Geoscience Reference
In-Depth Information
Fig. 1 Outgoing longwave
radiation as mean of 1979-2010
a January and February and
b July and August
(
a)
(
b)
moisture convergence and over a period of more than a month builds up the vertically
integrated moisture content of the atmosphere over south Asia and the oceans around to
about 45 kilograms per square metre which is needed for monsoon onset to take place—
Pearce and Mohanty (
1984
) and Joseph et al. (
2006
). There is a fine structure to the
changes in the warm pool over north Indian Ocean. About 40 days (8 pentads) before the
monsoon onset over the southern state of India (Kerala), the central Bay of Bengal (BoB)
develops high SST and to its south, in the area of large north-south SST gradient, a band of
convective clouds form near the equator. These clouds cause westerly low-level winds to
its south and this cloud-wind system moves north during the following few pentads cooling
the ocean and in many years bringing onset of monsoon rains over southeast Asia. Later a
similar warming occurs in central Arabian Sea when a cloud band forms over south
Arabian Sea close to the equator and, as it develops and moves north, the cross-equatorial
LLJ as documented by Joseph and Raman (
1966
) and Findlater (
1969
) forms and inten-
sifies. These changes herald the monsoon onset over Kerala, at the southern tip of India.
Figure
4
shows the warming and cooling of the BoB and the Arabian Sea in association
with the monsoon onset during a typical year 2003. Pentad zero is the pentad around
monsoon onset over Kerala.
The birth of LLJ coincides with the time of monsoon onset over Kerala (Joseph et al.
2006
), and it lasts during the four monsoon months June to September with major fluc-
tuations in its strength and spatial location during the active-break (AB) cycle of the
monsoon as shown by Joseph and Sijikumar (
2004
). Summer monsoon onset over south
Asia (Kerala) is found to be related to the time of transition across the equator of the ITCZ
over the Indian and west Pacific oceans from south of the equator to the north of it, which
is related to the large-scale SST anomalies in both these oceans around the equator as
