Geoscience Reference
In-Depth Information
An Analysis of Environmental
Dynamical Control of Tropical
Cyclone Intensity over the Bay of
Bengal during 1981-2010
S.D. Kotal* and S.K. Roy Bhowmik
India Meteorological Department, New Delhi - 110003, India
*e-mail: sdkotal.imd@gmail.com
1. Introduction
The intensity of tropical cyclones (TCs) is quite sensitive to SST, which mainly
determines the surface energy transfer from the ocean to the TC. However,
strong surface wind stress under a TC can generate strong turbulence in the
upper ocean and deepening the ocean mixed layer. This causes significant
decreases in SST due to entrainment of cooler water from the thermocline into
the mixed layer. The observed SST cold anomalies under TC core vary from
1° C to 6 °C (Sanford et al., 1987; Shay et al., 1992; Lin et al., 2003). The
reduction of SST can reduce enthalpy flux from the ocean to the atmosphere,
leading to a decrease in TC intensity. This feedback mechanism limits the TC
intensity, which also depends on the speed of movement of the TC. Wang and
Wu (2004) showed that the intense TCs are highly correlated with their speed
of movement and very intense TCs can only develop within the range of
translation speeds between 3 ms
-1
and 6 ms
-1
(in the Australian region) and
weaker cyclones form as the speed of movement increases. Zeng et al. (2007)
also showed the decreasing upper-bound intensity with increasing translational
speed and very intense TCs (with maximum wind speed greater than 65 ms
-1
)
which could only develop under a narrow range of translational speeds between
3 ms
-1
and 8 ms
-1
(in the western North Pacific).
Previous studies (Wu and Cheng, 1999; Emanuel et al., 2004; Wang and
Wu, 2004) also showed that the large-scale environmental dynamical forcing,
such as the vertical wind shear, is a key factor affecting TC intensity. It has
been long known that among different kinds of external forcing, the vertical
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