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with higher convection and heating of the atmosphere (in a deep layer) in the
tropical warm pool area, the FDO in SST difference (A-B) is hypothesized to
be the cause of the FDO in the 850 hPa wind in the equatorial box.
4. Decreasing Trend in MD Frequency and Monsoon LLJ
Earlier studies, like Dash et al. (2004), suggested that the decrease in MD
frequency may be due to the decrease in the horizontal and vertical wind shears
of the mean monsoon flow over India as well as over the Bay of Bengal and
decrease in the moisture and convection over the Bay of Bengal area. We find
that the decreasing trend in the frequency of MD is associated with the
decreasing trend in the strength of the Low Level Jetstream (Joseph and Raman,
1996; Findlater, 1969) passing through peninsular India during the period since
1950 as documented by Joseph and Simon (2005). Strong 850 hPa flow through
peninsular India as a LLJ is an important synoptic condition for the genesis of
MD (Sikka, 1977). It has been shown by Joseph and Sabin (2008) that the SST
of the equatorial central Indian ocean has warmed at a very rapid rate in the
global warming scenario since 1950 and this warming and the consequent
increasing deep convection there was hypothesized to weaken the monsoon
Hadley circulation leading to a decreasing trend in the strength of monsoon
LLJ. That equatorial Indian Ocean has warmed rapidly in the global warming
is shown by Fig. 3a. The mean SST June to September over the equatorial box
(5
o
S-5
o
N, 70
o
E-90
o
E) during 1950 to 2009 and its significant increasing linear
trend are shown in Fig. 3b. The increase in convection (decrease of outgoing
longwave radiation) consequent to the increasing SST may be seen in Fig. 3c.
The decreasing trend of the strength of the LLJ at 850 hPa through the area
10
o
N-20
o
N, 75
o
E-90
o
E may be seen in Fig. 3d.
5. Climate Change in the Intensity of Cyclones
Singh (2000) showed a linear increasing trend in the five-year moving average
of the annual frequency of severe cyclones of north Indian Ocean during the
period from 1891 to recent years.
The ratio between frequencies of severe TC
(systems with surface wind 48 knots and more) and TC which may be taken as
an Intensification Index has a prominent long-term increasing linear trend from
1891 to 2009 as may be seen in Fig. 4a. The plots of the frequency time series
separately for cyclones and severe cyclones of north Indian Ocean during the
same period are given in Fig. 4b. The two curves are close to each other in the
period where satellite data was used i.e. from 1965 to 2009. Our interpretation
is that during the satellite era, with the better monitoring capabilities available,
more cyclones were declared as having intensified into severe cyclones. Figure
4a also gives the linear trends in the non-satellite period 1891 to 1964 and the
satellite era 1965 to 2009 separately. The large shift in the trend curves show
the difference between the satellite era and the years before. It is inferred that
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