Influence of coastal Kelvin waves and local wind on the genesis and
characteristics of mesoscale eddies in the western Bay of Bengal
Abstract
The western region of Bay of Bengal (BoB) is of high importance because
of its variability in Western Boundary Current (WBC) pattern, numerous
eddy genesis and biogeochemical activities. The WBC is northward in
pre–Indian Summer Monsoon (ISM) season and southward in post–ISM
season with the presence of anticyclonic and cyclonic gyres
respectively. During ISM, the western BoB is dominated by various
cyclonic and anticyclonic eddies with discontinuous WBC. The seasonally
reversing wind pattern also favors the variation in the circulation
pattern of the bay. The mechanism of eddy genesis in the western BoB and
their characteristics specially during ISM, when the WBC is
discontinuous, is not studied well. Our study uses the Regional Ocean
Modeling System (ROMS) to investigate the role of coastal Kelvin wave,
which originated from the equatorial wave guide, and local wind on the
mesoscale eddies in the western BoB. To better understand the forcing
mechanism involved, river influx and modified wind fields are
incorporated in the experiments. The simulated Kelvin wave propagation
is consistent with the satellite altimetry observations. Two upwelling
and two downwelling Coastal Kelvin Wave (CKW) signals are identified.
First upwelling signal (UCKW1) exists from January to April whereas the
second signal (UCKW2) appears in September. Two downwelling signals are
seen from May to August (DCKW1) and October to December (DCKW2),
respectively. Depth of 20°C isotherm (D20) shows good correlation with
the remote forcing signals as well as surface wind variability. The CKW
signals during pre– and post–ISM (UCKW1 and DCKW2 respectively) are
dominant. The reversing weaker signals (DCKW1 and UCKW2) contribute to
the WBC discontinuity during ISM. Reduction in wind fields results in
weak WBC during pre– and post–ISM. Increase in wind field results in
strengthening of the weaker CKW modes rather than strengthening the WBC.
Average surface eddy kinetic energy along with eddy intensity, radius
and lifetime are sensitive to the change in wind field. The eddies are
seen to propagate clockwise (anticlockwise) in general due to decrease
(increase) in wind field. The study indicates that the CKWs and the
local wind have significant impact on seasonally reversing WBC, eddy
genesis and their characteristics in the western BoB.