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Abstract

In this study, we demonstrate that the enhanced storm track activity in the North Pacific midlatitudes during boreal winter over the past four decades was correlated with subtropical warming which strengthened midlatitude meridional temperature gradient. On the contrary, Arctic warming reduced the near-surface temperature gradient and had negative correlation with midlatitude storm track activity. The close relationship between temperature gradient and synoptic eddy activity is dynamically fundamental and can be seen in both long-term trends and the interannual variation. The observed trends in warming sea surface temperature (SST) in the subtropical North Pacific, ascending motion over the subtropical western North Pacific and anticyclonic circulation over the central and western North Pacific suggests a positive feedback between warming SST and atmospheric circulation. Numerical experiments further revealed that SST warming in the subtropical North Pacific and in the Indian Ocean could drive subtropical tropospheric warming and anticyclonic circulation in the North Pacific, as well as the strengthening of the North Pacific midlatitude storm track activity. The results suggest tropical and subtropical influence on North Pacific midlatitude winter weather.