\na rT\n2006 2008 2010 2012 2014 2016 2018 2020 0\nMe a\n1\nn tr a\n2\nns p\n3\nort\n4\n[Sv ]\n5 6 -0.3 -0.2 -0.1 0 0.1 0.2 0.3\nTrend [Sv year-1]\nFig. 4 Time series of transport anomaly [Sv], mean transport [Sv] and trends [Sv year−1] per distance\nbin[km]forOctober2005-April2019. Timeseriesof(a)total,(d)geostrophic,and(g)referencetransport\nanomaliesintheupper760m. Mean(b)total,(e)geostrophic,and(h)referencetransport[Sv]perdistance\nbin. Trends in (c) total, (f) geostrophic, and (i) reference transport [Sv year−1] per distance bin. Red and\ngreenlines(b)-(c)showthemeanandtrendsestimatedforall(237)availabletransectsandonlythose(60)\nvelocitytransectsthathaveacoincidenttemperature/salinitytransect,respectively. Filledsquares(c),(f),(i)\nindicatethattrendsaresignificantat95%confidenceafterthemodifiedMann-Kendalltest(p<0.05). Total\ntransport anomalies (a) are estimated based on the mean using all available transects (red line). Standard\nerrors for each line are shown in shaded areas in (b),(e),(h). Mean (± standard error) total, geostrophic\nandreferencetransportintegratedacrossDrakePassageareindicatedwithtextin(b),(e),(h). MeanEkman\ntransport (0-90 m) is also indicated in (h). Shaded rectangles show the approximate distance intervals\nfortheSubantarcticFront(SAF),PolarFront(PF),andSouthernACCFront(SACCF)(Gutierrez-Villanueva\netal.,2020).\n7\n4. Revised Figure 5\nThenewFig. 5isarevisionoftheoriginalperthesuggestionofreviewer1. Thetimeseriesshowtransport\nanoamliesinsteadoftransport. Thereisanewcolumnshowingtrends. ThecontentoftheoriginalFig. 6is\nnowshowninFig. 5c,f,i.\nDuring the revision, we identified an error in the binning of the transport time series by pair of sea surface\nheight(SSH)streamlinesandrecalculatedthetimeseries,meansandtrendsshowninthisfigure. Thetrends\nestimated using the streamwise coordinates show a much reduced pattern of acceleration/deceleration rel-\native to those estimated in passage coordinates (Fig. 4), with only 2 points showing significance. We use\nthis information, together with the means and trends of eddy kinetic energy (EKE) and eddy momentum\nflux(EMF),tomakeacaseforeddysaturationstateinsection3,afterpresentingthetimeseries,meansand\ntrendsoftransportincross-passagecoordinates. ThemodifiedFig. 5andtextinsection3areshownbelow:\nLines175-211: TheresultsbasedonbinningthetransportbydistanceindicatethatthedifferentACCfrontal\nregionsinDrakePassagehavetendedtocompensateeachotheroverthelast15yearssuchthatthenetDrake\nPassagetransportshowsnosignificantacceleration. Nonetheless,theaccelerationoftheregionbetweenthe\nfronts could be an effect of an increase in the meandering of the ACC frontal jets and eddy formation, an\naspect not captured when using the fixed cross-passage coordinate system. To explore this hypothesis we\nconstructed time series of transport anomalies and trends analogous to Fig. 4, but where the binning was\ndone per pair of streamlines (Fig. 5), using synoptic sea surface height (SSH) derived from a combination\nof daily altimetric sea level anomalies and mean geostrophic streamlines (Fig. 1a; see Methods). Contours\nof SSH provide a natural coordinate system (streamlines) for geostrophic flows. The streamlines allowed\nus to track the position of the fronts that comprise the ACC, which can vary (both in position and distance\nbetween each pair of streamlines) daily due to strong meandering of the fronts and eddy formation (Watts\netal.,2016;Gutierrez-Villanuevaetal.,2020).\nAs with the time series of transport anomalies in passage coordinates (Fig.4a,d,g), the transport anomalies\nin streamwise coordinates (Fig. 5a,d,g) are largest in northern Drake Passage, but are concentrated north\nof the Polar Front. Similar to the time series in passage coordinates (Fig. 4a,d,g), trends in the streamwise\ntime series are difficult to discern (Fig. 5a,d,g). The magnitude and pattern of the estimated trends for\nthe total transport in streamwise coordinates (Fig. 5c) are much reduced relative to passage coordinates\n(Fig. 4c), suggesting that at least some of the apparent acceleration in the zone between the Polar and\nSubantarcticFronts(Fig. 3,Fig. 4c)wasduetofrontsmeanderingandeddies. Whenbinnedbystreamline,\nthetransportanomalieswithinmeandersandeddiescontributetotheanomaliesofthefrontsratherthanthe\ninterfrontal zone. The main significant trend in the streamwise coordinate system is that the Subantarctic\nFront has significantly accelerated over the last 15 years (Fig. 5c). This result is only evident when using\nthe streamwise coordinate system, which is able to capture the effect of the steering of the fronts due to\nmeanderingandeddydetachment,anaspectthatismissedwhenusingthepassagecoordinates(Fig. 4).\nWewillexploretheincreasingmesoscaleactivityhypothesisinsection5byestimatingtrendsineddyenergy\nandmomentumflux. Inthefollowingsection,weexplorebuoyancychangesacrossDrakePassagethatmay\ncontributetoincreasededdyformation.\n2005-April 2019. Time series of (a) total, (d) geostrophic, and (g) reference transport anomalies in the\nupper 760 m. Mean (b) total, (e) geostrophic, and (h) reference transport [Sv]. Trends in (c) total, (f)\ngeostrophic,and(i)referencetransport[Svyear−1]. Redandgreenlines(b)-(c)showthemeanandtrends\nestimated for all (237) available transects and only those (60) velocity transects that have a concurrent\ntemperature/salinity transect, respectively. Filled squares (c),(f),(i) indicate that trends are significant at\n95%confidenceafterthemodifiedMann-Kendalltest(p<0.05). Totaltransportanomalies(a)areestimated\nbasedonthemeanusingallavailabletransects(redline). Standarderrorsforeachlineareshowninshaded\nareas in (b),(e),(h). Mean (± standard error) total, geostrophic and reference transport integrated across\nDrake Passage are indicated with text in (b),(e),(h). Ekman transport is indicated in (h) (see Methods).\nShaded rectangles show the streamwise intervals for the Subantarctic Front (SAF), Polar Front (PF), and\nSouthernACCFront(SACCF)(Gutierrez-Villanuevaetal.,2020).\n9\n5. New Figure 7\nThis figure showing depth-averaged EKE and EMF time series, means, and trends is moved from the sup-\nplementaryinformation tothemain text, aswe useit tosupportour caseforan eddysaturationstate. The\ndetailsofthecalculationhavebeenmovedfromthesupplementaryinformationtotheMethods(8.7Estimat-\ningeddykineticenergyandeddymomentumflux). ThenewFig. 7,thenewtextinsection3andmethods\nsectionareshownbelow:\nSection5. Trendsineddyactivity\nLines 241-282: As discussed in section 3, the acceleration of the transport in the region between the Sub-\nantarctic and Polar Fronts and the deceleration of the Polar Front jet (Figs. 3,4) potentially arise from an\nincreaseinthesteeringandmeanderingoftheACCfrontsduetoenhancedmesoscaleeddyactivity. Toexam-\ninethishypothesis,weconstructedtimeseriesofdepth-averagededdykineticenergyandeddymomentum\nfluxesperpairofstreamlines(<EKE>and<EMF>,respectively;seesupplementaryinformation).\nTime series of streamwise <EKE> show that eddy energy increases during the last quarter of the sampling\nperiod (Fig. 7a) in the zone between the Subantarctic Front and Polar Front (−0.60 m > SSH > −1.00\nm). This zone exhibits the largest time-mean (denoted by overbar) depth-averaged eddy kinetic energy\n(<EKE>), consistent with previous studies (Lenn et al., 2011; Gutierrez-Villanueva et al., 2020; Foppert\net al., 2017); <EKE> decreases significantly away from this region (Fig. 7b). Trends show a significant\nincrease in <EKE> in the south flank of the Subantarctic Front over the last 15 years (Fig. 7c), indicating\nan increase in the meandering of the front’s streamlines and eddy formation. Surprisingly, trends show a\nsignificantdecreaseof<EKE>inthePolarFront(Fig. 7c). Ourmeansareunalteredwhentheyarecalculated\nusingonlythose(60)velocitytransectsthathaveacoincidenttemperature/salinitytransect(Fig. 7b). Trends\nusingthecoincidenttransectsshowapositivepeakcenteredinthezonebetweentheSubantarcticandPolar\nFronts;thispeakliespolewardofthesignificantpositivetrendestimatedusingalltransects,althoughtrends\nusingthecoincidenttransectsareinsignificant(Fig. 7c). Ourresultsareconsistentwithaneddysaturation\nstate, i.e., the net ACC transport remains invariant to the increased wind forcing whereas the eddy activity\nincreases(HallbergandGnanadesikan,2001;MorrisonandHogg,2013).