Variation of the monsoon trough intensity in the South China Sea and western North Pacific ocean and its relationship to tropical cyclone activities

Using the NCEP/DOE AMIP-II reanalysis data from 1979 to 2005 and the characteristics of monsoon troughs in the western North Pacific, we established an intensity index and a location index to describe the activity of the monsoon troughs in three different regions and their impacts on tropical cyclones generated therein (MTTCs). The monsoon troughs was analyzed. (1) The established monsoon trough intensity index is positively correlated to the location index. (2) Monsoon trough intensity exhibits significant interannual variation, with obvious periods of 4–C5 years prior to 1994 and 2–C3 years afterwards. (3) The affecting factors are different with areas. The preceding SST anomaly results in anomalous atmospheric circulation, leading to the anomaly of monsoon trough intensity in different areas. (4) The frequency of cyclogenesis and location anomalies of the MTTC are closely related to the intensity and location of the monsoon trough. Most of the anomalously less MTTC years coincide with the years with a weak general monsoon trough and weak regional monsoon troughs. The anomalously more MTTC years are associated with both a strong general monsoon trough and a weak general monsoon trough combined with a strong one over the South China Sea. (5) The interseasonal variation of the intensity of monsoon troughs provides favorable conditions for TC generation and development. The monsoon trough is in the active periods of both quasi-biweekly 10 to 20 day and 30 to 60 day oscillations, being favorable for MTTC occurrence.