Intraseasonal oscillation of the southwest monsoon over Sri Lanka and evaluation of its subseasonal forecast skill

Abstract Sri Lanka, a small island country located near the southernmost end of the Indian subcontinent, is controlled by the southwest monsoon (SWM) during May to September, when it suffers the most accumulated rainfall in a year. Compared with extensive studies on the intraseasonal oscillation (ISO) of the Indian monsoon, less attention has been paid to the ISO of the SWM over Sri Lanka. Based on observational data, this study reveals that the leading mode of SWM rainfall shows a significant variability on a 10–35-day time scale, and it accounts for 66% of the fractional variance. The development of the intraseasonal rainfall anomaly is associated with a westward propagating anomalous cyclonic circulation. Furthermore, the skill of current dynamic models in simulating the SWM on the subseasonal time scale was evaluated by using the ECMWF (European Centre for Medium-Range Weather Forecasts) reforecast data from S2S (the Subseasonal to Seasonal Prediction project). In general, the model is more skillful in predicting the monsoonal wind index than the monsoonal rainfall index, with the skill for the former being beyond 30 days and the latter about two weeks. The forecast skills exhibit prominent interannual differences for both indices. It is suggested that a correct simulation of the large-scale circulation response to tropical convection is crucial for the subseasonal prediction of monsoonal rainfall over Sri Lanka. 摘要 斯里兰卡的雨季发生于5–10月间, 主要受西南季风的控制.本文发现该地区的西南季风降水存在很强的次季节变率, 主导周期为10–35天.降水的季节内变化与西传的异常气旋有关.进一步, 利用S2S比较计划中欧洲中心的数值预报模式 (ECMWF) 提供的回报试验数据, 评估了当今动力模式对斯里兰卡西南季风次季节变化的预报技巧.结果显示, 对季风指数的预测技巧超过30天, 而对降水指数的预测技巧大约两周, 且模式的预报技巧具有明显的年际差异.分析表明, 能否正确模拟出大尺度环流对热带对流的响应是影响斯里兰卡降水预测的重要因子.