江南雨季降水与前期西太平洋暖池热含量异常的关系及其可能机制

利用日本气象厅提供的历史海温资料、Hadley环流中心逐月海表温度(Sea Surface Temperature,简称SST)资料、美国NCEP/NCAR再分析资料以及江南地区逐旬降水资料,研究江南地区4—6月(江南雨季,亦泛称为华南前汛期)降水与前期暖池热含量异常的关系,并对可能的影响机制进行分析。研究结果表明,前期暖池热含量与江南雨季降水有密切的负相关关系,前期7—8月暖池关键区(130. 5°～150. 5°E,3. 5°～11. 5°N)热含量高(低)可以作为预报江南雨季旱(涝)的一个很好的指标。前期暖池热含量异常对4—6月环流和降水有重要影响。冷水年,菲律宾异常反气旋导致副高西伸加强,显著加强了其西侧暖湿气流向江南地区输送,高层辐散抽吸作用导致江南地区对流上升运动增强,暖水年相反,表明冷(暖)水年江南雨季降水偏多(少)。就影响机制而言,在前期夏季,关键区南侧存在异常强西风,导致在秋末形成了菲律宾异常反气旋,以及关键区附近(东侧)有冷(暖)海表温度异常发展,在当年春季和夏初该反气旋移到菲律宾以北。直到4月,次表层冷水团上传导致冷SST异常维持并加强了该异常反气旋,其西侧西南暖湿气流将水汽从南海和菲律宾海地区源源不断地向江南地区输送。同时,西印度洋暖海温和赤道印度洋东风异常也逐渐发展增强,在热带印度洋形成东西向异常垂直环流,其下沉支始终在西太平洋维持,导致了菲律宾异常反气旋的维持,并进一步引起江南地区的水汽辐合和上升运动。同时,副热带西风急流轴南压引起的高空强辐散,也有利于上升运动和对流活动在江南地区发展。正是上述过程和机制,导致了前期热含量异常偏低(高)时,我国江南雨季降水偏多(少)。

Relationship between the precipitation during the Jiangnan rainy seasons (JRS) and the preceding heat content anomalies over the western Pacific Ocean warm pool and its possible mechanism

In this research study,historical SST data provided by the Japan Meteorological Agency and Hadley Center,along with NCEP/NCAR reanalysis data,and the ten-day precipitation data in Jiangnan (25°-28°N,110°-120°E;112 stations),were used to examine the relationship between the yearly precipitation in Jiangnan from April to June(Jiangnan rainy season(JRS),also referred to as the first flood period in South China) and the preceding heat content anomalies over the western Pacific Ocean warm pool.Next,the possible mechanism for this relationship was investigated.In this study,consideration was given to the depth of the lower boundary of the thermocline,and the mean temperature in the 0 to 400 m range was defined as the value of the heat content in the upper level of the ocean.Due to the interdecadal increases in the SST,the linear trends of the calculated heat content and precipitation during the Jiangnan rainy season(JRS) were removed,and only the interannual relationship was discussed.Furthermore,the statistical methods which were used in this study included time-delay correlations,regression,and synthetic analyses.The results showed that the precipitation levels during the Jiangnan rainy seasons were negatively related to the heat content anomalies over the warm pool of the western Pacific Ocean.It was found that the heat content over the key areas (3.5°-11.5°N,130.5°-150.5°E) during the months of July and August could be regarded as a good precursor of the winter rainfall.The preceding heat content was found to have important influences on the precipitation and circulation from the month of April until the following June.During the cold years,it was been observed to be negative over northern Pacific Ocean and positive over the western Pacific Ocean warm pool at 500 hPa,which has been referred to as the anti-WP teleconnection waves or the enhancement of the subtropical highs.These are controlled by the anomalous anticyclone activities around the Philippines,and the anomalous cyclone activities at high latitudes over the western Pacific Ocean at the 850 hPa wind field,with distributions of significant anomalous anticyclonic water vapor flux around the Philippines.The anomalous anticyclone activities over the Phillipines tend to intensify the western extensions of the subtropical highs,which subsequently strengthen the transportation of the warm and wet water vapor flow from the west side of the anticyclone regions to Jiangnan.Also,with the divergences in the upper troposphere,the anomalous convective rising motions become enhanced in Jiangnan,and vice versa.These enhancement trends indicate that the precipitation which occurs during the Jiangnan rainy season will be more(less) during the cold(warm) years.In regard to the mechanism,the strong westerly anomalies to the south of the key areas during the previous summer will result in the development of anomalous anticyclone activities in the Philippines during the late autumn months,and cold(warm) SSTs to the west(east) of the key areas.Up until the month of April,the upwelling of the subsurface cold water will sustain the cold SSTA and enhance the anticyclone activities,which continuously transports water vapor to Jiangnan.Meanwhile,the anomalous zonal vertical circulations tend to become enhanced in the tropical Indian Ocean with the development of warm SSTs in the western Indian Ocean,and easterly anomalies in the Equatorial regions.Then,the sinking branch in the western Pacific Ocean sustains the anomalous anticyclone activities,which further induces the water vapor convergence and rising motions in Jiangnan.Moreover,the upper strong divergences which are induced by the southward movements of the subtropical westerly jet stream also favor the increasing convective rising motions in Jiangnan.Therefore,the above-mentioned processes lead to increased (lower) rainfall levels during the Jiangnan rainy season,which are known to be related to the preceding negative (positive) heat content anomalies over the key areas.