The storm-track suppression over the western North Pacific from a cyclone life-cycle perspective

Abstract

Abstract. Surface cyclones that feed the western part of the North Pacific storm track and experience a midwinter suppression originate from three regions: the East China Sea (∼30∘ N), the Kuroshio extension (∼35∘ N), and downstream of Kamchatka (∼53∘ N). In midwinter, in terms of cyclone numbers, Kuroshio (45 %) and Kamchatka (40 %) cyclones dominate in the region where eddy kinetic energy is suppressed, while the relevance of East China Sea cyclones increases from winter (15 %) to spring (20 %). The equatorward movement of the baroclinicity and the associated upper-level jet toward midwinter influence cyclones from the three genesis regions in different ways. In January, Kamchatka cyclones are less numerous and less intense, and their lifetime shortens, broadly consistent with the reduced baroclinicity in which they grow. The opposite is found for East China Sea cyclones, which in winter live longer, are more intense, and experience more frequently explosive deepening. The fraction of explosive East China Sea cyclones is particularly high in January when they benefit from the increased baroclinicity in their environment. Again, a different and more complex behavior is found for Kuroshio cyclones. In midwinter, their number increases, but their lifetime decreases; on average they reach higher intensity in terms of minimum sea level pressure, but the fraction of explosively deepening cyclones decreases and the latitude where maximum growth occurs shifts equatorward. Therefore, the life cycle of Kuroshio cyclones seems to be accelerated in midwinter with a stronger and earlier but also shorter deepening phase followed by an earlier decay. Once they reach the latitude where eddy kinetic energy is suppressed in midwinter, their baroclinic conversion efficiency is strongly reduced. Together, this detailed cyclone life-cycle analysis reveals that the North Pacific storm-track suppression in midwinter is related to fewer and weaker Kamchatka cyclones and to more equatorward intensifying and then more rapidly decaying Kuroshio cyclones. The less numerous cyclone branch from the East China Sea partially opposes the midwinter suppression. The cyclones passing through the suppressed region over the western North Pacific do not propagate far downstream and decay in the central North Pacific. The behavior of cyclones in the eastern North Pacific requires further analysis.