Variation Characteristics And Prediction Of Typhoon Activity Frequency Over The Western North Pacific

The inter-annual variability of typhoon activity frequency with its associated physical mechanisms is one of the important scientific issues in the field of typhoon research.In spite of great improvement on the prediction/forecast skills of typhoon track over the last several decades,it still remains a great challenge for the operational forecast and seasonal prediction of typhoon activity frequency.This study systematically investigated the typhoon activity frequency over the western North Pacific based on the typhoon best track datasets from the Joint Typhoon Warning Center(JTWC)and Japan Meteorological Administration(JMA),the reanalysis datasets from the National Centers for Environmental Prediction/National Center for Atmospheric Research(NCEP/NCAR)and the Japanese 55-year Reanalysis(JRA55),the monthly snow cover and sea surface temperature(SST)data from the National Oceanic and Atmospheric Administration(NOAA),and the sea ice data from the Met Office Hadley centre.Typhoon activity frequency is presented by three indices:genesis,occurrence and landing frequency.This study focused on the characteristics of the variations of typhoon genesis frequency and the associated physical mechanisms to develop a prediction model with promising predictive skills.In addition,the superiority of typhoon existence frequency in the representation of the regional circulation anomalies and the relationship between the landing frequency and the other two indices are also discussed.Primary results of this study are given as follows:(1)The relationship between the Pacific Ocean SST in boreal spring(April)and the typhoon genesis frequency(TGF)during July to October was investigated.In addition,the predictability of the TGF was examined and a prediction model was developed.It is found that the TGF is positively correlated to a tripole pattern of the preceding April SST anomalies in the North Pacific(NPT_{Ap r}),while it is negatively correlated to SST anomalies in the Coral Sea(CSST_{Ap r})off the east coast of Australia.TheNPT_{Ap r} leads to the forming of large anomalous cyclonic circulation over the North Pacific.The northeast trade wind is weakened by anomalous southwesterly wind,leading to decreased evaporation,which consequently results in anomalously warmer SST in the central tropical North Pacific.As such,the SST gradient sharpened by this warming effect in the central tropical North Pacific;this in turn helps maintenance of the cyclonic circulation anomaly over the western tropical Pacific,which favors the typhoon genesis in the western North Pacific(WNP).In the South Pacific,theCSST_{Ap r}supports the genesis of typhoon over the WNP by:(a)strengthening the cross-equatorial flows and enhancing the Inter-tropical Convergence Zone and(b)weakening the southeast and northeast trade winds that can maintain continuous warming in the central tropical Pacific.Both theNPT_{Ap r}andCSST_{Ap r}can have a persistent influence on the zonal winds along the tropical Pacific and induce favorable conditions for the genesis of typhoon in typhoon seasons.Using both theNPT_{Ap r}andCSST_{Ap r}as predictors,a Poisson regression model was developed to predict the TGF in the WNP during typhoon seasons,which displays a good predictive skill.(2)The“typhoon existence frequency”,defined as the total counts of typhoon occurrence at a specific time and region,was used for the statistical analyses of typhoon activity frequency,which can provide useful supplemental information on typhoon activity.The existence frequency can better reveal the information on regional circulation and build a solid linkage with large-scale circulations(e.g.,ENSO).Our results show that the typhoon genesis frequency and existence frequency have a similar annual cycle with a peak typhoon season during July-October.However,the location of the genesis frequency and the location of the existence frequency are different.Larger typhoon genesis frequency tends to locate in the southeastern part of the NWP.In contrast,typhoon existence frequency prevails in the northwest part,adjacent to the East Asian coastal areas,and has a closer relationship with weather and climate in China.During July-September,more landfalling typhoons are also found,in consistent with the seasonal variability of typhoon genesis frequency.(3)The Victoria mode(VM)of spring(February-April)SST anomaly(SSTA)in the North Pacific is found to be an important factor affecting the inter-annual variability of the typhoon genesis and existence frequencies over the WNP.The spring VM is found to be significantly positively correlated to the typhoon genesis and existence frequencies over the east of WNP(EWNP)during the following July-October.In contrast,the correlations of the spring VM with the typhoon activities over the west of WNP(WWNP)are found to be negative albeit insignificant.It shows a remarkable discrepancy between the two regions.The spring VM modulates the inter-annual variability of the SST in the tropical Pacific and forms a El Ni(?)o-like pattern owing to the wind-evaporation-SST(WES)mechanism;this affects the typhoon frequency during typhoon seasons.A positive SSTA in the central-eastern tropical Pacific leads to a greater SSTA gradient between the western and central-eastern tropical Pacific,which can further strengthen the westerly anomalies in the tropical western Pacific.In the EWNP,the enhanced westerlies tend to produce a cyclonic wind shear along the north flank of the anomalous westerlies,favoring the activities and maintenance of the WNP typhoon.In contrast,a cold SSTA and northward flows in adjacent to the island of Taiwan suppress the typhoon activities.In addition,possible regulatory effects of the ice-snow-atmosphere system in the North and South Hemisphere on the two key regions of SSTs(NPT_{Ap r}andCSST_{Ap r})were investigated.