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Abstract:
The North Pacific storm track activity is suppressed substantially under the excessively strong westerlies to form a distinct minimum in midwinter. This “midwinter minimum” of the storm track activity seems inconsistent with linear baroclinic instability theory. It has been intensively investigated for decades as a test case for the storm-track dynamics. However, the mechanisms controlling it are yet to be fully unveiled and are still under debate. Here we elucidate the detailed seasonal evolution of the climatological density of surface migratory anticyclones over the North Pacific, in comparison with its counterpart for cyclones, based on a Lagrangian tracking algorithm. We demonstrate that the frequency of surface cyclones over the North Pacific maximizes in midwinter, whereas that of anticyclones exhibits a distinct midwinter minimum under the upstream influence, especially from the Japan Sea. In midwinter, it is only on such a rare occasion that a prominent weakening of the East Asian winter monsoon allows a migratory anticyclone to form over the Japan Sea, despite the unfavorable climatological-mean conditions due to persistent monsoonal cold-air outbreaks and excessively strong upper-tropospheric westerlies. The midwinter minimum of the North Pacific anticyclone density suggests that anticyclones are likely the key to understanding the midwinter minimum of the North Pacific storm track activity as measured by Eulerian eddy statistics.
