Atlantic-origin water extension into the Pacific Arctic induced an anomalous biogeochemical event

Domain: earth_science
Published: 2023-11-02
Editorial outcome: accept

Abstract
The Arctic Ocean is facing dramatic environmental and ecosystem changes. In this context, an international multiship survey project was undertaken in 2020 to obtain current baseline data. During the survey, unusually low dissolved oxygen and acidified water were found in a high-seas fishable area of the western (Pacific-side) Arctic Ocean. Herein, we show that the Beaufort Gyre shrinks to the east of an ocean ridge and forms a front between the water within the gyre and the water from the eastern (Atlantic-side) Arctic. That phenomenon triggers a frontal northward flow along the ocean ridge. This flow likely transports the low oxygen and acidified water toward the high-seas fishable area; similar biogeochemical properties had previously been observed only on the shelf-slope north of the East Siberian Sea.

Introduction
In recent decades, the Arctic sea ice cover decreased 1. As the sea ice declines, the response of the upper ocean circulation to winds can be enhanced and the freshwater in the eastern Arctic basins is more likely to shift toward the Beaufort Gyre (BG) region in the western Arctic 2,3 (Fig. 1a), where freshwater accumulation has been observed 4,5. Bering Strait mooring observations from 1990 to 2019 show increasing northward flow of Pacific water (PW), spring/fall warming, and winter freshening 6. The anomalous PW influence is called Pacification 7. The PW transports a large quantity of nutrients; however, freshwater accumulation within the BG deepens the nutrient-rich PW 8 and may decrease phytoplankton production 9. In contrast, increased eddy activity due to sea ice loss could replenish nutrients and increase phytoplankton production 10. The freshening of PW inflow in winter may make nutrients available closer to the surface for spring phytoplankton bloom 6.

On the other side, recent changes in the eastern Arctic Ocean are frequently described in terms of Atlantification, which is related to the progression of anomalies carried by Atlantic water (AW) into the eastern Arctic 11 and intensified by the Arctic sea ice decline 12. Atlantification accompanies the salinification of halocline above the AW layer, weakening of the halocline stratification, shoaling of the AW layer with higher nutrients, and a possible increase in phytoplankton biomass by the vertical nutrients supply 7 and the phytoplankton transport by AW 13. Thus, the responses of ocean conditions to the sea ice loss are quite different between the western (Pacific) and eastern (Atlantic) Arctic 2,7,14.

The ocean circulation in the Arctic basins plays a vital role in the transports of PW, AW, and Arctic river runoff 15. The circulation mainly comprises the anticyclonic BG on the Pacific side, cyclonic ocean circulation on the Atlantic side, and Transpolar Drift (TPD), which is roughly the boundary of PW and AW (Fig. 1a). The boundary is climatologically located along the Lomonosov Ridge (LR). However, it shifted eastward to the Mendeleyev Ridge (MR) via the Makarov Basin (MB) with enlarged cyclonic ocean circulation and shrinkage of the BG in the Canada Basin (CB) when Arctic atmospheric circulation was characterized by a strong cyclonic mode, i.e., Arctic Oscillation (AO) 16 was in a positive phase (AO+) with its significant signature from the end of the 1980s to the mid-1990s 2,17. Subsequently, the AO indicated a negative phase (AO−) on average, and it again changed to AO+ after the mid-2010s. Accordingly, the above mentioned ocean circulation shift occurred with an eastward extension of Atlantic-origin water (Lower Halocline Water (LHW) and AW) along the shelf-slope of the East Siberian Sea (ESS) and Chukchi Sea 18. The Atlantic-origin water intrusion in 2017 pushed up the nutrient-rich PW and ESS water (ESSW), resulting in anomalously high surface phytoplankton blooms 19. The ESSW transport up to the Chukchi Sea (and further into the CB) is consistent with the previous modeling study, suggesting that the Eurasian runoff leaves the Siberian shelf at more eastern locations under AO+ 15. However, due to a lack of observed data, we have little knowledge about the changes in ocean environments, especially biogeochemical aspects, in the region toward the north of the ESS and the Chukchi Sea including the Chukchi Plateau (CP), an ocean ridge north of the Chukchi Sea. In this region, PW, AW, and ESSW complicatedly intersect (hereafter referred to as the intersectional water region).

In 2020, a multiship sampling campaign covering a broad research area in the western Arctic Ocean (Fig. 1b), including the intersectional water region, was conducted under the Synoptic Arctic Survey (SAS) 20. During the survey, unusually low dissolved oxygen and acidified water were observed on the CP for the first time. Herein, we describe these biogeochemically abnormal conditions and propose a mechanism driving these changes (Fig. 2a–c) by combining data obtained by the Research Vessel (R/V) Araon (Korea), R/V Mirai (Japan), and Canadian Coast Guard Ship (Canada) in the SAS project. Describing the presence and drivers of these biogeochemical anomalies is a vital preliminary step for Arctic ecosystem assessments 21,22; we can assess ecosystem responses only when we know the hydrographic changes underpinning them. This information will ultimately help to develop policies facilitating effective management, e.g., for potential fisheries in the future ice-free Arctic Ocean 23.

Results
Anomalous water found on the Chukchi Plateau
In 2020, we found low dissolved oxygen and thus, low oxygen saturation (Fig. 3a) in water on the southern portion of the CP (~75°N, 168°W). The low oxygen saturation water also corresponded to acidified water, which was corrosive to aragonite (i.e., saturation state Ωarg < 1; Fig. 3b). The time series of oxygen saturation in a water column (100–300 m) surrounding the CP (172–164°W and 73.5–76.5°N), which is obtained through ship-based observations during the summer and autumn of 2002–2020 (Supplementary Table 1), shows that the oxygen saturation was typically 70–80% (Fig. 3c). In contrast, water with oxygen saturation levels < 50% (i.e., oxygen concentrations below 180 µmol kg−1) was found in 2020. Such water had never been observed before, at least in the last two decades. Here, we term this water anomalously low oxygen saturation (dissolved oxygen) water or anomalous water.

The anomalously low oxygen saturation water exhibited extremely low Ωarg values (< 0.6) on the CP (Fig. 3). This suggests a high quantity of organic matter decomposition that used dissolved oxygen and produced CO2, resulting in significant decreases in oxygen saturation and Ωarg, respectively. However, there is no evidence of high-level suspended sediments and particles that might promote local microbial oxygen consumption and CO2 production, unlike the conditions observed in the ESS where much of terrestrial/marine organic matter is deposited on the bottom 24,25.

Historical data 26,27 indicate that the water with oxygen saturation as low as the anomalous water on the CP occupied the bottom along the shelf-slope north of ESS between 170° E and 170° W, hereafter referred to as ESS shelf-slope (Fig. 4a). In shallow shelves, currents and storm events replenish dissolved oxygen and diffuse CO2 in the bottom water 25. On the shelf-slope west of 170° E, AW would penetrate from the west along the slope. PW occupied east of 170° W. The AW and PW were likely well-ventilated in winter 28, so dissolved oxygen concentrations could be high. Consequently, the zone of the lowest oxygen saturation along the ESS shelf-slope is considered a region where AW and PW influences were minimal. Thus, it is recognized as a shadow (less ventilated) zone of the AW and PW inflows. The shadow zone is consistent with the lowest concentrations of an artificial inert gas, sulfur hexafluoride (SF6), in the ESS shelf-slope 29. The bottom salinity of the shadow zone (Fig. 4b) was similar to that of the anomalous water on the CP. In the cold Arctic seas, salinity mainly determines the water density. It is used to identify water masses that should move along the same density surfaces. In addition, biogeochemical characteristics of the lowest oxygen saturation water along the ESS shelf-slope were similar to those of the anomalous water on the CP (Supplementary Discussion 1 and Supplementary Figs. 1 and 2). Thus, the shadow zone of the ESS shelf-slope is the likely source of the anomalous water on the CP.

Historical data indicate that the lowest oxygen saturation levels along the ESS shelf-slope were observed in winter and summer. Thus, the shadow zone would be less ventilated throughout the year. Anomalously low oxygen saturation water on the CP was found in October 2020. Only with this observation it is not easy to describe the seasonality of the anomalous water. This will be examined using our numerical model below.

Changes in PW and AW circulation
We hypothesize that a change in ocean circulation in the intersectional water region north of ESS and Chukchi Sea caused the anomalous event on the CP, such as the appearance of low oxygen saturation water, which was likely derived from the ESS shelf-slope. The Arctic upper ocean circulation variation is mainly associated with the Beaufort High (BH) variability and AO 2. The BH induces a negative curl of winds that converges the surface freshwater within the BG, and the spatial distribution of freshwater determines the extent of the BG. On the other hand, when AO is positive, the BG shrinks to a smaller area in the western Arctic, and the cyclonic ocean circulation in the eastern Arctic expands toward the western Arctic 30. The wind curl over the CB was unprecedentedly negative in the second half of the 2000s and 2010s, and AO changed its phase from negative to positive in mid-2010s 2. During the 2000s and 2010s, although the BG extent evaluated via numerical simulations 31 and satellite data 19,32 generally increased until 2016 (the increase in sea surface height of the gyre had slowed over the 2010s 33), the gyre shrunk in 2017 and the cyclonic ocean circulation expanded concurrently under AO+ 19,31.

