Subseasonal Reversal of East Asian Surface Temperature Variability in Winter 2014/15
Although there has been a considerable amount of research conducted on the East Asian winter-mean climate, subseasonal surface air temperature (SAT) variability reversals in the early and late winter remain poorly understood. In this study, we focused on the recent winter of 2014/15, in which warmer anomalies dominated in January and February but colder conditions prevailed in December. Moreover, Arctic sea-ice cover (ASIC) in September–October 2014 was lower than normal, and warmer sea surface temperature (SST) anomalies occurred in the Niño4 region in winter, together with a positive Pacific Decadal Oscillation (PDO|+) phase. Using observational data and CMIP5 historical simulations, we investigated the PDO|+ phase modulation upon the winter warm Niño4 phase (autumn ASIC reduction) influence on the subseasonal SAT variability of East Asian winter. The results show that, under a PDO|+ phase modulation, warm Niño4 SST anomalies are associated with a subseasonal delay of tropical surface heating and subsequent Hadley cell and Ferrel cell intensification in January–February, linking the tropical and midlatitude regions. Consistently, the East Asian jet stream (EAJS) is significantly decelerated in January–February and hence promotes the warm anomalies over East Asia. Under the PDO|+ phase, the decrease in ASIC is related to cold SST anomalies in the western North Pacific, which increase the meridional temperature gradient and generate an accelerated and westward-shifted EAJS in December. The westward extension of the EAJS is responsible for the eastward-propagating Rossby waves triggered by declining ASIC and thereby favors the connection between ASIC and cold conditions over East Asia.
Key wordsEast Asia subseasonal temperature Arctic sea-ice Niño4 SST Pacific Decadal Oscillation
目前已有许多关于东亚冬季平均气候的研究, 但是, 在次季节尺度上, 前冬和后冬的气温变率有时出现反转. 本文主要关注2014/15年冬季东亚地区“1, 2月偏暖, 而12月偏冷”这一现象. 与此同时, 2014年秋季北极海冰偏少, 冬季Nino4区的海温偏高, 太平洋年代际振荡(PDO)呈正位相. 本文利用再分析资料和CMIP5的历史模拟资料, 发现冬季Nino4区暖海温异常(秋季北极海冰偏少)对东亚冬季气温次季节变率的影响受PDO正位相的调制. 结果表明, 当冬季Nino4区海温偏高时, 受PDO正位相的调制, 热带地表加热出现次季节延迟. Hadley环流和Ferrel环流在1, 2月份发展, 加强了热带和中纬度的联系. 同时, 东亚急流显著偏弱, 有利于1, 2月份东亚气温偏高. 在PDO正位相的调制下, 秋季北极海冰偏少与12月份西北太平洋的冷海温异常相关. 冷海温异常加大了经向温度梯度, 东亚急流加强西伸. 急流西伸有利于海冰异常引起的Rossby波列东传, 导致12月份东亚气温偏低.
关键词东亚 次季节气温 北极海冰 Nino4区海温 PDO
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This research was supported by the National Key R&D Program of China (Grant No. 2016YFA0600703), the National Natural Science Foundation of China (Grant Nos. 41505073 and 41605059), the Young Talent Support Program by China Association for Science and Technology (Grant No. 2016QNRC001), and the Research Council of Norway (Grant No. SNOWGLACE #244166).
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