\nTime series of streamwise <EMF> show that the largest negative values occur in the region between the\nSubantarcticandPolarFronts,whereasrelativelysmallerpositivevaluesdominateintheSubantarcticFront\n(Fig. 7d). The time-mean depth-averaged eddy momentum flux <EMF> reflects the pattern seen in the\ntime series; it is positive in the Subantarctic Front (SSH > −0.65 m) and negative in the region between\nthe fronts (−0.65 m > SSH > −1.00 m) (Fig. 7e). Within and south of the Polar Front (SSH < −1.00 m),\n<EMF>isagainpositivealthoughitsvalueisclosetozero(Fig. 7e). Positivevaluesimplyanorthwardflux\nof eastward momentum and negative values imply a southward flux of eastward momentum (Lenn et al.,\n2011). This interpretation suggests that, on average, the eddies in the interfrontal zone redistribute zonal\nmomentumtowardstheSubantarcticFrontandthePolarFront. Thepositivetrendof<EMF>inthesouth-\nernflankoftheSubantarcticFront(Fig. 7f)suggeststhateddieshavesignificantlyincreasedthenorthward\nflux of zonal momentum in the south flank of the Subantarctic Front over the last 15 years. Similar results\nare obtained when means and trends are calculated using only those transects that have a coincident tem-\nperature/salinitytransect(Fig. 7e,f).\n10\na b c\n0.2\n-0.40\nSAF SAF ]\n]m-\n-\n0\n0\n.\n.\n8\n6\n0\n0 0.15 2\n2\n-s\n[ H-1.00 0.1 m\nS S -1.30 PF All transects PF 0.05 [ > E K\n- - 1 1 . . 6 4 5 5 SACCF Subsampled SACCF E <\n0\n2006 2008 2010 2012 2014 2016 2018 0 0.03 0.06 -0.2 0 0.2\n<EKE> [m2 s-2]Trend 10-2 [m2 s-2 year-1]\nd e f\n0.2\n- - 0 0 . . 6 4 0 0 SAF SAF 0.1 2-s ]\n]m-0.80 2\n[ H S S -1.00 PF PF 0 m [ > F\n-1.30 -0.1 M\n-1.45 E <\n-1.65 SACCF SACCF -0.2\n2006 2008 2010 2012 2014 2016 2018 -0.01 0 0.01 -0.4 0 0.4\n<EMF> [m 2 s-2]Trend 10-2 [m2 s-2 year-1]\nFig. 7 Time series of eddy kinetic energy (EKE) and eddy momentum fluxes (EMF), mean and trends per\npairofseasurfaceheight(SSH)streamlines[m]forOctober2005-April2019. Timeseriesofdepth-averaged\ntotal (a) <EKE> and (d) <EMF> in the upper 760 m [m2 s−2]. Mean (b) <EKE> and (e) <EMF> [m2\ns−2]. Trendsin(c)<EKE>and(f)<EMF>[m2s−2year−1]. Redandgreenlines(b)-(c)showthemeanand\ntrendsestimatedforall(237)availabletransectsandonlythose(60)velocitytransectsthathaveaconcurrent\ntemperature/salinity transect, respectively. Filled squares (c),(f) indicate that trends are significant at 95%\nconfidence after the modified Mann-Kendall test (p < 0.05). Standard errors for each line are shown in\nshadedareasin(b),(e). ShadedrectanglesshowthestreamwiseintervalsfortheSubantarcticFront(SAF),\nPolarFront(PF),andSouthernACCFront(SACCF)(Gutierrez-Villanuevaetal.,2020).\nLines 530-547: To quantify trends in the eddy activity in Drake Passage and the eddy momentum transfer\nwith the ACC jets, we estimated time series of depth-averaged eddy kinetic energy and eddy momentum\nfluxfromthecorrectedOS38ADCPrecords(seesupplementaryinformationeq. 4). Wefocusontheperiod\nof October 2005 - April 2019 (237 velocity transects), which is the period employed to estimate trends in\ncross-transectvelocityandstreamwisetransport. FollowingGutierrez-Villanuevaetal.(2020),eddyvelocity\ntermspertransectperdepthbinwereestimatedasu′ =u−u,whereuanduarethe5-minvelocityvector\nandthetime-meanvelocityvector(seeMethodsinmainmanuscript). Subsequently,depth-averaged(0-760\nm)eddykineticenergy(EKE)andeddy-momentumflux(EMF)pertransectwerecalculatedas\n1 (cid:90) 0\nEKE=\n0.5∗(u′2 +v′2\n)dz, (6)\n760\n−760\n1 (cid:90) 0\nEMF= u′v′dz. (7)\n760\n−760\nNext,eachEKEandEMFtransectwasbin-averagedata∼12kmalong-trackdistanceandthenstreamwise\naveraged using the synoptic SSH to construct time series of <EKE> and <EMF> per transect per pair of\nstreamlines, where the angle brackets < · > denote the bin and streamwise averaging. The ∼ 12 km\nresolutionallowsustomeetthecriterionofatleastonepointperpairofstreamlinespertransect.\n12\n6. New Figure 8\nWe have included a schematic summarizing the response of the Antarctic Circumpolar Current (ACC) to\nchanges within the frontal regions over the last 15 years as discussed in the paper. The new Fig. 8 and the\nmodifiedtextinthediscussionareshownbelow:\nLines303-317: Trendsofdepth-averagedEKEestimatedfromthevelocitytransectsshowthateddyactivity\nhas increased over the last 15 years in the zone between the Subantarctic and Polar Fronts, peaking on the\nsouthern flank of the Subantarctic Front (Fig. 7c). The increased EKE is consistent with altimetry-based\nEKEtrendsobservedinACCeddyhotspotslikeDrakePassageoverthelasttwodecades(Mart´ınez-Moreno\net al., 2021; Hogg et al., 2015). Our interpretation is consistent with an eddy saturation state (i.e., eddy\nactivity increases as the wind stress increases with no net acceleration of the ACC; Hallberg and Gnanade-\nsikan,2001;MeredithandHogg,2006;Pataraetal.,2016;Meredithetal.,2012;DownesandHogg,2013;\nMundayetal.,2013;MorrisonandHogg,2013)andwithobservedbuoyancychanges(Fig. 6). Thewarming\nintheSubantarcticFront(redpatch,Fig. 8a)andwind-drivencoolingsouthofthePolarFront(bluepatch,\nFig. 8b) act to steepen the isopycnals across the fronts (dashed tilted, Fig. 8a,b), enhancing the available\npotential energy stored in the fronts that is subsequently released through baroclinic instabilities (Vallis,\n2017). Consequently,morebaroclinicinstabilitiestranslateintomoresteeringandmeanderingofthefronts\nandeddyformation(Fig. 8c),whichfeedintothevigorouseddyfieldinDrakePassage(Gutierrez-Villanueva\netal.,2020;Wattsetal.,2016;Foppertetal.,2017;Lennetal.,2011).\na b Intensified Ekman c\nTransport\nSubantarctic Front Polar Front SAF\nSSH SSH S Sh ou el t f h American\nWarming Cooling\nhtpeD edutitaL\nEddy\nPF\nUpwelling\nEddy saturation state\nSouth America Antarctica South America Antarctica Longitude\nFig.8TheresponseoftheAntarcticCircumpolarCurrenttobuoyancychangeswithintheACCfrontalregions\noverthelast15-years. Buoyancychangesacrossthe(a)SubantarcticFront(SAF)and(b)PolarFront(PF).\nRedpatchin(a)showswarmingandbluepatchin(b)showwind-drivencooling. Bluethickhorizontalarrow\nin(b)istheintensifiedequatorwardEkmantransportduetoincreasingwesterlywindstressτ (circles). Blue\nx\nverticalarrowsin(b)showtheenhancedwind-drivenupwellingandshoalingofisopycnals. Solidblacklines\nare the fronts isopycnals γ (γ < γ < γ ) in normal conditions; dashed lines are steepened isopycnals due\n1 2 3\nto the buoyancy changes in the fronts. Surface thick solid lines are the sea surface height (SSH) in normal\nconditions; thick dashed lines are the SSH of the steepened fronts’ SSH. (c) Eddy saturation state in Drake\nPassage. Solid gold and gray contours show the SAF and PF SSH streamlines, respectively, with arrow\nheadsshowingtheflowdirectionwithinthefrontaljets. Filledgoldandsilvervorticesaremesoscaleeddies\ndetachedfromtheSAFandPF,respectively.\n13\n7. Ekman transport\nWehaveincludedanestimateoftheEkmantransportinFigs. 