Geoscience Reference
In-Depth Information
Role of Ocean in the Variability of Indian Summer
Monsoon Rainfall
Porathur V. Joseph
Received: 2 September 2012 / Accepted: 6 April 2013 / Published online: 12 February 2014
Springer Science+Business Media Dordrecht 2014
Abstract Asian summer monsoon sets in over India after the Intertropical Convergence
Zone moves across the equator to the northern hemisphere over the Indian Ocean. Sea
surface temperature (SST) anomalies on either side of the equator in Indian and Pacific
oceans are found related to the date of monsoon onset over Kerala (India). Droughts in the
June to September monsoon rainfall of India are followed by warm SST anomalies over
tropical Indian Ocean and cold SST anomalies over west Pacific Ocean. These anomalies
persist till the following monsoon which gives normal or excess rainfall (tropospheric
biennial oscillation). Thus, we do not get in India many successive drought years as in sub-
Saharan Africa, thanks to the ocean. Monsoon rainfall of India has a decadal variability in
the form of 30-year epochs of frequent (infrequent) drought monsoons occurring alter-
nately. Decadal oscillations of monsoon rainfall and the well-known decadal oscillation in
SST of the Atlantic Ocean (also of the Pacific Ocean) are found to run parallel with about
the same period close to 60 years and the same phase. In the active-break cycle of the
Asian summer monsoon, the ocean and the atmosphere are found to interact on the time
scale of 30-60 days. Net heat flux at the ocean surface, monsoon low-level jetstream (LLJ)
and the seasonally persisting shallow mixed layer of the ocean north of the LLJ axis play
important roles in this interaction. In an El Ni ˜ o year, the LLJ extends eastwards up to the
date line creating an area of shallow ocean mixed layer there, which is hypothesised to
lengthen the active-break (AB) cycle typically from 1 month in a La Ni ˜a to 2 months in
an El Ni˜ o year. Indian monsoon droughts are known to be associated with El Ni ˜ os, and
long break monsoon spells are found to be a major cause of monsoon droughts. In the
global warming scenario, the observed rapid warming of the equatorial Indian ocean SST
has caused the weakening of both the monsoon Hadley circulation and the monsoon LLJ
