晚秋前冬海洋性大陆区域向外长波辐射年际变化及其与云贵高原降水异常的联系

利用美国国家海洋和大气管理局(NOAA)向外长波辐射(OLR)月平均资料、欧洲中期数值预报中心(ECMWF)ERA-interim月平均再分析资料、全球降水气候中心(GPCC)降水资料及中国气象局国家气象信息中心提供的中国756站逐日观测资料,通过定义一个海洋性大陆区域对流强度指数(I_OLR),分析了海洋性大陆区域(Maritime Continent,MC)近35年来11月—次年1月对流活动特征,并揭示了11月—次年1月海洋性大陆区域对流活动强度的年际变化与同期云贵高原降水的联系。结果表明:海洋性大陆区域对流活动除了有逐渐增强的趋势外,还存在3—5 a及8—10 a的振荡周期。当海洋性大陆区域对流活动偏弱(强)时,云贵高原西部降水偏少(多),东部降水偏多(少),高原东西部之间降水分布差异加大(减小)。引起云贵高原降水异常的原因有3个方面:一是在海洋性大陆区域与云贵高原间存在显著的异常垂直环流圈,当下沉(上升)支位于海洋性大陆区域时,上升(下沉)支将位于云贵高原地区。而云贵高原地形可能对云贵高原降水异常在东南部和西北部的差别的产生存在影响;二是海洋性大陆区域在对流层低层的辐散和对流层上层的辐合运动为热带和高原以东地区提供了异常的位涡强迫,直接导致对流层低层南海—孟加拉湾地区异常反气旋和对流层上层位于中国南方的异常气旋性环流的产生;三是由于海洋性大陆区域辐散运动作为位涡制造而激发的位涡扰动的能量从热带地区向云贵高原及其东侧频散并辐合,对云贵高原上空扰动异常的维持起到了重要作用。这些结果有利于深刻理解云贵高原冬季降水异常的形成机理以及为寻找降水异常预测因子提供了有用的线索。

Interannual variations of OLR in the Maritime Continent in late autumn and early winter and their relationships with precipitation anomalies over the Yunnan-Kweichow Plateau

Based on the monthly mean outgoing long wave radiation (OLR) data from NOAA, the ERA-interim reanalysis from ECMWF, the precipitation data from GPCC and station precipitation data from NMC, by defining a convection intensity index (I_OLR) in the Maritime Continent region (MC hereafter), we have investigated the characteristics of convections over MC and the relationships between the convection variations over MC and the precipitation anomalies over the Yunnan-Kweichow Plateau (Y-K Plateau hereafter) in the period of late fall to earlier winter including November, December, and January from 1979 to 2013. It is found that the intensity of convections in MC increases linearly during the past 3 decades. Besides this interdecadal change, the convections in MC change with periodicities of 3-5 years and 8-10 years. When the I_OLR is positive (negative), i.e., the convections region are weaker (stronger) in MC, the precipitation is found to be less (more) than normal in the west of the Y-K Plateau and simultaneously more (less) in the east. There are three mechanisms responsible for the precipitation anomaly in the Y-K Plateau. Firstly, the anomalous vertical circulation between MC and the Y-K Plateau connects the downdraft (updraft) over MC and updraft (downdraft) over the plateau. It seems that the terrain may play a role in inducing the precipitation anomaly differences between the southeastern and the northwestern parts of the Y-K Plateau. Secondly, the divergent wind components in both the lower and upper troposphere as a potential vorticity source directly induce the vorticity anomalies in the lower latitudes and in the southern part of China. Thirdly, the anomalous potential vorticity in the tropical region north of MC perturbs the atmosphere, dispersing wave energy from the tropical region northward into the Y-K Plateau, facilitating the maintenance of an anomalous cyclonic circulation there, and inducing rainfall anomalies in the Y-K Plateau. These results are helpful for us to better understand the formation mechanisms of precipitation anomaly over the plateau and for us to find some useful clues to precipitation prediction there in wintertime.