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极地海冰作为气候系统的重要组成部分,在气候演变中有着重要作用.有研究表明[1-3],海冰可通过改变表面反照率以及大气—海洋之间的热量交换而对表面温度产生影响.近几十年北极海冰的持续减少可能是导致"北极放大"现象的主要原因之一[3-4].同时,海冰的输出、冻结和消融对温盐环流、海洋层结等过程也起着重要作用[5-6].
20世纪70年代以来,北极海冰的持续减少已经成为一个不争的事实,海冰的时空变化特征也成为近年来科学界的研究热点.以往的研究指出,北半球海冰范围在每个季节都呈现趋势性减少,其中冬季下降速率最慢;夏季尤其是9月份,下降速率最快[7-9],并且,9月份北极海冰减少的速度在近十几年存在加速的现象[9-10].模式结果显示,本世纪中期至末期可能会出现夏季无冰的北冰洋[11].此外,就空间特征而言,北半球夏季海冰年际异常具有一定的整体性,即各个主要海区的海冰面积异常基本呈同相变化;而冬季海冰年际异常则表现出较强的区域性,巴伦支海、格陵兰海和白令海、鄂霍次克海等海域的海冰呈反相变化[12-13].不同海域海冰变化趋势的速度大小也不尽相同[9, 14].
北极海冰的变化受到动力学过程和热力学过程的共同影响.动力学过程主要包括大气环流和洋流的变化.热力学过程则主要包括温度、海洋热输送等,其中,北极地区气温的升高,强有力地推动了海冰覆盖面积的减少[7, 15].此外,海冰—反照率之间的正反馈过程也在夏季海冰面积缩减中扮演着重要角色[16].
过去的工作更多地关注北极夏季海冰的趋势变化,对北极冬季海冰的变化趋势及其空间特征的研究相对较少.本文从线性趋势的角度出发,分析了北半球3月海冰变化趋势的空间特征以及趋势本身在20世纪90年代中期前后差异的空间特征,并讨论了表面气温和表面风场对3月海冰趋势变化的影响.
Regional Features and Factors of the Northern Hemisphere Sea Ice Trends in March
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摘要: 利用海冰密集度资料和海冰范围指数,分析了3月北半球海冰在1979-1995年和1996-2014年2个时间段内的变化趋势以及趋势差异的区域特征.结果表明:北半球3月总海冰范围以稳定速率减少,速率无明显加速.而各个分海域的海冰变化表现出一定的区域差异.其中,巴伦支海海冰在2个时期内以稳定的速率减少;白令海海冰不减反增;鄂霍次克海海冰缩减速度在后期减小;圣劳伦斯湾的海冰范围在90年代中期以前显著增大,后期缓慢缩减.3月海冰趋势变化的空间特征与同期表面气温和表面风场的趋势变化相一致.Abstract: Based on the data of sea ice concentration and using the sea ice extent (SIE) index, the present study investigates the changes in the Northern Hemisphere sea ice trends in March during 1979-1995 and 1996-2014 and their regional features. The results show that although the negative slope of March SIE over the whole Northern Hemisphere did not experience obvious changes with time, pronounced regional differences were present in the sea ice trend and its change in the mid-1990s. The Barents Sea SIE exhibited a significant downward trend in both periods with a relatively steady rate. The SIE in the Bering Sea showed a positive trend and the slope became steeper after the late 1990s. The Sea of Okhotsk was subjected to ice loss during both periods with a notable increase in between. The SIE in the Gulf of St. Lawrence showed an upward trend during the earlier period and was dominated by inter-annual fluctuations during the later period. The trends and their changes in the mid-1990s of the March sea ice were consistent with those in surface air temperature and atmospheric wind.
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Key words:
- sea ice trend /
- regional difference /
- surface air temperature change /
- surface wind change .
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