Here, the circulation pattern and oxygen saturation distribution in 2017–2020 are compared with those in 2008–2016, selected as a period when the BG extent was larger than that in 2017–2020. These circulation patterns and oxygen saturation distributions are also compared with those obtained from historical data (temperature and salinity data 27 and oxygen data 26) that were mainly obtained during the 1950s to 1980s, when the wind curl over the CB had a positive anomaly on average and the AO was generally in a negative phase. In the Discussion section, we will further mention the hydrographic condition and oxygen saturation distribution in the mid-1990s during a highly positive phase of AO. The circulation pattern in each period is calculated as the dynamic height at a depth of 100 m, which is a suitable depth to examine the PW and AW circulation as used in previous studies 34 and described below. Because the observation points in each period are not spatially uniform, comparisons between hydrographic conditions with different data spacings may lead to a distortional feature. Thus, the observational data were interpolated to gridded points in each period for evaluating dynamic height (Fig. 5a–c), temperature and salinity related to the PW and AW circulation (Fig. 5d–f), and oxygen saturation distributions (Fig. 5g–i). The interpolation uncertainties inherent to each parameter were also estimated (Supplementary Discussion 2 and Supplementary Figs. 3 and 4).

In 2017–2020 (Fig. 5a), an anticyclonic circulation, the BG, occupied east of the CP (~168° W), and northward flows were predominant west of the plateau. The CP could be a western boundary of the BG 35. The northward flow west of the CP might be a branch of cyclonic ocean circulation extended from the Eastern Arctic. The BG shrunk toward the southeast compared to 2008–2016 (Fig. 5b, Supplementary Fig. 4a). In 2008–2016 (Fig. 5b), the westward flow of the southern part of the BG overshot the southern CP, and the return flow passed across the northern CP. The shrinkage of the BG in 2017–2020 is likely associated with the collapse of the BH in 2017 36. In this period, the area of the strong negative wind curl was confined to the southeastern CB 2. In the 1950s–1980s (Fig. 5c), the BG circulation was weak because the wind curl over the CB was not strongly negative in this period 2. A heavy and less mobile sea ice condition in this period might also inhibit the input of wind curl to the ocean 37. Because the AO was a negative phase in this period, the cyclonic ocean circulation would not expand to the study area.

The circulation patterns are associated with temperature and salinity distributions along 75–76° N across the CP (Fig. 5d–f). The BG transports cold PW that has entered the Arctic Ocean through the Bering Strait in winter and is characterized by a temperature minimum around salinity = ~33 38. Cold ESSW (salinity = 32–33) occupies the west of the cold PW 39. Layers below the cold waters are occupied by LHW derived from the eastern Arctic (salinity = 34–34.5) 38,40 and warm AW (temperature > 0 °C, salinity > 34.5) 41, both of which are transported by the cyclonic ocean circulation. In 2017–2020 (Fig. 5d), a strong front between the cold PW and eastern Arctic-derived LHW was found at depths of 100–150 m just west of the CP with a shoaling of the AW. This front caused a significant northward flow there (Figs. 2a and 5a). Such a frontal structure was not found in 2008–2016 (Fig. 5e) because the cold PW penetrated further west of the CP with the extension of the BG (Figs. 2b and 5b). In 2017–2020, shallow levels of LHW and AW at the west of the CP suggest enlarged intrusions of LHW and AW. The enlarged LHW and AW intrusions were presumably associated with the intensification of AW supply induced by the sea ice decline (Atlantification) 12 and AO+, which enhanced the cyclonic ocean circulation, in the second half of the 2010s. With the strengthening and expansion of the cyclonic ocean circulation and the shrinkage of the anticyclonic BG circulation, the LHW and AW could further flow along isobaths and spread into the CB 42,43. However, the spreading of these water masses, especially the LHW, was likely hampered by the BG, whose western boundary appeared over the CP when the gyre shrunk. Thus, there was a strong front between the PW and LHW in 2017–2020. In the 1950s–1980s (Fig. 5f), the cold PW with salinity = ~33 extended to the west of the CP. This cold PW would be carried by westward flows of the BG circulation, which was weak but widely expanded under AO− (Figs. 2c and 5c). The cold water with salinity = 32–33 found on the western side of the section would be derived from the ESS by northward flows of the BG circulation. The intrusions of LHW and AW were likely small because of AO− and the condition before Atlantification occurred, as shown in a deep AW layer and low temperature of the AW.

Oxygen saturation distribution determined by ocean circulation
The concurrent expansion of the cyclonic ocean circulation and shrinkage of the BG would determine the fate of the water originally located on the ESS shelf-slope. We examined the oxygen saturation distributions on isopycnal (i.e., the same potential density) surfaces of σθ = 27.7 (S ~ 34.5) in 2017–2020 and 2008–2016 (Fig. 5g, h) and σθ = 26.5 (S ~ 33) in the 1950s–1980s (Fig. 5i). Each isopycnal surface is located in a layer including the lowest oxygen saturation water in each case (Fig. 4a, Supplementary Figs. 1a and 2a). The isopycnal surface of σθ = 27.7 (S ~ 34.5) in the 1950s–1980s was slightly deeper than the level of the lowest oxygen saturation water on the ESS shelf slope (Fig. 4a). Thus, we selected a shallower isopycnal surface (σθ = 26.5, S ~ 33). The oxygen saturation distributions in 2008–2016 (Fig. 5h) and the 1950s–1980s (Fig. 5i) show that the area with the lowest oxygen saturation on the isopycnal surface appeared along the ESS shelf-slope. This area is thought to be a shadow zone. However, in 2017–2020 the area with the lowest oxygen saturation moved to the CP (Fig. 5g). The low oxygen saturation around the CP was the most pronounced in 2020, i.e., the anomalously low oxygen saturation water was found there (Fig. 3a). It further extended toward the north with increasing oxygen saturation. The migration of the area with the lowest oxygen saturation suggests that the lowest oxygen saturation water in the shadow zone was washed by an eastward penetration of LHW (i.e., higher oxygen saturation water) along the ESS shelf-slope. Thus, the shadow zone there was no longer maintained. The washed-out lowest oxygen saturation water was likely transported to the north by the strong frontal flow between the PW and LHW over the CP. Because the biogeochemical characteristics between the lowest oxygen saturation water along the ESS shelf-slope and the anomalously low oxygen saturation water on the CP were almost the same, the lowest oxygen saturation water was not considerably modified by mixing with the surrounding water during its migration to the CP. In contrast, the water was substantially modified after leaving the CP along the frontal northward flow. Assuming horizontal and vertical diffusivities of 102 44 and 10−6 45 m2 s−1, respectively, and estimating the horizontal and vertical gradients of oxygen concentration from the observed data over the CP in 2020, we found that oxygen concentrations increased along the frontal northward flow primarily through horizontal diffusion (O(10−6) μmol kg−1 s−1), which was an order of magnitude larger than vertical diffusion (O(10−7) μmol kg−1 s−1).

The CB’s oxygen saturation was lower in 2017–2020 than in 2008–2016 (Fig. 5g, h). This might be partly explained by the anomalously low oxygen saturation water spreading from the CP into the CB. However, the surrounding water would rapidly dilute the anomalous water through horizontal diffusion north of the CP. Therefore, it could not have contributed to the decreasing oxygen saturation throughout the CB. In 2008–2016, a source of high oxygen saturation seemed to develop in the northeastern CB. A tongue of high oxygen saturation extended southward from the source, which was probably transported by a southward flow of the eastern part of the BG. The source and the tongue of high oxygen saturation were not observed in 2017–2020. Although water with a high oxygen saturation is thought to originate in the eastern Arctic, further study is required to explain why the water was not delivered to the northeastern CB in 2017–2020. The lower oxygen saturation levels in the CB in 2017–2020 than in 2008–2016 were observed in the whole LHW (salinity = 34–34.5) layer (Supplementary Fig. 5a, b). In addition to the absence of high oxygen saturation source in the northeastern CB in 2017–2020, vertical mixing between the LHW layer and the above layer characterized by a vertical minimum of oxygen saturation might be another reason to reduce the oxygen saturation in the LHW layer (Supplementary Discussion 3 and Supplementary Fig. 5). Contrary to the CB, the MB indicated that the oxygen saturation was higher in 2017–2020 (Fig. 5g) than in 2008–2016 (Fig. 5h). This shift would result from the penetration of LHW with a high oxygen saturation to the MB by the expanded cyclonic ocean circulation in 2017–2020 under AO+.

Modeled transport of the lowest oxygen saturation water
A tracer experiment was conducted using a physical sea ice–ocean general circulation model to evaluate whether the lowest oxygen saturation water on the ESS shelf-slope was transported to the CP (see Methods). A virtual passive tracer was continuously provided on the ESS shelf-slope during two periods, namely, 2013–2016 and 2017–2020 (4 years each), to examine differences in the distribution of low oxygen saturation water between the two periods. A snapshot of the model output in December 2017 (Fig. 6a) shows that the lowest oxygen saturation water on the ESS shelf-slope, where the tracer concentration value was fixed to 1, is transported to the CP by a northward flow. Associated with this transport, the appearance of low oxygen saturation water passing through the CP has become more pronounced since 2017 (Fig. 6b), which is consistent with the year when the observed BG shrunk in 2017. Our numerical model also indicates the shrinking of the BG in 2017–2020 (Supplementary Fig. 6a) compared to 2013–2016 (Supplementary Fig. 6b). The appearance of low oxygen saturation water is not continuous throughout 2017 and beyond (Fig. 6b); instead, it is intermittent on a timescale of several months. The tracer concentration of the water seems to represent a seasonality with its increase in winter when winds are generally strong 46,47. Although we observed anomalously low oxygen saturation water on the CP only in October 2020, it might also have been present between 2017 and 2019 when observations were not conducted.

