Influence of the East Asian winter monsoon on the storm track activity over the North Pacific

Abstract

We investigate the relationship between the East Asian winter monsoon (EAWM) and synoptic eddy activity over the North Pacific Ocean. It is well known that the subtropical Pacific jet becomes stronger during strong monsoon periods than during weak monsoons. The storm track is generally expected to be active when the EAWM is strong, owing to the enhanced subtropical jet. Contrary to this expectation, synoptic eddy activity is significantly reduced in strong EAWM years and, conversely, significantly increased during weak years. In addition, we found that EAWM-induced lower level cooling brings about the reverse relationship: a stronger jet but with weaker storm activities. It is suggested that the local baroclinicity induced by the EAWM modulates the variation of both the jet and North Pacific storm track simultaneously in the interannual time scale. Because major lower level cooling is located in the western parts of both the subtropical jet and the storm track in longitudes and between the two central axes in latitudes, it induces the opposite sign of anomalous local baroclinicity at the two active regions and causes this reverse relationship. The wave seeding from East Asia to the North Pacific storm track is weakened owing to the anomalous lower level cooling in the northwestern Pacific, which also results in reduction of eddy generation and storm track activities.