Interactions between tropical cyclones and near-by monsoon gyres

In a recent study, interactions between Typhoon Megi (2010) and a near-by monsoon gyre in the western Pacific were identified as the cause of sudden norward movement of both. Classical Fujiwhara type interactions attracted the two systems to each other but they never merged into one system due to their different scales and characteristics. When the centers of the two systems are collocated, they moved northward together due to enhanced beta effect. In the current study, we investigated and identified more cases with similar phenomena. Between 2000 and 2011, twenty typhoons in the vicinity of Philippine and Taiwan with the largest tuning angles in their life spans are analyzed. Excluding cases in which the northward turning was more likely been affected by the upper level trough or cases with a weak monsoon trough, nine cases (including Megi in 2010) are examined in detail. All of them exhibit similar interactions as in the Megi-monsoon case. To further understand the mechanisms, we conducted idealized 2-D simulations for tropical cyclones (TCs) and monsoon gyres with different radial profiles and separation distances between them. Among mechanisms controlling the movement of TCs and the monsoon gyres, the radial vorticity gradients of both the TC and the monsoon gyre play the major role.