From 2017 to 2020, the mean velocity field in the cross-section from the top of the CP to its western slope (Fig. 6c) shows an overall northward flow, carrying the lowest oxygen saturation water from the ESS shelf-slope. In contrast, southward flows are predominant from 2013 to 2016 (Fig. 6d), and the lowest oxygen saturation water on the ESS shelf-slope hardly spreads to the CP. This southward flow is part of a meander around the southern CP generated when the plateau diverts a westward flow of the BG (Supplementary Fig. 6b). The northward flow in 2017–2020 is likely related to a frontal salinity structure, such as the salinity contours of 34.5 and 34.7, deepening toward the CP (Fig. 6c) as observed in 2017–2020 (Fig. 5d). These isohaline surfaces are shallower in 2017–2020 (Fig. 6c) than in 2013–2016 (Fig. 6d), suggesting an enhanced penetration of AW. In the observed data of 2017–2020 (Fig. 5d), LHW (salinity = 34–34.5) was also shallower than in previous years; however, the simulated isohaline surface of 34, i.e., the upper boundary of the LHW, is almost at the same level in 2013–2016 and 2017–2020. Thus, the model does not accurately reproduce the LHW penetration. The simulated penetration of LHW from the eastern Arctic might be underestimated because of the closed lateral boundary condition in the North Atlantic and insufficient vertical resolution. In addition, the lack of interannual variability and multi-decadal trends in riverine freshwater discharges might produce an unexpected model bias of stratification in the eastern Arctic (isohaline-surface structure of the LHW). Further approaches are needed to reproduce the penetration of properly stratified LHW to the CP, one of the key processes to form the northward flow that transports the lowest oxygen saturation water from the ESS shelf-slope.

Methods
Ship-based sampling campaign in 2020
The Synoptic Arctic Survey, a coordinated multiship and multination pan-Arctic ship-based sampling campaign, was planned to be implemented in 2020 to obtain a synoptic view of the totality of hydrographic and ecosystem changes occurring in the Arctic Ocean 20. However, several of these research cruises were canceled due to COVID-19. Even in such a situation, Canada/US, Japan, and Korea managed to conduct field expeditions that covered a wide area of the western Arctic Ocean (Fig. 1b). The Joint Ocean Ice Study/BG Exploration Project (Canada/US) was performed in the CB by CCGS Louis S. St-Laurent from September 14 to October 2, 2020 (see cruise report; https://www2.whoi.edu/site/beaufortgyre/wp-content/uploads/sites/108/2021/05/2020-79-LSSL-Cruise-Report-v2021-02-23.pdf). The R/V Araon (Korea) cruise operated in the Bering Strait, Chukchi, and Eastern Siberian seas, and the CB and MB during August 2020 as a part of the project titled “Korea-Arctic Ocean Warming and Response of Ecosystem (K-AWARE).” In between the study areas by the CCGS Louis S. St-Laurent and R/V Araon, a research cruise of R/V Mirai (Japan) was carried out in October 2020 as a part of the Arctic Challenge for Sustainability II (ArCSII) project (see cruise report; https://www.jamstec.go.jp/iace/e/report/pdf/2020.MR20-05.pdf).

Hydrographic data were obtained in each cruise using an SBE 911 plus conductivity-temperature-depth (CTD) system (Sea-Bird Scientific, Bellevue, WA, USA). The CTD system was set up with temperature, conductivity (salinity), and dissolved oxygen sensors. On the R/V Mirai, a nitrate sensor (Deep SUNA; Sea-Bird Scientific, Bellevue, WA, USA) was also attached to the system. The temperature sensor’s accuracy is ±0.001 °C. Sensor data for salinity, dissolved oxygen, and nitrate were corrected to be highly correlated with analytical values of water collected from Niskin bottles at the same locations and depths as the sensors.

Salinity samples from Niskin bottles were analyzed following the Global Ocean Ship-based Hydrographic Investigations Program (GO-SHIP) Repeat Hydrography Manual using a salinometer (AUTOSAL 8400B; Guildline Instruments, Smiths Falls, ON, Canada) and the International Association for the Physical Sciences of the Oceans (IAPSO) standard seawater as reference material 73. The precision of salinity measurements was ~0.001.

Dissolved oxygen samples from Niskin bottles were measured through Winkler titration based on the World Ocean Circulation Experiment Hydrographic Program method 74. The precision of dissolved oxygen measurements was ~0.2 μmol kg−1.

In this study, we also used data for nutrients, total alkalinity, and dissolved inorganic carbon (DIC) obtained from the R/V Mirai to calculate the following parameters: NO, PO, NO/PO, and Ωarg. These parameters were calculated using Ocean Data View (https://odv.awi.de) developed by R. Schlitzer (Alfred Wegener Institute for Polar and Marine Research). Hereafter, we describe the methods adopted by the R/V Mirai cruise.

Nutrient samples were analyzed according to the GO-SHIP Repeat Hydrography Manual 75 using certified reference materials produced by KANSO TECHNOS Co., Ltd. (http://www.kanso.co.jp/eng/production/index.html). Precisions were expressed as the coefficient of variation (CV), i.e., 0.13% for nitrate, 0.18% for nitrite, 0.14% for phosphate, 0.11% for silicate, and 0.29% for ammonium.

The total alkalinity of the samples was measured following a protocol developed using a spectrophotometric system 76. Its values were calibrated against certified reference materials from Dr. Dickson of the Scripps Institute of Oceanography. The precision of the total alkalinity measurement was 1.1 μmol kg−1.

Seawater DIC was determined using a coulometer (Model 3000; Nihon ANS Inc., Tokyo, Japan). Data were corrected using the same certified reference materials for the total alkalinity measurements. The precision of DIC was 0.63 μmol kg−1.

Ship-based observations before 2020
The R/V Mirai data utilized for this study were acquired in 2002, 2004, 2008, 2009, 2010, and 2012–2020. We also used the data obtained from CCGS Louis S. St-Laurent under a collaborative framework of the BG Exploration Project that started in 2003 and has been performed yearly since then. These two datasets allow us to study interannual changes in the BG and its impact on the marine environment of the western Arctic. This study also included data from the Chukchi Borderland Project 77 and International Siberian Shelf Study 78 conducted by the United States Coast Guard Cutter Polar Star (USA) in 2002 and Yacob Smirniskyi (Russia) in 2008. Together with the R/V Araon data, this information sheds light on the dynamics of water spreading in the intersectional water region north of the ESS and the Chukchi Sea. The above data from 2002 to 2020 were compared with the 1994 Arctic Ocean Section implemented by the CCGS Louis S. St-Laurent 52. All data utilized in this study, observation periods, and data sites are summarized in Supplementary Table 1.

As the data site of R/V Mirai in Supplementary Table 1 is temporarily closed because of a security incident, we provide the data as Supplementary Data 1.

Historical datasets from the 1950s to the 1980s
Historical datasets of temperature and salinity were compiled in the Environmental Working Group Joint US–Russian Atlas of the Arctic Ocean 27. Temperature and salinity data were collected from Russian and western drifting stations, icebreakers, and airborne expeditions between 1948 and 1993. The products of the datasets are gridded mean fields for decadal periods (the 1950s, 1960s, 1970s, and 1980s) and all periods from 1948 to 1993. This study used the product of the mean field from 1948 to 1993 (Nonetheless, the data were mainly obtained during the 1950s to 1980s). The Atlas is acquired from the site linked below.
https://nsidc.org/data/g01961/versions/1

Historical datasets of dissolved oxygen and other chemical parameters (e.g., nutrients) were compiled in the Hydrochemical Atlas of the Arctic Ocean 26. Hydrochemical data were collected from multiple cruises between 1948 and 2000. Herein, we used data from 1948 to 1993, i.e., the same period as the temperature and salinity data acquisition described above (Note that the hydrochemical data we used were concentrated in a period between 1960 to 1990). The temperature and salinity of the hydrochemical data (non-gridded data) were interpolated from the gridded mean field of the Environmental Working Group Joint US–Russian Atlas of the Arctic Ocean 27. The Hydrochemical Atlas is acquired from the site linked below.
https://doi.org/10.1594/PANGAEA.691332

Data gridding
Spatial maps of dynamic height and oxygen saturation (on a 0.5° × 2.5° latitude–longitude grid) and vertical sections of temperature and salinity (along a band of 75–76° N with a 1.0° longitude grid) in Fig. 5 were constructed using an Optimal Interpolation (OI) method 79 for the following three periods: the 1950s–1980s, 2008–2016, and 2017–2020. For gridding of the 1950s–1980s, we processed the data from the Environmental Working Group Joint US–Russian Atlas of the Arctic Ocean 27 and the Hydrochemical Atlas of the Arctic Ocean 26. The data from the ship-based observations listed in Supplementary Table 1 were used to create the gridded datasets for 2008–2016 and 2017–2020. Values were calculated at gridded points from averages weighted by the distance from each observation point. The weights were assumed to be proportional to the covariability of data. Based on the available data, they could be approximately represented by a Gaussian function with a length scale of 150 km. The interpolation uncertainty in each gridded point was derived from the standard deviation, covariability, and weights according to least squares procedures 79. The estimated uncertainties for the parameters shown in Fig. 5 are displayed in Supplementary Fig. 3.

Model experiment
A tracer experiment was conducted to visualize water transport from the shelf-slope north of the ESS using the Center for Climate System Research Ocean Component Model (COCO) version 4.9 80. The pan-Arctic regional COCO model used in this analysis has reasonably reproduced major fields of ocean circulation in the western Arctic 81,82. The model configuration, experimental design, and corresponding physical hydrographic/circulation results are the same as in the previous experiment 82. The horizontal resolution is approximately 5 km, enough to represent detailed ocean features such as eddies and local upwellings 83,84. Atmospheric forcing components were constructed from the Climate Forecast System Reanalysis version 2 (CFSv2) 6-hourly dataset of the National Centers for Environmental Prediction (NCEP) 85. A virtual passive tracer with a concentration value of 1 (assuming the lowest oxygen saturation water) was continuously given in a water column from 100 m to the seafloor within 100–200 m isobaths and 170° E–170° W throughout the experiment period for 2013–2020. Advection and diffusion of the tracer were calculated with ocean temperature and salinity. The tracer values in the entire model domain were reset to zero once on January 1, 2017, to compare tracer distributions between the following two periods: 2013–2016 and 2017–2020 (4 years each).

