The mean state and year-to-year variations of the tropospheric temperature fields are analyzed in light of their relationship with the establishment of the East Asian summer monsoon (EASM), and the Indian monsoon (INM). Primary data for the analysis include the National Centers for Environmental Prediction-National Center for Atmospheric Research (NCEP-NCAR) reanalysis from 1982 to 1996 and the European Center for Medium-Range Weather Forecast (ECMWF) Reanalysis (ERA) from 1979 to 1993. The analysis reveals that, in most of the analyzed years, the meridional temperature gradient in the upper troposphere reverses at, or before the onset time of the summer monsoon in both the EASM region and the INM region. On the average, the reversal time of the meridional temperature gradient in the EASM region (INM region) is concurrent with (one pentad earlier than) the onset time of the summer monsoon. A budget analysis shows that the strong horizontal warm advection and the diabatic heating are the main contributors to the upper tropospheric warming against the strong adiabatic cooling during the pre-onset period over the EASM region. Over the INM region, however, strong adiabatic heating due to subsidence is the major warming process against diabatic cooling, and the strong horizontal cold advection related to the persistent northwestlies to the southwestern periphery of the Tibetan Plateau. The results show that the seasonal shift of the South Asian High in the upper troposphere, and the establishment of the EASM and the INM are closely related to the seasonal warming, which results in the reversal of the meridional temperature gradient in the upper troposphere over the two monsoon regions.

The dominant pattern of summertime tropical and North Pacific sea surface temperature (SST) is characterized by an out-of-phase relationship between the tropics and the extratropics. This pattern, defined as the tropical-North Pacific mode (TNPM) in this study, is closely correlated with the variability of climate over Asia and the Pacific Ocean. A high TNPM index, with positive (negative) SST anomalies over the extratropics (tropics) of the Pacific, is linked to deep negative anomalies of tropospheric temperature over the extratropical Pacific, with shallow positive anomalies in the lower troposphere, and is also linked to deep positive temperature over Asia. It is also found that these anomalies of tropospheric temperature and SST are significantly related to the Asian-Pacific Oscillation (APO), an extratropical zonal-vertical atmospheric pattern connecting Asia and the Pacific. Indeed, when the variability of APO is removed, the above-described climate anomalies weaken significantly. Although the above relationships observed between atmospheric circulation and SST can be captured by general circulation models, sensitivity experiments show that the variations of summertime AsianPacific atmospheric circulation may not be mainly forced by the Pacific SST. Instead, the Asian land elevated heating seems to play a more important role in generating the climate anomalies, as shown by model-sensitivity experiments in which changes in topographic height are included. Moreover, the relative importance of Asian land and Pacific SST for the variations of Asian-Pacific climate in summer and winter is compared in this study. In winter the most dominant mode of Pacific SST exerts a stronger impact on the Asian-Pacific climate.

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The intensity of interannual variation of spring precipitation over southern China during 1979–2014 and possible reasons for it are investigated in this paper. There is a significant interdecadal change in the intensity of interannual variation of spring precipitation over southern China around 1995/96. The intensity of interannual variation of spring rainfall over South China is stronger during 1979–95 than that during 1996–2014. The possible reason may be the larger amplitude of the sea surface temperature anomaly (SSTA) in the western Pacific Ocean (WP) before 1995/96. The cooler (warmer) SSTA in WP may trigger an abnormal local anticyclone (cyclone) at lower levels. The anomalous southwesterly (northeasterly) flow at the northwestern flank of the WP anticyclone (cyclone) covers South China, transporting more (less) moisture to South China. Meanwhile, the anomalous winds converge (diverge) in South China at lower levels and diverge (converge) at upper levels, which causes the anomalous ascent (descent) to enhance (reduce) the precipitation over there. However, during 1996–2014, the intensity of interannual variation of spring rainfall over the middle and lower reaches of the Yangtze River valley becomes much stronger than that during 1979–95, which is related to the intensified interannual variation of the atmospheric circulation in the middle and high latitudes over Eurasia. The weak (strong) Siberian high and East Asian trough may reduce (enhance) the northerly wind from the middle and high latitudes. As a result, the middle and lower reaches of the Yangtze River valley are subjected to the anomalous southerly wind, favoring more (less) precipitation over there.