Toward role of westerly‐monsoon interplay in linking interannual variations of late spring precipitation over the southeastern Tibetan Plateau

Abstract

Abstract A recent study reported a notable westerly‐monsoon interplay (WMI) to the northeast of the Tibetan Plateau in boreal summer. However, this study introduces another striking and unique WMI over the southeastern TP (SETP) in late spring (May), emphasizing the critical roles of the upstream mid‐latitude westerly and the downstream Bay of Bengal (BOB) summer monsoon (BOBSM) in linking interannual variations of precipitation over SETP. In this study, the research SETP domain (22.5°–27.5°N, 87.5°–97.5°E) as well as a westerly index (WI) over the pivotal upstream westerly domain (22.5°–30°N, 65°–80°E) and a monsoon index (MI) over the pivotal downstream BOBSM domain (12.5°–22.5°N, 87.5°–100°E) are defined to investigate connections between the westerly and BOBSM and their combined effects on precipitation over SETP. The results indicate that the mid‐latitude westerly and BOBSM show opposite effects on precipitation over SETP. The weaker westerly and the stronger BOBSM correspond to enhanced SETP precipitation, and vice versa. The enhanced BOBSM acts as a predominant role on the strengthening of precipitation, while the decelerated westerly plays a secondary dynamical amplification role. To quantify their synergistic roles in linking SETP precipitation, a WMI index is defined. Statistical analysis suggests that this newly defined WMI index has a higher positive correlation with the precipitation over SETP. The correlation has a prominent increase compared to individual correlations between WI/MI and SETP precipitation, and dynamic conditions in high WMI index years are more favorable for the formation of SETP precipitation. The possible mechanism is that the BOBSM southerly strengthens and the southern branch of the upstream mid‐latitude westerly weakens in high WMI index years, resulting in an anomalous cyclone over the northwestern BOB and an anomalous anticyclone over the western TP. The anomalous cyclone resembles a Rossby wave response to the increased precipitation over SETP, and the precipitation in turn contributes to maintaining the BOB cyclonic anomaly and the above anticyclonic anomaly via the latent heating release. This positive feedback effect is conducive to enhanced SETP precipitation.

Keywords

• Bay of Bengal summer monsoon
• interplay
• late spring
• mid‐latitude westerly
• southeastern Tibetan Plateau