Enhanced wind mixing and deepened mixed layer in the Pacific Arctic shelf seas with low summer sea ice

Domain: earth_science
Published: 2024-11-29
Editorial outcome: major_revision

Abstract
The Arctic Ocean has experienced significant sea ice loss over recent decades, shifting towards a thinner and more mobile seasonal ice regime. However, the impacts of these transformations on the upper ocean dynamics of the biologically productive Pacific Arctic continental shelves remain underexplored. Here, we quantified the summer upper mixed layer depth and analyzed its interannual to decadal evolution with sea ice and atmospheric forcing, using hydrographic observations and model reanalysis from 1996 to 2021. Before 2006, a shoaling summer mixed layer was associated with sea ice loss and surface warming. After 2007, however, the upper mixed layer reversed to a generally deepening trend due to markedly lengthened open water duration, enhanced wind-induced mixing, and reduced ice meltwater input. Our findings reveal a shift in the primary drivers of upper ocean dynamics, with surface buoyancy flux dominant initially, followed by a shift to wind forcing despite continued sea ice decline. These changes in upper ocean structure and forcing mechanisms may have substantial implications for the marine ecosystem, potentially contributing to unusual fall phytoplankton blooms and intensified ocean acidification observed in the past decade.

Introduction
The Pacific Arctic continental shelves, comprising the northern Bering and Chukchi seas, are among the most productive high-latitude systems1–3 but also experiencing drastic climate changes4. Longer ice-free periods5,6, more solar heating7, and increasing Pacific water inflow8 through the Bering Strait promote ocean warming and freshening9,10 in this region. The state of the upper ocean is crucial to regional productivity since a strong stratification at the mixed layer base effectively isolates the convection of heat11,12 and nutrients13, hindering phytoplankton blooms in the euphotic zone. Several studies suggested that primary production should decrease with a freshwater lens14,15 under rapid warming and sea ice loss. However, satellite-based estimation showed increased primary production16,17 with greater light penetration and longer open water seasons in the Arctic18. The uncertainty of phytoplankton dynamics evaluated through a 20-year time series analysis specific to July as part of the Distributed Biological Observatory (DBO) in this region indicates productivity is equivocal, suggesting variability in sea ice regimes influences water column structure, nutrient cycling, and productivity seasonally19.

The northern Bering and Chukchi seas have multiple benthic biological hotspots20 (Supplementary Fig. 1a) and support a variety of upper trophic level species21. Earlier sea ice retreat weakens the contribution of ice algae to benthic food and promotes the region to become a more pelagic-dominated marine ecosystem22,23. The open water duration and upper mixed layer may greatly impact the exchange of heat, gas, nutrients, biological production, and the benthic communities on the shelf. In recent years, rapid climate change may have induced an northward shift of macrofaunal in this region3,4,20,24,25. Previous studies suggested that the mixed layer depth (MLD) in the western Arctic, especially in the Canada Basin, showed a shoaling trend from the 1970s to 2000s26–28. At the same time, cruise observations29,30 on a short timescale revealed that wind-induced mixing was enhanced and the mixed layer deepened in an increasingly ice-free Arctic Ocean31. Because the Pacific Arctic has now transitioned to a new normal with much less sea ice than 2–3 decades ago32, this leads to an open question about whether ocean dynamics may have shifted in recent years. Delineating the upper ocean dynamics on these highly productive shelves during the growing season is fundamental for understanding ongoing ecological changes under rapid sea ice decline.

Here, we analyze 23,320 shipboard CTD/XCTD profiles and model reanalysis results from 1996 to 2021 to depict the interannual variations and decadal trends of the upper ocean properties on the Pacific Arctic shelf seas, defined as depths of 15–500 m. Our results suggest a potential regime shift in the upper ocean dynamics: a reverse trend of MLD from shoaling to deepening after 2007, a well-known year with the second lowest summer sea ice coverage since the satellite monitoring era. This finding demonstrates another robust mechanism and consequence of sea ice loss on the upper ocean dynamics, in addition to a traditional understanding of shoaling mixed layers mainly due to more ice meltwater33–35. We further quantify the roles of wind mixing, atmospheric buoyancy flux, and sea-ice-derived meltwater for summer MLD development and variations. Our hypothesis is that in increasingly ice-free Pacific Arctic shelf seas with prolonged open water duration, wind forcing can sufficiently mix the upper water column and become a more dominant factor than atmospheric buoyancy and ice meltwater in determining the summer mixed layer.

Results
Sea ice regime shift
The sea ice in the Pacific Arctic shelf seas often started retreating at the end of March and completely melted or reached the annual minimum extent in September6,36,37. After 2007, this region became almost ice-free in August (Fig. 1a and b). The period from 1996 to 2006 could be considered as a transition with shrinking perennial ice cover in the Pacific Arctic5,38,39, and a new normal of low summer sea ice generally stabilized after 200740–42. Most areas of the northern Bering and Chukchi shelves did not show an early ice retreat or prolonged open water duration from 1996 to 2006 (Supplementary Fig. 2a and b). The year 2007 was a potential tipping point of ice regime shift, triggered by the transition of a negative Arctic Dipole index to a positive one43. The increasing positive Arctic Dipole index during 2007–2021 drove an enhanced anomalous oceanic heat flux and amplified ice–albedo feedback in the Pacific Arctic44,45. Besides, the negative trend of Artic Oscillation on the decadal time scale was also associated with increasing ice losses46,47. The Arctic Amplification48 driven by atmospheric circulation patterns, caused the seasonal sea ice to decline further over the long term without any sign of recovery49. The trigger time of ice retreat (i.e., day of a year when sea ice concentration drops below 15% for three consecutive days) advanced by a mean rate of −2.6 days yr−1 (p<0.1) in the Bering Sea and −3.0 days yr−1 (p<0.1) in the Chukchi Sea in 2007–2021 (Supplementary Fig. 2c). The open water duration (i.e., periods with sea ice concentration below 15%) increased significantly with a mean rate of +2.8 days yr−1 (p<0.1) in most areas (Supplementary Fig. 2d).

The states and dynamics of the upper water column might have been altered with continuous sea ice decline and longer ice-free days. We compiled 23,320 observed CTD/XCTD profiles from various sources (Supplementary Tables 1 and 2) in 1996–2021 to quantify the upper mixed layer depth in the Pacific Arctic shelves (see the “Methods” section for MLD definition; see Supplementary Fig. 1b for regional examples of potential density profiles and MLDs). Although no significant correlation was found between observed MLD and sea ice concentration, MLD was closely related to the trigger time of ice retreat (Fig. 1c). We defined the time difference between hydrographic sampling and ice retreat day in the corresponding year as Δt and retained 19,742 profiles with positive Δt values (i.e., hydrographic profiles were sampled after sea ice retreat) to calculate their correlations with MLDs in each grid cell (see the “Methods” section). The time difference Δt was positively correlated with the summer MLD in most of the Bering and Chukchi continental shelves (Fig. 1c). In other words, the closer hydrographic profiling to the annual ice retreat day, the shallower mixed layer with more content of sea-ice-derived meltwater. Theoretically, the mixed layer would deepen under sufficiently long wind mixing, and ice meltwater became less and less influential after sea ice retreat. Notably, a negative correlation was found in the subarctic southeastern Bering shelf, indicating that the physical mechanism that controls upper ocean dynamics there could be different from other regions of the Pacific Arctic.

Reversal of mixed layer depth trends
To assess the impacts of changing sea ice and atmospheric conditions on the upper ocean, we further calculated trends of the mean summer (June–September) MLDs using a total of 18,997 quality-controlled hydrographic profiles over two separate periods: 1996−2006 versus 2007–2021 (sampling years and locations refer to Supplementary Fig. 3). There were 2835 summertime profiles in the Bering shelf during 1996–2006 and 6554 profiles after 2007. The Chukchi shelf contained 2146 profiles in 1996–2006 and 7462 in 2007–2021, respectively. The time division was based on the cold/warm transformation50 in the Pacific Arctic and sea ice regime shifts analyzed above. Despite the sampling bias and data gaps of field observations in space and time scales, the consistency of MLD shoaling/deepening tends of sparse grid cells from the Bering to Chukchi shelves was notable (Fig. 2a and b). MLDs had a similar shoaling tendency in the southeastern Bering shelf during both periods, whereas higher-latitude regions showed exactly opposite trends in specific grid cells (Fig. 2a and b). The shoaling trends of MLDs on the northern Bering (−2.4 m yr−1 of 2 grid cells, p<0.1; Fig. 2a) and the Chukchi shelves (−1.1 m yr−1 of 5 grid cells, p<0.1) before 2007 agreed with previous studies27,28. However, after 2007, the mixed layer on the northern Bering and Chukchi shelves showed a remarkable deepening trend (up to 1.3 m yr−1, p<0.1), illustrated by reddish grid cells (Fig. 2b). A freshening trend was observed in some areas of the southeastern Bering shelf (−0.1 psu yr−1, p<0.1) before 2007, and then the water in the region gradually became saltier (up to 0.08 psu yr−1, p<0.1) (Supplementary Fig. 4a and c). The mixed layer temperature had a noticeable warming trend (0.6 °C yr−1, p<0.1) on the southeastern Bering shelf from 1996 to 2006, which extended to the whole middle Bering shelf (0.4 °C yr−1, p<0.1) over 2007–2021 (Supplementary Fig. 5a and c). In contrast, the mixed layer temperature and salinity on the northern Bering and Chukchi shelves did not show significant trends in both periods.

To confirm the trends revealed by unevenly distributed hydrographic profiles, we also analyzed daily temperature and salinity output from the Global Ocean Reanalysis and Simulations (GLORYS2V4) and evaluated the mixed layer properties using identical methods (Supplementary Fig. 1c). The reverse MLD trends on the northern Bering and Chukchi shelves during the two periods were broadly consistent with the observations (Fig. 2 and Supplementary Figs. 6a and 7a). The mixed layer shoaled at a mean rate of −0.47 m yr−1 (p<0.1) in 1996–2006 and then deepened at a mean rate of 0.33 m yr−1 (p<0.1) after 2007. Due to the randomness of ship-based observations and model errors, linear trends of the MLDs based on GLORYS2V4 were more moderate than observations. Lacking tidal mixing processes in the GLORYS2V4 might result in its poor performance on the Bering shelf (Supplementary Figs. 6b and 7b). Observation datasets revealed that the inner southeastern Bering shelf (<50 m) had an overall shoaling trend (Fig. 2b). In comparison, GLORYS2V4 model results showed no obvious trends in the same region (Fig. 2d). For the high-latitude Chukchi Sea, the summer MLD trends derived from observations were similar to the model results.

Roles of surface buoyancy flux versus wind mixing
Mechanistic understandings of surface buoyancy flux versus wind mixing are essential to disentangling upper ocean dynamics in the rapidly changing Pacific Arctic shelf seas. In principle, negative air–sea buoyancy fluxes, or net atmosphere-to-ocean heat and freshwater fluxes, make the surface ocean more stratified and relatively stable (Supplementary Fig. 8a). We calculated the air–sea heat and freshwater fluxes using ECMWF Reanalysis v5 (ERA5) data. Compared to freshwater flux (Fig. 3c), heat flux was a predominant factor in stratifying the upper ocean during 1996–2006 (Fig. 3a), while air–sea freshwater flux was stable during both periods (Fig. 3d).

Local winds over the Pacific Arctic could account for 30–50% of the synoptic scale variability51–53, and explain 10–20% MLD variance27,54. We evaluated monthly mean wind stress from May to September that spanned ice melting and open water periods based on ERA5. Significant changes in wind stress mostly occurred in open seas or marginal ice zones for both periods (Supplementary Fig. 9). The wind stress from the northern Bering to Chukchi shelves decreased in the season of June–August during 1996–2006 with a mean rate of −1.2 × 10−3 Pa yr−1. After 2007, the enhancement of wind stress started in May and lasted for nearly 4 months with a positive trend of 1.7 × 10−3 Pa yr−1, which agreed with prior results of climate models55,56 and observations57. The strengthened wind stress was related to wind speed-up over the Pacific Arctic shelf under Arctic Amplification58, because sea ice loss59 and surface warming55 decreased ocean surface roughness and atmospheric stability. The stronger pressure gradient between the Aleutian Low and Beaufort High also promoted geostrophic wind speed60,61 with a positive Arctic Dipole regime since 200745.

For high-latitude oceans, sea ice effects must be considered since the sea-ice-derived meltwater is an additional surface buoyancy source, and the presence of pack ice largely hinders direct wind influence on the mixed layer. To quantify the roles of wind mixing, atmospheric buoyancy input, and local ice meltwater on the upper mixed layer during the summertime, we applied a 1D sea ice-ocean mixed layer model62–64 to calculate wind-induced turbulent kinetic energy and surface buoyancy fluxes using daily model results of GLORYS2V4 (see the “Methods” section). We ascertained the evolution of MLDs from the ice-covered (i.e., sea ice concentration ≥ 15% and sea ice thickness > 0) to open waters. The Chukchi shelf contained complex water masses, including Alaska Coastal Water, ice meltwater, dense winter water, Bering Sea water, and their mixtures in summer65. Nevertheless, the timing and extent of advected ice meltwater and other water masses varied among regions and years66. Linear trends of turbulent kinetic energy and surface buoyancy fluxes were calculated using a 3-month average to smooth water mass advection and other short-timescale processes. Model results showed that the effects of wind-induced turbulent mixing decreased with a mean rate of −4.8 × 10−8 m3 s−3 yr−1 over the northern Bering to southeastern Chukchi shelf before 2006 (Fig. 4a) but significantly increased with a mean rate of 5.9 × 10−8 m3 s−3 yr−1 over almost entire Pacific Arctic shelves after 2007 (Fig. 4b). The increased ice meltwater buoyancy near the mean summer sea ice edge in northern Chukchi shelf implied the melting of perennial sea ice before 2006 (Fig. 4e). Although the Bering shelf received more surface buoyancy during both periods (Fig. 4c and d), the influence of sea-ice-derived meltwater in summertime significantly weakened during 2007–2021 because of earlier ice retreat (Fig. 4f). In summary, the reverse trends of the mixed layer after 2007 were likely attributed to a combination of enhanced wind mixing, elongated open water duration, and reduced ice meltwater input.

Methods
Ship-based observations
Vertical conductivity–temperature–depth (CTD) and expendable–conductivity–temperature–depth (XCTD) profiles from the NOAA World Ocean Database 2018 https://www.ncei.noaa.gov/products/world-ocean-database supplemented with ship-based observations from multinational Arctic expeditions (Supplementary Tables 1 and 2) are used to analyze the upper water column structure. Only profiles that have an accepted quality-control flag and contain information on position, date, depth, temperature, and salinity are retained. The accuracies of temperature and salinity measurements are 0.001 °C and 0.002 psu, respectively. The profiles with vertical resolution >5 m or without measurements in the upper 10 m are excluded on account of accurately describing the mixed layer (see the “Methods” subsection “Definition of mixed layer depth”). Profiles with noticeable noises or obvious errors of salinity (temperature) are also eliminated. After rigorous quality control procedures, there are a total of 23,320 hydrographic profiles from 1996 to 2021, with higher spatial coverage along the US Alaska coast (Supplementary Fig. 3).

