Since 1979, the total Arctic SIE decadal trend has shown that summer has the highest negative trend (-10.3±0.6% decade-1) followed by autumn (-5.0±0.3% decade-1), spring (-3.2±0.2% decade-1) and winter (-3.0±0.2% decade-1) (Table 1). Arctic sea ice declines at a rate of -56.9±2.5 ×103km2yr-1 throughout the year, with the maximum loss in summer and autumn and the minimum loss in spring. The interannual variation in the Arctic recorded a dipping minimum SIE in 2020 (3.74 million km2), which is 0.35 million km2 more than the lowest record of 2012 (3.39 million km2) 30. The decadal trend in seasonal Arctic Sea ice and temperature is examined in the following sections to further understand the decadal variation.
Seasonal variations in sea ice and temperature
In the last four decades, the SIC trend in Baffin Bay, the Eastern Greenland Sea, Barents, and the Kara Sea has shown a significant negative trend in all seasons (Fig. 1a-d). Summer experiences, maximum SIE losses throughout the Arctic except for a small region of the central Arctic. The marginal seas have been identified as the area experiencing the most significant loss in decadal SIC from spring to autumn. The SIC and SST have a substantial negative correlation throughout the year, whereas a significant negative correlation (r = −0.95, p <0.01) was observed in summer compared to the other seasons (Table S1). Notably, the Barents Kara region is not permanently frozen, and it has a higher SST (Fig. 1e-h). This is due to the inflow of warmer Atlantic waters, which keeps the area ice-free 11,31,32. Furthermore, the Arctic is warming rapidly, with temperatures rising by up to 0.5°C at its marginal seas.
There is accelerated heat and energy transfer from the ocean to the atmosphere aided by the lower trend in SIC, and it is primarily linked to the AA 14. Moreover, the increase in temperature during cold seasons (autumn and winter) is one of the possible reasons for the amplified warming in the Arctic 28. Similarly, the present study found differential atmospheric warming and cooling in different seasons in the Arctic by analyzing the seasonal and decadal trends of T2M (Fig. 1i-l). We observed a significant positive T2M trend throughout the year, with the highest trend during cold seasons compared to the warm season. During the summer, most of the heat observed from the atmosphere is used to melt the sea ice, resulting in a lower surface temperature (Fig. 1k). On the other hand, the T2M trends in autumn are higher due to the underlying mechanism through which absorbed latent oceanic heat is released back into the atmosphere during the summer. This contrasting behaviour of seasonal T2M is often attributed to the accelerated melting of sea ice, which keeps the surface air temperature near freezing in boreal summer 33,34. The enhanced heat flow from the exposed open sea surface, rather than ice-albedo feedback, is responsible for amplifying temperature during the cold season 35. However, the Arctic SIC shows a significant negative correlation with the T2M in all seasons, with the highest correlation in autumn (Table S1). Furthermore, in the following section, we have attempted to determine the mechanism responsible for temperature amplification in the Arctic.
Seasonal Variations in the Vertical Structure of the Atmosphere
The seasonal variations in air temperature (AT) at the vertical level in the Northern Hemisphere (30ºN-90ºN) are shown in Fig. 2. Warm seasons have a lower AT trend than cold seasons, which corresponds to the observations in the previous section. The question is to determine what drives significant warming in the mid- and upper Arctic atmosphere during the cold seasons. The temperature variation of the Arctic atmosphere on the surface and in the air aloft is influenced by the quantity of sea ice cover. As a result, during sea ice melting, surface amplification occurs14, resulting in stable atmospheric stratification at lower altitudes in the summer. A previous study also found that AT in the Arctic has a highly stable and stratified vertical structure 3.
During the spring season, the negative AT trend is confined to the upper atmosphere (∼250 hPa), which suggests that the influence of sea ice loss is constrained to the surface and lower levels of the atmosphere (Fig. 2a) due to limited vertical mixing 27. The summer AT trend recorded maximum warming in the mid-atmosphere relative to the lower and upper atmospheric AT (Fig. 2b). The positive ice-albedo feedback mechanism is more apparent during the summer. Increased atmospheric heat absorption by the ocean causes the open water to become warmer than the surrounding atmosphere.
In all other seasons, mid-tropospheric warming is most likely due to an imbalance in horizontal energy transmission 28. The ocean's heat accumulated in the summer was released in the subsequent seasons, as observed in autumn (Fig. 2c). The AT trend in the mid-troposphere of the Arctic during autumn is approximately 0.6°C per decade. The AT trend is positive during the cold seasons due to weak air stratification and greater vertical mixing. The upper troposphere (∼250 hPa) warms in the winter due to enhanced vertical mixing of heat and energy from the surface to the atmosphere aloft rather than ice-albedo feedback. Additionally, during the winter, the retreating sea ice and delayed freezing allow for the release of excess heat stored in the ocean 34,36,37. In the upper troposphere, a negative AT trend (60ºN-90ºN) is recorded in all seasons (Fig. 2a-c) except in winter, which shows a positive AT trend and winter warming (Fig. 2d).
Arctic atmospheric warming above the surface might also be influenced by other possible factors such as cloud cover 38, increased downward LW radiation 35, heat and energy transfer from the lower latitudes, and differential radiative forcing 28. The following section investigated the role of heat fluxes associated with seasonal variations in Arctic sea ice cover.
Inferring Arctic sea ice changes using THF, RHF, and NHF
This study examined the contribution of ocean-atmospheric heat fluxes to atmospheric warming and stability due to sea ice changes. Interannual and seasonal variations in sea ice cover were also investigated since they increase the interaction between the ocean, ice, and atmosphere.
