Mechanism of the Indian Ocean Tropical Cyclone Frequency Changes due to Global Warming - Monitoring and Prediction of Tropical Cyclones in the Indian Ocean and Climate Change - page 47

frequency increase in the Arabian Sea, because there is very little change in
precipitation and upward mass flux in the region. However, we can see a
significant decrease in vertical wind shear in the Arabian Sea, which probably
explain the TC frequency increase.
7. Discussion
We have noted that the CO 2 effect is 5.6% reduction of global TC frequency
when evaluated by the difference between CO 2 F run and P run, while it is
9.8% reduction when evaluated by the difference between F run and SSTF run
(Fig. 2). This may suggest that the CO 2 effect is larger for the warmer climate.
However, other possibility is that this is only a result of statistical uncertainty,
because the TC frequency estimated by a single 25-year run is subjected to a
large statistical uncertainty. The problem of statistical uncertainty is even more
serious for the regional TC frequency changes. In this study we estimated the
TC frequency change based on a pair of single 25-year run. Ideally, an ensemble
experiment with large number of members should be conducted for a reliable
estimate of regional TC frequency changes. Recently, Murakami et al. (2012a)
conducted such an ensemble experiment using the MRI-AGCM3.2. They
showed the overall TC frequency reduction with increases in central North
Pacific, Arabian Sea and western South Indian Ocean, which is basically
consistent with the present study.
In the present study, we analyzed the TC frequency in the Northern
Hemisphere (NH) and Southern Hemisphere (SH) separately during the
respective TC season (June to November for NH; January to April for SH).
However, we should note that TC seasons of the North Indian Ocean are pre-
monsoon period (May and June) and post-monsoon period (October to
December). Furthermore, a recent study by Murakami et al. (2012b) revealed
that there is a marked seasonality in the TC frequency change in the North
Indian Ocean. They showed that TC frequency will decrease in pre-monsoon
season but increase during monsoon season both in the Arabian sea and the
Bay of Bengal, while it will increase in the Arabian Sea but decrease in the
Bay of Bengal in the post-monsoon season. As a result, the annual mean TC
frequency will increase in the Arabian Sea but decrease in the Bay of Bengal.
This annual mean TC frequency changes are consistent with the present study.
8. Conclusions
Global TC frequency is projected to decrease by about 24% by the end of 21st
century. Both CO 2 effect and uniform SSTA effect cause a reduction of upward
mass flux, leading to the reduction of global TC frequency. In the Indian Ocean,
in addition to an overall decrease in TC frequency, increases in TC frequencies
are projected in the western-north part of the Arabian Sea and in the western-
south part of the South Indian Ocean. These increases of TC frequency are