Model reanalysis
Global Ocean Ensemble Physics Reanalysis (GLORYS2V4) from the Mercator Ocean is a 3D gridded product for the ocean’s physical state estimation. The data were produced by the Nucleus for European Modeling of the Ocean (NEMO3.1), which was constrained with data assimilation of satellite data and in situ observations83. GLORYS2V4 is forced by ERA-Interim, the fourth-generation ECMWF atmospheric reanalysis of the global climate, and has been superseded by the ERA5 after the year 2019. GLORYS2V4 provides daily mean potential temperature, salinity, sea ice concentration, and thickness during 1996–2021 with a 1/4° horizontal resolution and 75 vertical levels. The errors include both the instrument and model errors, with a depth uncertainty of 2% and a temperature uncertainty of 0.1−1 °C for different depths. The vertical resolution of the uppermost 40 m water column is less than 5 m. Because the averaged summer MLD based on hydrographic profiles rarely exceeds 35 m, it can be calculated from model results using the same method as observations.

Sea ice concentration
We calculate the sea ice index using daily sea ice concentration data from 1996 to 2021, which were derived with Nimbus-7 SMMR and DMSP SSM/I-SSMIS with AMSR-E Bootstrap Algorithm84 and archived by the National Snow and Ice Data Center. This sea ice product has a spatial resolution of 25 km and a general accuracy estimated to be ±3% and ±6% of the actual sea ice concentration in winter and autumn, respectively84. The day of the sea ice retreat is defined as the day of a year when the sea ice concentration of a grid cell drops below 15% for three consecutive days. The day of sea ice freeze is defined as the day of a year when the sea ice concentration of a grid cell exceeds 15% for three consecutive days after the annual ice retreat. Open water is defined as a sea ice concentration of <15% and open water duration is calculated as the time between the day of sea ice retreat and freeze. The open water area is the summed area of grid cells with <15% sea ice concentration. The monthly mean sea ice concentration SIC over the study region is calculated based on Eq. (1):

SIC = (Σ i=1 to n sic i × A i) / (Σ i=1 to n A i) (1)

where sic i is the sea ice concentration in grid cell i, A i is the area of grid cell i, and n is the total number of grid cells within the study region.

Atmospheric data
Daily mean atmospheric data are ECMWF reanalysis version 5 (ERA5) with a uniform 0.25° horizontal resolution. The wind stress is calculated from the eastward and northward components of the 10 m wind. The net air-sea heat flux is the sum of shortwave solar radiation, net longwave radiation, latent heat flux, and sensible heat flux (Supplementary Fig. 8a). The rates of evaporation and precipitation are used to compute air-sea net freshwater flux.

Definition of mixed layer depth
The vertical structural properties of the upper mixed layer are almost uniform, so theoretically, density, salinity, or temperature profiles can all be used as estimators of MLD85–87. Among them, density and temperature profiles are more commonly used because salinity profiles tend to be noisier and sparser88. The Pacific Arctic Ocean is a typical beta ocean, where the water column structure is mainly influenced by salinity89, so methods based on density rather than temperature are preferable for detecting mixed layer depth. The threshold difference method can provide more accurate MLD estimation than density gradient threshold, least squares regression, and integral methods87. Given the simplicity and successful applications26,27,54,90, this study chooses the density threshold method to estimate MLD. The density is converted from temperature, salinity, and pressure using the GSW Oceanographic Toolbox (www.TEOS−10.org). The density thresholds of 0.1, 0.125, and 0.15 kg m−3 are tested, and the changing tendency of MLD is insensitive to the criterion (Supplementary Fig. 7). Since we focus on the averaged changes of MLD over periods of months and longer, the threshold value of 0.125 kg m−3 is deemed appropriate54,90. We set the uppermost reference depth of 5 m because CTD profile data are generally less accurate within 0–5 m water depths. The mixed layer depth (H) can be inferred from Eq. (2), and σ 0 was the potential density at the depth referenced to the surface.

σ 0(z = −H) − σ 0(z = −5 m) = 0.125 kg m−3 (2)

1D sea ice-ocean mixed layer model
The parametric equations of 1D sea ice-ocean mixed layer model64 simulated the state of the mixed layer with given atmospheric forcing. Based on Lemke’s model64, we input the state of the mixed layer and sea ice from GLORYS2V4 model results to determine the contributions of turbulent kinetic energy and surface buoyancy flux for changing upper ocean (Supplementary Fig. 8b).

The determination of surface buoyancy flux in open sea and under sea ice cover is different. In the open sea, the surface heat flux Q T can be calculated straightforwardly by the net solar radiation Q sw, net longwave radiation Q lw, latent heat flux Q lat and sensible heat flux Q sen as Eq. (3).

Q T = (Q sw + Q lw + Q lat + Q sen) / (ρ water c p) (3)

According to the KTN model62,63, the turbulent kinetic energy TKE provided by wind stirring is balanced with the increase of the potential energy due to the surface fluxes Q and entrainment fluxes B, respectively. The surface buoyancy fluxes Q and entrainment buoyancy fluxes B are determined from Eq. (9a) and (9b) with thermal expansion α and haline contraction β coefficients.

Q = β Q S − α Q T g (9a)

B = β B S − α B T g (9b)

where g is the gravitational acceleration. Assuming a balance between energy production and dissipation integrated over the whole mixed layer in the KTN model, the turbulent kinetic energy induced by wind can be determined from Eq. (10) using an empirical parameter n = 0.294,95

TKE = B / 2 * [(1 + n) Q − (1 − n) |Q|] * h (10)

Statistical analysis of linear trends
In this work, we are interested in detecting long-term trends of the upper mixed layer properties from the ship-based observations and model results. In light of the sparse field data coverage in time and location, we bin MLDs derived from the observations of June–September in each 1°×2° grid cell to represent the state of the summer upper water column27,96. We only use the grid cell with sufficient sampling years to obtain robust decadal trends. For example, the grid cells on the shelf containing fewer than 5 years of data in 1996-2006 were excluded from the linear trend analysis. For 2007–2021, each grid cell required at least 6 years of data. According to the statistical results of Peralta-Ferriz and Woodgate (2015)27, the summertime (June–September) mixed layer depths in the Chukchi Sea shelf have a narrow, quasi-normal distribution. Therefore, the long-term trend of MLDs from ship-based observations was estimated as the best linear fit to the summer mean MLDs (June–September). Note that a sudden storm or water mass advection in a short timescale may affect the CTD profiling results on a daily scale, which causes uncertainty to the analyses.

For daily MLDs derived from the GLORYS2V4, we also use the mean values from June to September to calculate the linear trends in each 0.25°×0.25° grid cell. The linear trends of MLDs in Fig. 2c and d are estimated after removing the seasonal climatology. To get a better comparison with field observations, the linear trends of GLORYS2V4 model results in specific grids (Supplementary Figs. 6 and 7) were calculated without removing the seasonal cycle.

The daily wind stress and air–sea buoyancy flux is derived from ERA5 datasets. The linear trends of wind stress and air–sea net heat and freshwater fluxes from June to September in each 0.25°×0.25° grid cell are calculated after removing the seasonal climatology. The linear trends of wind-induced turbulent kinetic energy (TKE) are calculated for the time window from June to September. The linear trends of sea-ice-derived meltwater buoyancy are calculated under the time window from May to July since the sea ice started to retreat in May after 2007, and the study region was in open sea in August. The trends of sea ice retreat day and open water duration are calculated for the same 0.25°×0.25° grid cell.

