The Sea Ice Extent Anomaly in the North Pacific and Its Impact on the East Asian Summer Monsoon Rainfall

Abstract

The relationship between extreme anomalies of the spring sea ice extent over the Bering Sea and the Sea of Okhotsk and rainfall variability in the east Asian summer monsoon was examined through an analysis of observed data and modeling experiments. The results show that reduced sea ice extent leads to an enhanced summer monsoon rainfall in southeastern China. This relationship is well supported by the background atmospheric circulation changes and the stationary wave dynamics. A difference in the 500-hPa geopotential height composed from the NCEP?NCAR reanalysis data and model output between the light and heavy sea ice cases shows an anomalous high in the east Asian summer, which favors the invasion of a cold air mass into southern China and prevents the east Asian summer monsoon from advancing northward. Hence, the mei-yu front and its associated rainfall intensify and stay in southeastern China. The generation of the summer anomalous high and its interseasonal link to the spring sea ice extent anomalies can be accounted for by the stationary wave dynamics and the land surface process. In spring, the decrease in sea ice extent forces eastward-propagating wave activity flux and causes an anomalous high in Europe along with a decrease in precipitation. The decreased soil water content results in a higher land surface temperature and more sensible heat flux in summer, and this strengthens summer stationary wave activities in Europe. The eastward propagation of the wave energy and its intensification in east Asia are responsible for the anomalous high in the east Asian summer. In this process, the European land surface acts as a ?bridge? linking the spring sea ice extent anomalies with the east Asian summer monsoon.