Title: Changes in the relationship between Indian Ocean dipole and Indian summer monsoon rainfall in early and recent multidecadal epochs during different phases of monsoon
Authors: Palakkal P.V.H. Hrudya, Hamza Varikoden, Rajagopalan N. Vishnu
Journal: International Journal of Climatology
The monsoon rainfall over the Indian region, particularly during the months of June through September (known as Indian summer monsoon rainfall or ISMR), has been undergoing significant changes in its regional distributions for the last few decades. Since the regional variations of ISMR are highly influenced by different phenomena in the surrounding oceans, multiple studies have been carried out to find out the impacts of oceans (mainly the Indian and Pacific Oceans) on the regional variations of ISMR. However, the distribution of rainfall during summer monsoon period is different for the onset (the month of June), peak (July-August), and withdrawal (September) phases. Moreover, the impacts of the Indian Ocean on the ISMR variability have been increased in recent decades. The major phenomenon in the tropical Indian Ocean that affects the ISMR for the last few years is the Indian Ocean Dipole (IOD), which occurs as a part of the interaction between ocean and atmosphere. Hence, this work was carried out to explore the changes in the relationship between IOD and ISMR from early decades (1951-1980) to recent decades (1986-2015). The changes were analyzed separately for onset, peak, and withdrawal phases.
The impacts of IOD on ISMR have noticeably changed in recent years during the onset, peak and withdrawal phases. These changes are related to variations of the low level wind circulations and sea surface temperatures over the equatorial Indian Ocean. As a result, the moisture transport and thus the rainfall patterns over the Indian region are also undergoing significant variations during both the positive and negative IOD events. There is a considerable increase in ISMR over most of the Indian regions during positive IOD events in recent decades during all the phases. These changes are in accordance with the decrease in sea surface temperature and the strengthening of low level winds over the western equatorial Indian Ocean. The reverse condition (decrease in ISMR) is observed for negative IOD events, in accordance with the weakening of low level winds, but for the peak and withdrawal phases. These associations between the surface temperatures and the wind circulations over the equatorial Indian Ocean with the ISMR are clearly observed during all the three phases in recent years and different from one another. However, the peak and withdrawal phases show the most noticeable changes. On the whole, the understanding of ISMR variations within each phase and its influencing mechanisms are highly relevant for better understanding of climate variabilities and thus water management, especially for a country like India where a large portion of society depends upon agriculture.