Location and variation of the summertime upper-troposphere temperature maximum over South Asia

The upper-troposphere-temperature-maximum (UTTM) over South Asia is a pronounced feature in the Northern Hemisphere summer. Its formation mechanism is still unclear. This study shows that the latitude location of the upper-tropospheric warm-center (T) coincides with the subtropical anticyclone, and its longitude location is determined by the zonal distribution of vertical gradient of heating/cooling (Q (z) = a,Q/a,z), which is different from the Gill's model. Since both convective heating and radiation cooling decrease with height in the upper troposphere, the heating/cooling generates vertical northerly/southerly shear, leading to a warm/cold center being developed between heating in the east/west and cooling in the west/east. The location of the UTTM coincides with the South Asian High (SAH) and is between a radiation cooling in the west and the Asian-monsoon convection heating source in the east. The UTTM is sensitive to this convective heating: increased heating in the source region in a general circulation model causes intensification of both the SAH and UTTM, and imposing periodic convective heating there results in oscillations in the SAH, UTTM, and vertical motion to the west with the same period. Diagnoses of reanalysis indicate that such an inherent subtropical T-Q (Z) relation is significant at interannual timescale. During the end of the twentieth century, rainfall increase over South China is accompanied by an increasing northerly flow aloft and intensification in the SAH and UTTM to the west. Results demonstrate that the feedback of atmospheric circulation to rainfall anomalies is an important contributor to the regional climate anomaly pattern.