Abstract
The South Asian circulation and precipitation in spring shows a clear seasonal transition and interannual variation. We investigate how the North Atlantic sea surface temperature (SST) and Tibetan Plateau (TP) forcing affect this seasonal transition over South Asia on interannual timescale. Our results suggest that North Atlantic SST can affect the seasonal transition of South Asian monsoon via TP forcing in spring. The positive tripole pattern of North Atlantic SST anomaly during winter–spring can trigger a steady downstream Rossby wave train with cyclonic circulation over the southwestern TP. This forms a spring dipole mode of surface sensible heating and 10 m winds over the plateau, with a westerly (easterly) flow and positive (negative) surface sensible heating over its southern (northern) regions. A distinct land–air coupling configuration in May is then generated on the southwestern TP via such a positive TP dipole mode, which consists of anomalous positive precipitation, negative surface sensible heating and a baroclinic circulation structure with cyclonic circulation in the mid- to upper troposphere and a shallow anticyclonic circulation in the lower layer. The anticyclonic circulation is opposite to the summertime monsoon circulation. It weakens the cross-equatorial flow and water vapor transport to the South Arabian Sea and Bay of Bengal, resulting in in-situ precipitation reduction. Consequently, the seasonal transition in circulation over South Asia from winter to summer is delayed.
Highlights
In South Asia, the seasonal transition features as the onset of the monsoon associated with strong rainfall, which is usually considered to be a consequence of the atmospheric circulation response to the seasonal transition of the land–sea thermal contrast induced by the annual cycle of solar radiation (Webster et al 1998)
The partial correlation coefficient between V10I2 and seasonal transition index (STI) with removing the May Niño3.4 signal is − 0.54, nearly the same as the original correlation coefficient between V 10I2 and STI (− 0.55). These results imply that the spring dipole modes of sensible heating (SH) and V10 over the Tibetan Plateau (TP) affect the seasonal transition of the South Asian summer monsoon (SASM) and the associated precipitation over the South Arabian Sea (SAS), Bay of Bengal (BOB) and southwestern TP, which are independent of the El Niño–Southern Oscillation (ENSO)
Based on the observational analysis and numerical simulations, we find that the dipole modes of SH and V10 over the TP in spring are influenced by the winter–spring North Atlantic tripole SST anomaly (SSTA) pattern
Summary
In South Asia, the seasonal transition features as the onset of the monsoon associated with strong rainfall, which is usually considered to be a consequence of the atmospheric circulation response to the seasonal transition of the land–sea thermal contrast induced by the annual cycle of solar radiation (Webster et al 1998). Extensive researches have been conducted in exploring the factors affecting the interannual variability of the seasonal transition and precipitation over the SAS and BOB regions These include the El Niño–Southern Oscillation (ENSO) (Mao and Wu 2007; Deng et al 2016; Wang et al 2017), the interannual variability of the local sea surface temperature (SST) (Jiang and Li 2011; Yu et al 2012) and the timing of the South Asian High (SAH) establishment over the Indochina Peninsula (Wang and Guo 2012). The physical mechanisms of influence of spring TP forcing on the seasonal transition of the SASM and the associated circulation and precipitation over South Asia are investigated in Sect. A 2–9 years bandpass Lanczos filter was applied to these datasets after the linear trends had been removed to extract the signals of interannual variability (Sun et al 2019)
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