Sudden Track Changes of Tropical Cyclones in Monsoon Gyres: Idealized Numerical Experiments

Using the full-physics advanced research weather research and forecast (WRF-ARW) model, idealized numerical experiments are performed to reveal mechanisms of sudden track changes of tropical cyclones (TCs) in a large-scale monsoon gyre. It shows that a TC with initial position within the eastern semicircle of the monsoon gyre experiences a sudden northward track change only when it moves towards the monsoon gyre center and coalesces with with the monsoon gyre. While the beta effect makes the monsoon gyre move northwestward, its cyclonic circulation cyclonically rotates the beta gyres of the TC, leading to a reduction of the TC translation. As the TC center is collocated with monsoon gyre center, the increased TC size enhances the associated beta-induced energy dispersion and southwesterly winds in the southeastern periphery of the TC. As a result, TC movement accelerates and suddenly shifts northward. Two sensitivity numerical experiments are also conducted to examine the influence of the initial profiles of the monsoon gyre and the TC. For a monsoon gyre with weaker cyclonic relative vorticity, its cyclonic circulation cannot rotate the beta gyres to such a degree that the TC center is collocated with the monsoon gyre center, making the TC take a southwestward track. The northward turning angle depends on the strength of the initial TC. A TC with weaker strength make a northward shift with a smaller turning angle due to the more northward beta drift associated with the TC.