The relative roles of upper and lower tropospheric thermal contrasts and tropical influences in driving Asian summer monsoons

Abstract

[1] Summer thermal structure and winds over Asia show a larger land-ocean thermal gradient in the upper than in the lower troposphere, implying a bigger role of the upper troposphere in driving the Asian summer monsoon circulation. Using data from atmospheric re-analyses and model simulations, we show that the land-ocean thermal contrast in the mid-upper (200-500 hPa) troposphere (TCupper) contributes about three times as much as the thermal contrast in the mid-lower (500-850 hPa) troposphere (TClower) in determining both the strength and variations of Asian summer monsoon circulations. Tropical sea surface temperature anomalies associated with the annual cycle, El Niño-Southern Oscillation, decadal changes, and global warming all are accompanied with much larger variations and changes in TCupper than in TClower, partly due to enhanced latent heating aloft from convection. The variations and changes in TCupper and TClower are highly correlated with the strength of the South Asian Summer Monsoon (SASM) and the East Asian Summer Monsoon (EASM) in their respective sectors during the past 50-60 years. In particular, the weakening of the EASM since the 1950s is caused by the weakening mainly in TCupper and secondarily in TClower induced mainly by recent tropical surface warming, although spurious cooling over East Asia seen in reanalysis data may have enhanced this weakening. However, the strength of the SASM and EASM monsoons follows TCupper but decouples with TClower in the global warming case in the 21 st century. The results suggest that the TCupper plays a dominant role and provides an efficient mechanism through which tropical oceans can influence extratropical monsoons.

Drying of Indian subcontinent by rapid Indian Ocean warming and a weakening land-sea thermal gradient

“…The tropospheric temperature gradient (T trop ) post-1950s also indicates a trend similar to that in the surface temperatures. This is suggestive of a warm Indian Ocean having a strong hold on the whole troposphere, from the surface to the top, possibly due to enhanced convective mixing over the ocean [33][34][35] . Indeed, a separate analysis of the trend at each of these levels gave consistent results of a warming troposphere over the Indian Ocean, and a weakening thermal contrast between the subcontinent and the ocean.…”

“…This may be attributed to the fact that the tropical Indian Ocean is a highly convective region during the northern summer, as the mean SSTs are above the minimum convective threshold (26°C) and the southwesterly monsoon winds are conducive for enhanced moisture convergence 32 . Further warming of the Indian Ocean enhances the convection 23 , transferring surplus heat to the upper troposphere [33][34][35] .…”

“…Many studies have shown that warm SST anomalies are accompanied by large variations in the lower and upper troposphere due to enhanced latent heating aloft from convection over the ocean [33][34][35] . Furthermore, these changes are highly correlated with the strength of the monsoon circulation 31,34 .…”

“…Many studies have shown that warm SST anomalies are accompanied by large variations in the lower and upper troposphere due to enhanced latent heating aloft from convection over the ocean [33][34][35] . Furthermore, these changes are highly correlated with the strength of the monsoon circulation 31,34 . A more recent study using a coupled model framework also points out that a spatial extension of Indian Ocean warm pool could enhance the convection over the ocean while introducing a dry bias over land, by modulating the meridional Hadley circulation 36 .…”

Consistent response of Indian summer monsoon to Middle East dust in observations and simulations

“…Net atmospheric heating, which is the sum of LW, SW, sensible heat, and latent heat, demonstrates the atmospheric heating effect of dust from the near surface to 400 hPa except for an anomalous cooling effect at 900 hPa, which is caused by cloud effects. Dai et al (2013) showed that the south-north ocean-land thermal contrast in the mid-upper troposphere is more important for the ISM formation than in the lower troposphere. The dust-induced atmospheric heating in the mid-upper troposphere is studied by selecting two subgroups of simulations with "wet" and "dry" rainfall responses in the WHI region.…”