4hand5h,suchthattheEkman,geostrophic,\nand reference transports sum to the total. We have modified the Methods (Subsection 8.4) to include the\nEkmantransportcalculation. Thenewmethodssubsectionreads:\nSubsection8.4Total,geostrophicandreferencetransport\nLines 452-469: Thetotalcross-transectvelocityu ,definedasthatoftheADCPvelocities,canbedecom-\ntot\nposed into geostrophic u , reference u and Ekman u components (i.e. u = u +u +u )\ngeo ref Ekm tot geo ref Ekm\nwhenXBT/XCTDtransectsareavailable. WeestimatedtheEkmancomponentbysubtractingthegeostrophic\ncomponent profile from the total component profile (i.e., u = u −u ; Lenn and Chereskin, 2009)\nEkm tot geo\nwithin the Ekman layer (z ≤ z ). The Ekman layer was defined where the total shear showed an ex-\nEkm\nponentially decaying profile, after first averaging per transect and then averaging for the entire time series\n(LennandChereskin,2009). Forourobservations,thebaseoftheEkmanlayerwasz =−90msincethe\nEkm\nmeanshearvelocityreducestoaconstantvaluebelow90mandisingoodagreementwiththegeostrophic\nshear. Here, we interpolated the total velocity to the geostrophic velocity along-track and depth grid for\n(cid:82)L(cid:82)0\neachtransect. ThustheEkmantransportwascalculatedasU = u dzdx. Consequently,the\nEkm 0 zEkm Ekm\nreferencecomponentu wasdefinedastheaveragedresidualvelocitybetweenthebaseoftheEkmanlayer\nref\nandthedeepestbinz :\n0\n1 (cid:90) zEkm\nu = (u −u )dz. (3)\nref |z −z | tot geo\nEkm 0 z0\nTherefore,thereferenceDrakePassagetransportisU =\n(cid:82)L(cid:82)zEkmu\ndzdx.\nref 0 z0 ref\nIn “Compensating transport trends in the Drake Passage frontal regions yield no acceleration in net trans-\nport”, Gutierrez-Villanueva et al. presented the Drake Passage transport calculated from 15 years of direct\nmeasurements (2005-2019) for the top 760 m, based on shipboard ADCP surveys along with XBT/XCTD\nmeasurements(thatcoveredfrom1997to2019). Thekeyfindingisthatwhilethenettransportmeasured\nexhibits no acceleration, the key front regions do exhibit statistically significant trends that have opposite\nsigns. Theyfurthersuggestedthattheaccelerationofthefrontaljetsresultsfromanincreaseinthemesoscale\neddyactivity duetobuoyancy changesinthefronts, consistentwithaneddy saturationstate. Ibelieve this\nobservational work is of great value to the community and, to some degree, I think the authors can em-\nphasize a bit more on the strength and/or the importance of this unique observation, i.e., high-resolution\ndirect measurement of the current/transport across the full passage (for the upper ocean). However, there\nare some places I think the point is a bit unclear or potentially misleading, hence some clarifications are\nneeded.\nGeneral comments\n1: Thecompensatingtrendsinfrontalregions“explain”(L20)or“arelikelyresponsiblefor”(L69)the\nlack of trend in the net transport. Probably a minor wording issue but I am not sure if this causative\nimplication is appropriate. The net transport is made of the transports of the front regions, so it is\nsomewhatstrangetocalloneexplains/isresponsiblefortheother. Simplystatingtheresults,i.e.,no\ntrends in the net transport, but compensating trends in the frontal regions, is more appropriate to\nme.\nReply:Weagreewiththereviewerandhavemodifiedtheabstractto:\nLines17-20: Wefoundthat,althoughthenetDrakePassagetransportrelativeto760mshowsinsignificant\nacceleration,thenettransporttrendcomprisescompensatingtrendsacrosstheACCfrontalregions.\nWe also modified our discussion to be consistent with the abstract, and added a new figure (Fig. 8) which\nshows a schematic of the response of the ACC in Drake Passage to the buoyancy changes over the last 15\nyears. We refer the reviewer to the Major changes section (6. New Figure 8) in this response to read the\nmodifiedtextandnewFig. 8(Pages13,Lines303-317).\n2: I do not think it is accurate to state “SAF and PF jets have accelerated while the currents located\nbetweenthefronthavedecelerated...”(L17-18). IfonereadsFig. 3correctly,thestrongestincrease\nis located between SAF and PF, the southern half of the PF and the region between PF and SACCF\nshowdecreasingvelocity(L130-134;L185-186).\nReply: We thank the reviewer for raising this point. The confusion stems because of the projection of the\ntrendsin(now)Fig. 4(seeresponsetothenextcomment)indistanceacrossthepassage[km],whichdoes\nnotaccountforthemeanderingoftheACCfront’sstreamlinesandeddyformation. Wereferthereviewerto\nthe Major changes section (3. New Figure 4) in this response to read the modified text in section 3 (Pages\n6-7,Lines146-174). Also,wehavemovedFig. 5intosection3anddiscussedtheaccelerationoftheregion\n15\nbetween the fronts found in Fig. 3, 4 as an effect of an increase in the meandering of the ACC frontal jets\nand eddy formation. We refer the reviewer to the Major changes section (4. Revised Figure 5) to read the\nmodifiedtext(Pages8-9,Lines175-211).\n3: Not sure if “acceleration of frontal jets results from an increase of mesoscale eddy activity” is\nconsistent with eddy saturation state (L22-23). If an increase of mesoscale eddy activity leads to ac-\nceleration of the frontal jets, it should ultimately lead to a stronger net ACC transport, right? And\nwhatcontributesthedecreasingtrendinthesouthernpartofPFandnorthofSACCF?\nReply: We have modified our discussion about the interpretation of the eddy momentum flux across the\nfrontal regions. We refer the reviewer to the Major changes section in this response (5. New Figure 7) to\nread the modified text (Page 10, Lines 266-282), where we have addressed the reviewer’s comment. Like-\nwise,wehavemodifiedthesection6wherewediscussthetrendsinEMF.Themodifiedtextreads:\nLines 318-329: Moreover, the mean pattern of eddy momentum flux shows that eddies have acted to\nredistributemomentumacrosstheACCfrontalregionsoverthelast15years,byremovingmomentumfrom\nthe interfrontal zone and depositing it in the jets (Fig. 7e). The positive trend in eddy momentum flux\nat the southern boundary of the Subantarctic Front could help to accelerate the jet (Fig. 7f). The role\nof eddy momentum flux in redistributing momentum due to increased winds and its effect on the ACC\njets is an outcome not covered in the eddy saturation state hypothesis and warrants future work. Eddy-\npermitting models with sufficient resolution to resolve the ACC fronts in Drake Passage (Xu et al., 2020),\nforced with both realistic and idealized wind-forcing, provide an opportunity to study the potential eddy-\ndrivensharpeningoftheACCfrontsposedinthisstudy.\n4: I wonder if it helps to remove the mean from Fig 5, because it is difficult to see trend (which\nmight be the reason to have Fig. 6). More importantly, I think it is valuable to have a similar time\nseries plot from the geophysical coordinate perspective, showing over time the change of vertically\nintegrated velocity that corresponds to the trend in Fig. 3 (This plot can even include those sections\nthatdidnotcross1000misobathifthosesectionshavebeenprocessed,justnotincludedintransport\ncalculation).\nReply: We thank the reviewer for the suggestions. We refer the reviewer to the Major changes section\n(4. Revised Figure 5) to read the modified text (Pages 8-9, Lines 175-211) where we have addressed the\nreviewer’scomment.\n5: The ADCP based transport is calculated for the top 760 m, not to 1000 m as in the previous work\ncovering 4.5 years of the observations (Firing et al., 2011). I assume this is because many sections\ndo not extend to 1000 m, otherwise, one may argue it is valuable to have a separate time series that\ncover the top 1000 m to be consistent with the previous work (separating between geostrophic and\nreferencetransportat760mseemsoflessimportancetome,althoughthatisarguable).\nReply: We could not reliably extend our calculations to 1042 m given the large gaps in velocity data below\n970m. Wehaveprovidedsomeclarificationforthecriterionusedtodiscardvelocitytransects. Themodified\ntextreads:\ncompletedinlessthan4days,b)along-track/verticalgapsweretoolargetofill,orc)iftransectswithgaps\noutside the area enclosed by the gridded objectively mapped mean (see subsection 8.3). Following these\ncriteriaallowsustoestimatemeanandtrendsinboththenettotaltransportandtransportperdistancebin\nwith the same number of degrees of freedom (transects). From February 2005 - December 2019, 248 (of\n286totalcrossings)ADCPtransectsmetthesecriteria,with147transectsalongthemostcommonlyrepeated\nline (Supplementary Fig. S1). From October 2005 - April 2019, the common sampling period between the\nADCP and the XBT/XCTD transects (details below), 237 transects were available with 140 transect falling\nalongthemostrepeatedline.