Data availability
Data related to this paper are publicly available and can be downloaded from the following data sites (see also Methods and Supplementary information). R/V Araon data from https://kpdc.kopri.re.kr/search/694ee19e-1c0b-4a44-8fd6-e83c94992731; R/V Mirai data from http://www.godac.jamstec.go.jp/darwin/e or Supplementary Data 1; BGEP data from https://www2.whoi.edu/site/beaufortgyre/data/ctd-and-geochemistry/; AOS 1994 data from https://cchdo.ucsd.edu/cruise/18SN19940724; CBL 2002 data from http://psc.apl.washington.edu/HLD/CBL/CBL.html; ISSS 2008 data from https://cchdo.ucsd.edu/cruise/90JS20080815; Environmental Working Group Joint US–Russian Atlas of the Arctic Ocean 27 from https://nsidc.org/data/g01961/versions/1; Hydrochemical Atlas of the Arctic Ocean 26 from https://doi.org/10.1594/PANGAEA.691332. Source data are provided with this paper.

Code availability
The code of the numerical experiment is the same as that used in the previous study 82, which is based on the Center for Climate System Research Ocean Component Model (COCO) 80 found at https://ccsr.aori.u-tokyo.ac.jp/~hasumi/COCO/. The original code will be available upon request, and necessary information can be provided through the above website or the corresponding author.

