春季青藏高原表面感热加热的年际变化特征及其对印度夏季风爆发时间的影响

本文基于日本气象厅(JMA)的JRA-25再分析资料,分析了春季青藏高原表面感热加热年际变化的时空特征,及其对印度夏季风爆发过程的影响。EOF分析结果表明,春季高原感热加热的年际变化在高原中西部最为明显,这主要与局地地-气温差的年际变率有关。统计分析表明,当春季高原中西部表面感热偏强(弱)时,印度夏季风爆发偏早(晚),且高原中西部表面感热与ENSO事件无显著相关。春季高原中西部感热能够通过改变印度季风区对流层高层和低层的经向热力结构来影响印度夏季风的爆发时间。当春季高原中西部感热偏强时,造成的上升气流在高原以西的印度季风区北部下沉,通过绝热增暖引起局地对流层中上部的异常暖中心,令印度季风区对流层中上部平均温度经向梯度由冬至夏的季节性反转提早。同时,印度季风区北部的下沉运动能够抑制当地降水,令陆面温度升高,并通过非绝热过程造成对流层低层的异常暖中心,进一步增强了印度季风区的海陆热力对比。在印度季风区以北地区对流层高、低层异常增暖的共同作用下,印度夏季风提前爆发。

Interannual Variability of Surface Sensible Heating over the Tibetan Plateau in Boreal Spring and Its Influence on the Onset Time of the Indian Summer Monsoon

The JRA-25 reanalysis dataset provided by the Japan Meteorological Agency(JMA) was utilized to investigate the spatiotemporal characteristics of surface sensible heating over the Tibetan Plateau(TP) in boreal spring, as well as their effect on the onset of the Indian summer monsoon(ISM). The EOF results show that the most evident interannual variability of sensible heat occurs over the central-western TP in spring; this is primarily attributed to the interannual variability of the land-air temperature difference in situ. This is demonstrated by statistical analysis, which shows that the ISM builds up early(late) with strong(weak) spring sensible heat over the central-western TP, which is not significantly correlated with ENSO events. In fact, the spring sensible heat over the central-western TP could alter the ISM onset time by changing the meridional thermal structure in both the upper and lower troposphere over the ISM region. When the spring sensible heat over the central-western TP is stronger, local ascent is enhanced followed by descent to the west of the TP, which is to the north of the ISM region. Thus, the anomalous warming induced by the adiabatic process could accelerate the seasonal transition of the meridional mean temperature gradient in the middle and upper troposphere from winter to summer. Meanwhile, the subsidence anomaly to the north of the ISM region could prevent rainfall development in situ causing the land surface temperature to increase. Thereafter, the strengthened diabatic heating of the land leads to the warm center in the lower troposphere, increasing the land-sea thermal contrast in the ISM region. As a result, the anomalous warming in both the upper and lower troposphere contributes to the early onset of the ISM under the influence of the stronger spring sensible heat over the central-western TP.