\nWe found that both the transport bias and corrected mean net Drake Passage total transport are in close\nagreementwiththatofFiringetal.(2011)(SILines42-45). BecausewemodifiedSection3andaddedFig.\n4, we moved the discussion about extending the transport calculation to 970 m previously in Section 5 to\nsection 3. We refer the reviewer to Major changes section (3. New Figure 4) in this response where we\ndiscussedthemeantransportintheupper970m(Page6,Lines152-157).\nWefocusedontheupper760msinceitisthemaximumcommondepthbetweentheXBT/XCTDandcross-\ntransecttotalvelocityprofiles(Lines401-404).\nOther details\n1: L39. Ref 4 may be placed at the end of this sentence as it is about both ozone and greenhouse gas\nemissions (It should be mentioned that it is a modeling work). Not sure why ref 5 (also a modeling\nwork) is included here as it is mostly about ocean warming, not so much about wind or southern\nannularmode.\nReply:Wehaveremovedref. 5assuggestedandmodifiedthesentence:\nLines 41-43: Modeling studies show that the positive trend in the winds is a response to both depletion\nof the atmospheric ozone and increments in the greenhouse gas emissions over the last decades (Sigmond\netal.,2011;Saenkoetal.,2005;GillettandThompson,2003).\n2: L58, “... time series were not yet long enough” and L111. “... using short sampling periods”\nare not accurate for refs. 16 and 19. The geostrophic transports based on hydrographic surveys (ref\n16) cover a pretty long period (although less frequent). Ref 19 rely much on satellite SSH which is\nnot direct measurement of current. I think the strength or value of this ‘unique’ observation needs\nto be highlighted a bit more, i.e., high-resolution, direct measurements of the current across the full\nDrakepassageoveralongperiodoftime(Onelimitationisthatitcoversonlyupperpartofthewater\ncolumn).\nReply: We thank the reviewer for pointing out this mistake. We have modified the manuscript such that it\nreflectsthevalueoftheDrakePassagetimeseries.\nPassage transport (Cunningham et al., 2003; Chidichimo et al., 2014; Donohue et al., 2016; Koenig et al.,\n2014; Meredith and Hogg, 2006). Moreover, these studies were unable to explore trends within the ACC\nfrontal regions owing to the lack of direct current velocity measurements at a sufficient spatial resolution\nto resolve the frontal regions and mesoscale activity. The lack of long-term in-situ observations (velocity,\ntemperature, salinity) with high horizontal resolution in the SO hampers the exploration of trends in the\nACCtransportandpropertiesacrossthedifferentfrontalregions.\nSimilarly,wemodifiedthetextwithinsection2thatnowreads:\nLines 119-124: Our results are consistent with previous Drake Passage observational studies using coarser\nresolution in-situ observations (Cunningham et al., 2003; Chidichimo et al., 2014) and altimetry-derived\ngeostrophic currents (Koenig et al., 2014) which found no significant acceleration of the Drake Passage\ntransport.\nWealsomodifiedsection3inthemanuscripttoclarifythatourresultsarerepresentativeoftheupper760m:\nLines 68-73: This study uses a unique observational time series of year-round near-repeat upper-ocean\ntemperature,salinity,andvelocityacrosstheDrakePassage(Fig. 1a)toexplorewhethertheDrakePassage\ntransport in the upper 760 m shows significant acceleration in the last 15 years. The repeated upper-ocean\nvelocitytransectsprovidethemostefficientwaytomeasuretheupper-oceantransport(Xuetal.,2020).\n3: L113. Refs. 21, 22 find no acceleration in the Drake passage but their main result is that the\nwarming is leading to acceleration in zonal flow in much of the Southern Ocean, and I do not think\nlow-resolutionclimatemodelisthebestexamplestoshownoaccelerationinACCtransport(because\nwithouteddies,astrongerwindtendstodriveastrongerACCtransport).\nReply: We agree with the reviewer that coarse-resolution global climate models show no acceleration be-\ncausetheyareunabletoresolvemesoscaleactivity. Therefore, weremovedlines113-114fromtheoriginal\nmanuscripttoavoidconfusion.\n4: L114-L120. Ref. 23 found that “baroclinic and barotropic transports are not correlated; thus,\nmonitoring either baroclinic or barotropic transport alone may be insufficient to assess the temporal\nvariabilityofthetotalACCtransport”. Theydidnotimplythatthelackoftrendintheirmodeledtrans-\nport is due to two components compensating each other. Furthermore, the separation of geostrophic\nandreferencevelocityat760mhasdifferentmeaningfromthebaroclinic-barotropicseparationfor\nfull water column. On a different perspective, I think it is noteworthy that ref 23 carefully evaluated\nthe modeled ACC transport structure using various observations, including the result of the first 4.5\nyearsoftheADCPmeasurementthatarepresentedinthispaper. Theagreementbetweenmodeland\nthe ADCP measurement for the top 1000 m was one key reason that gives the confidence in their\nmodeledtransport. Thus,theobservationslikethisarealsoofgreatvaluetomodelingcommunityas\nwell. Ithinkthispointcouldbenotedsomewhereinthepaper.\nisons between the modeled ACC and the ADCP observations from (Firing et al., 2011) by Xu et al. (2020),\nwe moved this discussion into section 3 since we added Fig. 4 and included a few lines that address the\nreviewer’ssuggestion. WereferthereviewertotheMajorchangessection(3. NewFigure4)wherewehave\naddressed the reviewer’s comment in section 3 (Page 6, Lines 152-157). Likewise, we added text in the\ndiscussionthataddressesthereviewer’scomment. Theaddedtextreads:\nLines325-329: Eddy-permittingmodelswithsufficientresolutiontoresolvetheACCfrontsinDrakePassage\n(Xuetal.,2020),forcedwithbothrealisticandidealizedwind-forcing,provideanopportunitytostudythe\npotentialeddy-drivensharpeningoftheACCfrontsposedinthisstudy.\n5: L176. “[32]foundasimilar...”youmayrephrasethisas“asimilarcooling... [32]”.\nReply:Wemodifiedthesentenceperthereviewer’ssuggestion. Nowitreads:\nLines 231-233: A similar cooling trend in sea surface temperature in Drake Passage was found from 1993\nto2017(Augeretal.,2021).\n6: Fig.1Iassumepanelsb-disalongthecommonsection? Ifyes,itshouldbementionedintheFigure\ncaptionandnotedthatthelocationofthissectionisindicatedinS1,oraddaonelineinpanela.\nReply:Theclimatalogicalmeanswerecalculatedusingallavailabletotalvelocity,temperature/salinitytran-\nsects,i.e. includingalsothosetransectsawayfromthemostrepeatedline,fortheperiodsmentionedinthe\nfigure caption. Likewise, all time series and trends are calculated by employing all transects (see Fig. 1’s\ncaption).\nThispaperusesa15-yeartimeseriesofsubsurfacetemperature,salinity,andvelocityintheDrakepassageto\ninvestigatethestructureofthetransporttrendsoftheAntarcticCircumpolarCurrent. Thepaperpresentsthe\ntrendsintheoverallnettransportacrosstheDrakePassage,cross-passagevelocityandstreamwisetransport,\nandlinkthemwiththebuoyancychangesacrossthepassage.\nThese results hold significant importance as they offer a detailed description of the transport trends in the\nDrake Passage, particularly in relation to the ACC’s response to strengthening winds, which remains an\nactive research question. It is worth noting that although the Drake Passage is the most regularly sampled\nsection of the Southern Ocean, long-term time series of temperature, salinity, and velocity across the entire\n entire\nSouthernOceanarescarceandhighlyvaluable. Thepaperiswellwrittenandonlyafewminorissuesneed\ntobeaddressedorclarifiedinmyopinion.\nComments/Questions\n1: The paper frequently refers to the impact of wind on buoyancy and velocity change. Have the\nauthorsattemptedtocomputetrendsinwindsorwindstresscurlalongthetransect? Ifso,howcon-\nsistentarethesetrendswithFigure4andthechangesdescribedinSection4?