The seasonal sea ice growth or retreat is driven by the ocean-atmospheric heat exchange process influenced by the THF and RHF. Interannual Arctic sea ice variation is determined primarily by dynamic and thermodynamic mechanisms 39. The dynamic process involves the movement and accumulation of sea ice to the south, eventually melting. On the other hand, thermodynamics deals with the exchange of heat and energy between the ocean, ice, and atmosphere. Since several other mechanisms function simultaneously, these processes are independent. The earlier study revealed that one-third of the long-term variations in the oceanic heat content in the Arctic result from the fluctuation in ocean-atmosphere heat flux 40. In response to thermodynamic processes, our study focused on the seasonal change in sea ice.
The surface RHF and THF signs illustrate the direction of ice-ocean-atmosphere interactions. The RHF is governed primarily by the wind speed and AT near the surface, whereas integrals across the entire atmospheric column mainly determine the THF. Reduced sea ice can provide positive surface THF (upward), heat up the atmosphere, and indicate that ice drives the atmosphere. The negative THF (downwards) implies that the atmosphere drives sea ice. The negative NHF is associated with heat from the ocean and vice versa 41. Accelerated sea ice loss in the Arctic will result in more heat transfer from the ocean to the atmosphere, resulting in atmospheric warming amplification.
A composite analysis was carried out to explain the heat flow process that contributes to atmospheric warming and Arctic Sea ice melting. Arctic Sea ice loss occurs throughout the year, but we considered the heat flux changes only in the negative SIC phase in this study. During the negative SIC phase in spring, the peripheral Arctic seas have negative anomalies, with only the central Arctic experiencing positive anomalies (Fig. 3a). The increasing insolation causes the sea ice to melt with the onset of spring, resulting in a positive THF across the Arctic Ocean, whereas the RHF and NHF are observed to be negative (Fig. 3b-d). A positive LH anomaly has been found in the central Arctic, while a greater negative SH anomaly remains across the Arctic region (Fig. S1). The positive LH and SH fluxes imply that sea ice transfers heat to the atmosphere 42. Spring SIC had a significant (p <0.01) positive correlation with THF (r = 0.67), RHF (r = 0.52) and NHF (r = 0.80) (Table S1). Furthermore, the positive THF in the Arctic Ocean demonstrates that the ocean surface largely influences the atmosphere, resulting in a higher AT.
Arctic SIC varies most in the summer due to a persistently negative SIC anomaly (Fig. 4a). The summer months demonstrated a similar positive correlation with THF (r = 0.61), RHF (r = 0.86) and NHF (r = 0.67) as the spring months (Table S1). The RHF has the strongest correlation. This could be due to increased incident solar radiation, more open ocean absorbing heat, and considerable reduction in sea ice 15. In summer, the THF anomaly is weakly positive, while the RHF anomaly is found to be strongly positive in the marginal seas of the Arctic Ocean (Fig. 4b, c). NHF is largely negative during the negative phase of SIC (Fig. 4d). The LH anomaly is negative because most of the heat is absorbed by the ocean (Fig. S2), and it is significantly correlated with the SIC (r = 0.71, p <0.01). During summer, the atmosphere loses heat to the ocean, causing increased ocean warming 14. Hence, atmospheric warming is less than that of the ocean.
In early autumn, a weak positive ice anomaly is observed when the amount of sunlight decreases in the Arctic. However, from the analysis, autumn has the second-highest negative SIE trend (Table 1). The negative phase of SIC in autumn shows maximum sea ice loss in the region surrounding the Central Arctic (Fig. 5a). In contrast to the previous seasons, autumn shows a significant positive and negative correlation with the THF and RHF, respectively (Table S1). The RHF and THF anomalies show positive and negative anomalies, respectively (Fig. 5b, c). Similarly, the NHF does not correlate with the phenomenon that the atmosphere regulates sea ice, but strong positive LW and weak negative SW radiation were observed (Fig. S3). In autumn, the NHF pattern reverses from the previous summer, with positive regions becoming negative (Fig. 5d). The mechanism behind the pattern reversal is probably related to the fact that heat absorbed in the summer is released in the winter 14,43. This leads to maximum warming in the autumn and further delays in the subsequent winter when the sea ice freezes.
During the negative winter SIC phase, the composite map shows a weak negative SIC anomaly in the Arctic Ocean with a strong negative anomaly in eastern Greenland and the Barents Kara Sea region (Fig. 6a). The winter SIC primarily depends on the THF and NHF; therefore, increased ocean-atmosphere warming may delay ice formation. The THF anomaly shows a significant positive correlation with SIC (r = 0.35, p <0.1), which is coherent with the findings of spatial distribution (Fig. 6b). However, the RHF anomaly is positive, which is due to dark days (Fig. 6c). There is no correlation between winter SIC and RHF. The NHF anomaly is negative in the Arctic Ocean except in the Barents Kara Sea (Fig. 6d). The variations in the NHF are a cumulative response of the THF and RHF; thus, a decrease in RHF depicts lower NHF. As a result, the SIC and NHF have a lower positive correlation (r = 0.26) than the THF. The correlation analysis revealed significant positive and negative correlations with LH (r = 0.40, p <0.01) and SH (r = -0.26, p <0.10), respectively (Table 1). In winter, the sea-ice-air interaction is largely determined by the LW radiation (Fig. S4).