秋季喀拉海海冰对2020和2021年12月京津冀冷暖反相的影响

2020/2021和2021/2022年冬季京津冀气温呈明显相反的季节内变化特征,前者前冬气温异常偏低后冬偏高,而后者前冬气温极端偏高后冬转冷。这两年前冬冷暖反相的直接原因是亚洲冬季风环流异常。2020年12月欧亚地区为典型的经向环流,西伯利亚高压偏强,乌拉尔山高压脊亦偏强,造成京津冀上空对流层中低层气温一致性偏低,而2021年12月环流形势相反。这两年冬季均处在拉尼娜背景下,但夏秋季喀拉海海冰异常有明显差异,可能是京津冀这两年前冬气温异常相反潜在的外强迫信号。统计和个例分析结果均表明,喀拉海海冰偏多易导致前冬西伯利亚高压偏弱,青藏高原地区海平面气压和亚洲大陆中纬度地区500 hPa位势高度均为正距平,不利于冷空气活动,造成2021/2022年前冬京津冀气温偏高,反之海冰偏少造成2020/2021年前冬偏冷。

Opposite features of temperature anomalies in early winter of 2020/2021 and 2021/2022 in Beijing-Tianjin-Hebei Region and the Possible Influence from the sea ice in the Kara Sea

Winter temperature anomalies in 2020/2021 and 2021/2022 winters have remarkable opposite intra-seasonal variations in Beijing-Tianjin-Hebei region. In the former, it is quite colder in early winter while much warmer in later winter, but in the latter the temperature is extremely high in the early winter while much low in the later winter. The opposite Asia winter monsoon circulations in the two winters play the most direct role. In early winter of 2020/2021, the prevailing circulation pattern over Eurasian continent is the typical meridional type, with much high pressure over Siberia and strong geopotential height (GPH) ridge over the Ural Mountains. Under this pattern, uniformly lower temperatures could be found in the mid-lower troposphere over Beijing-Tianjin-Hebei region. While the circulation pattern is opposite in the early winter of 2021/2022. Both the two winters are under the background of La Ni?a events, but obvious differences appeared in the sea ice in the Kara Sea from previous summer to autumn, which may be the potential influencing factor for the opposite temperature anomalies in the region in the two winters. Both statistical results and individual case analysis indicate that denser sea ice in the Kara Sea is likely to lead to weaker Siberian high, higher pressure over the Qinghai-Tibet Plateau and higher GPH over the mid-latitudes of the Asian continent, which favor weaker cold air activity and finally result in a warmer early winter in Beijing-Tianjin-Hebei region in 2021/2022. On the contrary, less sea ice in the Kara Sea caused a colder early winter in 2020/2021.