A linear regression model is used to estimate the trends of variables in each grid cell of the study region, and only significant trends with p<0.1 are shown.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "Review of „Enhanced wind mixing and deepened upper mixed layer under the new normal Arctic of low summertime sea ice“\nThis paper describes the decadal changes in the ocean surface mixed layer depth in the Pacific Arctic. Both observations and reanalysis data are used to quantify the changes and explain the reason. In particular, the authors suggest that the mixed layer in the Pacific Arctic has been deepening since 2007 mainly due to enhanced wind-induced mixing.\nRainville et al. 2011 is one of the first papers that proposed that the Arctic shelf seas could experience deeper mixed layer in summer due to enhanced wind-induced mixing. As the current paper aligns with the idea of those studies, a review of related old studies is needed in the introduction.\nOne major concern is that the observations in the period of 2007-2021 demonstrate deepening in mixed layer only in 6 grid cells (Fig. 2b), while all other grid cells (data grid cell locations as indicated in Fig. 1c) did not experience statistically significant trends (Fig. 2b). Fig. S3 further shows that all the 4 benthic biological hotspot subdomains did not experience statistically significant deepening trends according to observations in the 2007-2021 period. Instead, only the model reanalysis data show statistically significant trend in three of the four subdomains for this period. Similarly inconsistent with the major claim of the study, in the 1996-2006 period, actually only one of the four subdomains shows shoaling trend in observations and only two of the four subdomains show shoaling trends in the reanalysis (Fig. S3). Considering all these facts, the combination of observations and model reanalysis as presented in the paper does not consistently lead to a convincing conclusion that a regime shift in the upper ocean dynamics in the Pacific Arctic has occurred.\nFurthermore, changes in mixed layer depth can be different in different regions of the Arctic Ocean because wind mixing is only one of the influencing factors and dominating factors can differ in different regions and warming scenarios. This point should be made clear and the paper title should reflect that this study is only about Pacific Arctic.\nA few other major comments:\nLines 116-120: Are the positive Artic Oscillation and the Arctic atmospheric dipole relevant to the Pacific Arctic sea ice change? Here the text is intended to explain the shift in sea ice in Pacific Arctic in 2007. In the current text, it is not obvious that the referred atmospheric circulation can explain the sea ice regime shift in the “Pacific Arctic area”.\nL132-145: It is not clear why the observed mixed layer depth is highly correlated with the time difference. Instantaneous mixed layer depth can be strongly influenced by storms that had just passed the region. Does the high correlation shown in Fig. 1c indicate that wind-driven mixing is not important for mixed layer depth? Furthermore, why is the negative correlation in the southeastern Bering Sea just due to “alpha” ocean, not due to the impact of wind-induced mixing?\nFig. S2f shows that there is a slight increase in upper ocean salinity in 1996-2006 in the reanalysis. And there is no trend in wind stress in this period. Then how to explain the shoaling trend of mixed layer in the Bering Sea in the reanalysis in this period?\nL200-202 These sentences are not clear.\nL250-256. Which factor plays the dominant role depends on the exact region. Even if wind-driven mixing plays a major role in Pacific Arctic, it does not prove that the same is true for the Arctic basin area.\nIn addition, as shown in Fig. S3, the variability among individual observations is a few tens of meters, while the total trend is about 5 meters over the last 15 years in those subdomains with strongest trend in the model (no significant trend in observations though). It would be necessary to discuss the importance of this simulated trend in comparison with the observed strong variability for marine ecosystems.\nRef:\nRainville, L., C.M. Lee, and R.A. Woodgate. 2011. Impact of wind-driven mixing in the Arctic Ocean. Oceanography 24(3):136–145, https://doi.org/10.5670/oceanog.2011.65.", "reviewer_2": "Review of ‘Enhanced wind mixing and deepened upper mixed layer under the new normal Arctic of low summertime sea ice’ by Wang et al.\nI find the primary conclusions of this paper to be about the balance between buoyancy forcing and wind stress for a melting Arctic. The conclusions and presentation of the study is overly focused on one time period where wind forcing overcomes buoyancy (2007-2021). The earlier time period (1996-2006) where wind stress did not overcome buoyancy forcing is an equally important result that should be integrated more into the paper. Overall, this is a study worth publishing if improvements are made to better support and present the conclusions.\nMajor comments:\n1) Abstract and title\na. Neither the abstract nor the title clarify that this paper is focused on the shelf regions exclusively. Reference to ‘Pacific Arctic’ at line 35 should be more specific; it could mean the entire Western Arctic. Reference to ‘biologically productive continental shelves’ at line 33 is not sufficient.\nb. Line 39-40 are the main dynamical conclusions of the paper but are incomplete as stated. There is a buoyancy forcing from sea ice melt, and the enhanced wind stress it able to overcome it.\nc. 1996-2006 is completely absent from the abstract, and no reference is given to a shoaling trend over this period or why it occurs.\n2) Some key information is missing, which effects how strong the conclusions are and the readability of the paper.\na. There is not a single example profile of stratification (or nutrients) from this region. It should be included in some form in the main paper in my opinion. Could Figure 4 be based on composite profiles? Is it already?\nb. Please add a timeseries of mixed layer depth to this paper (e.g., similar to Figure 1b for the sea ice). Figure 2 hints at the magnitudes of the mixed layer depth changes, but I’m still left wondering what the actual depths and changes are, especially with respect to the shallow topography. Line 291: ‘this deepened upper mixed layer’… by how much?\nc. Line 148-151: When does the summer mixed layer form? May? June? July? Do all June profiles have a summer mixed layer present? Are only profiles that have a summer mixed layer included in the statistics? Line 248: Explicitly show the time window in this paper. When does the summer mixed layer (potentially) form, and how long is the season over which it develops?\n3) The conclusions could be strengthened by making sure the hypothesis and explanations are applicable to both time periods, 1996-2006 and 2007-2021.\na. Same comment as (1) above, 1996-2006 is completely absent from the abstract.\nb. Line 85-88: As stated, this hypothesis does not explain why the mixed layer shoaled prior to 2007 (when the ice extent was also decreasing).\nc. Line 243-245: The conclusions regarding 1996-2006 need to be clearer. State that buoyancy forcing was sufficient to overcome and wind stress. During this time period, I would say that the ‘traditional’ view (as stated in this paper) of more melting leads to more stratification / shallower mixed layers is correct.\n4) Is the 1996-2006 time period appropriate? How much data is available from 1996-1998? Do the conclusions change if the time period is altered?\na. Line 128-130 is misleading and suggests that only the data in Table S1 is used. Lines 296-298 contradict this, as data from the world ocean database is also included in addition to the Table S1 data.\nb. A summary table or figure describing the entire dataset is needed. I suggest splitting Figure S5a into two maps, one for each time period, along with splitting Figure S5b into two panels, one for the Chukchi Sea and one for the Bering Sea. A bit more description at line 128-129 would be useful (e.g., total number of summertime profiles within each region and time period).\n5) Can the use of wind stress instead of surface stress be justified a bit more? This paper should be addressing the surface stress that the ocean feels accounting for the effects of sea ice cover, and not the wind stress that is imposed on any sea ice present. It’s possible the use of wind stress is justified, but that is not clear based on what is presented (analysis of the April – November time period).\na. Line 192-194: It seems more appropriate to focus the wind stress calculation on June – September as with mixed layer depths. Why are different months chosen?\nb. Line 214-215 & lines 222-224: Momentum input to the ocean is larger during periods of mixed ice concentration vs. open water. Treating everything as open water isn’t a great approach unless it can be justified. What is the ice concentration / coverage like when the summer mixed layer forms?\nAdditional comments:\n6) There’s a lot of emphasis on what the ‘traditional’ view is (lines 83, 250), and how this paper contradicts it. I think this needs to be toned down, and simply presented as both buoyancy forcing and wind forcing are potentially important. It will not take away from the significance of this paper.\na. I would argue that the ‘traditional’ view that interannual ice loss can increase buoyancy and shoal the summer mixed layer is in fact correct during 1996-2007.\nb. Line 37: Maybe rephrase ‘contradicting a common notion’. It’s an idea, but I don’t know that it’s pervasive.\nc. Line 41, 108, and elsewhere: Is this really a regime shift? The change is August ice extent between the two time periods does not qualify in my opinion. Even during 1996-2006, most of the region under consideration was seasonal ice. This contradicts lines 111-113 that claim MYI in the earlier period. I’m a bit confused about this argument.\nI do not find Figure 1c to be a new result (lines 136-139). Conceptually, it is true Arctic wide that the summer mixed layer forms in relation to ice melt / melt pond drainage and then gradually deepens during summer. It is not surprising that deeper mixed layers are associated with a longer time since mixed layer shoaling, or that mixed layers are shoaling earlier in the Arctic. The description here seems to have taken a simple concept and made it complicated.\nLine 123: define what you mean by trigger time, even if it is stated in the methods. Do lines 123-125 refer to a figure?\nLine 133: how is the day of sea ice retreat defined? State briefly even if it stated in the methods.\nLine 203-205: What is advection like in this area? The weakening wind stress in the western Bering Sea could induce a change in water properties that is then advected into the region where mixed layer shoaling is observed during 1996-2006. Is this plausible?\nLine 304: ‘station depths shallower than 10 m’ are excluded? This reads like profiles with a measurement in the upper 10 m are excluded, instead of excluding those without a measurement in the upper 10 m.\nIn table S1, ‘Sea storm 2011’ is listed as August – Sept 2009 (should be 2011?).\nFigure caption lables (a,b,c, etc) should be placed at the start of the sentence describing each panel, not at the end. These are difficult to read."}, "author_response": "Response to Reviewers\nReviewer #1 (Remarks to the Author):\nMajor comments:\n1) Rainville et al. 2011 is one of the first papers that proposed that the Arctic shelf seas\ncould experience deeper mixed layer in summer due to enhanced wind-induced mixing.\nAs the current paper aligns with the idea of those studies, a review of related old studies\nis needed in the introduction.\nResponse: Thank you for your suggestion. We have updated the introduction and added\nrelevant references as per your suggestion (References 29, 30, 31; Lines 1129-1134):\nLines 104-108: “Previous studies suggested that the mixed layer depth (MLD) in the\nwestern Arctic, especially in the Canada Basin, showed a shoaling trend from the 1970s\nto 2000s26-28. At the same time, cruise observations29-30 on a short time scale revealed\nthat wind-induced mixing was enhanced and the mixed layer deepened in an\nincreasingly ice-free Arctic Ocean31.”\n2) One major concern is that the observations in the period of 2007-2021 demonstrate\ndeepening in mixed layer only in 6 grid cells (Fig. 2b), while all other grid cells (data\ngrid cell locations as indicated in Fig. 1c) did not experience statistically significant\ntrends (Fig. 2b). Fig. S3 further shows that all the 4 benthic biological hotspot\nsubdomains did not experience statistically significant deepening trends according to\nobservations in the 2007-2021 period. Instead, only the model reanalysis data show\nstatistically significant trend in three of the four subdomains for this period. Similarly\ninconsistent with the major claim of the study, in the 1996-2006 period, actually only\none of the four subdomains shows shoaling trend in observations and only two of the\nfour subdomains show shoaling trends in the reanalysis (Fig. S3). Considering all these\nfacts, the combination of observations and model reanalysis as presented in the paper\ndoes not consistently lead to a convincing conclusion that a regime shift in the upper\nocean dynamics in the Pacific Arctic has occurred.\nResponse: Thank you for pointing out the limitations of the sparse observations and\nthe inconsistency between the model and observation. In the Pacific Arctic region, only\nlimited ship-based field observations were conducted, mostly in the summer months,\nso CTD/XCTD profile data were sparse. Nevertheless, observed hydrographic profiles\nare invaluable data to study ocean dynamics. We modified Fig. 1c and removed coastal\ngrids with total water depths of less than 15 m. All grids with colors or crosses had at\nleast 20 hydrographic profiles from 1996 to 2021. Despite that, not all marked grids in\nFig. 1c have sufficient data to calculate linear trends of mixed layer depths in 1996-\n2006 versus 2007-2021. To distinguish geographical locations with insufficient profiles\nversus no statistical significance, we replotted Fig. 2a and 2b (Line 568). The grids with\nsignificant linear trends (p<0.1) are shown in colored squares, and the grids with no\nsignificant trends are illustrated by black crosses; the blank grids are without sufficient\nprofiles. Based on the new results, the ship-based mixed layer indicates 12 deepening\ngrids in 1996-2006 (Fig 2a) and 7 shoaling grids in 2007-2021 (Fig 2b).\nThe relevant texts regarding model and observation comparisons have been rewritten\n(Lines 433-447): “To confirm the trends revealed by unevenly distributed hydrographic\nprofiles, we also analyzed daily temperature and salinity output from the Global Ocean\nReanalysis and Simulations (GLORYS2V4) and evaluated the mixed layer properties\nusing identical methods (Supplementary Fig. 1c). The reverse MLD trends on the\nnorthern Bering and Chukchi shelves during the two periods were broadly consistent\nwith the observations (Fig. 2 and Supplementary Fig. 6a & 7a). The mixed layer shoaled\nat a mean rate of -0.47 m yr-1 (p<0.1) in 1996-2006 and then deepened at a mean rate\nof 0.33 m yr-1 (p<0.1) after 2007. Due to the randomness of ship-based observations\nand model errors, linear trends of the MLDs based on GLORYS2V4 were more\nmoderate. Lacking tidal mixing processes in the GLORYS2V4 might result in its poor\nperformance on the Bering shelf (Supplementary Fig. 6b & 7b). Observation datasets\nrevealed that the inner southeastern Bering shelf had an overall shoaling trend (Fig. 2b).\nIn comparison, GLORYS2V4 model results showed no obvious trends in the same\nregion (Fig. 2d). For the high-latitude Chukchi Sea, the summer MLD trends derived\nfrom observations were similar to the models.”\nIn the revision, we updated GLORYS2V4 by adding additional model simulation data\nfor 2021, which was made available by the Copernicus database in recent months.\nTo further demonstrate and compare mixed layer depth calculations from the ship-based\nobservation and model reanalysis data, we added annual mean summer mixed layer\ndepths and linear trends at specific sites (Supplementary Fig. 5 and Fig. 6). The mixed\nlayer depths derived from observations and GLORYS2V4 are generally consistent with\neach other in the northern Bering and Chukchi Sea shelf during both shoaling (1996-\n2006) and deepening periods (2007-2021).\n3) Furthermore, changes in mixed layer depth can be different in different regions of\nthe Arctic Ocean because wind mixing is only one of the influencing factors and\ndominating factors can differ in different regions and warming scenarios. This point\nshould be made clear and the paper title should reflect that this study is only about\nPacific Arctic.