Abstract: Recent Antarctic surface climate change has been characterized by greater warming trends in West Antarctica than in East Antarctica. Although this asymmetric feature is well recognized, its origin remains poorly understood. Here, by analyzing observation data and multimodel results, we show that a west-east asymmetric internal mode amplified in austral winter originates from the harmony of the atmosphere-ocean coupled feedback off West Antarctica and the Antarctic terrain. The warmer ocean temperature over the West Antarctic sector has positive feedback, with an anomalous upper-tropospheric anticyclonic circulation response centered over West Antarctica, in which the strength of the feedback is controlled by the Antarctic topographic layout and the annual cycle. The current west-east asymmetry of Antarctic surface climate change is undoubtedly of natural origin because no external factors (e.g., orbital or anthropogenic factors) contribute to the asymmetric mode.
DISCUSSION
The model experiments conducted by CESM1 were not enough to fully confirm the roles of different regional SST forcings. Thus, we try to reinforce our logic weighted to the local atmosphere-ocean coupled feedback off West Antarctica through inference by comparing the regressed SST patterns among HadISST1, 38 CMIP5 models (CMIP5 MMM), and CESM1 onto their respective normalized EOF2 PC time series (fig. S9). In the tropics, there are large discrepancies between the observation and models, i.e., tropical central Pacific cooling in HadISST1, overall tropical and midlatitude cooling in CMIP5 MMM, and El Niño in CESM1. The large discrepancy in the tropical pattern associated with the Antarctic asymmetric mode implies that the dominant contribution of the tropical SST forcing to the upper-tropospheric anticyclonic circulation over West Antarctica cannot be generalized. By contrast, the consistent pattern of the warmer ABS SST is seen over the Southern Hemisphere high-latitude ocean. On the basis of this fact, it is natural to determine that the regional SST anomalies around Antarctica are the essential component for the asymmetry. The consistency between the observation and models over the high-latitude ocean enables us to think it reasonable to argue the harmony of the atmosphere-ocean coupled feedback off West Antarctica and the Antarctic terrain to generate the Antarctic west-east asymmetric natural variability.
In the asymmetric mode of Antarctic SATs, multidecadal variability is found in the long paleoclimate datasets of PAGES Antarctica2k, LOVECLIM, and TraCE-21K. This suggests that the enhanced asymmetric trend between West and East Antarctica during recent decades could be a manifestation of multidecadal variability. The linkage between the climatic conditions over the ABS and the Antarctic surface asymmetry at different time scales seems to determine time scales with either interannual or multidecadal variabilities. First, because the atmospheric Rossby wave bridge makes the connection between the tropics and west Antarctic surface climate (7, 11, 16), strong interannual variability in the tropics, such as ENSO, might contribute to variability in the Antarctic asymmetric mode. On the other hand, different seasonalities in the interaction between the atmosphere and ocean could alter the interannual variability because the interaction between sea level pressure and surface temperature over the Bellingshausen Sea has strong seasonality. Their correlation coefficients shift from negative during austral summer (r = −0.17 for February) to positive during austral winter (r = 0.53 for August). This seasonality contributes to the wintertime development of the asymmetric mode, including the increase in surface temperature and the high-pressure system over this region, but disturbs the persistence of asymmetry in the following warm season. Second, the long-term variability in the ocean over the West Antarctic coastal region seems to play a role in producing multidecadal periodicity. There have been some reports on long-term variability of the ocean in this region via ocean circulation changes (20, 21, 26). Possible roles of the ocean through the ASL have been suggested (20), but the relationship between the ocean and ASL is not immediately clear.
The climatic modes in this study suggest an important implication for future climate change in East Antarctica under global warming. The two future climate change experiments suggest that the explained variance in the first mode is much higher in the 21st century, while the second mode diminishes. The characteristics of the two modes strongly suggest that if global warming continues, a substantial temperature increase over East Antarctica may occur in addition to ongoing West Antarctic warming. The asymmetric mode will persist at its own pace in the future, even under global warming, but its role may not be as great as it is now. The intensified global warming over all of Antarctica in the future can induce massive melting of the ice shelves, even in East Antarctica. This explains why we have to keep an eye on Antarctica as global warming continues, despite the recent mitigation of warming in the eastern part of the region, due to the asymmetric nature of climate change.
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