The source of the midwinter suppression in storminess over the North Pacific

Abstract

Feature tracking techniques are employed to investigate why there is a relative mini- mum in storminess during winter within the Pacific storm track (the midwinter sup- pression). It is found that the frequency and amplitude of disturbances entering the Pacific storm track from mid-latitude Asia is reduced during winter relative to fall and spring, and that the magnitude of this reduction is more than sufficient to account for the midwinter suppression. Growth rates of individual disturbances are calculated and compared to expectations from linear theory for several regions of interest. While there are discrepancies between linear expectations and observed growth rates, the growth of disturbances within the Pacific storm track cannot explain why the mid- winter suppression exists. Furthermore, it is determined that the development of a wintertime reduction in storminess over mid-latitude Asia is consistent with linear expectations, which predict a wintertime minimum in Eady growth rates in this re- gion, mainly due to increased static stability. Several other mechanisms that may contribute to the initiation of the midwinter suppression over mid-latitude Asia are discussed, including the interaction between upper-level waves and topography, the behavior of waves upwind of the Tibetan plateau, and the initiation of lee cyclones.