Introduction

The changes in tropical cyclones that can be expected due to anthropogenic climate change still have several open aspects. One of these aspects is the possibility of changes in the geographical distribution of tropical cyclones. The projected increase of sea surface temperatures (SSTs) alone cannot be linked directly to an extension of the areas where tropical cyclones can be found, as the genesis and development of tropical cyclones depend also on atmospheric conditions like the atmospheric stability and the vertical wind shear. Previous studies have not found evidences of changes in the projected areas of formation of tropical cyclones

J.B. Elsner and T.H. Jagger (eds.), Hurricanes and Climate Change, 235

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

(IPCC, 2007; Lionello, 2002; Walsh, 2004). These results were based either on simulations with General Circulation Models (GCMs), which have a relatively low horizontal resolution or on one particular Regional Climate Model (RCM). The coarse resolution of GCMs limits their ability to simulate perturbations of the scale of tropical cyclones.

Recently, two cyclones have developed over unusual areas of the Atlantic Ocean, raising questions about this issue. In March 2004, the first documented South Atlantic hurricane, called Catarina, made landfall over the southern coast of Brazil (Pezza and Simmonds, 2005). Hurricane Vince (Franklin, 2006) formed next to Madeira Islands in the Atlantic Ocean and was the first detected hurricane to develop over this area. It was also the first known tropical cyclone to make landfall in Spain, though at this point it had lost the hurricane category. Both cyclones developed from an initially baroclinic cyclone, and the tropical transition occurred over SSTs lower than 26°C. The mechanisms of tropical transition and the potential impacts of large scale circulation changes on them have been analyzed in recent studies (Davis and Bosart, 2003; Pezza and Simmonds, 2005).

The Mediterranean Sea reaches already for present climate conditions rather high SST values at the end of summer and beginning of autumn. This has led to studies analysing observed Mediterranean cyclones with respect to tropical characteristics. Partially tropical characteristics have been found in some September and October cyclones (Reale and Atlas, 2001; Homar et al., 2003). Fita et al. (2007) make an analysis of seven observed Mediterranean tropical-like storms using observations and numerical simulations. A mechanism for the development of Mediterranean tropical cyclones, initiated by upper-level cut-off lows, has been proposed (Emanuel, 2005).

The future characteristics of Mediterranean cyclones have been analysed in several papers. Lionello et al. (2002, 2007) have used global climate model (GCM) results and one particular regional climate model (RCM), finding no evidence of more intense cyclones during early autumn months. Another study applied a different regional climate model to analyze Mediterranean cyclones and also did not find any clear tendency in the number of intense summer cyclones for future climate conditions (Muskulus and Jacob, 2005). A recent paper has found evidences of fully tropical cyclones developing over the Mediterranean Sea in future climate scenario simulations (Gaertner et al, 2007), making use of an ensemble of RCMs and an analysis method focused on the detection of tropical cyclones. Here we present additional results of this study.

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