Subseasonal shift in tropical cyclone genesis over the western North Pacific in 2013

摘要

The 2013 subseasonal asymmetry in tropical cyclone (TC) genesis over the western North Pacific (WNP) was investigated by using the 1979-2013 RSMC best track dataset. The genesis frequency of the 2013 WNP TCs between June-August (summer) and September-November (fall) manifested an abnormal temporal asymmetry: fewer typhoons (more tropical storms) in summer and more typhoons (normal tropical storms) in fall. The 2013 active summer-tropical storm genesis arose from both a failure of eastward extension of monsoon confluence region, especially in August and a lack of moisture supply for TC genesis over the eastern part of WNP, and consequently from fewer probability to reach typhoon intensity due to the westward movement of favorable location for genesis. Thereafter, the eastward extension of monsoon shear line in September and the establishment of monsoon gyre in October induced the eastward movement of favorable location for genesis which increased probability to reach typhoon intensity. The relative contribution of mid-level relative humidity to the positive GPI change played a major role in favorable condition for typhoon genesis in September (45.2%) and October (50.9%). The monsoon gyre pattern played a leading role in the most active fall-typhoon in 2013 contributing to the highest number of October-typhoon. The eastward-migration of convection mainly contributed to the subseasonal shift of TC genesis location following eastward movement of local SST warming from summer to fall under the La Nina-like neutral state. The enhanced active boreal summer intraseasonal oscillation (BSISO) in fall provided more favorable conditions for TC genesis showing about twice as many TCs occurred regarding BSISO in fall than those in summer. This spatiotemporal asymmetry in the large-scale circulations and moisture conditions between summer and fall accounted for the subseasonal shift of genesis location of TCs, and consequently for the active summer-tropical storm genesis and the active fall-typhoon genesis in 2013.