北极高密集度冰区海冰的多时间尺度变化特征及其极端低值事件分析

利用美国冰雪中心(NSIDC)高分辨率海冰密集度等多种数据,定义了北极高密集度冰区(High concentration ice region:HCIR)海冰变化指数,在此基础上研究了1989—2017年HCIR海冰多尺度变化特征及其极端低值事件的可能形成原因。结果表明:北极HCIR海冰密集度具有显著的单峰型季节变化特征,4月密集度最高,9月密集度最低,年较差达17.70%,兼有夏季融冰期短、冬季结冰期长且持续稳定的特点。HCIR海冰存在显著的年际年代际变化,在2007年发生了年代际转折以后,海冰变化指数的年际变化幅度和频次明显加强,且在2016、2012、2007、2011、2008和2010年依次出现海冰密集度极端降低事件;2016年9月初HCIR海冰密集度达到历史最低值,接近50%。对HCIR海冰密集度极端低值事件的统计研究表明,29年间共出现874天(次)极端低值事件,约占总频次的8%;空间上海冰密集度的降低主要出现在沿HCIR边界线一带,存在巴伦支海-喀拉海北缘的斯瓦尔巴群岛-北地群岛和东西伯利亚-波弗特海两个中心区域,该空间分布与气旋式大气环流引起的北冰洋Ekman漂流的辐散分布相一致。这表明HCIR海冰密集度的极端降低与极涡的动力作用有关,同时风场对海冰的动力辐散作用还会引起HCIR开阔水域的扩大,进一步加强海冰反照率的正反馈机制,使得热力和动力作用耦合起来共同影响HCIR海冰的加速融化。

Multi-Scale Variability of Sea Ice Concentration in the Arctic High Concentration Ice Region and Its Extremely Low Events

Based the National Snow and Ice Data Center high-resolution sea ice concentration(SIC)and other data,we definedanaveraged sea ice concentration index in the high concentration ice region(ASIC_HCIR),and studied the multi-scale variabilities of sea ice in the high concentration ice region(HCIR)from 1989 to 2017.Extremely low event of SIC(ELES)in HCIR and its possible causes were investigated as well.The results show that the ASIC_HCIR has a significant unimodal seasonal variation,with the maximum in April and the minimum in September.Annual range of the seasonal cycle for ASIC_HCIRis about 17.70%.It also has a short summer melting period and a long and stable winter icing period.ASIC_HCIR also has significant interannual and interdecadal variabilities.After the decadal shift in 2007,both amplitude and frequency of the ASIC_HCIR interannual variability significantly strengthened.ELES in HCIR occurred in turn in 2016,2012,2007,2011,2008 and 2010,respectively.The lowest SIC appeared at the beginning of September 2016,reaching a record low SIC close to 50%.Statistics on ELES in HCIR indicated that 874 days(times)of ELES,accounting for about 8%,occurred in the 29 years from 1989 to 2017.For ELES,the decrease of SIC mainly occurred in the marginal zonealong the boundary of the high concentration ice region,with two main areas the Barents sea-Svalbard Archipelago-Severnaya Zemlya zone and the eastern Siberian Sea-Beaufort Sea region.This spatial distribution is consistent with the divergence field of upper ocean Ekman drifting caused by a cyclonic atmospheric circulation,which suggested that the ELES in HCIRwas related to the dynamics of polar vortex.The dynamic divergence of wind field on sea ice would also cause wider open waters in HCIR,further strengthen the positive albedo feedback.The feedback enabled thermal and dynamic coupling,and then accelerated the sea ice melting in the HCIR.