Abstract
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 = ∂Q/∂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.
Highlights
In the boreal summer half of the year there exists in the upper troposphere of the Eurasian subtropics a huge high pressure system centered over South Asia southwest of the Tibetan Plateau (TP), which was formerly known as the Tibetan High (Ye and Gao 1979) but is referred to more frequently as the South Asian High (SAH)
The center of the climate mean SAH first appears in April over the South China Sea (SCS) before the onset of the Asian summer monsoon (ASM) (Liu et al 2012a, 2013a); it gets strengthened over the Indochina Peninsula during the AMS onset, which continues for more than 1 month from the end of April/early May over the Bay of Bengal (BOB) region until the end of May/early June over North India (Wu and Zhang 1998; Wang and Lin 2002); and it settles over the climate mean region until autumn
The classical dynamics has demonstrated that in response to an axisymmetric diabatic heating, the atmospheric circulation adopts two distinct regimes: the thermal equilibrium (TE) regime in extra-tropics and the angular momentum conservation (AMC) regime in tropics, and in the upper troposphere the maximum temperature and geopotential height should be located in subtropics
Summary
In the boreal summer half of the year there exists in the upper troposphere of the Eurasian subtropics a huge high pressure system centered over South Asia southwest of the Tibetan Plateau (TP), which was formerly known as the Tibetan High (Ye and Gao 1979) but is referred to more frequently as the South Asian High (SAH). Wu and Liu (2003) and Wu et al (2009) found that the summertime circulation configuration in the subtropics is a consequence of the atmospheric response to an organized continental-scale quadruple diabatic heating (LOngwave radiation cooling, surface SEnsible heating, COndensation heating, and Double dominant heating; called LOSECOD), together with local-scale sea breeze forcing and regional-scale mountain forcing. In this regard, the huge and strong SAH is due mainly to the large longitudinal span of the Eurasian continent and the large size and high elevation of the Iranian Plateau and the TP along the subtropics.
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