Interdecadal Change of East Asian Winter Monsoon and a Numerical Experiment on Its Possible Cause

In the recent more than 20 years, the surface air temperature (SAT) is obviously increasing over the globe. More and more scientists believe that it is caused by greenhouse effect. Many researches reveal that the warming trend is heterogeneous in space and time, and the most obvious warming appears in high latitude and winter. Under this heterogeneous warming background, the sea-land thermal contrast would be changed and then may lead to the variation of East Asian winter monsoon (EAWM). Based on this hypothesis, the interdecadal variation of EAWM is analyzed and the influence of the change of sea-land thermal contrast caused by greenhouse effect on the variation of EAWM is verified by a numerical experiment. In order to study the interdecadal variation of EAWM in the latest few decades, an annual index of EAWM intensity is defined, briefly I_WI, by means of the NCEP/NCAR reanalysis monthly mean sea level pressure (SLP) data during 1961—2000. The variation of the interdecadal component (11-year running mean value) of I_WI is investigated. The result shows that EAWM has an obvious interdecadal variation in the period of 1961—2000. It weakens from 1960s to early 1970s and strengthens after that, then obviously weakens from early 1980s again. This interdecadal variation of EAWM is consistent with the wintertime warming trend in the north of East Asia. To investigate the possible causes of the interdecadal variation of EAWM, the correlation coefficients of the interdecadal components of I_WI with the mean SAT over the east of East Asia and the west Pacific are calculated separately. The results show that on the interdecadal scale, the correlation of the I_WI with the SAT over the land is more significant than that over the sea. It reveals that the interdecadal warming in the northeast of East Asia is closely related to the interdecadal weakening of EAWM. So, a possible mechanism of the interdecadal variation of EAWM is that the warming over the East Asia and the weakening of sea-land thermal contrast between the East Asia and the West Pacific are led to by the greenhouse effect, and then the weakening of EAWM is induced. Because the greenhouse effect is mainly caused by the decreasing long-wave radiation effect of increasing greenhouse gases, to verify this possible influence of greenhouse effect on the interdecadal variation of EAWM, a sensible numerical experiment with decreased long-wave cooling rates over northeast of East Asia in the regional climate model (RegCM3) is performed. The simulation results show that the reduction of the eastwest orientation SLP difference and the weakening of northerly on the low level atmosphere in winter, i.e., the weakening of EAWM, could be led to by the decreasing of long-wave cooling rates over northeast of East Asia. Summing up the above diagnosing and simulation results, such conclusion can be drawn that the interdecadal weakening of EAWM might be a regional response to the global warming caused by greenhouse effect.