The year, 2024, marks the 40th anniversary of Chinese research expeditions in the polar regions and the 25th anniversary of its Arctic research expeditions. China has conducted 14 national Arctic research expeditions. With the increase of understandings on the global impacts of the changes of Arctic climate system, especially on China’s weather and climate, and demands for commercial utilization of the Arctic sea routes, Chinese scientists have made great progresses on in site and remote sensing observation technologies for Arctic Ocean, interaction mechanisms between atmosphere, sea ice, and ocean, the connection mechanism between the Arctic Ocean and other regions, and have achieved a series of research results. This study summarizes the research achievements by Chinese scientists in the above-mentioned aspects or beyond, identifies knowledge gaps, and based on this, discusses prospects and provides suggestions. From a perspective of observation, improving the observation capabilities of the Arctic Ocean in winter and the ocean under the ice, as well as floe-scale processes of sea ice and mesoscale and submesoscale processes of the ocean, is an urgent task to be addressed. Strengthening international cooperation is necessary for building a monitoring network for the Arctic marine environment. From a perspective of numerical simulation, the descriptive ability and parameterization scheme of sub-grid processes based on observational evidence need to be developed. From a perspective of cross-sphere interactions, in addition to the multi-media coupling within the Arctic Ocean that this review focuses on, the interaction between the Arctic Ocean and land or ice sheet (Greenland), especially the water cycle process, is also a scientific domain that needs to be considered, in the context of Arctic warming and humidification. From a perspective of climate effects, the physical mechanisms that affect the robustness of teleconnection need to be clarified.

The Southern Ocean is a critical component in the Earth system by dominating the global heat and anthropogenic carbon uptake and supplying heat to melt the largest ice sheet. Variability and changes in the water masses of the Southern Ocean are thus important to the global energy and water cycles, carbon cycling, and sea-level change. In this article, we review the recent progress on understanding the variability and changes in the four major water masses in the Southern Ocean, including Subantarctic Mode Water, Antarctic Intermediate Water, Circumpolar Deep Water and Antarctic Bottom Water. Subantarctic Mode Water and Antarctic Intermediate Water show statistically significant strong circumpolar shoaling, warming, and density reductions since 1970s, indicating that signals of global warming have entered the interior ocean. Meanwhile, strong regional variability of Subantarctic Mode Water and Antarctic Intermediate Water responding to surface buoyancy forcing and westerly winds is attracting more attention. Circumpolar Deep Water is an important modulator of heat content and nutrient concentrations on continental shelves around Antarctica and has made significant contributions to the basal melting of Antarctic ice shelves. Since the late 1950s, a long-term freshening trend in Antarctic Bottom Water in the Ross Sea and its downstream region has been observed and is mainly attributed to the accelerated basal melting of ice shelves in West Antarctica. The shrinking of Antarctic Bottom Water in the Weddell Sea during 1992–2020 has also been revealed and is attributed to reduced sea ice production over the southern Weddell continental shelf related to the Interdecadal Pacific Oscillation and the variability in the Amundsen Sea Low. Though significant advances have been achieved, there is an urgent need to enhance and improve both observations and model performances for better understandings and projections of the formation, transformation, and transport of the water masses in the Southern Ocean.

A halocline in the Arctic Ocean significantly slows the upward heat flux from deep warm water, thereby inhibiting the melting of surface sea ice. The western Arctic Ocean exhibits a double-halocline (DH) structure due to the complexity of the water mass. Using in situ measurements, we analyzed the vertical structural characteristics of DH and its interannual variation. The results indicated that the DH primarily occurs at the Northwind Ridge and the southern Canada Basin, extending westward to the Chukchi Abyssal Plain and northward to the northern boundary of the Canada Basin. From 2002 to 2022, there were changes in water masses that determined the structure of the DH. The significant increase in Pacific Water has resulted in 42% and 65% increases in freshwater and the heat content of the DH, respectively, along with a 14% reduction in stratification. Pacific Winter Water characterized by salinity of 33 has exhibited a gradually decreasing trend, suggesting that the lower halocline may be difficult to ventilate. The combined effects of Ekman pumping, mesoscale eddies, and positive buoyancy forcing (heat and freshwater input) from Pacific Water have altered the thickness and stratification of the DH. This study has enhanced our understanding of the evolution of vertical heat flux in the upper western Arctic Ocean.

As the Arctic undergoes rapid warming and sea ice continues to decline, Ocean-to-Ice Heat Flux (OIHF) plays a crucial role in regulating sea ice dynamics. This study investigates the seasonal variations in ocean-to-ice heat flux in north of the Fram Strait using the regional Arctic Ocean/sea ice reanalysis product from 1991 to 2020. The analysis reveals that the OIHF exhibits significant seasonal variability, with a pronounced peak during winter in north of the Fram Strait, driven by inflows of Atlantic water. Warm Atlantic water intrusion begins in October, reaches its peak in January and February, and results in a delayed increase in OIHF, with maximum flux observed 2−3 months later. These results highlight the significant role of Atlantic water inflows in influencing Arctic sea ice dynamics and emphasize the need for further investigation into the coupled ocean-atmosphere processes that govern seasonal fluctuations in OIHF.

