It has been recognized that a large proportion of North Atlantic tropical cyclones (TCs) originate from Africa easterly waves that propagate westward from the western coast of North Africa at around 15°N, and eventually developing into TCs (e.g., Landsea 1993). The peak of TC formation in the North Atlantic happens during the months from August to October, and the mean annual number is about 10 but more than 10 since 1995 (e.g., Goldenberg et al. 2001). The frequency of Atlantic TCs is observed to have significant variability on a number of different timescales (e.g., Goldenberg et al. 2001). Although previous studies (Shapiro 1982a, b; Gray 1984) have identified several factors (e.g., quasi-biennial oscillation, Sahel rainfall) that appear to influence Atlantic TC activity, El Nino / Southern Oscillation (ENSO) give a significant impact on Atlantic TC frequency on the interannual timescales (e.g., Gray, 1984; Shapiro 1987). During El Nino years, the vertical wind shear over the North Atlantic is increased. The increased vertical wind

J.B. Elsner and T.H. Jagger (eds.), Hurricanes and Climate Change, 323

doi: 10.1007/978-0-387-09410-6, © Springer Science + Business Media, LLC 2009

shear helps to prevent tropical disturbances from developing into TCs, resulting in fewer TC formations. In La Nina years, there are more TCs than average because the vertical wind shear is reduced.

Manabe et al. (1970) first noted that a general circulation model (GCM) could reproduce vortices similar to observed TCs. Subsequently, Tsutsui and Kasahara (1996) confirmed that the seasonal and geographic variations of TC-like vortices reproduced observational TC statistics reasonably well even in a GCM with the horizontal resolution of approximately 300 km. While Wu and Lau (1992) examined the ability of GCM to reproduce the interannual variability of TC frequency, Vitart et al. (1997, 1999) explored the possibility of a GCM to forecast seasonal TC activity using an ensemble of integrations for the period 1980-88. The latter showed that their GCM was able to simulate a realistic interannual variability of tropical storm frequency over the North Atlantic. Vitart and Anderson (2001) further demonstrated that a GCM also has the ability to simulate the interdecadal variability of North Atlantic tropical storm frequency. These studies have showed the potential skills of a GCM in reproducing observed interannual variability of TC frequency and genesis location. However, the resolution of a GCM used by the above-mentioned studies is relatively coarse, and hence the intensity of model TCs is much weaker than in observations, and also there is the deficiency in tracks of model TCs (e.g., Vitart et al. 1997; Camargo et al. 2005).

In global warming experiments, the simulations of TCs using a high-resolution GCM are now becoming feasible (Sugi et al. 2002; Yoshimura et al. 2006; Oouchi et al. 2006; Bengtsson et al. 2007). They have reported that the models are successful in simulating intense TCs. However, an air-sea interaction is not considered in the models, which is one of the important factors controlling TC intensity (e.g., Emanuel, 1986). Recently Iizuka and Matsuura (2008) have examined the relationship between ENSO and TC activity in the western North Pacific simulated in a high-resolution ocean-atmosphere coupled GCM (CGCM) with a nearly 60km horizontal resolution. They have demonstrated that the model has the ability to simulate the ENSO impact not only on TC frequency and genesis location as in the previous studies but also on TC track and intensity reasonably well. In this paper, we document the interannual variations in TC frequency over the North Atlantic simulated in the same CGCM. The paper is organized as follows. A description of the model and data are given in Section 2. The model climatology is shown in Section 3. Section 4 presents the relationship between ENSO and TC activity over the North Atlantic simulated in the model. A summary is given in Section 5.

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