\nResponse: Thank you for the suggestion. We agree that the title should reflect the exact\nstudy region, which has been revised accordingly. The revised title is: “Enhanced wind\nmixing and deepened upper mixed layer in the new normal Pacific Arctic Ocean with\nlow summertime sea ice” (Lines 1-2). We also agree that wind mixing is only one of\nthe influential factors, and dominant factors differ in location and period. The abstract\nhas been revised to emphasize this key point (Lines 34-38): “We attribute mixed layer\ndeepening to markedly lengthened open water duration and enhanced wind-induced\nmixing. Together, these factors increase cumulative wind energy input to an ocean less\nbuffered by sea ice. It is concluded that the relative importance of surface buoyancy\nflux versus wind forcing switched in two periods despite continued sea ice decline.”\nIn the revision, we carefully examined the relative contributions of surface buoyancy\nflux versus wind mixing in summer mixed layer evolution in the subsection “Roles of\nsurface buoyancy flux versus wind mixing” (Lines 575-685). The diagnostic 1-D mixed\nlayer model has been updated by using a 1-D sea ice-ocean mixed layer model and\nincluding sea-ice meltwater as an additional buoyancy source (Lines 1368-1454).\n4) Lines 116-120: Are the positive Artic Oscillation and the Arctic atmospheric dipole\nrelevant to the Pacific Arctic sea ice change? Here the text is intended to explain the\nshift in sea ice in Pacific Arctic in 2007. In the current text, it is not obvious that the\nreferred atmospheric circulation can explain the sea ice regime shift in the “Pacific\nArctic area”.\nResponse: The decadal-scale variability of the atmospheric circulation can explain the\nsea ice behavior in the Pacific Arctic Ocean (Bi et al., 2023). The Arctic Dipole (AD)\nindex was slightly negative before 2007 and became increasingly positive from 2007\nto 2021. The positive AD drives an enhanced anticyclonic Beaufort Gyre and\nTranspolar Drift and results in increased heat advection into the Arctic (Bi et al., 2021;\nPolyakov et al., 2023). The AD was also a major driver of the second record-low sea-\nice extent in summer 2007 during the satellite record (Wang et al., 2009). The Artic\nOscillation (AO) index has a statistically significant positive trend during the last 60\nyears (Jeong et al., 2022). However, from the late 1980s to the early 2010s, the AO\nindex showed a significant negative trend (Cohen et al., 2012). This negative trend of\nAO on the decadal time scale was associated with the Arctic amplification and\nincreasing ice loss (Lindsay et al., 2009; Cohen et al., 2014). Referred atmospheric\ncirculation can explain the shift in the sea ice regime on the Pacific Arctic shelves.\nWe have rewritten the main text in the subsection “Sea Ice Regime Shift” to better\ndemonstrate the impact of atmospheric circulation on sea ice changes (Lines 162-180):\n“The sea ice in the Pacific Arctic Ocean often started retreating at the end of March and\ncompletely melted or reached the annual minimum extent in September6,36,37. After\n2007, this region became almost ice-free in August (Fig. 1a & 1b). The period from\n1996 to 2006 could be considered a transition with largely seasonal ice in the Pacific\nArctic5,38,39, and a “new normal” of extremely low summertime sea ice generally\nstabilized after 200740-42. Most northern Bering and Chukchi continental shelves did\nnot show an early ice retreat or prolonged open water duration from 1996 to 2006\n(Supplementary Fig. 2a & 2b). The year 2007 was a potential tipping point, triggered\nby the transition of a negative Arctic Dipole index to a positive one43. The increasingly\npositive Arctic Dipole index during 2007-2021 drove an enhanced anomalous oceanic\nheat flux and amplified ice–albedo feedback44,45. Besides, the negative trend of Artic\nOscillation on the decadal time scale was also associated with increasing ice loss46,47.\nThe atmospheric circulation variability caused the seasonal sea ice to decline further\nover the long term without any sign of recovery48. The trigger time of ice retreat (i.e.,\nyear day when sea ice concentration drops below 15% for three consecutive days) \nadvanced by a mean rate of -2.6 days yr-1 (p<0.1) in the Bering Sea and -3.0 days yr-1\n(p<0.1) in the Chukchi Sea in 2007-2021 (Supplementary 2c). The open water duration\n(i.e., periods with sea ice concentration below 15%) increased significantly with a mean\nrate of +2.8 days yr-1 (p<0. 1) in most areas (Supplementary 2d).”\nBi, H. et al. Influences of summertime Arctic Dipole Atmospheric Circulation on sea\nice concentration variations in the pacific sector of the Arctic during different Pacific\nDecadal Oscillation Phases. Journal of Climate, 34(8): 3003-3019 (2021).\nBi, H., Liang, Y. & Chen, X. Distinct Role of a Spring Atmospheric Circulation Mode\nin the Arctic Sea Ice Decline in Summer. Journal of Geophysical Research:\nAtmospheres, 128(6), e2022JD037477 (2023).\nPolyakov, I. V. et al. Fluctuating Atlantic inflows modulate Arctic atlantification.\nScience 381, 972 (2023).\nWang, J. et al. Is the dipole anomaly a major driver to record lows in the Arctic sea ice\nextent? Geophysical Research Letter, 36(5): L05706 (2009).\nJeong, Y. C., Yeh, S. W., Lim, Y. K., Santoso, A. & Wang, G. Indian Ocean warming as\nkey driver of long-term positive trend of Arctic Oscillation. npj Clim Atmos Sci. 5, 56\n(2022).\nCohen, J. et al. Arctic warming, increasing snow cover and widespread boreal winter\ncooling. Environ. Res. Lett. 7, 014007 (2012).\nCohen, J. et al. Recent Arctic amplification and extreme mid-latitude weather. Nat.\nGeosci. 7, 627–637 (2014).\nLindsay, R. W., Zhang, J., Schweiger, A., Steele, M. & Stern, H. Arctic sea ice retreat\nin 2007 follows thinning trend. Journal of Climate, 22(1): 165-176 (2009).\n5) L132-145: It is not clear why the observed mixed layer depth is highly correlated\nwith the time difference. Instantaneous mixed layer depth can be strongly influenced by\nstorms that had just passed the region. Does the high correlation shown in Fig. 1c\nindicate that wind-driven mixing is not important for mixed layer depth? Furthermore,\nwhy is the negative correlation in the southeastern Bering Sea just due to “alpha”\nocean, not due to the impact of wind-induced mixing?\nResponse: We agree that storms can quickly deepen mixed layers, as Rainville et al.\n(2011) cruise observation had indicated. The presence of sea ice largely impedes wind\nenergy into the ocean and can buffer storm impact on the upper ocean. As sea ice melts,\nwind-induced turbulence can strongly affect the upper ocean structure. Theoretically, a\nlonger open water duration would extend the time window for wind mixing. The\npositive correlations imply that cumulative wind energy input to the upper ocean\nbecomes even more important than ice meltwater and surface buoyancy flux in\ndetermining mixed layer depth, as the 1-D sea ice-ocean mixed layer model diagnosis\nhas revealed (see the subsection “Roles of surface buoyancy flux versus wind\nmixing”). Southeast Bering Sea has a very short time window of sea ice cover, typically\nonly in the winter (see Fig 1a blue lines for March ice edge), and therefore, sea ice is\nless influential on that region. According to Carmack (2007), the upper layer stratified\nby heat is an alpha ocean (i.e., thermal dominance), and stratified by salinity is a beta\nocean (i.e., saline dominance). The southeastern Bering shelf is an alpha ocean where\nthe heat flux input to the ocean is a major factor for stratification (Ladd and Stabeno,\n2012; Stabeno et al., 2012).\nThe paragraph has been rewritten to better explain the correlations of mixed layer depth\nand the time difference between hydrographic sampling and ice retreat (Lines 211-285).\n“The states and dynamics of the upper water column might have been altered with\ncontinuous sea ice decline and longer ice-free days. We compiled 23,320 observed\nCTD/XCTD profiles from multiple cruises (Supplementary Table 1 & 2) in 1996-2021\nto quantify the upper mixed layer in the study region (see Methods for MLD definition;\nSee Supplementary Fig. 1b for regional examples of potential density profiles and\nMLDs). Although no significant correlation was found between observed MLD and sea\nice concentration, it was closely related to the trigger time of sea ice retreat (Fig. 1c).\nWe defined the timing difference between the sampling time of hydrography and sea\nice retreat day in the corresponding year as ∆𝑡 and retained 19,742 profiles with\npositive ∆𝑡 values (i.e., hydrographic profiles were sampled after sea ice retreat) to\ncalculate their correlations with MLDs in each grid (see Methods). The timing\ndifference ∆𝑡 was positively correlated with the summer MLD in most of the Bering\nand Chukchi continental shelves (Fig. 1c). In other words, the closer hydrographic\nprofiling to annual ice retreat, the shallower mixed layer with more content of sea-ice-\nderived meltwater. Theoretically, the mixed layer would deepen under a sufficiently\nlong wind stirring after sea ice retreat, and ice meltwater became less and less influential\ninto the ice-free period. Notably, a negative correlation was found in the sub-arctic\nsoutheastern Bering shelf, indicating that the physical mechanism that controls upper\nocean dynamics there could be different from other regions of the Pacific Arctic.”\n6) Fig. S2f shows that there is a slight increase in upper ocean salinity in 1996-2006 in\nthe reanalysis. And there is no trend in wind stress in this period. Then how to explain\nthe shoaling trend of mixed layer in the Bering Sea in the reanalysis in this period?\nResponse: Both upper ocean salinity and temperature increased in 1996-2006 based on\nthe GLORYS2V4 reanalysis (Supplementary Fig. 4b & 5b). An increase in salinity may\ndeepen the mixed layer, while temperature increases shoal the mixed layer. Although\nthere were no significant trends in wind stress (Supplementary Fig. 9) and wind-induced\nturbulent kinetic energy (Fig. 4a) on the southern Bering Sea shelf from 1996 to 2006,\nthis region may have received more surface buoyancy fluxes during this period (Fig 4c).\nTherefore, increasing atmospheric heat flux input was a dominant factor that caused the\nshoaling trend of mixed layer depth on the southern Bering shelf in 1996-2006.\n7) L200-202 These sentences are not clear.\nResponse: The original L200-202 sentence within the whole paragraph was to explain\nhow and why wind patterns changed based on our and previous studies. The Arctic\nwinds are integral to heat and energy transfer between the atmosphere and the ocean\n(Hughes and Cassano, 2015). The surface roughness of sea ice is usually higher than\nthat of open water, so winds tend to strengthen where sea ice diminishes (Jakobson et\nal., 2019). Besides, the increased surface temperature and turbulent heat flux will\ndecrease the low-level atmospheric stability as sea ice loss. The decrease in static\nstability further promotes stronger winds (Mioduszewski et al., 2018). In addition, the\nlarge decrease in sea level pressure (Aleutian low) also results in a mean geostrophic\nwind speed increase (Wang et al., 2021; Huang et al., 2022). The growth of wind in the\ntransition from sea ice to open ocean is associated with increased surface warming,\nreduced surface roughness, decreased static stability, and lowered sea level pressure.\nThis paragraph has been rewritten (Lines 585-598):\n“Local winds over the Pacific Arctic could account for 30-50% of the synoptic scale\nvariability50-52, and explain 10-20% MLD variance27,53. We evaluated monthly mean\nwind stress from May to September that spanned ice melting and open water periods\nbased on ERA5. Significant changes of wind stress mostly occurred in open sea or\nmarginal ice zone for both two periods (Supplementary Fig. 9). The wind stress from\nthe northern Bering to Chukchi Sea shelves decreased from June to August during\n1996-2006 with a mean rate of -1.2×10-3 Pa yr-1. With earlier sea ice retreat during the\nlater period than the previous, the wind stress had a nearly 4-month growth window\nstarting in May with a positive rate of 1.7×10-3 Pa yr-1, which agreed with prior results\nof climate models54,55 and observations56. The strengthened wind stress resulted from\nsea ice loss and surface warming54,57,58, which decreased ocean surface roughness and\natmospheric stability. The positive Arctic Dipole atmospheric circulation has been more\nanticyclonic since 200759,60, which increased the pressure gradient between the Aleutian\nLow and Beaufort High60 and further promoted wind growth61.”\nHughes, M., & Cassano, J. J. The climatological distribution of extreme Arctic winds\nand implications for ocean and sea ice processes. J. Geophys. Res. Atmos.120, 7358–\n7377 (2015).\nJakobson, L., Vihma, T., & Jakobson, E. Relationships between sea ice concentration\nand wind speed over the Arctic Ocean during 1979–2015. Journal of Climate, 32, 7783–7796 (2019).\nMioduszewski, J., Vavrus, S., & Wang ,M. Diminishing arctic sea ice promotes stronger\nsurface winds. Journal of Climate, 31(19), 8101–8119 (2018).\nWang, Q., Danilov, S., Mu, L., Sidorenko, D., & Wekerle, C. Lasting impact of winds\non Arctic sea ice through the ocean’s memory. The cryosphere, 15(10), 4703-4725\n(2021).\nHuang, Y., Bai, X., & Leng, H. Synoptic-scale variability in the Beaufort High and\nspring ice opening in the Beaufort Sea. Frontiers in Marine Science, 9, 929209 (2022).\n8) L250-256. Which factor plays the dominant role depends on the exact region. Even\nif wind-driven mixing plays a major role in Pacific Arctic, it does not prove that the\nsame is true for the Arctic basin area.\nResponse: Thanks for the suggestion. We have revised both the title and main text to\nspecify our study region as the Pacific Arctic Ocean. This paragraph has been rewritten\nto further clarify the point (Lines 748-987):\n“The traditional viewpoints explained that rapid sea ice loss should enhance upper\nocean stratification27,34 due to climate warming68 and/or ocean freshening69,70. In\ncontrast, as the Pacific Arctic shelves shifted to a new normal with extremely low\nsummer sea ice after 2007, the upper mixed layer reversed from shoaling to deepening,\nsuggesting that sea-ice-derived meltwater played a secondary role, whereas perfectly-\ntimed wind stirring during prolonged open water period dominated upper ocean\ndynamics. We proved that although Arctic sea ice cover continued its downward trend,\nthe regional upper water column may respond differently when wind-induced mixing\noutcompeted buoyancy forces in summer. Nevertheless, whether the mixed layer\ndeepening in recent years will continue to develop or is just a transition with sea ice\ndecline remains to be seen. Once the role of buoyancy flux takes a superior position,\nthe stratification might intensify again in a warmer and ice-free future Arctic71,72.”\n9) In addition, as shown in Fig. S3, the variability among individual observations is a\nfew tens of meters, while the total trend is about 5 meters over the last 15 years in those\nsubdomains with strongest trend in the model (no significant trend in observations\nthough). It would be necessary to discuss the importance of this simulated trend in\ncomparison with the observed strong variability for marine ecosystems.\nResponse: Thanks for your suggestion. We have made the following changes in the\nrevision. The new Supplementary Fig 1 presents the upper ocean structures at four\ndifferent Distributed Biological Observatory boxes using both observations\n(Supplementary Fig 1b) and GLORYS2V4 reanalysis (Supplementary Fig 1c).\nSupplementary Fig. 6 and Fig. 7 illustrate annual mean mixed layer depths at multiple\nsites in 1996-2006 and 2007-2021. The changes in upper ocean physical processes on\nthe marine ecosystem are rewritten (Lines 988-1004):\n“The marine ecosystem on the northern Bering and Chukchi shelves is sensitive to the\nchanges in the upper water column processes. The spring bloom timing on the Bering\nSea shelf was related to the seasonal ice breakup73. The spring blooms could occur\nearlier with sea ice retreat74, and marginal ice zone blooms have occurred more\nfrequently in recent years75. Strong wind mixing in the open water would deepen the\nupper mixed layer, bring nutrients into the euphotic zone, and further expand the\nblooms to the autumn, potentially supporting production over a longer seasonal scale76.