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

Lagrangian feature tracking techniques are employed to determine the cause of the midwinter suppression of the Pacific storm track. It is found that the frequency of disturbances entering the Pacific storm track from mid-latitude Asia is substantially reduced (>25%) during wintertime relative to fall and spring. In contrast, the seasonal cycle of both the amplitude and the growth rate of individual disturbances show little to no wintertime reduction. The source of the reduction in the number of disturbances within the Pacific storm track appears to be fewer wintertime lee cyclones emanating from the Altai-Sayan mountains in Mongolia and north central China. The frequency of lee cyclogenesis in the lee of the these mountains is reduced (>30%) in winter relative to fall and spring, and this may be caused by changes in the geographical location of the jet-stream, which is far removed from the orography in winter.