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 44

part of the west North Pacific Ocean, but this TC frequency increase is not
seen in Fig. 3c because of a larger TC frequency decrease due to SST effect as
shown in Fig. 3b.
The SST effect (Fig. 3b) is further divided into uniform SST effect (Fig.
3d) and non-uniform SST effect (Fig. 3e). The uniform SST effect is an overall
reduction of TC frequency with some regions of TC frequency increase, while
the non-uniform SST effect is a mixture of the regions of TC frequency increase
and decrease. The non-uniform SST effect does not change the global TC
frequency, but it causes a shift of active deep convection area and significantly
affects the regional TC frequencies.
Figure 4 shows the CO 2 effect and SST effect on TC frequency,
precipitation, upward mass flux, vertical wind shear and saturation deficit.
The effect of CO 2 increase is to decrease precipitation, upward mass flux and
TC frequency. On the other hand, the effect of SST increase is to increase
precipitation but to decrease upward mass flux and TC frequency. Despite the
opposite sign of the changes in precipitation, both CO 2 effect and SST effect
decrease upward mass flux and reduce TC frequency. We note, however, in
some regions the SST effect on upward mass flux do not agree well with that
on TC frequency. Such disagreement may be explained to some extent by the
changes in vertical wind shear or saturation deficit due to the SST effect as
shown in Figs 4i and 4j. For example, TC frequency decreases in the eastern
North Pacific and the Atlantic, although precipitation increases over the same
regions. The decrease of TC frequency in these regions may be explained by
the increase of vertical wind shear as shown in Fig. 4i.
6. Indian Ocean TC Frequency Changes
Figure 3c shows an overall reduction of TC frequency in the most part of the
Indian Ocean due to the CO 2 effect and SST effect. We note, however, significant
increases in TC frequency in the western-north part of the Arabian Sea and the
western-south part of the South Indian Ocean mainly due to the SSTA effect as
shown in Fig. 3c. There is a significant TC frequency increase in the Bay of
Bengal due to the SST effect (Fig. 3b), but this TC frequency increase is not
seen in Fig. 3c because of a large TC frequency decrease due to CO 2 effect
shown in Fig. 3a. It should be noted that the TC frequency increase in the
Arabian Sea and the Bay of Bengal in Fig. 3b is mainly due to the non-uniform
SST effect shown in Fig.3e. The uniform SST effect is to reduce the TC
frequency in both the Arabian Sea and the Bay of Bengal as shown in Fig. 3d.
The overall reduction of TC frequency in most part of the Indian Ocean
corresponds to the reduction of precipitation and upward mass flux due to the
CO 2 effect as shown in Figs 4b and 4c. On the other hand, the TC frequency
increase in the western part of the South Indian Ocean may be explained by the
increase of precipitation and upward mass flux due to the SST effect as shown
in Figs 4g and 4h. We note that similar explanation is not applicable to the TC