Antarctic sea ice multidecadal variability triggered by Southern Annular Mode and deep convection Communications Earth & Environment

Antarctic sea ice exerts great influence on Earth’s climate by controlling the exchange of heat, momentum, freshwater, and gases between the atmosphere and ocean. Antarctic sea ice extent has undergone a multidecadal slight increase followed by a substantial decline since 2016. Here we utilize a 300-yr sea ice data assimilation reconstruction and two NOAA/GFDL and five CMIP6 model simulations to demonstrate a multidecadal variability of Antarctic sea ice extent. Stronger westerlies associated with the Southern Annular Mode (SAM) enhance the upwelling of warm and saline water from the subsurface ocean. The consequent salinity increase weakens the upper-ocean stratification, induces deep convection, and in turn brings more subsurface warm and saline water to the surface. This salinity-convection feedback triggered by the SAM provides favorable conditions for multidecadal sea ice decrease. Processes acting in reverse are found to cause sea ice increase, although it evolves slower than sea ice decrease.

南极海冰通过调控大气与海洋之间的热量、动量、淡水及气体交换，对地球气候产生显著影响。自2016年以来，南极海冰范围经历了年代际尺度的小幅增长，随后出现大幅缩减。本研究利用一套300年的海冰数据同化重建数据集，以及两份美国国家海洋和大气管理局/地球物理流体动力学实验室（National Oceanic and Atmospheric Administration/Geophysical Fluid Dynamics Laboratory, NOAA/GFDL）模型模拟结果与五份耦合模式比较计划第六阶段（Coupled Model Intercomparison Project Phase 6, CMIP6）模型模拟结果，证实了南极海冰范围存在年代际变率。与南方环状模（Southern Annular Mode, SAM）相关的更强西风，增强了次表层海洋暖咸水的上升流。由此引发的盐度升高会削弱上层海洋层结，诱发深层对流，进而将更多次表层暖咸水输送至海表。这一由SAM触发的盐度-对流反馈，为南极海冰范围的年代际缩减提供了有利条件。研究同时发现，反向作用的过程会导致海冰范围扩张，尽管其演变速率慢于海冰缩减过程。