The onset and cessation of rainy season over the Hengduan Mountains

In this study, the onset and cessation of rainy season over the Hengduan Mountains (HM) are determined by an objective method. The onset and cessation of climatological rainy season are largely controlled by large-scale circulation systems, such as the Indian summer monsoon (ISM), 200 hPa subtropical jet, and the South Asian high. Both onset and cessation dates exhibit strong interannual variability. For late onset, the lower-level northeasterly anomalies over the north Indian Ocean (IO) delay the advance of moisture transported by ISM, and are excited by the positive SST anomalies over the southwest IO. In accord with late cessation, the equatorial west IO cooling adjusts the zonal overturn circulation and causes the west side wet anomalies, which trigger the north IO cyclonic wind anomalies and the northwest Pacific anticyclonic wind anomalies following Gill response. These two anomalous flows merge, turn southerly, and continuously transport moisture to the HM, prolonging the rainy season. At upper levels, the late onset and cessation related circulation anomalies are almost mirror images and highly resemble the Silk Road pattern. The anomalous upper and lower-level circulation cooperate and favor the dynamical and thermodynamical processes, which contribute to the vertical motion anomalies and finally affect the onset and cessation dates. Especially, the 200 hPa geopotential height anomaly center on the north side of the Arabian Sea (AS) is stronger than the other centers both for late onset and cessation, and coincides well with the nearby Rossby wave source, which are reinforced via the planetary vorticity stretching term due to the upper-level wind anomalies induced by the IO SST anomalies.

Automated summary

The onset and cessation of rainy season over the Hengduan Mountains (HM) are largely influenced by large-scale circulation systems and exhibit strong interannual variability. Delayed onset is caused by northeasterly anomalies over the north Indian Ocean and positive sea surface temperature (SST) anomalies over the southwest Indian Ocean. Delayed cessation is associated with cooling in the equatorial west Indian Ocean, triggering cyclonic wind anomalies and northwest Pacific anticyclonic wind anomalies. Upper and lower-level circulation anomalies cooperate to affect the onset and cessation dates, with the Arabian Sea playing a significant role.