\nHowever, the nitrate deficit in the upper water column may also intensify13,77 with a\nchanging mixed layer and notable biological utilization78. Altered timing and\nmagnitude of phytoplankton bloom79 and zooplankton grazing22, as well as nutrient\ndynamics, bring uncertainties to the foodweb energy flow, which could negatively\nimpact the strength of pelagic-benthic coupling3,32. Moreover, sea ice loss and longer\nopen water duration might promote rapid uptake of atmospheric CO\n2\nand amplify\nseawater acidification80,81, despite a strong biological uptake in the Chukchi Sea.\nIn summary, this deepened upper mixed layer modulated by large-scale climate change\nmay have long-lasting impacts on biogeochemical cycling, marine organisms, and the\necosystem as a whole in the new normal Pacific Arctic.”\nReviewer #2 (Remarks to the Author):\nMajor comments:\n1) a. Neither the abstract nor the title clarify that this paper is focused on the shelf\nregions exclusively. Reference to ‘Pacific Arctic’ at line 35 should be more specific; it\ncould mean the entire Western Arctic. Reference to ‘biologically productive continental\nshelves’ at line 33 is not sufficient.\nb. Line 39-40 are the main dynamical conclusions of the paper but are incomplete as\nstated. There is a buoyancy forcing from sea ice melt, and the enhanced wind stress it\nable to overcome it.\nc. 1996-2006 is completely absent from the abstract, and no reference is given to a\nshoaling trend over this period or why it occurs.\nResponse: Thank you for the suggestion! We agree that the title and abstract should\nreflect the exact study region, which has been revised accordingly. The revised title is:\n“Enhanced wind mixing and deepened upper mixed layer in the new normal Pacific\nArctic Ocean with low summertime sea ice” (Lines 1-2).\nWe also agree that the abstract should emphasize changes in the mixed layer depth\ntrends and the relative importance of wind versus surface buoyancy forces over the two\nperiods. The abstract has been rewritten (Lines 34-51):\n“The Arctic Ocean has undergone dramatic sea ice loss in recent decades, with earlier\nmelting and later freezing. The thickness of multiyear sea ice sharply decreased in the\nearly 2000s, and the ice regime shifted to thinner and more mobile seasonal ice.\nHowever, the possible changes in the upper ocean dynamics during this drastic\ntransformation have not been thoroughly investigated, especially for the biologically\nproductive Pacific Arctic shelves. Here, we quantified the summertime upper mixed\nlayer and dynamically related its development with sea ice and atmospheric forcing,\nusing a compilation of hydrographic observations and model reanalysis data from 1996\nto 2021. Before 2006, a shoaling summer mixed layer was associated with massive sea\nice loss and surface warming. However, the upper mixed layer reversed to a generally\ndeepening trend after 2007. We attribute mixed layer deepening to markedly lengthened\nopen water duration and enhanced wind-induced mixing. Together, these factors\nincrease cumulative wind energy input to an ocean less buffered by sea ice. It is\nconcluded that the relative importance of surface buoyancy flux versus wind forcing\nswitched in two periods despite continued sea ice decline. The changing upper ocean\ndynamics and dominant forcing factors may profoundly affect the marine ecosystem,\nwhich has seen unusual fall blooms and more severe ocean acidification, deeper and\nnorthward, in the recent decade.”\n2) Some key information is missing, which effects how strong the conclusions are and\nthe readability of the paper.\na. There is not a single example profile of stratification (or nutrients) from this region.\nIt should be included in some form in the main paper in my opinion. Could Figure 4 be\nbased on composite profiles? Is it already?\nb. Please add a timeseries of mixed layer depth to this paper (e.g., similar to Figure 1b\nfor the sea ice). Figure 2 hints at the magnitudes of the mixed layer depth changes, but\nI’m still left wondering what the actual depths and changes are, especially with respect\nto the shallow topography. Line 291: ‘this deepened upper mixed layer’… by how much?\nc. Line 148-151: When does the summer mixed layer form? May? June? July? Do all\nJune profiles have a summer mixed layer present? Are only profiles that have a summer\nmixed layer included in the statistics? Line 248: Explicitly show the time window in this\npaper. When does the summer mixed layer (potentially) form, and how long is the\nseason over which it develops?\nResponse: The potential density profiles and corresponding mixed layer depths at\nmultiple sites have been newly added in the supplementary materials as examples (see\nSupplementary Fig. 1b & 1c). Supplementary Fig. 1b shows the seawater potential\ndensity profiles averaged in the summer months (June-September) and other months\nfrom 1996-2021 based on ship hydrographic sampling. The original Figure 4 was not\nbased on composite profiles since that schematic was mainly used to illustrate the\ncombined effects of wind, atmospheric buoyancy flux, and ice melting water on the\nupper mixed layer. Fig. 5 (original Fig 4) has also been redrawn to better emphasize\nkey points of this study.\nThe time series of mixed layer depths from ship-based observations and GLORYS2V4\nreanalysis at specific sites have been added to the supplementary materials (see\nSupplementary Fig. 6 and Fig. 7). The magnitudes of mixed layer deepening in 2007-\n2021 can be found in Fig 2b (based on hydrographic observations) and Fig 2d (based\non GLORYS2V4 reanalysis), which varied in the range of 0-1.5 m yr-1.\nWe practically define the summer month as June to September, following the definition\nof previous studies (Peralta-Ferriz and Woodgate, 2015; Danielson et al., 2020). We did\nnot use a critical depth or state to define the formation of the summer mixed layer but\nrather focused on the change of the mean summer mixed layer within a fixed annual\ntime window (June-September). All hydrographic profiles sampled between June and\nSeptember were included in the analysis. The time window “June to September” has\nbeen clearly labeled in Fig. 2-Fig. 5.\nPeralta-Ferriz, C., & Woodgate, R. A. Seasonal and interannual variability of pan-arctic\nsurface mixed layer properties from 1979 to 2012 from hydrographic data, and the\ndominance of stratification for multiyear mixed layer depth shoaling. Progress in\nOceanography, 134, 19-53 (2015).\nDanielson, S. L., et al. Manifestation and consequences of warming and altered heat\nfluxes over the Bering and Chukchi Sea continental shelves. Deep-Sea Research II, 177,\n104781 (2020).\n3) The conclusions could be strengthened by making sure the hypothesis and\nexplanations are applicable to both time periods, 1996-2006 and 2007-2021.\na. Same comment as (1) above, 1996-2006 is completely absent from the abstract.\nb. Line 85-88: As stated, this hypothesis does not explain why the mixed layer shoaled\nprior to 2007 (when the ice extent was also decreasing).\nc. Line 243-245: The conclusions regarding 1996-2006 need to be clearer. State that\nbuoyancy forcing was sufficient to overcome and wind stress. During this time period,\nI would say that the ‘traditional’ view (as stated in this paper) of more melting leads to\nmore stratification / shallower mixed layers is correct.\nResponse: Thank you for this excellent suggestion. We have substantially modified our\nanalyses and texts accordingly. The unified concept and hypothesis on the relative\nimportance of surface buoyance forces versus wind mixing can explain what happened\nto the upper mixed layer in both periods.\nFirst, in the Introduction section, we have rewritten the hypothesis and explanations\nregarding upper mixed layer changes and potential mechanisms (Lines 113-159).\n“Here, we analyze 23,320 shipboard CTD/XCTD profiles and model reanalysis results\nfrom 1996 to 2021 to depict the interannual variations and decadal trends of the upper\nocean properties on the Pacific Arctic shelves. Our results suggest a potential regime\nshift in the upper ocean dynamics: a reverse trend of MLD from shoaling to deepening\nafter 2007, a well-known year with the second lowest summer sea ice coverage since\nthe satellite monitoring era. This finding demonstrates another robust mechanism and\nconsequence of sea ice loss on the upper ocean dynamics, in addition to a traditional\nunderstanding of the phenomenon33-35. We further quantify the roles of wind mixing,\natmospheric buoyancy flux, and sea-ice-derived meltwater on summer MLD\ndevelopment and variations. Our hypothesis is that in increasingly ice-free Pacific\nArctic shelves with prolonged open water duration, wind forcing can sufficiently mix\nthe upper water column and become a more dominant factor than atmospheric\nbuoyancy and ice meltwater in controlling the summer mixed layer.”\nSecond, we applied a new 1D sea ice-ocean mixed layer model for in-depth diagnosis.\nThis model incorporated sea-ice-derived meltwater as an additional buoyance source to\nthe upper ocean. We found that the shoaling mixed layer on the Bering Sea shelf from\n1996 to 2006 resulted from increased atmospheric heat flux to the ocean (Fig 3a),\nwhereas increased ice meltwater (Fig. 4e) strengthened the stratification (mixed layer\nshoaling) in the northern Chukchi Sea (Fig. 2c). These two phenomena and underlying\nmechanisms were consistent with the “traditional view”.\nThe abstract has been rewritten to emphasize key mechanisms of shifting forcing\nfactors, wind versus surface buoyancy, associated with sea ice loss (Lines 42-48):\n“Before 2006, a shoaling summer mixed layer was associated with massive sea ice loss\nand surface warming. However, the upper mixed layer reversed to a generally\ndeepening trend after 2007. We attribute mixed layer deepening to markedly lengthened\nopen water duration and enhanced wind-induced mixing. Together, these factors\nincrease cumulative wind energy input to an ocean less buffered by sea ice. It is\nconcluded that the relative importance of surface buoyancy flux versus wind forcing\nswitched in two periods despite continued sea ice decline.”\n4) Is the 1996-2006 time period appropriate? How much data is available from 1996-\n1998? Do the conclusions change if the time period is altered?\na. Line 128-130 is misleading and suggests that only the data in Table S1 is used. Lines\n296-298 contradict this, as data from the world ocean database is also included in\naddition to the Table S1 data.\nb. A summary table or figure describing the entire dataset is needed. I suggest splitting\nFigure S5a into two maps, one for each time period, along with splitting Figure S5b\ninto two panels, one for the Chukchi Sea and one for the Bering Sea. A bit more\ndescription at line 128-129 would be useful (e.g., total number of summertime profiles\nwithin each region and time period).\nResponse: Thanks for pointing out the unclarity and inconsistency of the data\ndescription. We have made multiple improvements in the main texts and supplementary\ninformation.\nFirst, the numbers of summertime profiles in different regions and periods were stated\nand plotted. See subsection “Reversal of Mixed Layer Depth Trends” and\nSupplementary Fig 3. This figure has been modified based on the reviewer’s\nsuggestions (see Supplementary Fig. 3). The detailed description of the dataset has been\nadded to the main text (Lines 288-293).\n“To assess the impacts of changing sea ice conditions on the upper ocean, we further\ncalculated trends of the summer (June to September) mixed layer using a total of 18,997\nquality-controlled hydrographic profiles over two separate periods: 1996-2006 versus\n2007-2021 (Supplementary Fig. 3). There were 2,835 summertime profiles in the\nBering Sea shelf during 1996-2006 and 6,554 profiles after 2007. The Chukchi Sea\nshelf contained 2,146 profiles in 1996-2006 and 7,462 in 2007-2021, respectively.”\nIf we altered the time period to 1996-2006, 1998-2006, and 2000-2006, the shoaling\ntrends of the mixed layer during different periods still existed in the Bering and Chukchi\nSea shelf (see Review Fig. 1).\nReview Fig. 1 Linear trends of upper mixed layer depth during different periods.\nThe linear trends of mean summer mixed layer depth (June to September) during (a)\n1996-2006, (b) 1998-2006 and (c) 2000-2006. Only the linear trends with p-value < 0.1\nwere shown. The grey line showed isobath of 50m, 250m, and 500m.\nThe observed CTD/XCTD profiles were compiled from the NOAA World Ocean\nDatabase 2018 (see Supplementary Table 2) and multinational Arctic expeditions (see\nSupplementary Table 1). We have added a summary of ship-based hydrographic\nprofiles from the World Ocean Database to the supplementary information\n(Supplementary Table 2).\n5) Can the use of wind stress instead of surface stress be justified a bit more? This paper\nshould be addressing the surface stress that the ocean feels accounting for the effects\nof sea ice cover, and not the wind stress that is imposed on any sea ice present. It’s\npossible the use of wind stress is justified, but that is not clear based on what is\npresented (analysis of the April – November time period).\na. Line 192-194: It seems more appropriate to focus the wind stress calculation on June\n– September as with mixed layer depths. Why are different months chosen?\nb. Line 214-215 & lines 222-224: Momentum input to the ocean is larger during periods\nof mixed ice concentration vs. open water. Treating everything as open water isn’t a\ngreat approach unless it can be justified. What is the ice concentration / coverage like\nwhen the summer mixed layer forms?\nResponse: Thank you for asking about the wind stress calculation. We have made\nmultiple adjustments per your suggestions. In the original work, we calculated the mean\nwind stress within a fixed time window (April to November) to account for both ice-\ncovered and open-water periods. As the reviewer pointed out, this simple calculation\nmay not be appropriate. Those wind stress trend subplots (original Fig 3a & 3b) have\nbeen removed. Instead, we have added trend analyses of monthly wind stress (May to\nSeptember) along with sea ice edge in 1996-2006 versus 2007-2021 (Supplementary\nFig. 9). The significant changes in wind stress were generally shown in the open sea or\nmarginal ice zone. Also, we did not consider the impact of sea-ice-derived meltwater in\nthe original 1D ocean mixed layer model. In our new analysis, we applied a 1D sea ice-\nocean mixed layer model to separate competing factors for the upper mixed layer\nevolution, including wind mixing (Fig 4a), surface buoyancy flux (Fig 4b), and sea-ice-\nderived meltwater (Fig 4c). The sea ice condition has been considered in our new\ndiagnosis. All the factors, except ice meltwater, are calculated based on the June to\nSeptember average, along with mixed layer depths. The sea-ice-derived meltwater\ntrends are calculated for May to July since the whole region becomes almost ice-free in\nAugust.\nAdditional comments:\n6) There’s a lot of emphasis on what the ‘traditional’ view is (lines 83, 250), and how\nthis paper contradicts it. I think this needs to be toned down, and simply presented as\nboth buoyancy forcing and wind forcing are potentially important. It will not take away\nfrom the significance of this paper.\na. I would argue that the ‘traditional’ view that interannual ice loss can increase\nbuoyancy and shoal the summer mixed layer is in fact correct during 1996-2007.\nb. Line 37: Maybe rephrase ‘contradicting a common notion’. It’s an idea, but I don’t\nknow that it’s pervasive.\nc. Line 41, 108, and elsewhere: Is this really a regime shift? The change is August ice\nextent between the two time periods does not qualify in my opinion. Even during 1996-\n2006, most of the region under consideration was seasonal ice. This contradicts lines\n111-113 that claim MYI in the earlier period. I’m a bit confused about this argument.\nResponse: We agree with the reviewer's comments. In the revised paper, the statement\n“contradicting a common notion” has been removed. Instead, we emphasized a key\nmechanism, which is the relative importance of surface buoyancy flux versus wind\nforcing, which switched over two periods in the abstract and main text.\nThe original description of multiyear ice was incorrect. The subsection “Sea Ice Regime\nShift” has been rewritten (Lines 162-169).\n“The sea ice in the Pacific Arctic Ocean often started retreating at the end of March and\ncompletely melted or reached the annual minimum extent in September6,36,37. After\n2007, this region became almost ice-free in August (Fig. 1a & 1b). The period from\n1996 to 2006 could be considered a transition with largely seasonal ice in the Pacific\nArctic5,38,39, and a “new normal” of extremely low summertime sea ice generally\nstabilized after 200740-42. Most northern Bering and Chukchi continental shelves did\nnot show an early ice retreat or prolonged open water duration from 1996 to 2006\n(Supplementary Fig. 2a & 2b).”