Geoscience Reference
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time scales. One of the major factors causing drought in ISMR is known to be the El Ni
˜
o,
but it is not clear how monsoon and El Ni˜o are physically related. The shallow MLD area
created over the west Pacific Ocean by the El Ni˜ o is hypothesised to cause long break
monsoon spells (long AB cycle) and thus create large negative anomalies in monsoon
rainfall of India. Modelling studies are required to verify this empirical finding. The paper
has also given a mechanism for the large inter-annual variability of ISMR during the
30-year-long dry epochs like 1961-1990 occurring during the cold phase of the Pacific
decadal oscillation/Atlantic multidecadal oscillation. This mechanism involves the inter-
action between the monsoon convective heat source and the southwards displaced mid-
latitude westerlies (and the subtropical jetstream) over south Asia. Modelling studies are
required to test this empirical finding also.
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