\nReply: We thank the reviewer for suggesting estimating trends in wind stress or wind stress curl. We did\nnotincludetheminthemainmanuscriptorsupplementaryinformationsinceitiswelldocumentedthatthe\nwindstressovertheSouthernOcean(asshownbytheSouthernAnnularMode, SAM,thelargestempirical\nmodeofwesterlywindstressintheregion)hasincreased(Marshall,2003;FogtandMarshall,2020)forthe\nlastfourdecadeswhichcoverourperiodofstudy. Howeverweincludethetrendsinthewindstresscurlas\nthey are important when calculating trends in spice and heave components of temperature and salinity in\nDrakePassage(seeFigS4).\nτ [N m−3]. Negative values indicate Ekman suction (upwelling). (b) Trend in wind stress curl [N m−3\nyear−1] for the 2005-2019. Negative values indicate increasing Ekman suction. Trends enclosed with gray\nsolidcontourare95%statisticallysignificantafterthemodifiedMann-Kendalltest.\nWealsomodifiedsection4toincludetheresultsshowninthesupplementaryfigureinthemanuscript. The\nmodifiedtextnowreads:\nLines 228-231: The heave-driven cooling potentially stems from a more negative wind-stress curl (Ekman\nsuctionorupwelling)overtheDrakePassageareaduringthelast15years(SupplementaryFig. S2),consis-\ntentwithprevioustrendsestimatedforashorterperiodoftime(Meehletal.,2019).\n2: L144: “islocated”?\nReply: In response to a suggestion from Reviewer 1, we have added a new figure in section 3 (Fig. 4) and\nitssubsequentdescriptioninSection3(seeourfollowingresponse). Hence,thistypohasbeendeleted.\n3: Figure3andSection3mayconfusereaders. WhileFigure3setsthemethodologyforseparatingthe\ntotal,reference,andgeostrophiccomponents,andshowsinasimplewaythatthecompensationofthe\nACC frontal regions, it presents results that, at first glance, seem to contradict the final conclusions.\nSpecifically, it shows a deceleration of velocities in the SAF and PF zones, as well as an acceleration\nin the region between them, whereas the abstract mentions that the subantarctic and polar jets have\nacceleratedwhilethecurrentsbetweenthemhavedecelerated. Whilethismightbeconfusingatfirst\nread, this confusion could be partly avoided by adding a few similar words as the opening of section\n5. Ileavethisdecisionuptotheauthors.\nReply:Wethankthereviewerforraisingthismatter. Inresponsetoasimilarcommentraisedbyreviewer1,\nwe have moved Fig. 5 into section 3 and discussed the acceleration of the region between the fronts found\n21\nin Fig. 3, 4 as an effect of an increase in the meandering of the ACC frontal jets and eddy formation. We\nrefer the reviewer to the Major changes section (4. Revised Figure 5) in this response to read the modified\ntext(Pages8-9,Lines175-190)wherewehaveaddressedthereviewer’scomment.\n4: L173-175: ThedifferenceinmagnitudebetweenFig. 4andFig. S2issurprising. Doesthatquestion\ntherobustnessofthetrends?\nReply:Werespectfullydisagreewiththereviewer. WhilethedifferenceinmagnitudebetweenFig. 4andFig.\nS2isstark,statisticallysignificanttrendsshowninthefiguresaresimilarbothinsignandspatialdistribution.\nThisgivesusconfidencethatthelong-termtrendsarerobust. Nevertheless,wehavedecidedtoremoveFig.\nS2fromthesupportinginformationandlines173-176inthetexttoavoidanyconfusion.\n5: L283: “torise”\nReply:Wehavecorrectedthistypo. Nowthesentencereads:\nLines 335-339: Giventhesustainedincreaseinglobalgreenhouseemissions,thewesterlywindstressover\ntheACCisexpectedtocontinuetorise,whichwouldacttoincreasethepolewardeddytransportandthusthe\nupwelling of deep warm waters along isopycnals outcropping near the Antarctic continental shelf (Palo´czy\netal.,2018;Tamsittetal.,2016).\n6: L299: Is there any specific criterion used do discard the transects that significantly deviated from\nthemaincourse?\nReply:WehavemodifiedtheMethodssectiontobetterreflectthespecificcriteriausedtodiscardtransects.\nThemodifiedtextreads:\nLines 350-361: Transects were discarded if one of the following criteria were met: a) transects were not\ncompletedinlessthan4days,b)along-track/verticalgapsweretoolargetofill,orc)iftransectswithgaps\noutside the area enclosed by the gridded objectively mapped mean (see subsection 8.3). Following these\ncriteriaallowsustoestimatemeanandtrendsinboththenettotaltransportandtransportperdistancebin\nwith the same number of degrees of freedom (transects). From February 2005 - December 2019, 248 (of\n286totalcrossings)ADCPtransectsmetthesecriteria,with147transectsalongthemostcommonlyrepeated\nline (Supplementary Fig. S1). From October 2005 - April 2019, the common sampling period between the\nADCP and the XBT/XCTD transects (details below), 237 transects were available with 140 transect falling\nalongthemostrepeatedline.\n7: L338-340: How many of the transects between October 2005 and December 2019 fall into the\nmost sampled line? I am just wondering how the zonal spreading of the transects could impact the\ncross-transectvelocity,temperatureandsalinitytrends. Morespecifically,canthatexplainpartofthe\ndifferences in the trends from the longer and shorter time periods (Fig. 3a vs Fig Fig.3b; Fig.4 vs Fig\nS2).\nReply:BetweenOctober2005-April2019,only26(45%)ofthe69temperature/salinitytransectsand140\n(60%) of the 237 and ADCP transects fall along the most repeated line. We did not calculate trends for\n22\ntemperature/salinity for the most repeated line due to the small percentage of transects and gaps in the\ntime series of as much as 2 years. In response to the reviewer’s comment, we estimated trends in the total\ntransport per distance bin using the transects falling along the most repeated line and they were similar\nin spatial distribution, magnitude, and sign compared to those calculated using all transects available (Fig.\nR1). Weconcludethat thezonalspreadingofthetransectsdoesnotimpactsignificantlythe totaltransport\ntrendsfoundinthisstudy.\n]m\nk[\nh\ntro\nN 400 PF\nm\no\nrf\ne\ncn\na tsiD 600\nAll transects\nMR line\n-0.2 -0.1 0 0.1 0.2\nTrend [Sv year-1]\nFig.R1Trendsincross-transecttotaltransportperdistancebin[Svyear−1]fromOctober2005-April2019.\nRed and green lines are trends estimated using all available (237) velocity transects and only those (140)\nvelocity transects that fall along the most repeated line. Shaded areas show the error bars estimated from\ntheleast-squaresfit. Filledsquaresanddotsindicatethattrendsaresignificantat95%confidenceafterthe\nmodifiedMann-Kendalltest(p<0.05). ThemeanlocationoftheSubantarcticFront(SAF),PolarFront(PF),\nandSouthernACCFront(SACCF)areindicatedbyshadedareas.\nWemodifiedsection3inthemainmanuscripttomakeourresultsclear:\nLines171-174: Similarpatternsarefoundwhentotaltransporttrendsarecalculatedwiththetotalvelocity\ntransectsthatcoincidewiththetemperature/salinitytransects(Fig. 4c),orusingthetransectsfallingalong\nthemostrepeatedcrossing(notshown).\n8: L425-428: WhatisthemotivationbehindtheuseoftwodifferentdatasetssourcesfortheMeanDy-\nnamic Topography and Sea Level Anomaly? This could introduce discrepancies between the datasets\nthatcouldbeavoidedwiththeuseofthefullSSHsignalfromSSALTO/DUACS.