Introducing a new classification scheme for warm core cyclones

Some of the most intense storms in the mid-latitudes are cyclones that develop a warm core extending through the full depth of the troposphere, a structure more commonly associated with tropical cyclones. However, not all warm-core mid-latitude cyclones form and intensify in the same way, and distinguishing between different types has long been a scientific challenge. In this study, we propose a general classification method based on potential intensity, a measure of how favorable the oceanic and atmospheric environment is for sustaining diabatically driven storms fuelled by surface fluxes. We apply it to a 42-year climatology of deep warm-core cyclones in the Mediterranean Sea, a basin whose high cyclogenetic activity and well-documented population of tropical-like cyclones make it particularly well suited for testing such a framework. Three categories of cyclones emerge. First, classical tropical cyclones (39\% of cases) develop in environments where the ocean is already warm enough to fuel intense evaporation and deep convection. Second, CYCLOP (CYClones from Locally Originating Potential intensity, 26\% of cases) represent a recently identified category of cyclones that do not find a ready-made favorable environment, but instead create the conditions for their own intensification through atmospheric dynamics that cool the troposphere. Third, warm seclusions (31\%) develop a similar warm-core structure predominantly through an atmospheric, baroclinic process, with little contribution from the ocean. The three categories occur at different times of year: tropical cyclones peak in September–October when sea surface temperatures are highest, CYCLOP peak in winter when strong upper-level cold intrusions are most frequent, and warm seclusions are most common in spring. Independent atmospheric diagnostics confirm that the three categories differ in terms of convective activity, precipitation, baroclinic instability, and air-sea coupling. They should nonetheless be interpreted as a practical partition of a continuum rather than as physically strict discrete populations. This framework provides a physically consistent way to categorize deep warm-core mid-latitude cyclones, applies in principle to any basin where they occur, and has implications for understanding their past behavior and projecting their evolution under climate change.