\n7) I do not find Figure 1c to be a new result (lines 136-139). Conceptually, it is true\nArctic wide that the summer mixed layer forms in relation to ice melt / melt pond\ndrainage and then gradually deepens during summer. It is not surprising that deeper\nmixed layers are associated with a longer time since mixed layer shoaling, or that mixed\nlayers are shoaling earlier in the Arctic. The description here seems to have taken a\nsimple concept and made it complicated.\nResponse: Thank you for the comment. We argue Figure 1c is important for two\nreasons. First, this simple correlation analysis used pure observation evidence to prove\nthe concept regarding the time evolution of mixed layers after the ice retreat. Second,\nthis figure is useful for logically leading to follow-up analyses of summer mixed layer\ntrends and underlying physical mechanisms.\n8) Line 123: define what you mean by trigger time, even if it is stated in the methods.\nDo lines 123-125 refer to a figure?\nResponse: The definition of trigger time of ice retreat has been stated in the main\ncontent. The origin lines 123-125 refer to the results from Supplement Fig. 2c &2d,\nmarked in the revised paper (Lines 175-180).\n“The trigger time of ice retreat (i.e., year day when sea ice concentration drops below\n15% for three consecutive days) advanced by a mean rate of -2.6 days yr-1 (p<0.1) in\nthe Bering Sea and -3.0 days yr-1 (p<0.1) in the Chukchi Sea in 2007-2021\n(Supplementary Fig. 2c). The open water duration (i.e., periods with sea ice\nconcentration below 15%) increased significantly with a mean rate of +2.8 days yr-1\n(p<0. 1) in most areas (Supplementary Fig. 2d).”\n9) Line 133: how is the day of sea ice retreat defined? State briefly even if it stated in\nthe methods.\nResponse: See the previous one. The term “day of sea ice retreat” has the same\ndefinition as “trigger time of ice retreat”.\n10) Line 203-205: What is advection like in this area? The weakening wind stress in the\nwestern Bering Sea could induce a change in water properties that is then advected into\nthe region where mixed layer shoaling is observed during 1996-2006. Is this plausible?\nResponse: We agree with the reviewer's comments that the advection of water mass\nmay impact the mixed layer in the region. However, the 1D sea ice-ocean mixed layer\ncannot account for the advection of water masses. We acknowledged such a limitation\n(Lines 672-677).\n“The Chukchi shelf contained complex water masses, including Alaska Coastal Water,\nice meltwater, dense winter water, Bering Sea water, and mixtures in summer65.\nNevertheless, the timing and extent of the advection of ice meltwater and water masses\nvaried among regions and years66. Linear trends of competing factors for changing the\nupper mixed layer were calculated using three-month-mean variables to smooth water\nmass advection.”\n11) Line 304: ‘station depths shallower than 10 m’ are excluded? This reads like profiles\nwith a measurement in the upper 10 m are excluded, instead of excluding those without\na measurement in the upper 10 m.\nResponse: The sentence has been modified (Lines 1261-1265).\n“The profiles with vertical resolution greater than 5m or without measurements in the\nupper 10 m are excluded on account of accurately describing the mixed layer (see\nMethods section ‘Definition of mixed layer depth’).”\n8) In table S1, ‘Sea storm 2011’ is listed as August – Sept 2009 (should be 2011?).\nResponse: The expedition date of ‘Sea storm 2011’ has been corrected in the\nSupplementary Table 1.\n9) Figure caption labels (a,b,c, etc) should be placed at the start of the sentence\ndescribing each panel, not at the end. These are difficult to read.\nResponse: All figure caption labels have been modified per your suggestion."}, {"round": 2, "reviewer_comments": {"reviewer_1": "This is my second review of this manuscript. The manuscript was substantially improved compared to the original version,\nespecially for the explanation of the MLD changes. However, some places need clarification, and a careful proofreading is\nstill required.\n1. Line37-38: Fig 4e does not show imprint of increased melting in accordance with “massive sea ice loss”\n2. Line 40: the MLD increase in the recent period was also contributed by the reduction in sea ice meltwater as shown in Fig\n4f. Why is this effect neglected in your conclusion?\n3. Line 85-86: if you write “another” here, it would be better to write explicitly at this place what is the first mechanism.\n4. L88, role … for\n5. L98: do you mean that sea ice became seasonal during this period?\n6. L99, most area of the …\n7. L102, what is “tipping point” in this context?\n8. L105, which regions do you refer to here, for “enhanced anomalous oceanic heat flux and amplified ice–albedo feedback”\n9. L107, is it “variability” that caused long-term trend?\n10. L109, -> day of a year\n11. L119, I do not see what is excluded in (a)\n12. 126,144, do you mean “the day when ice is free”?\n13. 129, With no - without (and at other places)\n14. 128, grid - grid cell (and at other places)\n15. L137, what does “it” refer to?\n16. 145, remove “a”\n17. 147, into … - with sea ice retreat\n18. 153, mixed layer - MLD\n19. 166, and then … - and then the water in the region …\n20. 170, why “freshening” trend? I would rather say there is no significant salinity trend\n21. 186 models - model results\n22. 208 remove two\n23. 209, - in the season of June …\n24. 211, why is there growth window for wind stress?\n25. 212, - a positive trend\n26. 213-214: do you mean that “winds” are strengthened by surface warming? The strengthening in winds is different from a\nstrengthening in wind stress.\n27. 215-216: The Arctic Dipole has two active centers, not just “anticyclonic”\n28. 217: You did not illustrate “wind growth” or increase of wind speed in the paper. Wind stress is not only influenced by\nwinds and an increase in wind stress does not prove an increase in wind speed.\n29. Fig S9 does not show a systematic increase in wind stress in June to Sept in the latter period, while the TKE increased\nmore substantially. Why?\n30. 227, - their mixtures?\n31. 229, - other water …\n32. L230, could you explain what variables are 3-month-mean in your analysis? In Method, you mentioned that daily data\nwere used.\n33. L236-237, this rather tells that changes in ice meltwater play a role! Or, without the changes in sea ice meltwater, MLD in\nthe latter period would have been different. So it is not just that wind mixing played the role.\n34. 241-242, - Linear trends of buoyancy flux associated with …\n35. 264, for the first period, you did not show “increasing sea-ice-derived meltwater”\n36. 268, - trend in MLD\n37. 267-269, please rewrite this sentence properly\n38. 270, - fluxes into the …\n39. 274, due to upper ocean warming?\n40. 277, as commented above, the reduction in meltwater could contribute to the increase in MLD\n41. 303, showed – showing\n42. the description of the 1D model should be improved. The most importantly, it should explain the solution procedure, what\nis done/solved first, the second step, the third ….\n43. 594, 596, how are T/Sbottom and d_T, d_S determined?\n44. 607, the left and right sides of the equation have different units. Time should be divided on the left side.\n45. What is deltaS_whole? How is it determined?\n46. L611, the units on the left and right sides of the equations are different\n47. 625, time should be divided to get “rate”\n48. 630, mixed layer – seawater in the mixed layer\nPlease proofread the text thoroughly.", "reviewer_2": "The manuscript has been improved and I am in favor of publication after two issues are addressed.\nMajor comments\n‘Pacific Arctic Ocean’ is a much larger region than what is studied here and includes the entirety of the Canada Basin\n(to ~85°N). I suggest replacing with ‘Northern Bering and Chukchi Seas’ or 'Pacific Arctic Shelf Seas' throughout. This is a\nproblem in the title and lines 49, 95, 196, 262, 301, and likely other places I have missed.\nLines 159-171: The updated figure 2 is very useful, and shows that most locations do not correspond to a significant trend for\nmixed layer depth (e.g., 69 grid boxes out of 76 grid boxes). But Figure 2 is presented focusing on only those few boxes with\na significant trend, which I think is misleading. Too much of a concrete conclusion is stated from very inconsistent evidence.\nA more honest description of these results needs to be included. This does not affect the conclusions that can be drawn from\nthe paper overall, as these are primarily based on GLORYS and ERA5 analysis.\nAdditional comments\nLine 35: ‘its development’ should be more specific, e.g., its interannual to decadal evolution\nLine 85: ‘another robust mechanism’ and ‘understanding of the phenomenon’ I’m not sure what you’re referring to here.\nLine 153: suggest changing to ‘trends of the mean summer…’ so that it’s clear how trends are calculated without referring to\nthe methods.\nLine 152-161: It would be useful to state here that the mixed layer shoals in May, if this is true for the entire region. I am\ngetting this impression from other parts of the paper, but it would be good to clarify here. This would clarify that even though\nthe mixed layer is shoaling earlier with time, that does not affect the trend statistics during June – September.\nLine 182-185: where is the ‘inner southeastern Bering shelf’? A box or arrow in Figure 2 would help.\nFigure 5a-b: What is indicated by the ‘E’ and downward arrow, Ekman pumping? For winds and Ekman pumping, it would\nbe better to indicate overall values for the summer period somehow as a monthly analysis (Figure S9) shows no increase\nduring e.g., September. Or indicate the monthly variability depicted in Figure S9 in this summary schematic.\nFigure 5c-d: only a suggestion - in panels c and d, it would be more intuitive to keep the terms (TKE, meltwater etc.) on the\nsame side of the balance in both panels, and change which side is up / down between the two.\nLine 530-541: GLORYS is used to show the mixed layer depth change, but ERA5 for diagnosis of what’s causing that\nchange. Does GLORYS use ERA5 forcing?\nLine 648 – 658: Am I correct that for the observations, the mean value from June – September in each year is first calculated, and then a linear trend of those mean values is determined? How does the seasonal cycle bias these results, if observations\nare in June for one year and September for another? Does this explain why so many grid boxes have an insignificant trend?\nSupplement line 39: typo on ‘2007-201’"}, "author_response": "Response to Reviewers\nReviewer #1 (Remarks to the Author):\n1) Line37-38: Fig 4e does not show imprint of increased melting in accordance with\n“massive sea ice loss”.\nResponse：Thank you for the suggestion. The perennial ice in the Pacific Arctic shelves\nsteadily decreased and reached its second record-low extent in 2007 (Comiso, 2006;\nKwok, 2007). Fig 1a and Fig 4e indicated that massive sea ice loss mainly occurred in\nthe northern Chukchi Sea shelf. The sentence has been revised to be more proper (Lines\n35-36): “Before 2006, a shoaling summer mixed layer was associated with sea ice loss\nand surface warming.”\nComiso, J. C. Abrupt decline in the Arctic winter sea ice cover. Geophysical Research\nletters 33, L18504 (2006).\nKwok, R. Near zero replenishment of the Arctic multiyear sea ice cover at the end of\n2005 summer. Geophysical Research letters 34, L05501 (2007).\n2) Line 40: the MLD increase in the recent period was also contributed by the\nreduction in sea ice meltwater as shown in Fig 4f. Why is this effect neglected in your\nconclusion?\nResponse：We agree with the reviewer's comments. In the revised abstract, the role of\nreduced sea ice meltwater has been emphasized (Lines 37-39): “We attribute mixed\nlayer deepening to markedly lengthened open water duration, enhanced wind-induced\nmixing and ice meltwater reduction.”\n3) Line85-86: if you write “another” here, it would be better to write explicitly at this\nplace what is the first mechanism.\nResponse：Thank you for the suggestion. The phenomenon and mechanism of\ntraditional understanding have been clarified in the revise paper (Lines 83-86): “This\nfinding demonstrates another robust mechanism and consequence of sea ice loss on the\nupper ocean dynamics, in addition to a traditional understanding of shoaling mixed\nlayer mainly due to more ice meltwater33-35.”\n4) Line88, role … for\nResponse：The sentence has been modified (Lines 86-87): “We further quantify the\nroles of wind mixing, atmospheric buoyancy flux, and sea-ice-derived meltwater for\nsummer MLD development and variations.”\n5) Line98: do you mean that sea ice became seasonal during this period?\nResponse：The annual Arctic sea ice minimum extent declined rapidly during 1996 to\n2006. The perennial ice cover in the Chukchi Sea shelf continued to shrink and\ngradually replaced with seasonal sea ice which grows in the fall and winter, and melts\nin the summer. The northern movement of mean summer sea ice edge represents the\nshrinking of perennial ice cover (see Fig. 1a). The sentence has now been rewritten\n(Lines 96-99): “The period from 1996 to 2006 could be considered as a transition with\nshrinking perennial ice cover in the Pacific Arctic5,38,39, and a “new normal” of\nextremely low summertime sea ice generally stabilized after 200740-42.”\n6) Line99, most area of the …\nResponse：The sentence has been modified (Lines 99-101): “Most areas of the\nnorthern Bering and Chukchi shelves did not show an early ice retreat or prolonged\nopen water duration from 1996 to 2006 (Supplementary Fig. 2a & 2b).”\n7) Line102, what is “tipping point” in this context?\nResponse：The year 2007 was a potential tipping point of ice regime shift. The sentence\nhas been modified (Lines 101-103): “The year 2007 was a potential tipping point of ice\nregime shift, triggered by the transition of a negative Arctic Dipole index to a positive\none43.”\n8) Line105, which regions do you refer to here, for “enhanced anomalous oceanic\nheat flux and amplified ice–albedo feedback”.\nResponse：The region in the cited references and main context refer to Pacific Arctic\nshelf seas. The specific region is added to the sentence (Lines 103-105): “The\nincreasingly positive Arctic Dipole index during 2007-2021 drove an enhanced\nanomalous oceanic heat flux and amplified ice–albedo feedback in the Pacific Arctic\nshelves44,45.”\n9) Line107, is it “variability” that caused long-term trend?\nResponse：Recently, a number of studies have shown that Arctic sea ice declines are\nrelated to intense warming in the Arctic, which known as Arctic amplification (Serreze\net al., 2011; Previdi et al., 2021; Rantanen et al., 2022). The extreme Arctic\ntemperatures are generally attributed to the atmospheric circulation patterns occurring\nin the northern hemisphere (Overland and Wang, 2010; Moore et al., 2018). As\nmentioned in the main context, the variabilities of Arctic Dipole index (Bi et al., 2021;\nPolyakov et al., 2023) and Artic Oscillation (Lindsay et al., 2009; Cohen et al., 2014)\nwere associated with the Arctic amplification and increasing ice loss. Besides, warm\nair masses carried on by extra-tropical cyclones have an increased tendency to invade\nthe whole Arctic Ocean from the Fram Strait on the Atlantic side to the Bering Strait\non the Pacific side (Moore, 2016; Kim et al., 2017). Therefore, we think that the long-\nterm decreasing trend of sea ice are related to the patterns of variability in atmospheric\ncirculation. The sentence has been rewritten to be proper (Lines 106-108): “The Arctic\nAmplification48 driven by atmospheric circulation patterns caused the seasonal sea ice\nto decline further over the long term without any sign of recovery49.”\nBi, H. et al. Influences of summertime Arctic Dipole Atmospheric Circulation on sea\nice concentration variations in the pacific sector of the Arctic during different\nPacific Decadal Oscillation Phases. Journal of Climate, 34(8): 3003-3019 (2021).\nCohen J., et al. Recent Arctic amplification and extreme mid-latitude weather. Nat.\nGeosci. 7, 627–637 (2014).\nKim, B. M., et al. Major cause of unprecedented Arctic warming in January 2016:\ncritical role of an Atlantic windstorm. Scientific Reports 7, 40051 (2017).\nLindsay R W., Zhang J., Schweiger A., Steele, M. & Stern, H. Arctic sea ice retreat in\n2007 follows thinning trend. Journal of Climate, 22(1): 165-176 (2009).\nMoore, G. W. K. The December 2015 North Pole warming event and the increasing\noccurrence of such events? Scientific Reports 6(1), 39084 (2016).\nMoore, G. W. K., Schweiger A., Zhang J. & Steele M. Collapse of the 2017 winter\nBeaufort High: a 587 response to thinning sea ice? Geophysical Research Letters\n45, https://doi.org/10.1002/2017GL076446 (2018).