Climatology Model Climate

SST and vertical wind shear are key environmental variables that control tropical cyclone development, in particular, over the North Atlantic (e.g., Gray 1968; Gray

1975; Gray et al. 1993). Thus, we first present the climatology of the model SST and vertical wind shear averaged from August to October in Fig. 1. The vertical wind shear in this study is defined as the magnitude of the vector difference of the 200 and 850 hPa horizontal winds. The warm SST regions exceeding 26°C are spread over the entire tropical regions in the model. The overall patterns and magnitudes of the model warm SST regions are in agreement with the observations, although the model SSTs, in particular, over the eastern parts of the Atlantic are warmer compared with the observation (Figs. 1a and b) partially due to a lack of low-level marine stratus in this model. The similar deficiency is found also in other CGCMs (e.g., Mechoso et al. 1995). The model SSTs over the mid-latitude oceans are also warmer compared with the observation due to insufficient resolution of ocean model to simulate the Gulf Stream and a lack of sea ice model in this CGCM. The overall patterns and magnitudes of surface wind fields in the model are in general agreement with the observations (Figs. 1a and b).

In observations, the area of weak vertical shear with values less than 10 m/s extends from the east of the Caribbean to northwest Africa. The regions of strong vertical shear are found to both south and north of the weak vertical shear area. The

Fig. 1 Climatology of observed (a) and model SST (b) averaged from August to October, along with surface wind vector. Regions with values more than 26°C are shaded and contour interval is 2°C. (c)-(d) Same as in (a)-(b) but for magnitude of vertical wind shear defined as the vector difference of the 200 and 850 hPa horizontal winds. Regions with values less than 10 m/s are shaded and contour interval is 2 m/s

Fig. 1 Climatology of observed (a) and model SST (b) averaged from August to October, along with surface wind vector. Regions with values more than 26°C are shaded and contour interval is 2°C. (c)-(d) Same as in (a)-(b) but for magnitude of vertical wind shear defined as the vector difference of the 200 and 850 hPa horizontal winds. Regions with values less than 10 m/s are shaded and contour interval is 2 m/s overall pattern of model vertical wind shear is similar to the observation, but the magnitude over the observed weak vertical shear region is stronger, in particular, over the Caribbean and the Gulf of Mexico. As shown in the next, this model bias in vertical wind shear could affect the distribution in genesis location of model TCs.

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