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3. FDO in Cyclones, Equatorial Wind and
SST of Indian Ocean
Xavier and Joseph (2000) reported a decadal scale variation in VWS over NIO
which possibly influenced the decadal oscillation in the frequency of TC.
Pattanaik (2005) suggested that it is not SST but the large scale atmospheric
circulation that is responsible for the interdecadal variability of tropical cyclones
of NIO. The October to December (main season of TC) 850 hPa zonal wind
averaged over an equatorial box bounded by latitudes 2.5
o
S and 7.5
o
N and
longitudes 50
o
E and 100
o
E had a FDO during 1948 to 2009 (NCEP reanalysis
data used) in phase with that in TC frequency as shown in Fig. 2a which gives
11-year moving averages of 850 hPa wind in the equatorial box along with the
October-December cyclone frequency and the vorticity in the cyclone genesis
area of Bay of Bengal (7.5
o
N-15
o
N, 80
o
E-92.5
o
E). The 850 hPa wind difference
between HFP and LFP in this FDO is given in Fig. 2b. Change in Sea Surface
Temperature (SST) of tropical Indian Ocean appears to be related to the FDO
in cyclones. Figure 2c gives areas A and B whose SST difference (A-B) is
shown in Fig. 2d as 11-year moving average. Since higher SST is associated
Fig. 2:
(a) The 11-year moving average of tropical cyclones, equatorial wind and vorticity
over the genesis region. (b) Difference in equatorial wind between the HFP and LFP.
The equatorial box selected for wind is represented by the rectangular box. (c)
The two areas A and B selected for the SST. (d) The 11-year running mean of the SST
difference between the area selected ( A-B).
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