Research On Effects Of The Monsoon Trough Upon The Genesis Of Tropical Cyclones In The Western North Pacific And Related Mechanisms

In the context of data on western North Pacific cyclones provided by ShanghaiInstitute of Typhoons, daily means of NCEP/DOE AMIP-â…¡reanalysis, daily OLRdata as well as NOAA Extended Reconstructed SST V2 and based on cyclonesselected from the 1979-2005 general survey, established is a sequence of tropicalcyclones (TC) generated in the monsoon trough (MTTC hereinafter) over the westernNorth Pacific, followed by analysis of impacts of the anomaly in the monsoon troughon MTTC activities, the development of index of the monsoon trough activities andstudy of fundamental climate law and possible mechanism for MTTC occurrence ingroups, thereby constructing a physically conceptual model of the monsoon troughinfluencing TC. The main results are as follows.(1) The total of TCs is 672 over the South-China Sea and western NorthPacific during May-October, 1979-2005, of which MTTC is 491, making up 73.1ï¼…of the total and 79.2ï¼…of the onland typhoons. As a result, grasp of the law of MTTCactivities means the grasp of the principal laws of western North Pacific TC andChina-affecting TC.(2) The 1979-2005 TC yearly frequencies experience the following stages; 1)normal (1979-1987), high frequencies (1988-1994) and low frequencies (1995-2005),with 1994 as the turning point from high to low frequencies of MTTC. MTTCexhibits quasi-4 and quasi-2 year periods, particularly the former that is quiteremarkable.(3) By comprehensive consideration of the circulation situation, monsoontrough feature and TC genesis position, we divide the monsoon troughs into a rangeof annual types, i.e., normal, SW, NW, and eastward of mean. For the abnormalannual monsoon trough features the TC genesis location, annual frequency, track andlanding position vary greatly in comparison to the normal type.(4) The proposed indexes of the monsoon trough intensity and position areable to objectively indicate the season-dependent strength and position of themonsoon trough, with the strength displaying noticeable interarmual and intra-seasonal variation, and the strength-impacting factors varying from one place toanother. And the intensity of the monsoon trough, especially in the South-China Sea,bears a close relation to the anomaly of MTTC frequency.(5) The anomaly of MTTC occurring in groups is responsible mainly for theanomaly of its annual frequency, with the yearly occurrence in groups decreased(increased) corresponding to the drop (rise) in the yearly frequency. That the monsoontrough is in the active period both of 10-20 and 30-60 day oscillation presents afavorable situation for the MTTC occurrence in groups. The subtropical highpositioned in the zonal direction, as well as the related phases of 30-60 day oscillationbetween Somali and western Pacific cross-equatorial flows all contribute to theMTTC happening in groups, with its previous change used as a reference to themedium-and short-range forecasts of in-group happening MTTC.(6) On the basis of the diagnoses constructed is a conceptual model of theanomaly in the western North Pacific monsoon troughs influencing MTTC behaviors.