Studys On The Intensity Changes Of Tropical Cyclone And Their Influencing Factors Over Northwest Pacific

Factors of what determine the maximum potential intensity (MPI)that a tropical cyclone (TC) can achieve given the thermodynamic stateof the atmosphere and the ocean and what prevent the TCs fromreaching their MPIs had become two important problems in the area ofunderstanding of TC intensity changes.Datum including intensity of Northwest Pacific TCs and weeklymean SST of 22years from 1982 to 2003, were compared in this paper.By analyzing the given SST data and its corresponding upper limit of TCintensity, an empirical relation between SST and TC maximum intensitywas determined. In succession, relative intensity of TC, which means thedegree of similitude of real TC intensity by estimated and the TC MPIdetermined by the SST, was calculated according to the above formula.Meanwhile, the result was compared with the instances on NorthAtlantic.The results showed that far more TCs occurred on Northwest Pacificthan on North Atlantic. The maximum intensities of TCs on NorthwestPacific were much less than their MPIs in the average level of 55.6%MPI. 55.5% TCs had reached 50% of their corresponding MPI,whileonly 19.3% get their 80% MPI. The intensity of Northwest Pacific TCs which in more west or more north areas trended to get more close totheir MPIs. However, the relative intensity of TCs in the area of SouthChina Sea were comparatively weaker probably in the influence oflandform differences. In the cause of the weather system, like theenvironmental stream field and so on, the most northern positions wherethe Northwest Pacific TCs attained to their maximum relative intensity,were a bit more south than that of Atlantic. In active seasons of theNorthwest Atlantic TCs, the average intensity of TCs from September toNovember is greater than that from June to August.Furthermore, it showed that there were significant interannualchanges on the TC relative intensity. The macroscale circulation in yearsof EL Nino or La Nina affected the number, location and relativeintensity of TCs to a certain extent. The mean relative intensity of TCsappeared to be a bit greater in EL Nino years than common years, whileless in La Nina years. The result showed that the increase of TCintensity was not completely confined to the SST by calculating theirsensitivity. The vertical shear of environmental wind was probably animportant factor confining the intensifying of TC. Weak environmentalwind shear was good for TCs to reach their MPI. As for the TCs withrelative intensity reached more than 80% in their maximum intensities,the vertical environmental wind shear in the past 12 hours had mostimportant effect on their intensity changes.