The effect of anomalous snow cover over the Tibetan Plateau upon the South Asian summer monsoon is investigated by numerical simulations using the NCAR regional climate model (RegCM2) into which gravity wave drag has been introduced．
the sensitivity of the South Asian early summer monsoon to snow cover anomaly over the Tibetan Platea
The main results are summarized as follows
The heavier than normal snow cover over the Plateau can obviously reduce the shortwave radiation absorbed by surface through the albedo effect, which is compensated by weaker upward sensible heat flux associated with colder surface temperature, whereas the effects of snow melting and evaporation are relatively smaller.
The anomalies of surface heat fluxes can last until June and become unobvious in July．
The decrease of the Plateau surface temperature caused by heavier snow cover reaches its maximum value from late April to early May．
The atmospheric cooling in the mid-upper troposphere over the Plateau and its surrounding areas is most obvious in May and can keep a fairly strong intensity in June.
In contrast, there is warming to the south of the Plateau in the mid-lower troposphere from April to June with a maximum value in May.
The heavier snow cover over the Plateau can reduce the intensity of the South Asian summer monsoon and rainfall to some extent, but this influence is only obvious in early summer and almost disappears in later stages．
Deepen the east Asian trough
Induce off-equatorial ascending Rossby wave responses and land surface cooling in northeast Asia
Numerical simulations demonstrate that the thermal effects of large-scale orography, including the Tibetan and Iranian Plateaus (TIP), are crucial for the formation of the East Asian and South Asian summer monsoons (SASM) because the surface SH of the TIP is the major driver of the water vapor transport required for the genesis of the north branch of the SASM.
in the upper troposphere
Despite climate warming, the atmospheric heat source over the TP, particularly the spring SH, exhibits a clear weakening trend from the 1980s to 2000s.
However, few studies have focused on possible causes of the interannual variation of atmospheric heat source over the TP.
The atmospheric circulation patterns associated with the TH over the TP in June are different from those in July and August.
Enhanced WMC convection induces an anticyclone to the south of the TP, which favors water vapor transport to the southeastern TP and thus an increase in precipitation.
Enhanced convection over the southeastern TP may exert a positive feedback on local precipitation through pumping more water vapor from the southern boundary.