Multi-Scale Climatic Characteristics Of Tropical Cyclone Genesis Productivity Over The Western North Pacific

Great progresses had been made in the forecast and prediction of tropical cyclone(TC)activity over the past several decades,but the operational forecast/prediction of TC genesis remains a great challenge.Understanding of the TC genesis in the western North Pacific(WNP)has become an important scientific issue and a hot topic in tropical atmospheric research.Currently,most studies qualitatively understood the TC genesis from the changes of large-scale environment factors associated with TC activity,but few study focuses on its precursors.Tropical cloud clusters(TCCs)are referred to as the precursor for TCs and can develop into a TC under favorable circumstances.However,the efficiency TCC developing into TC at various time scales and the associated physical mechanisms are still unclear.In this study,TC genesis productivity(TCGP)calculated as the ratio of TC and TCC frequency is used to define the efficiency.The characteristic of TCGP at decadal,interannual and intra-seasonal scales and the associated key factors and plausible reasons will be systematically investigated.Main conclusions are as follows:The first three dominant modes obtained from EOF analysis of boreal summer(MayOctober)TCC frequency are found to be a whole-basin consistent,zonal dipolar and meridional dipolar pattern respectively.The dominant modes show great effect on TC and TCGP in a meridional,southeast-northwest and southeast-northwest dipolar pattern.Under the first dominant mode,the mid-level relative humidity(high-level divergence)is the important factor for TCC(TC/TCCGP).In contrast,the low-level convergence(relative vorticity and humidity)found to be critical to TCC(TC/TCGP).The different modulation of the first and last two modes on TC/TCGP pattern may arise from sea surface temperature(SST).The high-level dipolar divergence associated with strong southerly in the western WNP may be induced by strong India-Pacific SST gradient in a Pacific decadal oscillation(PDO)-like pattern.Enhanced lowlevel relative vorticity over the eastern WNP and decreased mid-level relative madidity over the western WNP are produced by enhanced zonal Pacific SST gradient in an El Ni?o-Southern Oscillation(ENSO)-like pattern.On the interdecadal scale,decreasing TCGP is companied by the PDO from warm to cold phase since 1998.In the western WNP,TCGP changes are mainly contributed to more TCCs and less TCs,which is induced by high local SST.Warm SST shows significant effect on TCC genesis but does nothing with its development.In the eastern WNP,TCGP changes are mainly contributed to more TCCs and less TCs,which is induced by strong vertical wind shear.Enhanced vertical wind shear shows a distinct impact on TCCs and its development,favorable for TCCs but not conducive to its development.In cold PDO phase,the strong vertical wind shear in the eastern WNP is induced by great westerlies anomaly caused by enhanced zonal SST gradient with warm(cold)SST in the western(central-eastern)Pacific.On the intra-seasonal scale,boreal summer(May-October)intraseasonal oscillation(BSISO)exhibits significant effect with increasing(decreasing)TCGP in the active(inactive)phase.Associated with active(Inactive)phase,enhanced(weakened)intra-seasonal low-level vorticity and mid-level humidity are conducive to(unfavorable for)TCC genesis and its development.It's worth noting that TCC's development is much more sensitive to the environment than its genesis,leading a great(weak)TCGP in the active(inactive)phase.Additionally,the interaction between BSISO and synoptic wave contributes to TCGP seriously.There is a strong(weak)amplitude synoptic wave-train associated with low-level cyclonic(anticyclonic)synoptic wind anomaly in the active(inactive)BSISO phase which is favorable(unfavorable)for TCGP.