Why Tropical Cyclone Recurves?
In Hong Kong, tropical cyclone (TC) attracts immense public concern during TC season every year. Although the intensity of a TC is of significant interest, the impact of a TC to Hong Kong depends much on where it is going, or its track. In Northern Hemisphere, recurvature of a TC is defined as the situation when a TC transits from a mainly westward track to a northward and sometimes even an eastward track. In order to forecast TC track and in particular, whether it will recurve, one must understand how a TC moves and how different factors influence its motion.
TC motion can be thought of as the cumulative effect of many factors, including the environmental wind and the presence of other tropical cyclones nearby. The environmental wind is usually of primary importance to determining TC motion and it can be represented by the winds at a certain altitude, or the mean of the winds at different altitudes.
What determines the environmental wind? Synoptic-scale weather systems are very important in influencing the environmental wind. The most prominent ones are upper level trough at mid-latitude and subtropical ridge. A trough can alter the environmental wind to carry a component flowing more towards the north or northeast direction such that the cyclone recurves, or at least takes a more northerly track. A strong subtropical ridge usually steers a TC to move along its periphery. If the ridge retreats eastward, there is a high chance for the TC to move along the southwestern or western flank of the ridge and track northward.
Fig.1 The environmental winds surrounding a tropical cyclone causing it to track north to northeastward.
Besides the above, there are also other factors affecting the TC motion, for example, the interaction of TC with terrain and the presence of other tropical cyclones nearby. All these factors affect each other, making the forecast of TC motion a challenging task.