P. N.
Vinayachandran*1 and J. Kurian*1, *2
*1Centre for Atmospheric and Oceanic Sciences,
Indian Institute of Science, Bangalore, India,
vinay@caos.iisc.ernet.in
*2 Present affiliation. Department of Atmo
spheric and Oceanic Sciences, UCLA, USA
ABSTRACT: The
Indian subcontinent divides the north Indian Ocean into two tropical basins,
namely the Arabian Sea and the Bay of Bengal.
The Arabian Sea has high salinity whereas the salinity of the Bay of
Bengal is much lower due to the contrast in freshwater forcing of the two
basins. The freshwater received by the
Bay in large amounts during the summer monsoon through river discharge is
flushed out annually by ocean circulation. After the withdrawal of the summer
monsoon, the Ganga – Brahmaputra river plume flows fi rst along the Indian
coast and then around Sri Lanka into the
Arabian Sea creating a low salinity pool
in the southeastern Arabian Sea (SEAS).
In the same region, during the pre-monsoon months of February – April, a warm
pool, known as the Arabian Sea Mini Warm
Pool (ASMWP), which is distinctly warmer
than the rest of the Indian Ocean, takes shape.
In fact, this is the warmest region in the world oceans during this
period. Simulation of the river plume and its movement as well as its implications
to thermodynamics has been a challenging probl em for models of Indian Ocean.
Here we address these issues using an ocean general circulation model – first
we show that the model is capable of reproducing fresh plumes in the Bay of
Bengal as well as its movement and then
we use the model to determine the
processes that lead to formation of the ASMWP.
Hydrographic observations
from the western Bay of Bengal have shown the presence of a fresh plume along
the northern part of the Indian coast during summe r monsoon. The Indian Ocean
model when forced by realistic winds and climatological rive r discharge
reproduces the fresh plume with
reasonable accuracy. The fresh plume does not advect along the Indian coast
until the end of summer monsoon. The North Bay Monsoon Current, which flows
eastward in the northern Bay, separates the low salinity water from the more
saline southern parts of the bay and thus plays an important role in the fresh
water budget of the Bay of Bengal. The model also reproduces the surge of the
fresh-plume along the Indian coast, into the Arabian Sea during
northeast monsoon.
Mechanisms that lead to the formation of the Arabian Sea Mini Warm Pool
are investigated using several numerical experiments. Contrary to the existing
theories, we find that salinity effects are not necessary for the formation of
the ASMWP. The orographic effects of the
Sahyadris (Western Ghat s) and resulting
reduction in wind speed leads to the formation of the ASMWP. During November – April, the SEAS
behave as a low-wind heat-dominated regime where the evolution of sea surface
temperature is solely determined by atmospheric forcing. In such regions the
evolution of surface layer temperature is not
dependent on the characteristics of the subsurface ocean such as the
barrier layer and temperature inversion.
