Evolution of Tropical Cyclones in North Atlantic mid-latitudes - a Reanalysis of 1987-2003

Abstract

Tropical cyclones may undergo a major transition in their structure when they encounter circumstances that do not support the tropical structure of such a cyclone. This is called extratropical transition (ET). However, ET can take place in a number of different ways. To gain insight in these different ways of phase transition, this study uses a relatively high-resolution reanalysis dataset called MERRA and categorizes all tropical cyclones between 1987 and 2003 based on their thermal symmetry and core nature. The study also review the trajectory these tropical cyclones follow throughout the mid-latitudes. Generally, tropical cyclones that enter mid-latitudes follow a curved path from different places in the tropics towards the East American Coast, along which they tend to turn eastward along the Gulf Stream and when the influence of the westerlies becomes important. The amount of tropical cyclones that enter the mid-latitudes correlates well with the amount of tropical cyclones in general (r = 0.77), although it does not correlate well with the amount of cyclones that enters Europe (r = 0.46). Six different categories of phase cycles are found. 1) Tropical cyclones that undergo almost no transition. Cyclones going through this phase cycle are generally weaker in pressure and winds and do not reach far north. 2) Tropical cyclones that become thermally asymmetric while retaining their warm lower core. These are relatively strong and reach far north. 3) Tropical cyclones that become thermally asymmetric and then grow cold. Their tracks are close to the East American coast and are relatively strong. 4) Tropical cyclones whose core cools down and then start developing the asymmetric structure of an extratropical cyclone. These cyclones are highly variable in strength and track. 5) Tropical cyclones that seclude a shallow warm core after having had the extratropical characteristics of asymmetry and a cold-core nature. These cyclones are relatively strong and generally come far north. 6) Tropical cyclones that seclude a deep warm core after having had a shallow warm seclusion. These cyclones penetrate farthest north and are the strongest in pressure and winds.