Study On Midwinter Suppression Of North Pacific Storm Track And Its Possible Mechanism

Based on the reanalysis data of ECMWF (ERA-40). detailed investigation is conducted of the midwinter suppression of north pacific storm track and its possible mechanism. Therefrom conclusions are drawn as follows:(1) If the strength of north pacific storm track over subject areas in midwinter of some year exhibits weaker than that in previous late autumn and in after early spring, we call this phenomenon as midwinter suppression of north pacific storm track. This suppression occurs most prominent at the upper levels of troposphere, second significant at middle levels and unclear at lower levels. And it also exhibits obvious interannual and interdecadal variability:In the study time of interest, the suppression shows a gradually increase trend from1960s to80s, in which it reaches its peak at around the middle and late70s to early80s, then weakens or disappeares from late80s to early90s, but increases again from late90s to21century.(2) Results of energy analysis show that the seasonal variation of eddy kinetic energy can well reflect the phenomenon of midwinter suppression of the north pacific storm track, which is mainly affected by the baroclinic energy conversion term at middle and lower tropospheric levels, the barotropic energy conversion term and divergence term of eddy ageostrophie geopotential flux at upper levels. In midwinter, by adding up the stronger loss resulted from the larger decrease of the baroclinic energy conversion and eddy ageostrophie geopotential flux and the weaker gain resulted from the smaller decrease of the barotropic energy conversion, the remarkable net loss of the eddy kinetic energy during the midwinter is possibly the direct reason for midwinter suppression of north pacific storm track.(3) Inverstigation shows that jet stream and downstream development effect may play important roles in the suppression. In the midwinter of strong suppression years, the east-Asia subtropical west jet stream at upper levels of troposphere is stronger and narrower than that in late autumn and in early spring, but more southward and parallel to the latitude cycle. As a result, the poleward and upward heat fluxes at the lower and middle levels, so that the baroclinic energy conversion, over the entrance area of north pacific strom track decrease largerly, and so does the gain of eddy energy by strom track because of the hampering of the upward spread of baroclinic disturbance. At the same time, the gain of eddy energy from energy dispersion of upstream disturbance also greatly reduces. Therefore, all of the above factors lead the possibility of the occurrence of suppression to increase.(4) This article is highlighted by the following innovations:it reveals more facts of the phenomenon of midwinter suppression of north pacific storm track; detailed investigation is first time conducted of the energy conversion characteristics of midwinter suppression; it reveals the fact that jet stream and downstream development effect may play important roles in the suppression.