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东亚冬季风年代际变化可能成因的模拟研究
Interdecadal Change of East Asian Winter Monsoon and a Numerical Experiment on Its Possible Cause
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摘要: 利用NCEP/NCAR再分析月平均海平面气压资料, 定义了一个东亚冬季风强度指数 (IWI), 并发现20世纪60年代到70年代初期东亚冬季风强度减弱, 其后冬季风强度有所增强, 80年代初期以后东亚冬季风强度又开始减弱。年代际时间尺度上, 冬季陆地表面气温与IWI的相关性比海洋与IWI相关性好, 冬季大陆东部的年代际增温与东亚冬季风的年代际减弱之间可能存在密切的联系。利用区域气候模式 (RegCM3) 进行敏感性数值模拟试验发现:减小冬季东亚大陆东北部的长波辐射降温率, 将导致东西向海平面气压差的减小以及低层北风减弱, 反映了东亚冬季风的减弱。近40年来, 特别是20世纪80年代以后东亚冬季风的年代际减弱趋势很可能是东亚区域气候对温室效应的一种区域响应。Abstract: 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 IWI, 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 IWI 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 IWI 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 IWI 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.
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图 1 1961—2000年逐年东亚冬季风强度指数 (细折线) 和11年滑动平均值 (粗曲线) 的变化以及逐年强度指数的线性拟合趋势 (虚线)
Fig. 1 The change of the IWI (thin curve) and its 11 years running mean value (thick curve) from 1961 to 2000, as well as the linear trend of the IWI (dashed line)
图 2 1961—2000年10 m高度上北风显著区域 (25°~50°N, 115°~145°E) 冬季平均经向风逐年 (细折线) 及其11年滑动平均值 (粗曲线) 的变化
Fig. 2 Change of annual mean 10 m meridional wind over prominent northern wind region (25°~50°N, 115°~145°E) in winter (thin curve) and its 11 years running mean value (thick curve)
图 3 模拟区域及其地形分布图 (阴影和等值线为海拔高度, 单位:m)
Fig. 3 The simulation region and its topography (the shaded and contours denote altitude, unit:m)
图 4 冬季敏感性试验 (DLR) 与控制试验 (CTL) 的模式最低层长波辐射降温率差值分布
(阴影区为正值, 单位:℃/d)
Fig. 4 The difference of long wave radiation cooling rate in the lowest level between DLR and CTL in winter
(the shaded area is positive, unit:℃/d)
图 5 敏感性试验 (DLR) 与控制试验 (CTL) 的冬季平均地面气温差值分布
(阴影区表示气温差≥0.5 ℃, 单位:℃)
Fig. 5 The difference of surface air temperature between DLR and CTL in winter
(the shaded area denote difference≥0.5 ℃, units:℃)
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