\nOverland, J. E. & Wang, H. Large scale atmospheric circulation changes are associated\nwith the recent loss of Arctic sea ice. Tellus A 62(1), 1-9 (2010).\nPolyakov, I. V. et al. Fluctuating Atlantic inflows modulate Arctic atlantification.\nScience 381, 972 (2023).\nPrevidi, M., Smith, K. L. & Polvani, L. M. Arctic amplification of climate change: a\nreview of underlying mechanisms. Environmental Research Letters 16, 9 (2021).\nRantanen M., et al. The Arctic has warmed nearly four times faster than the globe since\n1979. Communications Earth and Environment 3, 168 (2022).\nSerreze M C. & Barry R G. Processes and impacts of Arctic amplification: A research\nsynthesis. Global and Planetary Change 77, 85-96 (2011)\n10) Line109, -> day of a year\nResponse：The sentence has been modified (Lines 109-110): “day of a year when sea\nice concentration drops below 15% for three consecutive days”.\n11) Line119, I do not see what is excluded in (a)\nResponse：We thank the reviewer for pointing out the confused description here. We\nhave removed the description of study area (water depth) in Fig. 1a (“excluding\nregions shallower than 15 m or deeper than 500 m”) but rather emphasize this point in\nthe main content (Lines 78-80): “Here, we analyze 23,320 shipboard CTD/XCTD\nprofiles and model reanalysis results from 1996 to 2021 to depict the interannual\nvariations and decadal trends of the upper ocean properties on the Pacific Arctic shelf\nseas (depths of 15-500m).”\n12) 126,144, do you mean “the day when ice is free”?\nResponse：The definition of ice retreat time in this paper is the day of a year when sea\nice concentration drops below 15% for three consecutive days (Lines 109-110 and\nMethod “Sea ice concentration”). When the sea ice concentration is less than 15%,\nthe\ngrid cell is generally considered as an open sea. The “annual ice-retreat time” in Lines\n126 and 144 refers to the ice retreat time in each year.\n13) 129, With no - without (and at other places)\nResponse：Those words have been modified (Lines 108, 129 & 197).\n14) 128, grid - grid cell (and at other places)\nResponse：Those phrase have been modified throughout the manuscript (Lines 128,\n129, 142, 162, 165, 166, 167, 169, 196, 197, 200, 252, 261, Methods and\nSupplementary).\n15) L137, what does “it” refer to?\nResponse：The “it” in Line 137 refers to the mixed layer depth (MLD). The sentence\nhas been modified (Lines 137-139): “Although no significant correlation was found\nbetween observed MLD and sea ice concentration, MLD was closely related to the\ntrigger time of sea ice retreat (Fig. 1c).”\n16) L145, remove “a”\nResponse：The sentence has been modified (Lines 146-147): “Theoretically, the\nmixed layer would deepen under sufficiently long wind mixing.”\n17) L147, into … - with sea ice retreat\nResponse：The sentence has been modified (Lines 147-148): “ice meltwater became\nless and less influential with sea ice retreat.”\n18) L153, mixed layer – MLD\nResponse：The sentence has been modified (Lines 153-155): “To assess the impacts\nof changing sea ice conditions on the upper ocean, we further calculated trends of the\nmean summer (June to September) MLD using a total of 18,997 quality-controlled\nhydrographic profiles over two separate periods”.\n19) L166, and then … - and then the water in the region …\nResponse：The sentence has been modified (Lines 171-172): “then the water in the\nregion gradually became saltier (up to 0.08 psu yr-1)”.\n20) L170, why “freshening” trend? I would rather say there is no significant salinity\ntrend\nResponse: The sentence has been modified to be more proper (Lines 175-177): “In\ncontrast, the mixed layer temperature and salinity on the northern Bering and Chukchi\nshelves did not show significant trends in both periods.”\n21) L186 models - model results\nResponse：The sentence has been modified (Line 191-192): “For the high-latitude\nChukchi Sea, the summer MLD trends derived from observations were similar to the\nmodel results.”\n22) L208 remove two\nResponse：The sentence has been modified (Lines 214-215): “Significant changes in\nwind stress mostly occurred in open seas or marginal ice zones for both periods.”\n23) L209, - in the season of June …\nResponse：The sentence has been modified (Lines 215-217): “The wind stress from\nthe northern Bering to Chukchi shelves decreased in the season of June to August\nduring 1996-2006.”\n24) L211, why is there growth window for wind stress?\nResponse：The original Line 211 was to clarify that wind stress increased in different\nareas of Pacific Arctic shelves in the season of May to August during 2007-2021. We\nhave rewritten the sentences to eliminate misunderstandings (Lines 217-220): “After\n2007, the enhancement of wind stress started in May and lasted for nearly 4 months\nwith a positive trend of 1.7×10-3 Pa yr-1, which agreed with prior results of climate\nmodels55,56 and observations57.”\n25) L212, - a positive trend\nResponse：The sentence has been modified (Line 218).\n26) L213-214: do you mean that “winds” are strengthened by surface warming? The\nstrengthening in winds is different from a strengthening in wind stress.\nResponse：Thanks for your question. The magnitude of wind stress at the ice/ocean\nsurface is calculated as the square of the wind speed at 10 m above the sea surface\nmultiplies air density and drag coefficient. We attributed the increase of wind stress to\nan increase in wind speed of the study region. One of the explanations for the\nstrengthened surface wind speed in Pacific Arctic Ocean is the reduction in surface\nroughness caused by sea ice loss (Jakobson et al., 2019). Besides, the stronger surface\nwinds are linked to decreased atmospheric stability, which follows from increased\ntemperature of Arctic Amplification (Seo and Yang, 2013; Mioduszewski et al., 2018).\nIn addition, the stronger sea level pressure gradient between the Aleutian Low and the\nBeaufort High promoted geostrophic winds (Zhang et al., 2016; Huang et al., 2022).\nThe paragraph has been rewritten to further clarify the points (Lines 219-224):\n“The strengthened wind stress was related to wind speed-up over the Pacific Arctic\nshelf under Arctic Amplification58, because sea ice loss59 and surface warming60\ndecreased ocean surface roughness and atmospheric stability. The stronger pressure\ngradient between the Aleutian Low and Beaufort High also promoted geostrophic wind\nspeed61,62 with the positive Arctic Dipole regime since 200745.”\nHuang, Y., Bai, X. & Leng, H. Synoptic-scale variability in the Beaufort High and\nspring ice opening in the Beaufort Sea. Frontiers in Marine Science, 9, 929209\n(2022).\nJakobson, L., Vihma, T. & Jakobson, E. Relationships between sea ice concentration\nand wind speed over the Arctic Ocean during 1979–2015. Journal of Climate, 32,\n7783–7796 (2019).\nMioduszewski, J., Vavrus, S. & Wang, M. Diminishing arctic sea ice promotes stronger\nsurface winds. Journal of Climate, 31(19), 8101–8119 (2018).\nSeo, H. & J. Yang. Dynamical response of the Arctic atmospheric boundary layer\nprocess to uncertainties in sea-ice concentration. J. Geophys. Res.\nAtmos, 118, 12383–12402 (2013).\nZhang, J., et al. Mesoscale climatology and variation of surface winds over the\nChukchi–Beaufort coastal areas. Journal of Climate 29 (8), 2721–2739 (2016).\nOverland, J. E., Francis, J. A., Hanna, E. & Wang M. The recent shift in early summer\nArctic atmospheric circulation. Geophysical Research Letters 39 (19), L19804\n(2012).\n27) L215-216: The Arctic Dipole has two active centers, not just “anticyclonic”\nResponse: Thank you for pointing out the loose expression on the Arctic Dipole,\nwhich\nhas been corrected. The sentence has been rewritten (Lines 222-224): “The stronger\npressure gradient between the Aleutian Low and Beaufort High promoted geostrophic\nwind speed61,62 with a positive Arctic Dipole regime since 200745.”\n28) L217: You did not illustrate “wind growth” or increase of wind speed in the paper.\nWind stress is not only influenced by winds and an increase in wind stress does not\nprove an increase in wind speed.\nResponse: Thank you for reviewer’s suggestion. The original L213-217 sentences\nwithin the whole paragraph was to explain the potential causes of increased wind stress\nafter 2007. Based on the calculation of wind stress, we speculated that the strengthened\nwind stress was related to the increase in wind speed. Several lines of evidence from\nthe literature can support the phenomenon and mechanism of increased wind speed in\nthe Pacific Arctic Ocean. The paragraph has been rewritten to clarify the point (Lines\n219-224).\n29) Fig S9 does not show a systematic increase in wind stress in June to Sept in the\nlatter period, while the TKE increased more substantially. Why?\nResponse: Ocean surface currents driven by the increased wind stress will result in\nincreased levels of turbulent mixing. The increasing exposure and duration of open\nwater to stable surface wind stress will also promote more transfer of momentum from\nthe atmosphere to the ocean (Rainville et al., 2011; Muilwijk et al., 2024). The\nsubstantial increased TKE is the combination of reduced sea ice cover and increased\nwind stress.\nThe turbulent kinetic energy (TKE) calculated using 1D sea ice-ocean mixed layer\nmodel is related to the surface buoyancy flux and the state of mixed layer (see Methods\n1D sea ice-ocean mixed layer model). The differences in salinity and temperature\nbetween the mixed layer and the entrainment zone, and the value of MLD affect the\nTKE. The wind stress is not equal to the momentum that cause turbulent mixing in the\nwater column. Besides, the increased wind stress in May leads to a potential deepening\nof the mixed layer. There, the MLD value in May has a chain reaction on the\ndetermination of TKE in the season of June to September.\nIn addition, tides, eddies, submesoscale dynamics, and lateral process can also generate\ninternal waves that increase turbulent mixing (Rippeth and Fine, 2022), although those\nprocesses are beyond the scope of the present study.\nRainville, L., Lee, C. M. & Woodgate, R. A. Impact of wind-driven mixing in the Arctic\nOcean. Oceanography 24(3), 136-145 (2011).\nMuilwijk, M., Hattermann, T. Martin T., & Granskog, M. A. Future sea ice\nweakening amplifies wind-driven trends in surface stress and Arctic Ocean spin-\nup. Nature Communications, 15, 6889 (2024).\nPippeth, T. P. & Fine, E. C. Turbulent mixing in a changing Arctic Ocean.\nOceanography 35(3-4), 66-75 (2022).\n30) L227, - their mixtures?\nResponse：The sentence has been modified (Lines 233-235): “The Chukchi shelf\ncontained complex water masses, including Alaska Coastal Water, ice meltwater,\ndense winter water, Bering Sea water, and their mixtures in summer66.”\n31) L229, - other water …\nResponse：The sentence has been modified (Lines 235-236): “Nevertheless, the\ntiming and extent of advected ice meltwater and other water masses varied among\nregions and years67.”\n32) L230, could you explain what variables are 3-month-mean in your analysis? In\nMethod, you mentioned that daily data were used.\nResponse：Thanks for your question. We applied a 1D sea ice-ocean mixed layer model\nto separate and diagnose competing factors for the upper mixed layer evolution,\nincluding wind mixing induced turbulent kinetic energy TKE (Fig 4a), surface\nbuoyancy flux (Fig 4b), and sea-ice-derived meltwater (Fig 4c). The linear trends of\nTKE and surface buoyancy flux are calculated based on the mean value from June to\nSeptember, along with MLDs. The sea-ice-derived meltwater trends are determined by\nthe mean value from May to July since the whole region becomes almost ice-free in\nAugust. The sentence has been modified (Line 236-238): “Linear trends of turbulent\nkinetic energy and surface buoyancy fluxes were calculated using 3-month-mean to\nsmooth water mass advection and other short time scale processes.”\n33) L236-237, this rather tells that changes in ice meltwater play a role! Or, without\nthe changes in sea ice meltwater, MLD in the latter period would have been different.\nSo it is not just that wind mixing played the role.\nResponse：Thank you for your excellent suggestion. We agreed that the reduced sea-\nice-derived meltwater indeed played an important role. We emphasize the role of ice\nmeltwater in the abstract and conclusions.\nAbstract (Lines 37-39): “We attribute mixed layer deepening to markedly lengthened\nopen water duration, enhanced wind-induced mixing, and ice meltwater reduction.”\nConclusions (Lines 282-286): “In contrast, as the Pacific Arctic shelves shifted to a\nnew normal with extremely low summer sea ice after 2007, the upper mixed layer\nreversed from shoaling to deepening, suggesting that sea-ice-derived meltwater was\nless effective in maintaining strong summer stratification.”\n34) L241-242, - Linear trends of buoyancy flux associated with …\nResponse: Thank you for your suggestion. Figure 3 shows the linear trends of air-sea\nheat and freshwater flux based on ERA5 forcing data (Methods: Equations 3 and 4).\nThe determination of surface buoyancy flux is different in the open sea and ice cover\nregion. To avoid ambiguity, we have revised the caption of Figure 3 as “Linear trends\nof air-sea buoyancy fluxes associated with heat and freshwater” (Line 250).\n35) L264, for the first period, you did not show “increasing sea-ice-derived meltwater”\nResponse: The perennial ice in the Pacific Arctic Ocean steadily decreased and reached\nits second record-low extent in 2007 (Comiso, 2006; Kwok, 2007). The sea-ice-derived\nmeltwater did not show significant changes in the Bering Sea shelf potentially due to\nthe seasonal sea ice regime has stabilized prior to 1996. The advance of sea ice retreat\ntiming in the Bering Sea shelf after 2007 cause the variability of ice meltwater\nbuoyancy. In the northern Chukchi shelf, the sea-ice-derived meltwater show\nsignificant increasing trend near the mean summer sea ice edge (see Fig. 4e and Fig. 1a\nred dotted line). The increased meltwater buoyancy likely resulted from the melting of\nperennial ice in the Chukchi shelf. The description of regional increasing sea-ice-\nderived meltwater in the first period was mentioned in the revised paper (Lines 242-\n244): “The increased ice meltwater buoyancy near the mean summer sea ice edge in\nnorthern Chukchi shelf implied the melting of perennial ice before 2006 (Fig. 4e).”\n36) L268, - trend in MLD\nResponse：The sentence has been modified (Line 277): “a deepening trend in MLDs\nfacilitated by early ice retreat and enhanced wind stress”.\n37) L267-269, please rewrite this sentence properly.\nResponse：The sentence has been rewritten (Lines 275-278): “As the pattern of thinner\nand seasonal sea ice became a new normal after 2007, the exposure of open water to\nsurface wind stress was greatly extended, which resulted in a deepening trend in MLD\nfacilitated by early ice retreat and enhanced wind stress (Fig. 5b & 5d).”\n38) L270, - fluxes into the …\nResponse：The sentence has been modified (Lines 278-279): “Although long-term\nobservations indicated increasing Pacific water inflow with more heat and freshwater\nfluxes into the region.”\n39) L274, due to upper ocean warming?\nResponse: The sentence has been modified (Lines 281-282): “The traditional\nviewpoints explained that rapid sea ice loss should enhance upper ocean\nstratification27,34 due to upper ocean warming69 and/or freshening70,71.”\n40) L277, as commented above, the reduction in meltwater could contribute to the\nincrease in MLD.\nResponse：The main purpose of origin Lines 274-279 was to explain that the effect of\nwind mixing outcompetes ice meltwater buoyancy and reversed the mixed layer from\nshoaling to deepening in later period. We agreed that the contribution of reduced ice\nmeltwater in deepening the mixed layer should not be neglected. The paragraph has\nbeen rewritten to be more proper (Lines 282-287): “In contrast, as the Pacific Arctic\nshelves shifted to a new normal with extremely low summer sea ice after 2007, the\nupper mixed layer reversed from shoaling to deepening, suggesting that sea-ice-derived\nmeltwater was less effective in maintaining summertime strong stratification.\nMeanwhile, the wind stirring during prolonged open water period gradually dominated\nupper ocean dynamics.”\n41) L303, showed – showing\nResponse：The word has been modified (Line 315-316): “Fig. 5 Schematic diagrams\nshowing the dynamic changes of the upper mixed layer under different sea ice\nregimes.”\n42) the description of the 1D model should be improved. The most importantly, it\nshould explain the solution procedure, what is done/solved first, the second step, the\nthird ….