The Variation Of The East Asian Winter Monsoon And Its Relation With Sea Surface Temperature

By using the ERA-40monthly reanalysis data and the1950-2000observational SSTs from global, extratropical, tropical oceans, Indian Ocean-Pacific at tropics, tropical Indian Ocean and tropical Pacific, with each of them to conduct experiments by means of the NCAR CAM3as the global atmospheric model, we address the SSTA impact on inter-annual and inter-decadal change of East Asian winter monsoon (EAWM) and the principal members of its circulation systems. The following are of note.(1) With the method of EOF to define the intensity index of each single system, an index of the East Asian winter monsoon is defined. The new index exhibits distinct inter-annual variability and takes into account variations of Siberian high, East Asian trough, and meridional shear of upper-tropospheric zonal wind. Results show that the index can capture well the continuous weakening of the EAWM since the mid-1980s. It is also statistically significantly correlated with variations of both atmospheric circulation fields and surface temperature during winter. Modeling shows that variations in SST of Indian Ocean-Pacific at tropics exert pronounced impacts upon the inter-decadal weakening process to be in the mid-late1980s, in better agreement with observations.(2) The East Asian subtropical Westerly Jet (EAWJ) strengthens for the period from1957to2001. SSTAs from tropical oceans, especially tropical Pacific, impact on the variation of the EAWJ significantly. When SST from tropical Pacific rises, the East Asian subtropical Westerly Jet (EAWJ) strengthens.(3) The EOF analysis is applied to obtain the dominant modes of the East Asian trough (EAT), the leading EOF (EOF1) of500-hPa geopotential height over East Asia depicts well the variation of the EAT intensity and the second EOF mode (EOF2) depicts the tilt of the EAT axis. The numerical experiment with different SSTs indicates that the SSTA has lower impact upon the change of the EAT intensity. But the simulated tilt of the EAT axis by means of observational SSTs of global/tropical oceans, Indian Ocean-Pacific at tropics and tropical Pacific shows an better agreement with observations, thereby demonstrating that SSTA in these ocean regions play an active role in the inter-annual and inter-decadal variability of the tilt of the EAT axis. The tropical Pacific is the key basin.(4) The Siberian high weakens for the period from1957to2001, and it has an inter-decadal weakening process to be in the early1980s. It is failing to simulate the variation of Siberian high in the numerical experiments with different SSTs. In addition, the Siberian high is closely related to the Arctic Oscillation (AO) index. The inter-decadal strengthening process of Arctic Oscillation may be the main cause of the inter-decadal variability of Siberian high in the early1980s.