\nReply:Weusethemeandynamictopographybecauseitisdirectlytiedtotheocean’ssurfacegeostrophiccir-\nculation. ThefullSeaSurfaceHeightsignalfromSSALTO/DUACSisreferencedtotheellipsoidandcontains\nthe geoid signal which could introduce discrepancies in defining anomalies such as mesoscale meanders\nand eddies, therefore affecting our binning and attribution methodology. The mean dynamic topography\nremoves the geoid model such that the mean topography is defined exclusively by the mean major surface\ncurrents. We have modified subsection 8.6 to justify our choice of using daily maps of sea level anomalies\nplusthemeandynamictopography. Thetextnowreads:\nwe employed a synoptic streamwise coordinate system (sea surface height; SSH) in a similar way as used\nby Gutierrez-Villanueva et al. (2020) for eddy heat flux. We used a combination of the mean dynamic\ntopography(Maximenkoetal.,2009)andtheSSALTO/DUACSdailymapsofsealevelanomaliesfromAVISO\ntotrackthepositionoftheACCstreamlines. Themeandynamictopographyisdirectlytiedtothelarge-scale\nsurfacegeostrophiccirculationconstrainedby20yearsofin-situobservationssuchasdrifterscorrecteddue\ntowind/Ekmancurrents, altimetry, Argotemperature/salinityprofiles, andconductivity-temperature-depth\n(CTD)casts(Maximenkoetal.,2009). Thedailymapswereobtainedfrommultiplesatellitealtimetersand\nobjectivelymappedtoa0.25◦×0.25◦Cartesiangrid(Ducetetal.,2000). Thesealevelanomaliesarerelative\ntoatwenty-yearmeanoftheseasurfaceheightfield. Addingthemeandynamictopographytothesealevel\nanomaliesproducesmapsofdynamicalSSHthatenabletrackingoftheshiftingandmeanderingoftheACC\nstreamlines and mesoscale eddies. We used the daily maps from February 1999 to December 2019, which\ncovers our period of interest, and we subtracted a 0.6 m constant from the daily maps following Gutierrez-\nVillanueva et al. (2020) (and references therein), which does not affect the position of the fronts and their\nhorizontalgradients.\n9: L447-449: Doesthatmeanthatprofilesfallingintoclosedcontoursarediscarded?\nReply:No,themethoddevelopedinGutierrez-Villanuevaetal.(2020)binstheprofilesfallingwithinclosed\ncontours(eddies)intothepairofopencontoursenclosingtheclosedcontours. WereferthereviewertoAp-\npendixAinGutierrez-Villanuevaetal.(2020)wherethebinningofdataprofilesisdetailedwithexamples.\n10: L459: Youmightmeana insteadofa .\n0 1\nReply: We thank the reviewer for pointing out this typo. We replaced a for a so it is consistent with our\n0 1\nmethodology. Themodifiedtextreads:\nLines 552-554: The trends were estimated by least-squares fitting yˆ = a +a t to each time series; the\n0 1\nsecondterma ontheright-handsideoftheequationisthetrend.\n1\n11: Figure 3. caption: “Trends in (a) and (b) are estimated using all available (237) velocity tran-\nsectsandonlythose(60)velocitytransectsthathaveacoincidenttemperature/salinitytransectused\nto compute the geostrophic u component in (c), respectively.” Please split this sentence into two\ngeo\nparts: Trendsin(a)... whiletrendsin(b)...\nReply: We thank the reviewer for the suggestion. We wish to refer the reviewer to Major changes section\n(2. RevisedFigure3)toreadthecorrespondingchangestoFigs. 3wherewehaveaddressedthereviewer’s\ncomment(Page5).\nThispaperanalysesa15-yeartimeseriesofDrakePassagemadeupofunderwayADCPmeasurementsand\nXBT/XCTD deployments. The data has been collected during crossings of ARSV Laurence M. Gould during\ntransits from South America to the Antarctic Peninsula. The measurements cover the top 1000 m, or so,\nof the water column with analysis restricted to between depths of 20-40 m and 760 m, due to instrument\nconstraintsandqualitycontrolprocessing. Theresultingtransectsclearlyshowthe3mainjetsoftheAntarc-\ntic Circumpolar Current (ACC) and the steeply inclined isopycnals that accompany them. Analysis of least\nsquare trends shows that there are no meaningful trends in the Drake Passage transport, consistent with\npreviouspublications. However,therearestatisticallysignificanttrendsinthecross-transectvelocity,which\ncompensate when the area-integrated cross-transect transect is calculated. The locations of these trends\nindicatesastatisticallysignificantaccelerationofthePolarFrontandSubantarcticFront,withcompensating\ndeceleration between the fronts. This is interpreted as being a result of increased wind stress leading to\nlocally steepened isopycnals and increased Eddy Kinetic Energy (EKE), with the more vigorous eddy field\nthensharpeningandacceleratingthejetsduetotheirmomentumtransport.\nMyexpertiseisnotinanalysingandprocessingobservationaldata. Thismakesitdifficultformetocomment\\ontheseaspectsofthepaper. Themethodssectionappearsthoroughandwellthoughtout. Itissupportedby\ncitationstopreviousapplicationsofsimilartechniqueswithsimilarcorrections,etc,beingfoundnecessaryin\nthecurrentpaper. Thestatisticaltestsareconsistentlyappliedandsystematicallyreported. Withmyinexpert\neye,thisallseemsquitereasonable.\nThe paper makes a valuable contribution to understanding how the Southern Ocean and ACC has and/or\nwillrespondtoclimatechange. TheirresultsareconsistentwithpreviousonesshowingnoincreaseinACC\ntransport. However, they are able to add the important detail that this may be because of compensation\nbetween different regions increasing/decreasing their velocity. The authors link this to eddy-driving of the\njetsviaeddymomentumfluxesandprovidequantificationofthesame.\nGeneral comments\n1: There are some points missing from the discussion that I think should be included. The observa-\ntionsonlycoverthetop760mofthewatercolumn, amountingto61%ofthebaroclinictransportor\n47%ofthetransportincludingthenear-bottomflow(lines202-204). However,thereisnodiscussion\nofwhethertheresultsarelikelytoholdoverthewholewatercolumn. Isitlikelythatthesamepattern\nofspatiallylocalisedaccelerationsanddecelerationshold?\nReply: In response to a similar comment raised by reviewer 2, we have provided some clarification of our\nresultsastohowrepresentativetheyareforthewholewatercolumn. WereferthereviewertoMajorchanges\nsection(3. RevisedFig. 3)inthisresponsetoreadingthemodifiedtext(Pages4,Lines132-134).\n2: The second thing that might warrant some discussion is whether the conclusions would hold up-\nstream/downstream of Drake Passage. A recent paper, Shi et al. (2021), indicates that there might\nbe acceleration of the northern edge of the ACC elsewhere in the Southern Ocean. Can the authors\nresults shed any light on whether this might affect the circumpolar transport of the ACC? Or is this\n25\ntoospeculative,givenlimitedSouthernOceanobservations?\nReply:WedonotthinkwecantietheresultsbyShietal.(2021)withoursbecausetheyemployedacoarse-\nresolution global climate model that are incapable of resolving eddies, and therefore unable to resolve any\neddy-drivenaccelerationinDrakePassageandtheACCasdetectedinourhigh-resolutionobservations.\nMinor Comments\nTherewereafewminorthingsInoticedwhilstreadingthepaperthattheauthorsmaywishtoconsider.\n1: Lines73-76: Thissentenceseemsincomplete,didtheauthorsmeantopointtowardsFigure1?\nReply:Wethankthereviewerforpointingthisout. Themodifiedsentencereads:\nLines 82-85: Climatological mean sections of cross-transect velocity u , potential temperature θ (tem-\ntot\nperature hereinafter), and salinity S across Drake Passage using all available velocity (237) and tempera-\nture/salinity(114)transects(seeMethods)areshowninFig. 1(b-d).\n2: Figure 5b: I found it hard to tell the lines apart, because there colour is the same, even with one\nhavingprominentmarkers. Differentcoloursmightbebetter.\nReply: We thank the reviewer for pointing us to this issue. In response to comments by reviewers 1 and 2,\nwe have moved Fig. 5 into section 5. We wish to refer the reviewer to Major changes section (4. Revised\nFigure5)toseetherevisedFig. 5wherewehaveaddressedthereviewer’scomment(Page9).\n3: Lines 148-151 and Figure 3b: I was puzzled as to the difference between Figure 3a and 3b until I\ngot to this point. It might be better to put this information in the figure caption or move it earlier in\ntheparagraph,sincereadersmaylookatthefigurebeforereadingthefullparagraph(asIdid).\nReply:Wethankthereviewerforthesuggestion. WewishtoreferthereviewertotheMajorchangessection\n(2. RevisedFigure3)toseethechangestoFig. 3(Page5).\nAuger,M.,Morrow,R.,Kestenare,E.,Sall´ee,J.-B.,andCowley,R.(2021). SouthernOceanin-situtempera-\nturetrendsover25yearsemergefrominterannualvariability. Nat.Commun.,12(1):1–9.\nChidichimo,M.P.,Donohue,K.A.,Watts,D.R.,andTracey,K.L.(2014). Baroclinictransporttimeseriesof\ntheAntarcticCircumpolarCurrentmeasuredinDrakePassage. J.Phys.Oceanogr.,44(7):1829–1853.\nCunningham,S.A.,Alderson,S.G.,King,B.A.,andBrandon,M.A.(2003). Transportandvariabilityofthe\nAntarcticCircumpolarCurrentinDrakePassage. J.Geophys.Res.Oceans,108(C5).