Climatology ofTCs

Various definitions have been proposed to detect TCs simulated in GCMs (e.g., Camargo and Zebiak 2002). In the present study, we used the following definitions to identify TCs simulated in the CGCM:

1. The grid point with minimum sea level pressure in a 9 x 9 grid-point box is defined as the center of a TC

2. The difference in sea level pressure between the center and each grid point on the boundaries at 4 grid points away from the center is greater than 5 hPa

3. Maximum surface wind speeds at10mina9 x 9 grid-point box around the center are greater than 17 m/s

4. The difference in the vorticity of surface winds between the center and each grid point on the boundaries at 4 grid points away from the center is greater than 1.5 x 10~4V1

5. The lifetime is more than 2 days

6. The genesis location is located to the south of 40°N

7. The vorticity of surface winds around the center has a value greater than 5.0 x 10~4 s_1 at least 4 times throughout its lifetime.

Figure 3 present the genesis locations of model TCs, along with the observed ones during the period from 1930 to 2000. The more model TCs are formed over the northeastern part of the North Atlantic, along the western coast of Africa, and along the eastern coast of North America, while there are less model TCs over the Caribbean and the Gulf of Mexico. The reason for this discrepancy is not clear,

Model

Figure 3 present the genesis locations of model TCs, along with the observed ones during the period from 1930 to 2000. The more model TCs are formed over the northeastern part of the North Atlantic, along the western coast of Africa, and along the eastern coast of North America, while there are less model TCs over the Caribbean and the Gulf of Mexico. The reason for this discrepancy is not clear,

Model

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100W 80W 60W 40W 20W

100W 80W 60W 40W 20W

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Fig. 3 Density of total TC genesis number in (b) model and (a) observations over the 1930-2000. TC genesis density is shown in each 5° x 5° latitude-longitude grid box. Contour interval is 5 and values more than 10 are shaded

Fig. 4 Seasonal frequency of observed (thick line) and model (dashed line) TCs

but in the model, a trough or cold-core low in the upper atmosphere produces convection, occasionally developing into a warm-core tropical cyclone, which seems to contribute to the excess formation of model TCs in the mid-latitude of the North Atlantic compared with the observations. Less model TCs formed over the Caribbean and the Gulf of Mexico appear to be related to the stronger vertical wind shear compared with the observation (Figs. 1c and d).

Figure 4 shows the climatological seasonal cycle of frequencies of the simulated TCs. The annual number of the model TC is about 10, which is consistent with the observation. The peak of the observed North Atlantic TC season happens during the months of August to October with its maximum in September, while the frequency of model TCs is more in July and August compared with the observation.

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