The Relationship Between North Pacific Storm Track And East Asian Monsoon During Winter

Based on the latest daily reanalysis data of the US National Center for Atmospheric Research of National Centers for Environmental Prediction (NCEP/NCAR), detailed investigation is conducted of the relationship and its mechanism between north Pacific storm track and East Asian monsoon during winter. Therefrom conclusions are drawn as follows:(1) Compared to the significant enhancement of strength of the north Pacific storm track in the late1980s, it has been weakened and less than the climate average for about ten years or more since2000. In general, while the strength of storm track is stronger, its occupied area is bigger. The meridional and zonal shifts of the storm track are quasi-consistent, i.e., whether consistently northeastward or southwestward. Evidence shows that the strength of storm track is negatively correlated with the East Asian Winter Monsoon (EAWM) intensity.(2) When the EAWM is stronger, the temperature meridional gradient decreases in East Asian, leading to the weakening of atmosphere baroclinicity in the high latitudes, which finally results in the weakening and southward movement of north pacific storm track. Results of energy analysis show that both the baroclinic energy conversion and barotropic energy conversion are stronger when the EAWM activity is weaker than usual. It indicates that there are more eddy kinetic energy generated and consumed as well in the weaker winter monsoon years, but the quantity of generated eddy kinetic energy is far more than the consumed eddy kinetic energy. So the eddy kinetic energy is surplus in weaker winter monsoon years, and the storm track activity is stronger consequently.(3) The patterns of abnormal synoptic-scale transient dynamical forcing in the upper troposphere, middle troposphere and lower troposphere are different from each other, and it’s strongest in the upper atmosphere. When the transient dynamical forcing in300hPa is stronger, the convergence of E vector near the jet stream induces the weakening of westerly jet. Meanwhile, E vector divergence zone to the north results in the enhancement of westerly jet in temperate belt. When the storm track activity is active, the transient dynamical forcing in500hPa from20°N to40°N contributes to the rise of geopotential height above East Asian, and the transient dynamical forcing in1000hPa contributes to the rise of geopotential height above Aleutian region.(4) The patterns of abnormal synoptic-scale transient thermal forcing in the upper troposphere, middle troposphere and lower troposphere are in general similarity, which is most powerful in the lower atmosphere. When the storm track activity is stronger, the transient thermal forcing in850hPa to the north of40°N contributes to the warming of East Asian. The transient thermal forcing to the south of40°N contributes to the cooling of ocean, which decreases the land-sea thermal difference, and weakens the EAWM eventually.