\nDonohue, K. A., Tracey, K. L., Watts, D. R., Chidichimo, M. P., and Chereskin, T. K. (2016). Mean Antarctic\nCircumpolarCurrenttransportmeasuredinDrakePassage. Geophys.Res.Lett.,43(22):11,760–11,767.\nDownes, S. M. and Hogg, A. M. (2013). Southern Ocean Circulation and Eddy Compensation in CMIP5\nModels. J.Clim.,26(18):7198–7220.\nDucet, N., Le Traon, P.-Y., and Reverdin, G. (2000). Global high-resolution mapping of ocean circulation\nfromTOPEX/PoseidonandERS-1and-2. J.Geophys.Res.,105(C8):19477–19498.\nFiring, Y. L., Chereskin, T. K., and Mazloff, M. R. (2011). Vertical structure and transport of the Antarctic\nCircumpolarCurrentinDrakePassagefromdirectvelocityobservations. J.Geophys.Res.,116(C8).\nFogt,R.L.andMarshall,G.J.(2020). TheSouthernAnnularMode: Variability,trends,andclimateimpacts\nacrosstheSouthernHemisphere. WIREsClim.Chg.,11(4):e652.\nFoppert, A., Donohue, K. A., Watts, D. R., and Tracey, K. L. (2017). Eddy heat flux across the Antarc-\ntic Circumpolar Current estimated from sea surface height standard deviation. J. Geophys. Res. Oceans,\n122(8):6947–6964.\nGillett, N. P. and Thompson, D. W. (2003). Simulation of recent Southern Hemisphere climate change.\nScience,302(5643):273–275.\nGutierrez-Villanueva,M.O.,Chereskin,T.K.,andSprintall,J.(2020). Upper-oceaneddyheatfluxacrossthe\nAntarcticCircumpolarCurrentinDrakePassagefromobservations: time-meanandseasonalvariability. J.\nPhys.Oceanogr.,50(9):2507–2527.\nHallberg, R. and Gnanadesikan, A. (2001). An Exploration of the Role of Transient Eddies in Determining\ntheTransportofaZonallyReentrantCurrent. J.Phys.Oceanogr.,31(11):3312–3330.\nHogg,A.M.,Meredith,M.P.,Chambers,D.P.,Abrahamsen,E.P.,Hughes,C.W.,andMorrison,A.K.(2015).\nRecenttrendsinthesouthernoceaneddyfield. J.Geophys.Res.Oceans,120(1):257–267.\nKillworth,P.D.andHughes,C.W.(2002).TheAntarcticCircumpolarCurrentasafreeequivalent-barotropic\njet. Journalofmarineresearch,60(1):19–45.\nKoenig,Z.,Provost,C.,Ferrari,R.,Senn´echael,N.,andRio,M.-H.(2014). VolumetransportoftheAntarctic\nCircumpolar Current: Production and validation of a 20 year long time series obtained from in situ and\nsatelliteobservations. J.Geophys.Res.Oceans,119(8):5407–5433.\nOceanogr.,39(3):768–779.\nLenn,Y.-D.,Chereskin,T.K.,Sprintall,J.,andMcClean,J.L.(2011).Near-surfaceeddyheatandmomentum\nfluxesintheAntarcticCircumpolarCurrentinDrakePassage. J.Phys.Oceanogr.,41(7):1385–1407.\nMarshall, G. J. (2003). Trends in the Southern Annular Mode from Observations and Reanalyses. J. Clim.,\n16(24):4134–4143.\nMart´ınez-Moreno, J., Hogg, A. M., England, M. H., Constantinou, N. C., Kiss, A. E., and Morrison, A. K.\n(2021). Global changes in oceanic mesoscale currents over the satellite altimetry record. Nat. Clim.\nChang.,11(5):397–403.\nMaximenko, N., Niiler, P., Centurioni, L., Rio, M.-H., Melnichenko, O., Chambers, D., Zlotnicki, V., and\nGalperin,B.(2009). Meandynamictopographyoftheoceanderivedfromsatelliteanddriftingbuoydata\nusingthreedifferenttechniques. J.Atmos.OceanicTechnol.,26(9):1910–1919.\nMeehl, G.A., Arblaster, J.M., Chung, C.T., Holland, M.M., DuVivier, A., Thompson, L., Yang, D., andBitz,\nC.M.(2019). SustainedoceanchangescontributedtosuddenAntarcticseaiceretreatinlate2016. Nat.\nCommunic.,10(1):1–9.\nMeredith, M. P., Garabato, A. C. N., Hogg, A. M., and Farneti, R. (2012). Sensitivity of the Overturning\nCirculationintheSouthernOceantoDecadalChangesinWindForcing. J.Clim.,25(1):99–110.\nMeredith,M.P.andHogg,A.M.(2006). CircumpolarresponseofSouthernOceaneddyactivitytoachange\nintheSouthernAnnularMode. Geophys.Res.Lett.,33(16):L16608.\nMorrison, A. K. and Hogg, A. M. (2013). On the Relationship between Southern Ocean Overturning and\nACCTransport. J.Phys.Oceanogr.,43(1):140–148.\nMunday, D. R., Johnson, H. L., and Marshall, D. P. (2013). Eddy Saturation of Equilibrated Circumpolar\nCurrents. J.Phys.Oceanogr.,43(3):507–532.\nPalo´czy,A.,Gille,S.T.,andMcClean,J.L.(2018). OceanicHeatDeliverytotheAntarcticContinentalShelf:\nLarge-Scale,Low-FrequencyVariability. J.Geophys.Res.Oceans,123(11):7678–7701.\nPatara,L.,Bo¨ning,C.W.,andBiastoch,A.(2016). VariabilityandtrendsinSouthernOceaneddyactivityin\n1/12°oceanmodelsimulations. Geophys.Res.Lett.,43(9):4517–4523.\nSaenko,O.A.,Fyfe,J.C.,andEngland,M.H.(2005). Ontheresponseoftheoceanicwind-drivencirculation\ntoatmosphericCO2increase. Climatedynamics,25(4):415–426.\nShi, J.-R., Talley, L. D., Xie, S.-P., Peng, Q., and Liu, W. (2021). Ocean warming and accelerating Southern\nOceanzonalflow. Nat.Clim.Chg.,11(12):1090–1097.\nSigmond,M.,Reader,M.C.,Fyfe,J.C.,andGillett,N.P.(2011). DriversofpastandfutureSouthernOcean\nchange: Stratosphericozoneversusgreenhousegasimpacts. Geophys.Res.Lett.,38(12).\nTamsitt, V., Talley, L. D., Mazloff, M. R., and Ceroveˇcki, I. (2016). Zonal variations in the Southern Ocean\nheatbudget. J.Clim.,29(18):6563–6579.\nsecondedition.\nWatts,D.R.,Tracey,K.L.,Donohue,K.A.,andChereskin,T.K.(2016). Estimatesofeddyheatfluxcrossing\nthe Antarctic Circumpolar Current from observations in Drake Passage. J. Phys. Oceanogr., 46(7):2103–\n2122.\nXu, X., Chassignet, E. P., Firing, Y. L., and Donohue, K. (2020). Antarctic Circumpolar Current Transport\nThroughDrakePassage: WhatCanWeLearnFromComparingHigh-ResolutionModelResultstoObserva-\ntions? J.Geophys.Res.Oceans,125(7):e2020JC016365.\nReviewer #1 (Remarks to the Author):\nThe authors carefully and adequately addressed all my previous concerns. I only have one or two minor\nsuggesfions.\n1. It seems useful to quanfify the trends by adding some numbers. Specifically, to add the trend (in\nSv/year) for the accelerafion between the SACCF and PF, and decelerafion of the southern Flank of the\nPolar Front in lines 166-174. This would be a good contrast to the (lack of) trend of the net transport\n(Figure 3a). I assume this is essenfially a parfial summafion of the trends in Figure 4c. It would be even\nbefter to add a figure (for example, in the supplementary) that is similar to Figure 3a but for the\naccelerafion/decelerafion parts, to highlight the trends easier than Figure 4a (but I am leaving to the\nauthor to decide if they want to add or not).\n2. Given the noise or variability as shown in Figures 3a and 4a, it seems useful if the authors can\ncomment a bit on the uncertainfies of the esfimated transports. The authors have discussed the error on\nvelocifies in Secfion 8 lines 371-376. It would be useful to have some esfimates on the uncertainty of the\ntransports, even if it is a rough esfimate.\nReviewer #2 (Remarks to the Author):\nThe authors have addressed all my comments. I only have one minor suggesfion which is rearranging the\nx labels for Figure 7 (b) and (c), so that they are not so close to each other to avoid confusion. Same\ncomment for 7(e) and (f).\nResponse to Reviewers’ Comments\nNCOMMS-23-17942A\nManuel O. Gutierrez-Villanuevaa, Teresa K. Chereskin, and Janet Sprintall\namog002@ucsd.edu\nOctober 30, 2023\nThankyouforreviewingourmanuscript. Below,wehaveprovideddetailedpoint-by-pointreplies. Wehave\nusedboldanditalicfontfortheReviewers’commentswhereasourresponsesarewritteninnormalfont. In\nourresponses,weusedtextinbluetoindicatethattexthasbeenincorporatedintotherevisedmanuscript.\nTextinredintheaccompaniedmanuscriptwithtrackchangesindicatesthattexthasbeenremovedfromthe\noriginalmanuscript.\nComments by Reviewer 1\nThe authors carefully and adequately addressed all my previous suggestions.\n1: It seems useful to quantify the trends by adding some numbers. Specifically, to add the trend (in\nSv/year)fortheaccelerationbetweentheSACCFandPF,anddecelerationofthesouthernFlankofthe\nPolarFrontinlines166-174. Thiswouldbeagoodcontrasttothe(lackof)trendofthenettransport\n(Figure 3a). I assume this is essentially a partial summation of the trends in Figure 4c. It would be\nevenbettertoaddafigure(forexample,inthesupplementary)thatissimilartoFigure3abutforthe\nacceleration/decelerationparts,tohighlightthetrendseasierthanFigure4a(butIamleavingtothe\nauthortodecideiftheywanttoaddornot).