Peer review rounds
[{"round": 1, "reviewer_comments": {"reviewer_1": "During the revision the authors removed the wrong statement about the impact of the low oxygen saturation water over the Chukchi Plateau on the large area of the Canada Basin, and now focus mainly on the phenomenon over the Chukchi Plateau itself. I find that the paper is convincing now.\nI have one major comment on the explanation for the penetration of low oxygen saturation water from ESS shelf-slope to Chukchi Plateau. The authors attribute the penetration to the shrinkage of Beaufort Gyre and the Arctic Atlantification. This is also reflected in the paper title. The explanation is not fully correct. The shrinkage of the Beaufort Gyre is certainly one reason, but the Atlantification is not. The other main cause besides Beaufort Gyre shrinkage is the enhanced cyclonic circulation mode of the upper Arctic Ocean under positive Arctic Oscillation (possibly facilitated by cyclones intrusion as well). On average, the Arctic Oscillation was in a positive phase in the second half of the 2010s. Positive Arctic Oscillation releases surface freshwater (low salinity waster) from the Eurasian Basin and Makarov Basin, thus enhancing the cyclonic circulation mode (Morison2021, Wang2022). This can enhance eastward penetration of halocline water and Atlantic Water, and the low oxygen saturation water originally over the ESS shelf-slope as well. Dynamically, it is the shrinkage of the anticyclonic Beaufort Gyre and the expansion of the cyclonic circulation together that caused the phenomenon described in this paper. The propagation of the signal of Atlantification toward the Canada Basin could also be strengthened by the stronger cyclonic circulation mode, but the Atlantification itself is not the dynamic cause of the penetration of low oxygen saturation water to the Chukchi Plateau. Certainly, both the shrinkage of the Beaufort Gyre and the enhancement of the cyclonic circulation mode are intensified by Arctic sea ice decline (Wang2022).\nThe shift of the boundary between Pacific derived and Atlantic derived waters towards the Mendeleyev Ridge occurred in the 1990s following the strongly positive Arctic Oscillation (the strongest on average for the past six decades). Hydrography observations have shown this (Carmack1995, Morison1998, Steele1998). In a period with weak Beaufort Gyre, the penetration of the boundary to the Canada Basin under positive Arctic Oscillation is even stronger (Wang2021). It is worth mentioning the extreme event in the 1990s in the Discussion section.\nIt is required to polish the English writing.\nLit:\nMorison, J., Kwok, R., Dickinson, S., Andersen, R., Peralta-Ferriz, C., Morison, D., et al. (2021). The Cyclonic Mode of Arctic Ocean Circulation. J. Phys. Oceanog. 51, 1053-1075. doi: 10.1175/JPO-D-20-0190.1\nWang Q and Danilov S (2022) A Synthesis of the Upper Arctic Ocean Circulation During 2000-2019: Understanding the Roles of Wind Forcing and Sea Ice Decline. Front. Mar. Sci. 9:863204. doi: 10.3389/fmars.2022.863204\nCarmack, E. C., Macdonald, R., Perkin, R. G., McLaughlin, F. A., and Pearson, R. J. (1995). Evidence for Warming of Atlantic Water in the Southern Canadian Basin of the Arctic Ocean: Results from the Larsen-93 Expedition. Geophys. Res. Lett. 22, 1061–1064. doi: 10.1029/95GL00808\nMorison, J., Steele, M., and Andersen, R. (1998). Hydrography of the Upper Arctic Ocean Measured from the Nuclear Submarine U.S.S. Pargo. Deep-Sea. Res. I 45, 15–38. doi: 10.1016/S0967-0637(97)00025-3\nSteele, M., and Boyd, T. (1998). Retreat of the Cold Halocline Layer in the Arctic Ocean. J. Geophys. Res. - Ocean. 103, 10419–10435. doi: 10.1029/98JC00580\nWang Q., Danilov S., Sidorenko D., Wang X. (2021). Circulation Pathways and Exports of Arctic River Runoff Influenced by Atmospheric Circulation Regimes. Front. Mar. Sci. 8, 1153. doi: 10.3389/fmars.2021.707593\nSome minor comments are given below.\nLines33-35. Sea ice decline could freshen some areas of the Arctic Ocean, but it could also increase upper ocean salinity in other parts of the Arctic Ocean due to its impact on ocean circulation (Wang2022). The current sentence neglects the second aspect and should be revised.\nL36-37. The knowledge about PW inflow has been updated in Woodgate2021. Please update your sentence. Importantly, it is wrong to use two individual years to express trend. A trend estimate for the observation period was provided by Woodgate2021.\nLit:\nR. A. Woodgate and C. Peralta-Ferriz, Warming and freshening of the Pacific inflow to the Arctic from 1990-2019 implying dramatic shoaling in Pacific Winter water ventilation of the Arctic water column, Geophysical Research Letters, vol. 48, e2021GL092528, 2021. doi: https://doi.org/10.1029/2021GL092528\nL44, with - by\nL45, do you mean “salinification” of the halocline?\nL45, do you mean “weakening of the ocean stratification” in the halocline?\nL48, Wang2022 mentioned above well explains the different responses of the western and eastern Arctic to Arctic sea ice retreat.\nL50, in a - in the\nL51-56, the previous modelling study (Wang2021) details the response of the boundary between Pacific-Atlantic origin waters and the circulation of runoff water to different modes of Arctic atmospheric circulation, which explains observations mentioned in this paragraph.\nL62, was - were\nL63, - never been studied\nL62-63, Was this appearance of unusual water described before? These sentences leave an impression that this phenomenon has been described in past studies. In this case, please add citations. If not the case, then please revise these sentences.\nL97, “Pacific/Atlantic boundary region” is confusing. For example, it is clearer to say “… in the boundary region between Pacific and Atlantic derived waters”. In other places in the paper as well.\nL98 on a - on the\nL103, by - through\nL119, at the depths or between the depth ranges of ?\nL128, characterizing - resulting in\nL131, remove “at”\nL151, - only with this …\nL185-187, “The westward (eastward) flow of the southern (northern) branch of the gyre overshot (returned to) the southern (northern) Chukchi Plateau.” This sentence is hard to follow. Please revise.\nL187, shrink - shrinkage\nL188-189, “The extent of the Beaufort Gyre is governed by the strength of the Beaufort High, as described above.” This sentence is not fully correct. The extent of the Beaufort Gyre corresponds to the spatial distribution of freshwater (low salinity), which is largely determined by the spatial pattern of wind curl. Note that “wind curl” is different from the strength of Beaufort High. Or, „strength of Beaufort High“ is not clear: the spatial scale of wind curl or the magnitude of wind curl. Also see comment below.\nL189-193. This part of the text needs revision because the explanation is not fully correct.\nThe accumulation and release of freshwater in the Canada Basin is governed by wind-driven convergence and divergence anomalies, for which wind curl is a good indicator (Fig3B of Wang2022). The wind curl anomaly was not strongly negative before 2000 (Fig3A of Wang2022) so we do not expect a strong Beaufort Gyre for that period. The collapse of the Beaufort High caused a (southeast) shrinkage of the wind curl negative anomaly in the second half of the 2010s, which drives the shrinkage of the Beaufort Gyre.\nYou mentioned the impact of sea ice compactness in the past. Yes, sea ice can impact the ocean surface stress and thus ocean surface Ekman transport, but wind curl is a first order indicator for the evolution of the Beaufort Gyre. As shown by the blue line in Fig. 3A of Wang2022, we do not expect a stronger Beaufort Gyre averaged over the few decades before 2000. Therefore, in Fig. 1e, the forcing of the “BH” (the magnitude of wind curl) on the ocean should be weaker than in Fig. 1d. The difference in the BG between d and e is not only due to sea ice decline, but also due to the difference in wind curl.\nIn Fig. 1c, the recent shrinkage of the Beaufort Gyre and the penetration of LowOxy water onto CP is not only due to the collapse of BH, but also due to positive Arctic Oscillation, which strengthens the upper ocean cyclonic circulation (Morison2021, Wang2022). Both the impacts of Arctic Oscillation and shrinkage of BH on the ocean are intensified by sea ice decline (Wang2022).\nL199, remove “was”\nL207, L208, of salinity - with salinity\nL209, the influences on what?\nL212, characterizing - influencing\nL216, - decline\nL216-219, The Arctic Atlantification previously identified in the “eastern Eurasian Basin” can be largely explained by sea ice decline through its impacts on ocean surface stress and upper ocean circulation. See the explanation in Wang2022, and the argument therein that the contribution from salinity changes in the Barents Sea associated with sea ice decline is relatively small.\nL223 “cyclonic atmospheric circulation“, here it would be better to also refer to the positive phase of the Arctic Oscillation, which enhanced cyclonic upper ocean circulation in the second half of the 2010s.\nL223, - - change to “With the strengthening and expansion of the cyclonic circulation and the shrinkage of the anticyclonic circulation, the LHW and AW that carry signals of Arctic Atlantification could further flow …\"\nL246-247, As commented above, it is rather the concurrent expansion of the cyclonic circulation and shrinkage of the Beaufort Gyre that determines the fate of the water originally located on the ESS shelf-slope.\nL267, would not be - was not\nL272, - increase along the northward flow primarily …\nL281-282, change to “… in the northeastern Canada Basin.”\nL283, remove “be”\nL287, LHW passes Lomonosov Ridge to enter the Makarov Basin. Why do you mention “Mendeleev Ridge” here?\nL275-287, In Fig. 4g and 4h, the oxygen saturation in the interior of the Canada Basin is lower in g than in h. To what an extent the displayed difference in oxygen saturation is influenced by the choice of isopycnal layer for showing the plots? Could you show a vertical transect of oxygen saturation through the Canada Basin for the two periods separately? The oxygen saturation in the interior of the Canada Basin is influenced not only by horizontal advection, but also by vertical motion.\nL299, The shrinkage of the Beaufort Gyre took place over years. If you see that the western boundary of the Beaufort Gyre (northward flow) is located over the Chukchi Plateau in 2017, it means that the process of the shrinkage had started before 2017 already. Arctic Oscillation was positive in 2015, which can contribute to the release of freshwater from the Makarov Basin and Chukchi Plateau, thus the eastward shrinkage of the western boundary of the Beaufort Gyre.\nL300, Fig5b shows some seasonality.\nL316, Does the model reproduce the observed changes in the strength and spatial extent of the Beaufort Gyre shown in Fig.4?\nL317-319, The concurrent shrinkage of the Beaufort Gyre and the strengthening of the cyclonic circulation facilitate the transport of the low oxygen saturation water from the ESS shelf-slope to the Chukchi Plateau. I do not see dynamic impacts of Atlantification on the transport of this water.\nL333-338, check the grammar of this long sentence.\nL360-361, - resulting in quicker and wider formation of low oxygen saturation water\nL394 – scales\nL406 “strong ventilation in the winter and higher alkalinity of AW also…“ Check grammar.", "reviewer_3": "I believe this draft of the manuscript is appropriate for prompt publication in Nature Communications. The organization is clear; the findings are supported; sufficient context and discussion are provided to aid a wide readership in understanding the conclusions and implications; the reporting of previously unobserved biogeochemical anomalies in response to changing Arctic circulation are noteworthy and a plausible mechanism is identified and supported with observational and model analyses. The authors made defensible revisions in response to my prior review, including presenting new numerical model analyses in support of their hypothesis. I've provided several minor comments below for the authors' further consideration, if they choose.\nAbs,L30. I suggest instead \"This flow likely transports the low oxygen and acidified water towards the fishable area; similar biogeochemical properties had previously been observed only on the shelf and slope of the East Siberian Sea.\"\nInt,L45. Perhaps \"upwelling\" instead of \"uplifts.\"\nInt, L63. I suggest deleting this sentence, and instead adding to the end of the previous sentence:\n\"...boundary region for the first time.\" Or in the next paragraph, you could write \"Here, we describe these biogeochemically anomalous conditions for the first time, and propose a mechanism driving these changes by combining data...\".