Geoscience Reference
In-Depth Information
which has been related to the observed rapid decreasing trend in the seasonal number of
monsoon depressions.
Keywords Monsoon onset
Active-break cycle
Rapid warming of
Indian Ocean
Decadal variability of monsoon
Tropospheric biennial
oscillation
Long break monsoon spells
1 Introduction
The continental land mass (Eurasia) north of India gets heated up, and a giant low pressure
area is created there in the low levels of the atmosphere in summer as the Sun moves to the
northern hemisphere. Six months later with the Sun overhead the southern oceans, radi-
ative heat loss cools this continental land mass when the giant low pressure area gets
replaced by a large high pressure area. In response to these large-scale surface pressure
changes on the continental scale, we get the summer and winter monsoons of south Asia.
That continental scale land-sea thermal contrast (a huge land and sea breeze system) is the
main reason that monsoons exist held sway since the time of Halley (
1686
). That the ocean
with its Intertropical Convergence Zone (ITCZ) and the deep convective (cumulonimbus)
clouds associated with it plays a major role in generating the deep monsoon wind systems
due to the heating of a deep layer of the atmosphere by the latent heat released in the ITCZ
clouds is increasingly being considered as the main driving force of the monsoons. The
modelling study by Chao and Chen (
2001
) and the discussions in the following section on
monsoon onset over south Asia show that ocean is as important as the land in the Asian
summer monsoon. Figures
1
,
2
show that, as the ITCZ with its cloud systems moved from
south of the equator to north of it in the annual cycle, the winter monsoon wind system
changes to the summer monsoon wind system. Figure
2
b shows the low-level jetstream
(LLJ) at 850 hPa level as a mean of several July and August months.
Summer monsoon rainfall is a precious natural resource for the countries of south Asia,
but it is highly variable in space and time. Prediction of monsoon rainfall 7-10 days in
advance (medium range) has great applications in agricultural operations. Planners par-
ticularly in the government require monthly and seasonal predictions (long range) and also
predictions of inter-annual and decadal scale variability of monsoon. Recent research has
shown that monsoon variability on these time scales is highly dependent on the ocean
below, particularly on sea surface temperature (SST). We have to understand the coupling
between the atmosphere and the ocean on these time scales by data analysis, process
studies and ocean-atmosphere coupled modelling. In this paper, some aspects of the role of
the ocean in the variability of the Indian Summer Monsoon Rainfall (ISMR) of June to
September will be presented and discussed.
2 Monsoon Onset Over India
In January, the centre of the warm pool of the tropical oceans lies over southwest Pacific
Ocean. In the annual cycle, the central region of the warm pool gets shifted to the north
Indian Ocean by May as may be seen from Fig.
3
. This large-scale change in the ocean
SST is necessary for monsoon onset over south Asia. The warm pool of May attracts
