Occurrences of Binary Tropical Cyclone and their characteristc track-modes in the western north pacific

Some climatological features of occurrences of binary tropical cyclone systems in the western North Pacific

Suguru Ishijima* and Nathaniel Servando**

* Faculty of Sciences, University of the Ryukyus,Okinawa, Japan **Philippine Atmospheric Geophysical and Astronomical Services Administaration , Quezon City, Philippines

A climatological survey was made on the occurrences of BTS in the western North Pacific based on which the features of relative and actual tracks of the binary vortices are extracted by applying the centroid analysis . The 45 year recording data of tropical cyclones(storm and typhoons) indicate that the BTS occurs at the rate of nearly twice a year in the months of May through November. Their occurrences are most frequently found in the central region south of Japan and east of the Philippines with initial positions of vortices of BTS in the E-W direction. The relative motion of the vorticies of BTS is characterized with their separation distances and rotation type. Cyclonic Approaching (CA) type of motion only occupies 21% of the whole number of BTS and the majority belongs to Cyclonic Approaching-Departing(CAD) type . Anticyclonic Rotation type exists only 10% . Interaction between the vortices in the CA mode starts around 13 to 14 latitude-degree distance and their merging occurs with the separation distance of about 2.6 latitude-degree distance. The escape from the CA type orbital track to CAD type occurs with separation distance in the range of 6-13 latitude -degree distance. Intensity ratio of the weaker vortex versus stronger one is 0.65. The averated size ratio is less than one, while being greater than 1 for some cases. Some evidence is found to show that intensity and size ratios is effective in showing the mutual interaction between the two of BTS vortices.