热带西北太平洋10~30 d振荡对南海夏季风影响

采用1958—2011年NCEP/NCAR再分析资料以及ERSST海温资料，分析热带西太平洋夏季对流10~30 d振荡对南海夏季风的影响。在年际变化尺度上，热带西北太平洋夏季10~30 d振荡强度指数 (TWPI) 与南海夏季风强度有很好的正相关关系。在TWPI增强年份，海温主要呈El Ni?o分布，南海周边区域增强的异常西风产生强的正涡度切变，导致异常气旋性环流，为季风槽的增强提供了热量和水汽，从而增强南海夏季风强度。反之，在TWPI减弱年份，海温主要呈La Ni?a分布，南海夏季风强度减弱。在不同的年代际背景下，垂直切变和水汽-对流的总体变化是影响TWPI总体变化的重要因子，但不能影响南海夏季风强度的总体变化。海陆热力对比的总体变化是导致南海夏季风强度总体变化的主要影响因素。

Impact of 10-30 day Oscillation Intensity over the Tropical Northwest Pacific Ocean on the South China Sea Summer Monsoon

Based on the NCEP reanalysis data and ERSST sea surface temperature (SST) data, using statistical methods, 10-30-day significant sub-seasonal variability periods are extracted from the summer (Jun-Aug) convective in the tropical western Pacific, and these oscillations have different effects on the South China Sea summer monsoon intensity at different scales. At inter-annual time scale, a positive significant correlation is found between the intensity variation of 10-30-day oscillation over the tropical northwest Pacific Ocean regions (TWPI), and the correlation coefficient is 0.635. Influences of TWPI on the South China Sea summer monsoon intensity are mainly regulated by ENSO. Because of the asymmetric response of the lower troposphere Northwest Pacific atmospheric circulation to ENSO, TWPI is much more significant in El Ni?o developing years than in La Ni?a years. During strong TWPI years, the SST anomalies are El Ni?o pattern, which induces anomalously enhanced westerly in the South China Sea, the Philippines and the tropical northwest Pacific Ocean. The westerly anomalies generate strong positive vorticity shear, resulting in abnormal cyclonic circulation, and enhance TWPI and summer monsoon intensity through the wind-evaporation feedback mechanism. On the contrary, in La Ni?a years, the anticyclonic anomaly circulation result in TWPI weakening and the monsoon strength weakening. Under different inter-decadal backgrounds, TWPI does not show a significant change in the decadal trend, mainly slightly weaker (1958-1976), slightly stronger (1977-1993) and slightly weaker (1994-2011) change. The trend of the summer monsoon intensity inter-decadal changes are more obvious, namely, much stronger, slightly weaker and much weaker. The overall change of vertical shear and water vapor-convection are consistent with TWPI, but are not consistent with the summer monsoon. Vertical shear of wind field and water vapor-convection play important roles on the inter-decadal variation of TWPI, but not for the summer monsoon. The thermal contrast between sea and land is the key factor that leads to the inter-decadal change in the South China Sea summer monsoon.