Role of regional thermal contrast over West Asia in interannual variation in atmospheric moisture transportation over the Indian Ocean and neighboring areas at summer monsoon onset

Abstract

AbstractThe low‐level temperature contrast over West Asia influences the interannual variation in water vapor transportation over the northern and equatorial Indian Ocean and neighboring monsoon area. A composite analysis that takes into account the thermal contrast over West Asia during the monsoon seasonal transition is performed based on the reanalysis and merged observational precipitation data sets. The positive (negative) low‐level thermal contrast anomaly over the Iranian Plateau (IP) strengthens (weakens) the thermal contrast over the Arabian Sea. The low‐level westerly anomaly develops earlier in the positive IP thermal contrast years than in the negative years. As a result, water vapor transport varies. This variation in water vapor transport in turn has an influence on the abrupt increase in precipitation over South Asia and the Arabian Sea and the decrease over equatorial East Africa. The variation in low‐level temperature over the IP precedes the variation of precipitation over these regions by a few pentads. A numerical experiment using the Lagrangian particle dispersion model agrees with the results of the composite analysis. Particles are emitted from the western tropical Indian Ocean region from the preonset to onset period. Results of numerical experiments concerning positive IP thermal contrast years show that particles can be transported into South Asia and the Arabian Sea before the climatological Asian summer monsoon onset pentad. However, small amounts of particles arrive in South Asia and the Arabian Sea at the onset period in negative IP years. The transport into equatorial East Africa becomes weak earlier in positive IP years.