Loss of multiyear ice (MYI) is of great importance for Arctic climate and marine systems and can be monitored using active and passive microwave satellite data. In this paper, we describe an upgraded classification algorithm using the data from the scatterometer and radiometer sensors onboard the Chinese Haiyang-2B (HY-2B) satellite to identify MYI and first-year ice (FYI). The proposed method was established based on K-means and fuzzy clustering (K-means+FC) and was used to focus on the transition zone where the ice condition is complex due to the highly commixing of MYI and FYI, leading to the high challenge for accurate classification of sea ice. The K-means algorithm was applied to preliminarily classify MYI using the combination of scatterometer and radiometer data, followed by applying fuzzy clustering to reclassify MYI in the transition zone. The HY-2B K-means+FC results were compared with the ice type products [including the Ocean and Sea Ice Satellite Application Facility (OSI SAF) sea ice type product and the Equal-Area Scalable Earth-Grid sea ice age dataset], and showed agreement in the time series of MYI extent. Intercomparisons in the transition zone indicated that the HY-2B K-means+FC results can identify more old ice than the OSI SAF product, but with an underestimation in identifying second-year ice. Comparisons between K-means and K-means+FC results were performed using regional ice charts and Sentinel-1 synthetic aperture radar (SAR) data. By adding fuzzy clustering, the MYI is more consistent with the ice charts, with the overall accuracy (OA) increasing by 0.9%–6.5%. Comparing against SAR images, it is suggested that more scattered MYI floes can be identified by fuzzy clustering, and the OA is increased by about 3% in middle freezing season and 7%–20% in early and late freezing season.

The Antarctic Slope Current (ASC) links the processes occurring in the coast area with the global ocean by modulating the flow across the continental slope, which influences global overturning circulation and the mass balance of the Antarctic ice shelves. This paper aims to explore the effects of wind and buoyancy fluxes on the long-term changes in the ASC using observation and reanalysis datasets. From 1993 to 2022, the ASC accelerated in all seasons, particularly in austral autumn, and was accompanied with the advancement of the strong flow. The positive buoyancy flux anomaly generates a low-density anomaly, which is accumulated by the prevailing surface easterly, maintaining a sharp density front along the continental slope. The heat flux intensifies the positive trend of buoyancy flux in summer and autumn, increasing the input of the lighter density anomaly into the ocean and advancing the strong flow of ASC. Compared with the annual mean, the additional acceleration of the ASC in autumn is mainly due to the contribution of the barotropic component, which could be explained by the local momentum input from the weakly enhanced local surface wind. The acceleration of the ASC is primarily driven by the positive trend of the buoyancy flux, while the influence of the wind on its seasonal variability becomes more significant due to the contribution of the local surface wind in autumn.

We use a high-resolution numerical simulation to analyze the generation and evolution of semidiurnal internal tides in the Weddell-Scotia Confluence. Our results indicate that two ridges near the Antarctic Peninsula, the South Scotia Ridge (SSR) and the Philip Ridge (PR), are energetic sources of semidiurnal internal tides. The strongest semidiurnal barotropic to baroclinic energy conversion occurs around the crest of the SSR, reaching 0.3 W/m². The depth-integrated, tidally averaged semidiurnal internal tide energy fluxes that radiated from the SSR reach about 2 kW/m. The northward semidiurnal internal tide energy fluxes entering the Scotia Sea are stronger than the southward energy fluxes entering the Powell Basin. For the SSR region between the South Orkney Plateau and Elephant Island, the area-integrated barotropic to baroclinic conversion rate is 0.71 GW, of which 0.56 GW (79%) dissipates locally. The dissipation of internal tides occurs mainly in water depths of less than 1 000 m. The dissipation rate over the SSR is as high as 10⁻⁷ W/kg. The energy fluxes create an anticlockwise gyre between the SSR and PR. The horizontal kinetic energy and available potential energy oscillate sinusoidally with peaks occurring at quarter-wavelength intervals. This energy pattern suggests the formation of standing waves due to interference between the semidiurnal internal tides originating from the SSR and PR. The results of this study show that internal tides and related mixing need to be considered in the analysis of water mass transformation in the Southern Ocean.