\nResponse: The 1D sea ice-ocean mixed layer model in the Methods has been\nrewritten (Lines 607-676). Detailed modifications have been made to the model's\nparametric equations (the determination of entrainment velocity), solution procedures,\nvariable units, etc. The flow chart of 1D sea ice-ocean mixed layer model was added to\nillustrate the solution steps (supplementary Figure. 8b). We also modified “Code\navailability” and made the code public in ZENODO repository (Lines 758-761): “All\nMatlab scripts used for data analysis and 1D sea ice-ocean mixed layer model\ncalculation and auxiliary data files are available at the ZENODO\n(https://doi.org/10.5281/zenodo.12174955).”\n43) L594, 596, how are T/Sbottom and d_T, d_S determined?\nResponse: The mean temperature and salinity in the mixed layer (MLT, MLS),\nentrainment zone (𝑇∗, 𝑆∗) and sea bottom (𝑇 , 𝑆 ) can be obtained from\n𝑏𝑜𝑡𝑡𝑜𝑚 𝑏𝑜𝑡𝑡𝑜𝑚\nGLORYS2V4 model results. 𝑑 and 𝑑 are the e-folding depths of thermocline and\n𝑇 𝑆\nhalocline. The determination of 𝑑 and 𝑑 based on the mean temperature and\n𝑇 𝑆\nsalinity in the mixed layer entrainment zone and sea bottom from GLORYS2V4.\n𝑇 and 𝑆 𝑑 and 𝑑 were previously used to determine the upwelling\n𝑏𝑜𝑡𝑡𝑜𝑚 𝑏𝑜𝑡𝑡𝑜𝑚 𝑇 𝑆\nvelocity and entrainment velocity in the origin method. We have modified the\ncalculation of entrainment velocity 𝑊 in the revised paper (Lines 632-637). The\n𝑒\nentrainment velocity 𝑊 is now given by the deepening rate of mixed layer using daily\n𝑒\nGLORYS2V4 model results and the Ekman pumping/suction velocity. The variables of\n𝑇 and 𝑆 𝑑 and 𝑑 are no longer needed.\n𝑏𝑜𝑡𝑡𝑜𝑚 𝑏𝑜𝑡𝑡𝑜𝑚 𝑇 𝑆\n44) L607, the left and right sides of the equation have different units. Time should be\ndivided on the left side.\nResponse: Thank you for pointing out this error. we have modified the calculation of\nentrainment velocity 𝑊 (Lines 631-636): For the 1D model, the entrainment velocity\n𝑒\n𝑊 is given by the deepening rate of mixed layer and the upwelling velocity 𝑊\n𝑒\nas Equation (5):\ndh\n𝑊 = +𝑊 (5)\n𝑒\ndt\n45) What is deltaS_whole? How is it determined?\nResponse: 𝑆 was the salt content of water column, ∆𝑆 represented the\n𝑤ℎ𝑜𝑙𝑒 𝑤h𝑜𝑙𝑒\nchange of salt content in the water column.\ndSwhole\nrepresented the change rate of salt\ndt\ncontent, which was determined by the daily 𝑆 results from GLORYS2V4. ∆𝑆\n𝑤ℎ𝑜𝑙𝑒 𝑤h𝑜𝑙𝑒\nwas used to the determine the upwelling velocity and entrainment velocity in the origin\nmethod. In the modified method, these variables are no longer needed. The entrainment\nvelocity 𝑊 is given by the deepening rate of mixed layer from daily GLORYS2V4\n𝑒\nmodel results and the Ekman pumping/suction velocity. \n46) L611, the units on the left and right sides of the equations are different\nResponse: Thank you for pointing out this written error. We have modified the\ncalculation of entrainment velocity 𝑊 in the revised paper, and this function was\n𝑒\ndeleted.\n47) L625, time should be divided to get “rate”\nResponse: Thanks again for pointing out this error, we have corrected the equation\n(Line 647):\n𝑄 = (𝐸 −𝑃)×𝑀𝐿𝑆+(𝑀𝐿𝑆−𝑆 )×𝑆𝐼𝐶 ×\ndhice𝜌\n/𝜌 (7)\n𝑠 𝑖𝑐𝑒 𝑖𝑐𝑒 𝑤𝑎𝑡𝑒𝑟\ndt\n48) L630, mixed layer – seawater in the mixed layer\nResponse: The paragraph has been rewritten (Lines 652-653): “Sea ice salinity\n𝑆 = 5 𝑝𝑠𝑢 and density 𝜌 = 917 kg m-3 are constants and 𝜌 is seawater\n𝑖𝑐𝑒 𝑖𝑐𝑒 𝑤𝑎𝑡𝑒𝑟\ndensity in the mixed layer.”\nReviewer #2 (Remarks to the Author):\nMajor comments:\n1) ‘Pacific Arctic Ocean’ is a much larger region than what is studied here and\nincludes the entirety of the Canada Basin (to ~85°N). I suggest replacing with ‘Northern\nBering and Chukchi Seas’ or 'Pacific Arctic Shelf Seas' throughout. This is a\nproblem in the title and lines 49, 95, 196, 262, 301, and likely other places I have missed.\nResponse: Thanks for your excellent suggestion. We have replaced the ‘Pacific Arctic\nOcean’ to “Pacific Arctic continental shelves” in whole article, both the title and main\ntexts.\nTitle: Enhanced wind mixing and deepened upper mixed layer in the new normal\nPacific Arctic Shelf Seas with low summertime sea ice.\nLine 47: The Pacific Arctic Shelf, comprising the northern Bering and the Chukchi seas.\nLine 80: upper ocean properties on the Pacific Arctic shelf seas (15-500m).\nLine 88: in increasingly ice-free Pacific Arctic shelf seas\nLine 94: The sea ice in the Pacific Arctic shelf seas\nLine 205: rapidly changing Pacific Arctic shelf seas.\nLines 271-272: the changing upper ocean dynamics in the Pacific Arctic shelf seas.\nLine 310: as a whole in the new normal Pacific Arctic shelf seas\n2) Lines 159-171: The updated figure 2 is very useful, and shows that most locations\ndo not correspond to a significant trend for mixed layer depth (e.g., 69 grid boxes out\nof 76 grid boxes). But Figure 2 is presented focusing on only those few boxes with\na significant trend, which I think is misleading. Too much of a concrete conclusion is\nstated from very inconsistent evidence. A more honest description of these results needs\nto be included. This does not affect the conclusions that can be drawn from the paper\noverall, as these are primarily based on GLORYS and ERA5 analysis.\nResponse: Thanks for your excellent suggestion that results in much improved\npresentation of observation results (Fig. 2). In the revised paper, we point out the\nlimitation of linear tends calculated by field observation data. Although we\nacknowledge the imperfection of unevenly distributed CTD/XCTD casts, the\nconsistency of shoaling/deepening tends in sparse grid cells is the starting point of this\nwork. We modified the concrete conclusions of MLD in the main content. However, we\nthought the description of regional mixed layer salinity and temperature trends is\ntenable, since the significant changes existed in large number of grid cells\n(Supplementary Fig. 4 & 5).\nThe paragraph has been modified as followed (Lines 160-172): “Despite the sampling\nbias and data gaps of field observations in space and time scales, the consistency of\nMLD shoaling/deepening tends of sparse grid cells from the Bering to Chukchi shelves\nwas notable (Fig. 2a & 2b). MLDs had a similar shoaling tendency in the southeastern\nBering shelf during both periods, whereas higher-latitude regions showed exactly\nopposite trends in specific grid cells (Fig. 2a & 2b). The shoaling trends of MLD on the\nnorthern Bering (-2.4 m yr-1 of 2 grid cells, p<0.1; Fig. 2a) and the Chukchi shelves (-\n1.1 m yr-1 of 5 grid cells, p<0.1) before 2007 agreed with previous studies27,28. However,\nafter 2007, the mixed layer on the northern Bering and Chukchi shelves showed a\nremarkable deepening trend (up to 1.3 m yr-1, p<0.1) illustrated by reddish grid cells\n(Fig. 2b). A freshening trend was observed in some areas of the southeastern Bering\nshelf (-0.1 psu yr-1, p<0.1) before 2007, and then the water in the region gradually\nbecame saltier (up to 0.08 psu yr-1) (Supplementary Fig. 4a & 4c).”\nAdditional comments:\n3) Line 35: ‘its development’ should be more specific, e.g., its interannual to decadal\nevolution.\nResponse: Thank you for your suggestion. The sentence has been modified (Lines\n33-34): “Here, we quantified the summertime upper mixed layer and dynamically\nrelated its interannual to decadal evolution with sea ice and atmospheric forcing”.\n4) Line 85: ‘another robust mechanism’ and ‘understanding of the phenomenon’ I’m\nnot sure what you’re referring to here.\nResponse: In the revised paper, we have clarified the phenomenon and mechanism of\ntraditional understanding (Lines 83-86): “This finding demonstrates another robust\nmechanism and consequence of sea ice loss on the upper ocean dynamics, in addition\nto a traditional understanding of shoaling mixed layer with more ice meltwater33-35.”\n5) Line 153: suggest changing to ‘trends of the mean summer…’ so that it’s clear how\ntrends are calculated without referring to the methods.\nResponse: Thanks for your suggestion. The sentence has been modified (Lines\n153-155): “we further calculated trends of the mean summer (June to September) MLD\nusing a total of 18,997 quality-controlled hydrographic profiles over two separate\nperiods.”\n6) Line 152-161: It would be useful to state here that the mixed layer shoals in May,\nif this is true for the entire region. I am getting this impression from other parts of the\npaper, but it would be good to clarify here. This would clarify that even though\nthe mixed layer is shoaling earlier with time, that does not affect the trend statistics during\nJune – September.\nResponse: Thanks for your suggestion. Considering Pacific Arctic shelves span more\nthan 10 latitudinal degrees and ice conditions vary from year to year (Fig 1b), mixed\nlayer may not always shoal in May. The definition of the summertime MLD in the\nseason of June to September followed previous studies for the whole Arctic (Peralta-\nFerriz and Woodgate, 2015) and also specific to the Pacific Arctic shelves (Danielson\net al., 2020).\nPeralta-Ferriz, C., & Woodgate, R. A. Seasonal and interannual variability of pan-arctic\nsurface mixed layer properties from 1979 to 2012 from hydrographic data, and the\ndominance of stratification for multiyear mixed layer depth shoaling. Progress in\nOceanography, 134, 19-53 (2015).\nDanielson, S. L., et al. Manifestation and consequences of warming and altered heat\nfluxes over the Bering and Chukchi Sea continental shelves. Deep-Sea Research\nII, 177, 104781 (2020).\n7) Line 182-185: where is the ‘inner southeastern Bering shelf’? A box or arrow in\nFigure 2 would help.\nResponse: The inner shelf is the region with water depth less than 50m in the\nsoutheastern Bering Sea (Kachel et al., 2002). The description of the inner southeastern\nBering shelf water depth is added (Lines 182-183): “Observation datasets revealed that\nthe inner southeastern Bering shelf (< 50m) had an overall shoaling trend (Fig. 2b).”\nKachel, K. B., et al. Characteristic and variability of the inner front of the southeastern\nBering Sea. Deep-Sea Research II 49, 5889-5909 (2002).\n8) Figure 5a-b: What is indicated by the ‘E’ and downward arrow, Ekman pumping?\nFor winds and Ekman pumping, it would be better to indicate overall values for the\nsummer period somehow as a monthly analysis (Figure S9) shows no increase during\ne.g., September. Or indicate the monthly variability depicted in Figure S9 in this\nsummary schematic.\nResponse: The origin symbol ‘E’ represents the transfer of momentum from the\natmosphere to the ocean by wind stirring, which is quantified by turbulent kinetic\nenergy TKE in the paper. We revised the schematic diagram (Line 314; Fig 5) and\nremoved the arrows and symbol ‘E’ to avoid potential misunderstanding. The spiral line\nbelow the mixed layer is used to represent the process of turbulent mixing driven by\nwind stirring. The density of spirals implied the mean strength of turbulent kinetic\nenergy, which increase in the “new normal” of low sea ice. The drawing of wind stress\nhas also been modified according to your suggestion. We use different colors to\nrepresent the regional significant increased (red), regional significant decreased (blue)\nand no changes (grey) monthly wind stress based on the supplementary Figure. 9.\n9) Figure 5c-d: only a suggestion - in panels c and d, it would be more intuitive to\nkeep the terms (TKE, meltwater etc.) on the same side of the balance in both panels,\nand change which side is up / down between the two.\nResponse: We have modified Figure. 5c & 5d based on your suggestion.\n10) Line 530-541: GLORYS is used to show the mixed layer depth change, but ERA5\nfor diagnosis of what’s causing that change. Does GLORYS use ERA5 forcing?\nResponse: According to the quality information document for Global Ocean\nReanalysis Products (http://marine.copernicus.eu/documents/QUID/CMEMS-GLO-\nQUID-001-025-011-017.pdf), GLORYS2V4 are forced by ERA-Interim reanalysis\nproducts. ERA-Interim is the fourth generation ECMWF atmospheric reanalysis of the\nglobal climate covering the period from January 1979 to August 2019. ERA-Interim\nhas been superseded by the ERA5 reanalysis and users are advised to use ERA5 forcing\nbased on data provider, the Copernicus Service. The description of ERA-Interim and\nits relationship with ERA5 is mentioned in Model reanalysis in Methods (Lines 553-\n555): “GLORYS2V4 are forced by ERA-Interim, the fourth generation ECMWF\natmospheric reanalysis of the global climate and has been superseded by the ERA5 after\nyear 2019.”\n11) Line 648 – 658: Am I correct that for the observations, the mean value from June\n– September in each year is first calculated, and then a linear trend of those mean\nvalues is determined? How does the seasonal cycle bias these results, if observations\nare in June for one year and September for another? Does this explain why so many\ngrid boxes have an insignificant trend?\nResponse: Your understanding is correct. In the Pacific Arctic region, only limited\nship-based field observations were conducted. Given changing spatial sampling from\nyear to year, CTD/XCTD profile data were sparse and unevenly distributed. It is\ndifficult to extract meaningful results from field observations without performing some\nspatial and temporal averaging.\nOur research aims to explain the phenomenon of reversed mixed layer from shoaling to\ndeepening between 1996-2006 and 2007-2021.The time span is shorter relative to prior\nstudies of Danielson et al (96 years from 1922-2018) and Peralta-Ferriz and Woodgate\n(33 years from 1979-2012). The amount of monthly field observations in this paper is\ninsufficient to eliminate the seasonal cycle bias in an accurate way. The maximum\nsampling years for specific months during 2007-2021 in all 1°×2° grid cells of the study\nregion are 11 years, with large spatial and temporal differences. Besides, according to\nthe statistical results of Peralta-Ferriz and Woodgate (2015), the summertime (June to\nSeptember) mixed layer depths in the Chukchi Sea shelf have a narrow, quasi-normal\ndistribution. Therefore, we used the mean values from June to September in each year\nto represent the state of summertime mixed layer.\nThe mixed layer may have significant changes on order days and on small space scales,\nin additional to a longer seasonal scale (Peralta-Ferriz and Woodgate, 2015). A sudden\nstorm or the advection of ice meltwater in short timescale may affect the field\nobservations. The insignificant trends in the grid cells partly due to the randomness of\nthe observations.\nWe added descriptions of linear trends from field observation data in Statistical\nanalysis of linear trends in Methods, including solution procedures, hypothesis and\nlimitation (Lines 688-702): “In this work, we are interested in detecting long-term\ntrends of the upper mixed layer properties from the ship-based observations and model\nresults. In light of the sparse field data coverage in time and location, we bin MLDs\nderived from the observations of June to September in each 1°×2° grid cell to represent\nthe state of the summer upper water column27,93. We only use the grid cell with\nsufficient sampling years to obtain robust decadal trends. For example, the grid cells on\nthe shelf containing fewer than 5 years of data in 1996-2006 were excluded from the\nlinear trend analysis. For 2007-2021, each grid cell required at least 6 years of data.\nAccording to the statistical results of Peralta-Ferriz and Woodgate (2015)27, the\nsummertime (June to September) mixed layer depths in the Chukchi Sea shelf have a\nnarrow, quasi-normal distribution. Therefore, the long-term trend of MLDs from ship-\nbased observations was estimated as the best linear fit to the mean MLDs of\nsummertime (June to September). It needs to be known that a sudden storm or the\nadvection of ice meltwater in a short timescale may affect the CTD profiling on a daily\nscale, which causes uncertainty to the results.”\nIn addition, the linear trends of MLD from GLORYS2V4 is recalculated after removing\nthe seasonal cycle in Fig. 2c & 2d. The linear rends of MLD from GLORYS2V4 model\nresults in Supplementary Fig.6 and Fig.7 was calculated without removing the seasonal\ncycle for better comparisons with observations.\n12) Supplement line 39: typo on ‘2007-201’\nResponse: The typo has been corrected."}, {"round": 3, "reviewer_comments": {"reviewer_1": "The authors have addressed all my questions and I suggest the paper can be accepted for publication.\n(Remarks on code availability)\nI had a brief look at the code. The matlab routines have a good readability and are useful for the community.", "reviewer_2": "I recommend publication of this paper, and have no further comments.\n(Remarks on code availability)\nI briefly viewed that the linked code existed but will not download and view it further. This appears sufficient."}, "author_response": ""}]

Decision letter