\nInt,L66-68. I think it would be more accurate to say that describing the presence and drivers of these biogeochemical anomalies is a vital preliminary step--we can't assess ecosystem responses until we recognize the hydrographic changes underpinning them.\nRes,L127. See also comment regarding S1,L38. Transmissometry could provide a snapshot of suspended particulate matter, but I don't think it provides enough evidence to rule out episodic particle flux events or other contributing factors to water column and sediment oxygen demand. So I personally think it would be more accurate to say that there is not evidence of high levels suspended sediments and particles that might enhance local oxygen uptake, unlike the conditions observed in the ESS. Thus you expect that biogeochemical anomalies transported from elsewhere are an important factor in the observed low oxygen conditions. That makes the same point without claiming that you can rule out local changes in oxygen demand, which would be hard to prove. Your discussion around Line 352 seems to relate to this assertion as well, so it might be worth bringing up earlier.\nRes,L185-187. I find this sentence to be complicated to understand with all the parentheticals.\nWould you please rewrite this as two sentences, one for each branch of the flow?\nFig. 5 and associated discussion, ~L316. It is also not unusual for regional ocean models to have biases in their vertical structure (poor characterization of onshore shoaling of isopcynal layers) due to either model and bathymetry resolution or issues with boundary forcings. You noted in particular that the Beaufort High collapsed in 2017--so my expectation is that the underlying pycnocline should have shoaled, as observed. If either that pressure or changing ocean boundary conditions with increased Atlantic water inputs were not accounted for in the model forcings, I imagine that could result in the similar isopycnal structure you see in the two model periods compared to the offset observed in observations. If you check and this is what happened, I suggest revising the text to acknowledge this as a model limitation that helps explain the discrepancy you noted on Line 316.\nDis,L337. \"which [had previously] only ever appeared\"\nOverall I think the discussion of potential ecological impacts is more compelling in this draft. I have two suggestions to consider:\nDis,L374. Pacific cod and Greenland cod were recently suggested to be the same species based on genetic evidence. There are few Arctic observations attributed to either\n(https://obis.org/taxon/254538) but any general population connectivity would appear to span the Chukchi and Beaufort Seas. I believe the Pacific cod fishery is roughly an order of magnitude larger than either species you mention here, and consequently could be of greater commercial importance in a warming Arctic (just as you mention Atlantic cod might become). Thus oxygen loss that could \"only\" happen in this region could still be be ecologically important to the broader Arctic and subarctic ecosystem.\nDis, Line 380. Environmental hypoxia is often designated as a threshold--and that value of ~60 umol kg-1 is heavily tilted towards warm water systems, particularly the Gulf of Mexico. Cold water fish tend to have higher hypoxia thresholds (https://www.nature.com/articles/s41586-020-2721-y), implying ecological consequences could occur at higher O2. Using the hypoxia traits in the supplemental table of this reference for Greenland cod, this species might be expected to experience hypoxia at 75%-85% of O2sat (at 0C). In other words, the recent anomalously low O2 might be expected to limit connectivity between the Greenland and Pacific populations (if they really are the same species). If you choose, you could use this or hypoxia thresholds for other coldwater species to demonstrate that current low O2 levels already have the potential to cause ecological impacts compared to prior conditions.\nS1,L38. I think that transmissometry does not say much about prior carbon export events or diffusive sediment oxygen fluxes. So I think the phrase \"could not\" expresses too much certainty.\nS2,L60. Throughout, I suggest \"gridded\" instead of \"grid\" datasets. Also, in general I suggest \"uncertainties\" rather than \"errors,\" because it sounds like each of these variances are driven by real environmental phenomenon rather than methodological problems."}, "author_response": "Responses to comments of Reviewer #1 on “Beaufort Gyre shrinkage and Atlantification induced an anomalous biogeochemical event in the western Arctic Ocean” by S. Nishino, J. Jung, K.-H. Cho, W. J. Williams, A. Fujiwara, A. Murata, M. Itoh, E. Watanabe, M. Aoyama, M. Yamamoto-Kawai, T. Kikuchi, E. J. Yang, and S.-H. Kang (Paper # NCOMMS-22-30054A).\nWe would like to express our gratitude once again for valuable comments from the reviewer, which improved our paper significantly. We have revised the manuscript in line with suggestions from the reviewer. Our respective responses are shown below.\nReviewer #1 (Remarks to the Author):\nDuring the revision the authors removed the wrong statement about the impact of the low oxygen saturation water over the Chukchi Plateau on the large area of the Canada Basin, and now focus mainly on the phenomenon over the Chukchi Plateau itself. I find that the paper is convincing now.\nI have one major comment on the explanation for the penetration of low oxygen saturation water from ESS shelf- slope to Chukchi Plateau. The authors attribute the penetration to the shrinkage of Beaufort Gyre and the Arctic Atlantification. This is also reflected in the paper title. The explanation is not fully correct. The shrinkage of the Beaufort Gyre is certainly one reason, but the Atlantification is not. The other main cause besides Beaufort Gyre shrinkage is the enhanced cyclonic circulation mode of the upper Arctic Ocean under positive Arctic Oscillation (possibly facilitated by cyclones intrusion as well). On average, the Arctic Oscillation was in a positive phase in the second half of the 2010s. Positive Arctic Oscillation releases surface freshwater (low salinity waster) from the Eurasian Basin and Makarov Basin, thus enhancing the cyclonic circulation mode (Morison2021, Wang2022). This can enhance eastward penetration of halocline water and Atlantic Water, and the low oxygen saturation water originally over the ESS shelf-slope as well. Dynamically, it is the shrinkage of the anticyclonic Beaufort Gyre and the expansion of the cyclonic circulation together that caused the phenomenon described in this paper. The propagation of the signal of Atlantification toward the Canada Basin could also be strengthened by the stronger cyclonic circulation mode, but the Atlantification itself is not the dynamic cause of the penetration of low oxygen saturation water to the Chukchi Plateau. Certainly, both the shrinkage of the Beaufort Gyre and the enhancement of the cyclonic circulation mode are intensified by Arctic sea ice decline (Wang2022).\nResponse:\nWe deleted the word “Atlantification” from the title. We largely modified the sections of Introduction and Results to include the viewpoint of Arctic Oscillation, a positive phase of which can cause the enhanced cyclonic circulation mode of the upper Arctic Ocean. In the Discussion section, we mentioned shortly the Atlantification whose signal could be carried by the cyclonic circulation.\nThe shift of the boundary between Pacific derived and Atlantic derived waters towards the Mendeleyev Ridge occurred in the 1990s following the strongly positive Arctic Oscillation (the strongest on average for the past six decades). Hydrography observations have shown this (Carmack1995, Morison1998, Steele1998). In a period with weak Beaufort Gyre, the penetration of the boundary to the Canada Basin under positive Arctic Oscillation is even stronger (Wang2021). It is worth mentioning the extreme event in the 1990s in the Discussion section.\nResponse:\nIn the Discussion section, we described the oxygen saturation distribution obtained from the 1994 Arctic Ocean Section (lines 370-381, see also Supplementary Fig. 7). This observation was conducted under a condition that the shift of the boundary between the Pacific and Atlantic waters toward the Mendeleyev Ridge occurred in the 1990s following the strongly positive Arctic Oscillation.\nIt is required to polish the English writing.\nResponse:\nWe requested native speakers of English to proofread our English writing.\nSome minor comments are given below.\nLines33-35. Sea ice decline could freshen some areas of the Arctic Ocean, but it could also increase upper ocean salinity in other parts of the Arctic Ocean due to its impact on ocean circulation (Wang2022). The current sentence neglects the second aspect and should be revised.\nResponse:\nWe revised the sentence as follows (see lines 33-35).\n“As the sea ice decline, the response of the upper ocean circulation to winds can be enhanced and the freshwater in the eastern Arctic basins is more likely to shift toward the Beaufort Gyre region in the western Arctic (Wang and Danilov, 2022; Wang et al., 2019).”\nL36-37. The knowledge about PW inflow has been updated in Woodgate2021. Please update your sentence. Importantly, it is wrong to use two individual years to express trend. A trend estimate for the observation period was provided by Woodgate2021.\nResponse:\nWe revised the sentence as follows (see lines 36-38).\n“Bering Strait mooring observations from 1990 to 2019 show increasing northward flow of Pacific water (PW), spring/fall warming, and winter freshening (Woodgate and Peralta-Ferriz, 2021).”\nL44, with - by\nResponse: Corrected (see line 46).\nL45, do you mean “salinification” of the halocline?\nL45, do you mean “weakening of the ocean stratification” in the halocline?\nResponse:\nWe revised the sentence as follows (see lines 46-49).\n“Atlantification accompanies the salinification of halocline above the AW layer, weakening of the halocline stratification, shoaling of the AW layer with higher nutrients, and a possible increase in phytoplankton biomass by the vertical nutrient supply (Polyakov et al., 2020) and the phytoplankton transport by AW (Oziel et. Al., 2020).”\nL48, Wang2022 mentioned above well explains the different responses of the western and eastern Arctic to Arctic sea ice retreat.\nResponse:\nWe added Wang and Danilov (2022) to a reference that studied the different responses of the western and eastern Arctic to Arctic sea ice retreat.\nL50, in a - in the\nResponse:\nCorrected. But we have changed the phrase “in the boundary region between the PW and AW influences” to “in the region toward the north of ESS and the western Chukchi Sea including the Chukchi Plateau where PW, AW, and ESSW complicatedly intersect (hereafter referred to as the intersectional water region)” to be more accurate. Please see lines 69-71.\nL51-56, the previous modelling study (Wang2021) details the response of the boundary between Pacific-Atlantic origin waters and the circulation of runoff water to different modes of Arctic atmospheric circulation, which explains observations mentioned in this paragraph.\nResponse:\nThis paragraph was largely modified to start with the description of the study by Wang et al. (2021). Please see lines 52-71.\nL62, was - were\nResponse: Corrected (see line 75).\nL63, - never been studied\nResponse:\nWe have changed the phrase “never studied before” to “for the first time” to be more accurate. Please see lines 74-75.\nL62-63, Was this appearance of unusual water described before? These sentences leave an impression that this phenomenon has been described in past studies. In this case, please add citations. If not the case, then please revise these sentences.\nResponse:\nWe revised the sentence as follows (see lines 74-75).\n“During the survey, unusually low dissolved oxygen and acidified water were observed on the Chukchi Plateau, an ocean ridge north of the western Chukchi Sea, for the first time.”\nL97, “Pacific/Atlantic boundary region” is confusing. For example, it is clearer to say “… in the boundary region between Pacific and Atlantic derived waters”. In other places in the paper as well.\nResponse:\nHere, we simply changed the phrase “Pacific/Atlantic boundary region” to “Chukchi Plateau”. Elsewhere, as noted above, we have changed the phrase “the boundary region between the PW and AW influences” to “the intersectional water region”. Please see lines 69-71.\nL98 on a - on the\nResponse: Corrected (see line 111).\nL103, by - through\nResponse: Corrected (see lines 115 and 132).\nL119, at the depths or between the depth ranges of ?\nResponse:\nWe have changed the phrase “at the depths of 100 and 300 m” to “between the depth range of 100 and 300 m”. Please see line 131.\nL128, characterizing - resulting in\nResponse:\nWe have changed the phrase “the organic matter decomposition characterizing the anomalous water” to “high-level suspended sediments and particles that might promote local microbial oxygen consumption and CO production”. Please see lines 137-138.