Relationship between SST in the North Pacific and Northeast Cold Vortex

Given the close relationship between the anomalous atmospheric circulation and anomalous SST, we calculate the correlation between NECVI in Meiyu period and global SST in the preceding six months (from December of the preceding year to May of the present year) and in the corresponding period (June-July of the present year). It is found that NECVI is most significantly correlated with SST in the North Pacific (not shown). A small negative correlation area occurs in the central North Pacific in preceding December, then enlarges month by month, extends to northwest, and finally covers the entire Northwest Pacific in July with the center lying at 40°N-45°N, indicating that when the NECV in Meiyu period is stronger, SST in the above-mentioned region is usually significantly lower in the preceding or present year, which favors the development of the anomalous descending motion in this region. In the meantime, the thermodynamic property of the East Asian continent shifts from winter to summer (the East Asian continent is a cold source in winter and wind blows from land to sea; while it is a heat source in summer and wind blows from sea to land). Since the western North Pacific SST in summer is lower in strong NECV years and the land-sea thermodynamic difference between the western North Pacific and East Asian continent is mainly zonal, there is anomalous easterly in lower-levels between the western North Pacific and East Asian continent. The lower SST in the western North Pacific also increases the thermal difference between this region and middle and low latitudes in East Asia, thus strengthening the upper-level westerly jet. When NECV happens, an anomalous closed circulation is formed between the western North Pacific and middle and high latitudes in East Asia, which favors the further strengthening of NECV. Therefore, the anomalous SST in the North Pacific in the preceding year may be a factor resulting in the anomalous

NECV in Meiyu period.

It can be seen without effort that the land-sea thermal contrast facilitates NECV in summer, while that inhibits NECV in winter, which may be the reason why the NECV happens more frequently in summer than in other seasons.

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