The oceanic general circulations in the Ross Sea, where the southernmost ocean is located, play an important role in the climate system. Yet, the energy cycle of oceanic circulations in the Ross Sea is still unclear. By employing an eddy-permitting coupled regional ocean-sea ice-ice shelf model, this study investigates the oceanic energy cycle in the Ross Sea. Based on the Lorenz Energy Cycle framework, the spatiotemporal distributions of kinetic energy and available potential energy within the Ross Sea are quantitatively analyzed. The power pathways and magnitudes of energy conversion are also quantified. The simulated results show that the Mean Available Potential Energy (MAPE) is the largest energy reservoir of about 527.62 PJ (1 PJ = 10¹⁵ J), followed by the Eddy Available Potential Energy (EAPE), the Mean Kinetic Energy (MKE), and the Eddy Kinetic Energy (EKE) of about 19.20 PJ, 1.04 PJ, and 0.82 PJ, respectively. In the sub-ice-shelf cavity, the maximal MAPE is up to about 177.81 PJ, and the EAPE, MKE, and EKE are about 2.58 PJ, 39.87 TJ (1 TJ = 10¹² J), and 23.05 TJ, respectively. The inputs to the regional energy reservoirs are mainly from the sea surface momentum and buoyancy fluxes. The baroclinic pathway plays a dominant role in the conversion of energy to EKE, both in the open ocean and in the sub-ice-shelf cavity. The energy conversion from EAPE to EKE in the open ocean and the sub-ice-shelf cavity is about 2.86 GW (1 GW = 10⁹ J) and 162.18 MW(1 MW = 10⁶ J), respectively. In addition, the kinetic energy is directed from EKE to MKE in the Ross Sea, and such an energy flow in the barotropic pathway is opposite from that in the Southern Ocean.

A three-dimensional coupled sea ice-ice shelf-ocean numerical model is developed for the Prydz Bay, Antarctica, using the Regional Ocean Modeling System with a grid resolution of approximately 2 km. The influence of the grounding giant iceberg D15 on the distribution of sea ice and polynyas in the Prydz Bay is analyzed through two numerical experiments. Iceberg D15, grounded off the western edge of the West Ice Shelf (WIS), obstructs the southwestward transport of sea ice along the east coast of Prydz Bay, causing sea ice to accumulate to the east of the iceberg and form multi-year fast ice. Grounding of Iceberg D15 also decreases sea ice coverage off its south edge and creates ice-free openings in spring near Davis Station and Zhongshan Station, facilitating the accessibility of vessels to the research stations. These simulated sea ice patterns closely match current satellite observations. When Iceberg D15 is removed, the previously blocked sea ice north of the iceberg, which moved westward, shifts southwesterly along the coastline, leading to a reduction in sea ice thickness during winter and spring, as well as lower sea ice concentrations in spring across large areas north and west of the iceberg. In contrast, the sea ice thickness increases considerably southwest of the WIS, extending to the front of the Amery Ice Shelf during seasons covered by sea ice. The increase in sea ice concentration can also extend to as far as 75°E in spring. Without Iceberg D15, which previously contributed to the ice barrier of Barrier Polynya (BP), the shape of BP changes, the area of BP and Davis Polynya (DP) decreases, and the polynya off the northwest edge of the WIS near 83°E expands. These polynya patterns are much similar to the satellite remote sensing observations before Iceberg D15 was grounded. From April to October, the total area of BP and DP decreases by 2.83 × 10⁴ km² (60%) and 2.20 × 10³ km² (20%), respectively, while the total sea ice production decreases by 4.11 × 10¹⁰ m³ (66%) and 1.52 × 10¹⁰ m³ (52%) compared to the experiment with iceberg. These results indicate the substantial effects of grounding giant icebergs on the spatio-temporal distribution of sea ice, the area of polynyas, and sea ice production. High-resolution Antarctic coastal numerical models, typically with grid scales of kilometers, are sufficient to represent large icebergs, and adding the grounding giant icebergs is crucial for producing realistic simulations of sea ice and polynyas.

Sea ice and snow are the most sensitive and important crucial components of the global climate system, affecting the global climate by modulating the energy exchange between the ocean and the atmosphere. The sea near Zhongshan Station in Antarctica is covered by landfast sea ice, with snow depth influenced by both thermal factors and wind. This region frequently experiences katabatic winds and cyclones from the westerlies, leading to frequent snow blowing events that redistribute the snow and affects its depth, subsequently impacting the thermodynamic growth of sea ice. This study utilized the one-dimensional thermodynamic model ICEPACK to simulate landfast sea ice thickness and snow depth near Zhongshan Station in 2016. Two parameterization schemes for snow blowing, the Bulk scheme, and the ITDrdg (ITD/ridges) scheme are evaluated for their impact on snow depth. The results show that simulations using snow blowing schemes more closely align with observed results, with the ITDrdg scheme providing more accurate simulations, evidenced by root mean square errors of less than 10 cm for both snow depth and sea ice thickness. Snow blowing also impacts the thermodynamic growth of sea ice, particularly bottom growth. The sea ice bottom increases by 9.0 cm using the ITDrdg scheme compared to simulations without the snow blowing, accounting for 12.5% of total sea ice bottom growth. Furthermore, snow blowing process also influences snow ice formation, highlighting its primary role in affecting snow depth. Continued field observations of snow blowing are necessary to evaluate and improve parameterization schemes.