\nResponse: Corrected (see line 141).\nL151, - only with this …\nResponse: Corrected (see lines 159-160).\nL185-187, “The westward (eastward) flow of the southern (northern) branch of the gyre overshot (returned to) the southern (northern) Chukchi Plateau.” This sentence is hard to follow. Please revise.\nResponse:\nWe revised the sentence as follows (see lines 207-208).\n“In 2008–2016 (Fig. 4b), the westward flow of the southern part of the Beaufort Gyre overshot the southern Chukchi Plateau (CP), and the return flow passed across the northern CP.”\nL187, shrink - shrinkage\nResponse: Corrected (see line 209).\nL188-189, “The extent of the Beaufort Gyre is governed by the strength of the Beaufort High, as described above.” This sentence is not fully correct. The extent of the Beaufort Gyre corresponds to the spatial distribution of freshwater (low salinity), which is largely determined by the spatial pattern of wind curl. Note that “wind curl” is different from the strength of Beaufort High. Or, „strength of Beaufort High“ is not clear: the spatial scale of wind curl or the magnitude of wind curl. Also see comment below.\nResponse:\nWe revised the sentence as follows (see lines 177-178).\n“The Beaufort High induces a negative curl of winds that converges the surface freshwater within the Beaufort Gyre (BG), and the spatial distribution of freshwater determines the extent of the BG.”\nL189-193. This part of the text needs revision because the explanation is not fully correct.\nThe accumulation and release of freshwater in the Canada Basin is governed by wind-driven convergence and divergence anomalies, for which wind curl is a good indicator (Fig3B of Wang2022). The wind curl anomaly was not strongly negative before 2000 (Fig3A of Wang2022) so we do not expect a strong Beaufort Gyre for that period. The collapse of the Beaufort High caused a (southeast) shrinkage of the wind curl negative anomaly in the second half of the 2010s, which drives the shrinkage of the Beaufort Gyre.\nResponse:\nWe revised the sentences as follows (see lines 213-217).\n“In the 1950s–1980s (Fig. 4c), the Beaufort Gyre circulation was weak because the wind curl over the Canada Basin was not strongly negative in this period (Wang, and Danilov, 2022). A heavy and less mobile sea ice condition in this period might also inhibit the input of wind curl to the ocean (Yang, 2009). Because the Arctic Oscillation was a negative phase in this period, the cyclonic ocean circulation would not expand to the study area.”\nRegarding the collapse of the Beaufort High, we mentioned this point in the Discussion section. Please see lines 364-369.\nYou mentioned the impact of sea ice compactness in the past. Yes, sea ice can impact the ocean surface stress and thus ocean surface Ekman transport, but wind curl is a first order indicator for the evolution of the Beaufort Gyre. As shown by the blue line in Fig. 3A of Wang2022, we do not expect a stronger Beaufort Gyre averaged over the few decades before 2000. Therefore, in Fig. 1e, the forcing of the “BH” (the magnitude of wind curl) on the ocean should be weaker than in Fig. 1d. The difference in the BG between d and e is not only due to sea ice decline, but also due to the difference in wind curl.\nResponse:\nWe revised Fig. 1e so that the forcing of the Beaufort High (the magnitude of wind curl) on the ocean was weaker than in Fig. 1d. Also, an arrow representing the status of the Arctic Oscillation was added to the top of Fig. 1c-e.\nIn Fig. 1c, the recent shrinkage of the Beaufort Gyre and the penetration of LowOxy water onto CP is not only due to the collapse of BH, but also due to positive Arctic Oscillation, which strengthens the upper ocean cyclonic circulation (Morison2021, Wang2022). Both the impacts of Arctic Oscillation and shrinkage of BH on the ocean are intensified by sea ice decline (Wang2022).\nResponse:\nAs mentioned above, an arrow representing the status of the Arctic Oscillation (AO) was added to the top of Fig. 1c-e. In Fig. 1c, the arrow indicates a positive AO.\nIn the first paragraph of the Discussion section, we described that the Arctic sea ice decline intensified both the Beaufort Gyre and cyclonic ocean circulation (Wang and Danilov, 2022).\nL199, remove “was”\nResponse: Corrected (see line 221).\nL207, L208, of salinity - with salinity\nResponse: Corrected (see lines 237 and 239).\nL209, the influences on what?\nResponse:\nWe used the term “intrusions” instead of “influences” (see line 240).\nL212, characterizing - influencing\nResponse:\nThe sentence including this word was deleted because it contained a description of the Atlantification.\nL216, - decline\nResponse:\nThe sentence including this word was deleted because it contained a description of the Atlantification.\nL216-219, The Arctic Atlantification previously identified in the “eastern Eurasian Basin” can be largely explained by sea ice decline through its impacts on ocean surface stress and upper ocean circulation. See the explanation in Wang2022, and the argument therein that the contribution from salinity changes in the Barents Sea associated with sea ice decline is relatively small.\nResponse:\nIn association with the sea ice decline, the shift of freshwater from the cyclonic ocean circulation region to the Beaufort Gyre region develops Atlantification in the eastern Arctic (Wang and Danilov, 2022). This point was described in lines 358-360 in the Discussion section. And the description on the Barents Sea was removed.\nL223 “cyclonic atmospheric circulation“, here it would be better to also refer to the positive phase of the Arctic Oscillation, which enhanced cyclonic upper ocean circulation in the second half of the 2010s.\nResponse:\nWe revised the sentence as follows (see lines 229-231).\n“The enlarged LHW and AW intrusions were presumably associated with the intensification of AW supply induced by the sea ice decline (Atlantification; Wang et al., 2020) and the positive phase of the Arctic Oscillation, which enhanced the cyclonic ocean circulation, in the second half of the 2010s.”\nL223, - - change to “With the strengthening and expansion of the cyclonic circulation and the shrinkage of the anticyclonic circulation, the LHW and AW that carry signals of Arctic Atlantification could further flow …”\nResponse:\nWe revised the sentence as follows (see lines 231-234).\n“With the strengthening and expansion of the cyclonic ocean circulation and the shrinkage of the anticyclonic Beaufort Gyre circulation, the LHW and AW could further flow …”\nL246-247, As commented above, it is rather the concurrent expansion of the cyclonic circulation and shrinkage of the Beaufort Gyre that determines the fate of the water originally located on the ESS shelf-slope.\nResponse:\nWe revised the sentence as follows (see lines 256-257).\n“The concurrent expansion of the cyclonic ocean circulation and shrinkage of the Beaufort Gyre would determine the fate of the water originally located on the ESS shelf-slope.”\nL267, would not be - was not\nResponse: Corrected (see line 276).\nL272, - increase along the northward flow primarily …\nResponse: Corrected (see line 281).\nL281-282, change to “… in the northeastern Canada Basin.”\nResponse: Corrected (see line 289).\nL283, remove “be”\nResponse: Corrected (see line 292).\nL287, LHW passes Lomonosov Ridge to enter the Makarov Basin. Why do you mention “Mendeleev Ridge” here?\nResponse:\nWe revised the sentence as follows not to mention the Mendeleev Ridge (see lines 300-302).\n“This shift would result from the penetration of LHW with a high oxygen saturation to the Makarov Basin by the expanded cyclonic ocean circulation in 2017–2020 under the positive Arctic Oscillation.”\nL275-287, In Fig. 4g and 4h, the oxygen saturation in the interior of the Canada Basin is lower in g than in h. To what an extent the displayed difference in oxygen saturation is influenced by the choice of isopycnal layer for showing the plots? Could you show a vertical transect of oxygen saturation through the Canada Basin for the two periods separately? The oxygen saturation in the interior of the Canada Basin is influenced not only by horizontal advection, but also by vertical motion.\nResponse:\nThe lower oxygen saturation levels in the Canada Basin in 2017–2020 than in 2008–2016 were observed in the whole LHW (salinity = 34–34.5) layer (see lines 293-295). We added vertical sections of oxygen saturation in 2017–2020 and 2008–2016 as Supplementary Fig. 5a, b. We also added the description on the vertical mixing of oxygen saturation using a conservative tracer, NO (Supplementary Fig. 5c, d). Please see lines 295-299 and Supplementary Discussion 3.\nL299, The shrinkage of the Beaufort Gyre took place over years. If you see that the western boundary of the Beaufort Gyre (northward flow) is located over the Chukchi Plateau in 2017, it means that the process of the shrinkage had started before 2017 already. Arctic Oscillation was positive in 2015, which can contribute to the release of freshwater from the Makarov Basin and Chukchi Plateau, thus the eastward shrinkage of the western boundary of the Beaufort Gyre.\nResponse:\nWe added the description of delayed oceanic response to surface forcing as follows (see lines 208-213).\n“The shrinkage of the Beaufort Gyre (BG) in 2017–2020 would be a response of ocean circulation to the change in the Arctic Oscillation (AO) phase in the mid-2010s. The time scale of the delayed oceanic response to surface forcing (a few years) is consistent with the previous studies (Yoshizawa et al., 2015). That is, a positive AO in the second half of the 2010s caused a southeast shrinkage of the negative anomaly area of wind curl over the Canada Basin (Wang and Danilov, 2022), which drove the shrinkage of the BG with a delay of several years from the change in the AO phase.”\nPlease also see lines 312-315 as follows.\n“Associated with this transport, the appearance of low oxygen saturation water passing through the Chukchi Plateau has become more pronounced since 2017 (Fig. 5b), which is consistent with the year when the observed Beaufort Gyre shrunk in 2017 after a several-year delay from the change in the Arctic Oscillation phase in the mid-2010s.”\nL300, Fig5b shows some seasonality.\nResponse:\nWe revised the sentence as follows (see lines 318-320).\n“The tracer concentration of the water seems to represent a seasonality with its increase in winter when winds are generally strong (Martin et al., 2014; Stopa et al., 2016).”\nL316, Does the model reproduce the observed changes in the strength and spatial extent of the Beaufort Gyre shown in Fig.4?\nResponse:\nWe added the simulated sea surface height as Supplementary Fig. 6 and described as follows (see lines 315-316).\n“Our numerical model also indicates the shrinking of the Beaufort Gyre in 2017–2020 (Supplementary Fig. 6a) compared to 2013–2016 (Supplementary Fig. 6b).”\nL317-319, The concurrent shrinkage of the Beaufort Gyre and the strengthening of the cyclonic circulation facilitate the transport of the low oxygen saturation water from the ESS shelf-slope to the Chukchi Plateau. I do not see dynamic impacts of Atlantification on the transport of this water.\nResponse:\nWe deleted this sentence concerning the Atlantification. Your proposed sentence above was added to the first sentence of the Discussion section (see lines 355-357).\nL333-338, check the grammar of this long sentence.\nResponse:\nThis sentence was deleted because it contained an incorrect explanation as you pointed out earlier in this review.\nL360-361, - resulting in quicker and wider formation of low oxygen saturation water\nResponse: Corrected (see lines 403-404).\nL394 – scales\nResponse: Corrected (see line 442).\nL406 “strong ventilation in the winter and higher alkalinity of AW also…“ Check grammar.\nResponse:\nWe revised the sentence as follows based on the native speakers’ proofread (see lines 453-454).\n“strong winter ventilation and AW's high alkalinity contribute to the sink of atmospheric CO in the AW gateway area.”\nAgain, we deeply appreciate valuable comments of the reviewer, which improved our paper significantly. We have revised the manuscript in line with suggestions from the reviewer.\nOur respective responses are shown below.\nReviewer #3 (Remarks to the Author):\nI believe this draft of the manuscript is appropriate for prompt publication in Nature Communications. The organization is clear; the findings are supported; sufficient context and discussion are provided to aid a wide readership in understanding the conclusions and implications; the reporting of previously unobserved biogeochemical anomalies in response to changing Arctic circulation are noteworthy and a plausible mechanism is identified and supported with observational and model analyses. The authors made defensible revisions in response to my prior review, including presenting new numerical model analyses in support of their hypothesis. I've provided several minor comments below for the authors' further consideration, if they choose.