Reexamination of Tropical Cyclone Formation in Monsoon Troughs over Western North Pacific

Monsoon troughs (MTs) and monsoon gyres (MGs) are two specific types of large-scale patterns favorable for tropical cyclone (TC) formation over the western North Pacific (WNP). This study reexamines the TC formation by treating the MT as a large-scale background for TC activity during May-October. Over the 11-year (2000-2010) period, on average 8.3 TC formation events per year are identified to occur within MTs, accounting for 43.1% of the total TC formation events in the WNP basin. This percentage is much lower than those reported in previous studies. Further analysis indicates that TC formation events in monsoon gyres were included at least in some previous studies. The MT includes a monsoon confluence zone where westerlies meet easterlies and a monsoon shear line where the trade easterlies lie north of the monsoon westerlies. In this study, the large-scale flow pattern associated with TC formation in MT is composited based on the reference point in the confluence zone where both of the zonal and meridional wind components are zero with positive vorticity. While previous studies found that many TCs form in the MT confluence zone, this study shows that nearly all of the TC formation events occur in the MT shear region since the MT shear region is associated with stronger low-level relative vorticity and higher mid-level relative humidity than the MT confluence zone. The prevailing easterly vertical shear of zonal wind and barotropic instability may be also conducive to TC formation in the MT shear region through the development of synoptic-scale tropical disturbances in the MT that are necessary for TC formation.