\nReply:Wehaveaddedtexttotheendofparagraph4insection4thatdiscussesthesumofthetrendsfound\ninFig. 4c,f,i. Theaddedtextreads:\nLines 175-180: The geostrophic and reference transport trends (Fig. 4f,i) summed across Drake Passage\nyield −0.10 Sv year−1and 0.04 Sv year−1, respectively, and are in close agreement with the insignificant\ntrends calculated from the time series of net Drake Passage transport (Fig. S1). The sum of the total trans-\nport trends (Fig. 4c) yields 0.08 Sv year−1, which is of opposite sign (but insignificant) compared to that\nestimatedfromthenettransporttimeseries(Fig. S1).\nWehavedecidednottoincludethesuggestedfigurebythereviewerinthesupplementaryinformation,since\nwebelievethetrendsinFig. 4alreadyshowthatthecompensatingtrendscoincidewiththedifferentfrontal\n1\nregions. Nonetheless,wehavecalculatedthetrendsintotal,geostrophic,andreferencetransportperfrontal\nregion and included it in this response (Fig. R1). We have included the regions between the SAF and the\nSouth American continental slope (SA slope) and the region between the SACCF and the Antarctic slope\n(ANT slope). Trends in transport (Fig. R1) for each component show that the Polar Front compensates for\nthe acceleration found in the region between the Subantarctic Front and Polar Front (as shown in Fig. 4);\nthese two regions show the largest trends in Drake Passage frontal regions. The sum of the trends for each\ncomponentyieldsnosignificanttrendinnettotaltransport,consistentwithourresults.\na\n] 1.5 All transects\n1 - 1 Subsampled\nr\na 0.5\ne\ny\nv 0\nS\n[\nd\n-0.5\nn -1\ne\nr\nT-1.5 Total\nb\n0.4\n]\n1\n- 0.2\nr\na\ne\ny\nv 0\nS\n[\nd\nn-0.2\ne\nr\nT\nGeostrophic\n-0.4\nc\n1\n]\n1\n- 0.5\nr\na\ne\ny\nv 0\nS\n[\nd\nn-0.5\ne\nr T Reference\n-1\nA\nSl o p e S A F P F\nS\nA C C\nT\nF Sl o p e S u m\nS N\nA\nFig. R1 Trends in (a) total, (b) geostrophic, and (c) reference transport per frontal region [Sv year−1] for\ntheperiodofOctober2005-April2019. Filledcirclesarestatisticallysignificanttrends(p<0.05)afterthe\nmodifiedMann-Kendalltest. Redandgreencirclesin(a)aretrendsestimatedusingalltransectsandthose\ntransectswithacoincidenttemperature/salinity(XBT/XCTD)transect,respectively. SAF,PFandSACCFare\nthe Subantarctic Front, Polar Front, and Southern ACC Front. SA slope is the region between SAF and the\nSouth American continental slope. ANT Slope is the region between SACCF and the Antarctica continental\nslope. Sumistheintegralofthetrendsperfrontalregions.\n2\n2: Given the noise or variability as shown in Figures 3a and 4a, it seems useful if the authors can\ncommentabitontheuncertaintiesoftheestimatedtransports. Theauthorshavediscussedtheerror\nonvelocitiesinSection8lines371-376. Itwouldbeusefultohavesomeestimatesontheuncertainty\nofthetransports,evenifitisaroughestimate.\nReply: We thank the reviewer for the point raised here. In addressing it, we found an inaccuracy in the\nMethods section where we described the velocity data processing (lines 369-371). The velocity data used\nfor the means and trends in transport were 15-min averages (i.e., 4.5 km along-track resolution) instead of\n5-min (1.5 km resolution). The oversight occurred because there are two ADCP datasets that differ only in\ntheir time resolution. We thought that all calculations were made with the 5-min datasets, but the earlier\ncalculationsofthemeans(Figs. 1andS1)andtrends(Figs. 2-5,andFig. S2)employedthe15-minaverages.\nForthetrendsineddyactivity(Fig. 7),the5-mindatasetswereemployed. Therefore,wehavemodifiedthe\nmethodssectiontoclarifythatweusedthe15-minaverages. Thetextreads:\nLines 372-383: Velocities were transformed from ship-relative to absolute ocean currents using GPS posi-\ntionandattitudemeasurements. Theabsolutecurrentvelocitieswerefurtheraveragedto15-minresolution,\ni.e. ∼ 4.50 km along-track resolution (assuming a ship’s speed of 5 m s−1). Barotropic tidal currents were\nremoved from the absolute velocity by subtracting the tidal prediction of the TPXO7.2 tide model (Egbert\netal.,1994). Aslablayerwasassumedfortheupper46mwheretheOS38ADCPdidnotsample. Thesingle\nping velocity uncertainty is 23 cm s−1; thus the uncertainty of a 300 ping average is 1.3 cm s−1. Combined\nwith the error in ship speed from GPS (0.8 cm s−1) the error in absolute velocity in a 15-min average is\n1.5 cm s−1. Averaging over 83 min (i.e., ∼ 25 km along-track assuming an average ship speed of 5 m s−1)\nreducestheerrorσ to0.65cms−1.\ntot\nSimilarly, we have modified the methods for calculating the eddy kinetic energy and momentum fluxes to\nstatethatthe5-minaverageswereused. Thetextreads:\nLines 542-546: To quantify trends in the eddy activity in Drake Passage and the eddy momentum transfer\nwith the ACC jets, we estimated time series of depth-averaged eddy kinetic energy and eddy momentum\nfluxfromthe5-min(100pings;1.5kmalong-trackresolution)OS38ADCPrecordscorrectedfortransducer\nmisalignmentangle(seesupplementaryinformationeq. 4).\nWe have included a discussion about the uncertainty in the transport estimates in subsection 8 (Total,\ngeostrophic and reference transport) estimated using the absolute error velocity σ (Lines 382-383). The\ntot\naddedtextreads:\nLines 447-452: The error in absolute transport in the upper 760 m per 25-km along-track bin (dx) is\n√\nestimatedfromtheabsolutevelocityerror(σ )asσ ∗dx∗H/ N,whereH =760mandN isthenumber\ntot tot\nof observations (transects), yielding 8×10−3 Sv for 237 transects for the October 2005-April 2019 period.\n√\nSimilarly,thestandarderrorinabsolutenetDrakePassagetotaltransport,estimatedasσ ∗L∗H/ N,is\ntot\n0.28Sv.\nTheauthorshaveaddressedallmycomments.\n1: IonlyhaveoneminorsuggestionwhichisrearrangingthexlabelsforFigure7(b)and(c),sothat\ntheyarenotsoclosetoeachothertoavoidconfusion. Samecommentfor7(e)and(f).\nReply: We have modified Figure 7 to re-arrange the x-labels in subplots (b)-(d) and (e)-(f). The modified\nfigureisshownbelow.\na b c\n0.2\n-0.40\nSAF SAF\n-0.60 0.15 2 ]\n]m\n[\nH-\n-\n1\n0\n.\n.\n8\n6\n0\n0 0.1\n2\n-\nm\ns\nS\nS PF PF\n[\n>\n-1.30 All transects E K\n-1.45 Subsampled 0.05 E <\n-1.65 SACCF SACCF\n0\n2006 2008 2010 2012 2014 2016 2018 0 0.03 0.06 -0.2 0 0.2\n<EKE> [m2 s-2] Trend 10-2\n[m2 s-2 year-1]\nd e f\n0.2\n-0.40\nSAF SAF ]\n-0.60 0.1 2 -s\n]m-0.80 2\n[ H\nS S\n-1.00\nPF PF\n0 m [\n> F\n-1.30 -0.1 M E\n-1.45 <\n-1.65 SACCF SACCF -0.2\n2006 2008 2010 2012 2014 2016 2018 -0.01 0 0.01 -0.4 0 0.4\n<EMF> [m 2 s-2] Trend 10-2\n[m2 s-2 year-1]\nFig.7Timeseriesofeddykineticenergy(EKE)andeddymomentumfluxes(EMF),meanandtrendsperpair\nofseasurfaceheight(SSH)streamlines[m]forOctober2005-April2019. Timeseriesofdepth-averagedtotal\n(a) <EKE> and (d) <EMF> in the upper 760 m [m2 s−2]. Mean (b) <EKE> and (e) <EMF> [m2 s−2].\nTrendsin(c)<EKE>and(f)<EMF>[m2s−2year−1]. Redandgraylines(b)-(c)showthemeanandtrends\nestimated for all (237) available transects and only those (60) velocity transects that have a concurrent\ntemperature/salinity transect, respectively. Filled squares (c),(f) indicate that trends are significant at 95%\nconfidence after the modified Mann-Kendall test (p < 0.05). Standard errors for each line are shown in\nshadedareasin(b),(e). ShadedrectanglesshowthestreamwiseintervalsfortheSubantarcticFront(SAF),\nPolarFront(PF),andSouthernACCFront(SACCF)(Gutierrez-Villanuevaetal.,2020).\nEgbert, G. D., Bennett, A. F., and Foreman, M. G. (1994). TOPEX/Poseidon tides estimated using a global\ninversemodel. J.Geophys.Res.,99(C12):24821–24852.\nGutierrez-Villanueva,M.O.,Chereskin,T.K.,andSprintall,J.(2020). Upper-oceaneddyheatfluxacrossthe\nAntarcticCircumpolarCurrentinDrakePassagefromobservations: time-meanandseasonalvariability. J.\nPhys.Oceanogr.,50(9):2507–2527.\n5"}]

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