\nAbs,L30. I suggest instead \"This flow likely transports the low oxygen and acidified water towards the fishable area; similar biogeochemical properties had previously been observed only on the shelf and slope of the East Siberian Sea.\"\nResponse:\nWe revised the sentence as follows (see lines 29-31).\n“This flow likely transports the low oxygen and acidified water toward the fishable area; similar biogeochemical properties had previously been observed only on the shelf-slope north of the East Siberian Sea.”\nInt,L45. Perhaps \"upwelling\" instead of \"uplifts.\"\nResponse:\nThe uplifts of nutrient-rich water are associated with shoaling of the AW layer. Thus, we revised the sentence as follows (see line 48).\n“shoaling of the AW layer with higher nutrients”\n\"...boundary region for the first time.\" Or in the next paragraph, you could write \"Here, we describe these biogeochemically anomalous conditions for the first time, and propose a mechanism driving these changes by combining data...\".\nResponse:\nWe deleted the sentence and revised the rest of the text as follows (see lines 74-77).\n“During the survey, unusually low dissolved oxygen and acidified water were observed on the Chukchi Plateau, an ocean ridge north of the western Chukchi Sea, for the first time. Herein, we describe these biogeochemically abnormal conditions and propose a mechanism driving these changes (Fig. 1c–e) by combining data…”\nInt,L66-68. I think it would be more accurate to say that describing the presence and drivers of these biogeochemical anomalies is a vital preliminary step--we can't assess ecosystem responses until we recognize the hydrographic changes underpinning them.\nResponse:\nWe revised the sentence as follows (see lines 78-81).\n“Describing the presence and drivers of these biogeochemical anomalies is a vital preliminary step for Arctic ecosystem assessments (refs. 21,22); we can only assess ecosystem responses once we recognize the hydrographic changes underpinning them.”\nRes,L127. See also comment regarding S1,L38. Transmissometry could provide a snapshot of suspended particulate matter, but I don't think it provides enough evidence to rule out episodic particle flux events or other contributing factors to water column and sediment oxygen demand. So I personally think it would be more accurate to say that there is not evidence of high levels suspended sediments and particles that might enhance local oxygen uptake, unlike the conditions observed in the ESS. Thus you expect that biogeochemical anomalies transported from elsewhere are an important factor in the observed low oxygen conditions. That makes the same point without claiming that you can rule out local changes in oxygen demand, which would be hard to prove. Your discussion around Line 352 seems to relate to this assertion as well, so it might be worth bringing up earlier.\nResponse:\nWe deleted the description of transmissometry and its figure, and revised the rest of the text as follows (see lines 137-139). Here, we brought up the discussion on the organic mater deposition with references.\n“there is no evidence of high-level suspended sediments and particles that might promote local microbial oxygen consumption and CO production, unlike the conditions observed in the ESS where much of terrestrial/marine organic matter is deposited on the bottom (refs. 24,25).”\nWould you please rewrite this as two sentences, one for each branch of the flow?\nResponse:\nWe revised the sentence as follows (see lines 207-208).\n“In 2008–2016 (Fig. 4b), the westward flow of the southern part of the Beaufort Gyre overshot the southern Chukchi Plateau (CP), and the return flow passed across the northern CP.”\nFig. 5 and associated discussion, ~L316. It is also not unusual for regional ocean models to have biases in their vertical structure (poor characterization of onshore shoaling of isopcynal layers) due to either model and bathymetry resolution or issues with boundary forcings. You noted in particular that the Beaufort High collapsed in 2017--so my expectation is that the underlying pycnocline should have shoaled, as observed. If either that pressure or changing ocean boundary conditions with increased Atlantic water inputs were not accounted for in the model forcings, I imagine that could result in the similar isopycnal structure you see in the two model periods compared to the offset observed in observations. If you check and this is what happened, I suggest revising the text to acknowledge this as a model limitation that helps explain the discrepancy you noted on Line 316.\nResponse:\nWe added the description about conceivable reasons why the LHW penetration is not well reproduced in the model (see lines 335-339).\n“The simulated penetration of LHW from the eastern Arctic might be underestimated because of the closed lateral boundary condition in the North Atlantic and insufficient vertical resolution. In addition, the lack of interannual variability and multi-decadal trends in riverine freshwater discharges might produce an unexpected model bias of stratification in the eastern Arctic (isohaline-surface structure of the LHW).”\nDis,L337. \"which [had previously] only ever appeared\"\nResponse: Corrected (see lines 356-357).\nOverall I think the discussion of potential ecological impacts is more compelling in this draft. I have two suggestions to consider:\nDis,L374. Pacific cod and Greenland cod were recently suggested to be the same species based on genetic evidence. There are few Arctic observations attributed to either\n(https://obis.org/taxon/254538) but any general population connectivity would appear to span the Chukchi and Beaufort Seas. I believe the Pacific cod fishery is roughly an order of magnitude larger than either species you mention here, and consequently could be of greater commercial importance in a warming Arctic (just as you mention Atlantic cod might become). Thus oxygen loss that could \"only\" happen in this region could still be be ecologically important to the broader Arctic and subarctic ecosystem.\nResponse:\nAs you pointed out, Pacific cod and Greenland cod are the same species (Carr et al., 1999, doi:10.1139/cjz-77-1-19), but they are generally considered to be sedentary and nonmigratory species (Mikhail and Welch, 1989, doi:10.1007/BF00002475). In the Chukchi Sea, Pacific cod is expected to expand north and eastward if warming conditions continue, based on observational (ref. 60) and model (ref. 61) studies. However, such expansion might be inhibited by the anomalously low oxygen saturation water (less than the hypoxia threshold for the Pacific cod), which has already appeared on the Chukchi Plateau and is assumed to be enlarged in the future. This point is described in lines 428-431.\nDis, Line 380. Environmental hypoxia is often designated as a threshold--and that value of ~60 umol kg-1 is heavily tilted towards warm water systems, particularly the Gulf of Mexico. Cold water fish tend to have higher hypoxia thresholds (https://www.nature.com/articles/s41586-020-2721-y), implying ecological consequences could occur at higher O2. Using the hypoxia traits in the supplemental table of this reference for Greenland cod, this species might be expected to experience hypoxia at 75%-85% of O2sat (at 0C). In other words, the recent anomalously low O2 might be expected to limit connectivity between the Greenland and Pacific populations (if they really are the same species). If you choose, you could use this or hypoxia thresholds for other coldwater species to demonstrate that current low O2 levels already have the potential to cause ecological impacts compared to prior conditions.\nResponse:\nRegarding the hypoxia threshold for cold-water fishes, we added descriptions, including the abovementioned sentences, as follows (lines 425-431).\n“Cold-water fishes tend to have higher hypoxia thresholds (ref. 58). The Greenland cod, which distributes from Alaska to the western coast of Greenland, has a hypoxia threshold of ~40 % in oxygen saturation at 1 °C, and it would be higher oxygen saturation levels with ocean warming (ref. 59). The same species in the Chukchi Sea, i.e., the Pacific cod, is expected to expand north and eastward if warming conditions continue, based on observational (ref. 60) and model (ref. 61) studies. However, such expansion might be inhibited by the anomalously low oxygen saturation water, which has already appeared on the Chukchi Plateau and is assumed to be enlarged in the future.”\nS1,L38. I think that transmissometry does not say much about prior carbon export events or diffusive sediment oxygen fluxes. So I think the phrase \"could not\" expresses too much certainty.\nResponse:\nWe deleted the description of transmissometry and its figure as mentioned above. Here, we describe an assumption that the low oxygen saturation water is delivered from a remote region based on a conservative tracer, NO, as follows (see lines 38-42 in the Supplementary Information (SI)).\n“The low oxygen saturation corresponded with low NO (Supplementary Fig. 1b), where NO is defined as 9[NO −] + [O ] (µmol kg−1) and used as a quasi-conservative tracer that is 3 2 independent of biological processes (ref. S1). Thus, the low oxygen saturation and low NO water are assumed to be delivered from a remote region rather than formed by local microbial oxygen consumption.”\nS2,L60. Throughout, I suggest \"gridded\" instead of \"grid\" datasets. Also, in general I suggest \"uncertainties\" rather than \"errors,\" because it sounds like each of these variances are driven by real environmental phenomenon rather than methodological problems.\nResponse:\nWe changed the term “grid” to “gridded” throughout the main text and SI. Also, we changed the term “error” to “uncertainty”.\nWe thank the editor and reviewers very much for their kindness and generosity with their time.\nWe appreciate the time and effort that the editor and reviewer dedicated to providing feedback on our manuscript. We have incorporated most of the suggestions made by the reviewer. Our respective responses are shown below.\nReviewer #1 (Remarks to the Author):\nThank the authors for their further effort to improve the manuscript. The dynamic processes are well explained now. I have only a couple of very minor comments:\nLine 209-213\nThe shrinkage of the BG in 2017-2020 is associated with the collapse of the BH, that is, the area of the strong negative wind curl was confined to the southwest Canada Basin in this period.\nResponse:\nWe revised the sentences as follows (see lines 213-215).\n“The shrinkage of the BG in 2017–2020 is likely associated with the collapse of the BH in 2017. In this period, the area of the strong negative wind curl was confined to the southeastern CB.”\nLine 380-381\nShrinkage of the BG with strong northward flow along the CP is needed to transport the low oxygen saturation water toward the CP.\n“strengthening the BG” could also mean enlargement (westward extension) of the BG, which will not lead to the transport toward the CP.\nResponse:\nWe corrected the text as the reviewer indicated (see lines 382-384).\nThe authors need to proofread the text carefully in the next step, or the journal may check the language before publication?\nResponse:\nWe requested native speakers of English to proofread our English writing.\nIn some places the writing could be improved. Here just a few examples:\nL33 decline – declines\nResponse: Corrected (see line 33).\nL59 Arctic Oscillation (AO) was in a positive phase …\nResponse: Corrected (see lines 58-59).\nL75 “western” Chukchi Sea should be “eastern” Chukchi Sea?\nResponse: As the reviewer pointed out, “western Chukchi Sea” and “eastern Chukchi Sea” are ambiguous and confusing, so we simply termed “Chukchi Sea” (see line 70).\nL80 … we can assess ecosystem response only when we know hydrographic changes …\nResponse: Corrected (see line 80).\nL120, better add a “water” before “or”\nResponse: Corrected (see line 126).\nL192 … the AO was largely in a negative phase\nResponse: Corrected (see line 197).\nL364-367 “2017” is within the period of “after the mid-2010s”. Rewrite these two sentences to make the flow logic.\nResponse: We changed “after the mid-2010s” to “after 2017” (see line 369).\nL393-394, I cannot understand this sentence.\nResponse: We rewrote the sentence as follows (see lines 396-398).\n“In this period, the water with a higher oxygen saturation would be transported to the CP, and therefore, the lowest oxygen saturation water might no longer be observed on the CP.”"}]

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