Abstract
By using multiple data sources and numerical simulation results from the atmospheric general circulation model (AGCM) of CAM4.0 (Community Atmosphere Model, version 4), we investigated the effect of thermal forcing over the Tibetan Plateau (TP) on the onset of the summer monsoon in the Arabian Sea (AS) and India. Results indicate that the spatial distribution of diabatic heating over the TP in May is a southeastern–northwestern inverse pattern. This diabatic heating shows a robust negative relationship with the onset date of the summer monsoon over the AS and India. A positive diabatic heating seesaw pattern can enhance the ascending (descending) motion over the southeastern (northwestern) TP, corresponding to above (below) normal in- situ precipitation. Temperature budget diagnosis reveals that the adiabatic heating due to anomalous vertical motion and relevant horizontal advection of temperature in the mid-upper troposphere are responsible for the warming over the TP. Consequently, the reversal of the meridional temperature gradient from winter to summer over South Asian regions occurs earlier than the climate-mean state. Furthermore, the diabatic heating over the TP induces an enhanced and westward-extended South Asian High (SAH), which together with the easterly along the southern flank of the SAH superimpose on the low-level westerly flow over the AS and India, results in intensive upper-level divergence-pumping and upward motion. This anomalous circulation configuration in lower and upper levels further facilitates an earlier onset of the summer monsoon in AS and India. These findings are corroborated in the sensitivity runs based on CAM4.0.
Highlights
The onset of the summer monsoon usually signals the seasonal transition from winter to summer in the Northern Hemisphere, characterized by the onset of the rainy season, large-scale convection, and abrupt transitions of largescale atmospheric circulation (Taniguchi and Koike 2006)
Using station datasets from 80 meteorological stations over the Tibetan Plateau (TP), JRA-55 data, and OLR and sea surface temperature (SST) from NOAA during 1979–2016, the connections between the thermal forcing of the TP and summer monsoon onset over South Asia, including the Arabian Sea (AS), India, and Bay of Bengal (BOB), were investigated
The relationship between the diabatic heating over the TP and the onset of the BOB summer monsoon mainly appears over the southern slopes of the TP. (2) The seesaw pattern of the diabatic heating anomaly over the TP in May can trigger an anomalous response of vertical motion over the TP
Summary
The onset of the summer monsoon usually signals the seasonal transition from winter to summer in the Northern Hemisphere, characterized by the onset of the rainy season, large-scale convection, and abrupt transitions of largescale atmospheric circulation (Taniguchi and Koike 2006). Previous studies have mainly attributed the substantial interannual variation of the SASM onset to the following factors: (1) the arrival of a wet phase of a low-frequency intraseasonal oscillation and Madden–Julian Oscillation (Taraphdar et al 2018); (2) the seasonal northward movement of the Intertropical Convergence Zone (Saha and Saha, 1980); (3) the generation of monsoon onset vortexes over the BOB and AS; (4) the role of SST anomalies and air–sea interactions, i.e., the AS warm pool, ENSO (Liu et al 2014b; Mao and Wu 2006; Vinayachandran et al 2007); (5) the establishment of the timing of the SAH over the Indochina Peninsula (Liu et al 2013; Wei et al 2014; Wu et al 2015); and (6) the mechanical and thermal effects of the TP (Mao and Wu 2006; Wu and Zhang 1998).
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