The Interdecadal Weakening of the Relationship Between Indian Ocean Sea Surface Temperature and Summer Precipitation in Central Asia

The close relationship between the Indian Ocean Basin mode (IOBM) and summer precipitation in Central Asia (CA) has been documented in several studies. Nonetheless, this relationship has weakened since the 1990s and varies with the Atlantic Multi-decadal Oscillation (AMO) phase transition. During the cold phase of the AMO (1970-1998), precipitation in CA was significantly positively correlated with the IOBM. Conversely, during the warm phase of the AMO (1999-2019), this correlation became insignificant. The decrease in the interannual variation in the IOBM resulted in the weakening of the atmospheric heat source variation over the North Indian continent and the south-north movement of the subtropical westerly jet (SWJ). Along with the southerly SWJ, the IOBM exhibited only a weak positive correlation with precipitation in southern CA after the 1990s. This remarkable contrast in the impact of IOBM during different phases of the AMO offers intriguing possibilities for improving climate prediction in CA. The dominant role of the IOBM in influencing interannual changes in summer precipitation in CA (CAP) has been indicated in previous studies. However, such a high correlation is unstable and varies with the AMO. When the AMO is in its positive phase, the eastward Rossby waves triggered by the warm SSTs in the North Atlantic Ocean would lead to cyclonic anomalies and a southward shift in the SWJ over CA. Simultaneously, the warm SST anomaly in the Indian Ocean (IO) that is related to positive IOBM events is predominantly concentrated in its northern regions. This results in a smaller thermal difference between the tropical IO and the northern Indian continent, causing only a slight reduction in precipitation and the release of the latent heat of condensation in the eastern part of the North Indian continent. Consequently, this leads to weak cyclone anomalies in southern CA, and the SWJ moves weakly from north to south. Therefore, the southward SWJ during the positive AMO phase and the weaker south-north movement of the SWJ associated with the weakened interannual variation in IO-SST, collectively contribute to the absence of a robust relationship between the IOBM and CAP after the 1990s. The relationship between the summer Indian Ocean Basin Mode (IOBM) and Central Asian precipitation has weakened since the late 1990s The weakening of the interannual variation in the IOBM and the south-north movement of the SWJ primarily causes the unstable relationship The interdecadal shift of the Atlantic Multi-decadal Oscillation partly accounts for the instability in the relationship