Environmental control of tropical, subtropical, and extratropical cyclone development over the North Atlantic Ocean: Idealized numerical experiments

摘要

The North Atlantic Ocean in autumn is characterized by a variety of cyclone activity: tropical, subtropical, and extratropical cyclones are meridionally distributed from low to high latitudes, and cyclones are more active in the western part than in the eastern part. To examine to what degree the environmental fields explain the geographical distribution of the various cyclones, idealized numerical experiments are conducted using climatological environmental fields at different longitudes and latitudes. The experiments qualitatively reproduce the geographical distribution of development and types of cyclones, which enables an intercomparison of different types of cyclones on the basis of their structure, energy budget analysis, and sensitivity experiments. It is notable that subtropical cyclones can develop spontaneously in the climatological environment which does not have an upper-tropospheric disturbance in the initial fields. While the simulated subtropical cyclones are sensitive to initial conditions, some of them have structures that are in good agreement with those of observed subtropical cyclones: a shallow warm-core structure, concentrated convection near the cyclone centre embedded in synoptic-scale cloud on the north and east sides, and a narrow mid-tropospheric potential vorticity anomaly near the cyclone centre under a wide upper-tropospheric anomaly on the west side. The subtropical cyclones exhibit characteristics intermediate between those of tropical cyclones and extratropical cyclones, notably multi-scale structures with tropical characteristics on a small scale embedded in extratropical characteristics on a large scale.