Analysis of the anomalous strength and location of the ITCZ affecting the formation of northern Pacific typhoons

Using the NCEP/NCAR reanalysis dataset from 1959–2004, the location and strength of the ITCZ (Inter-Tropical Convergence Zone), as well as their relations with typhoons in the northwestern Pacific were studied. It was found that the pentad location and strength of the ITCZ had close relations with the typhoon frequency. Higher latitude location or strengthened ITCZ were found to be favorable for the occurrence of typhoons over the Northwestern Pacific. An index was defined for ascertaining the location of the ITCZ. It was found that the index defined with the maximum value of pentad and monthly meridional shear of zonal wind speed could better describe the location of ITCZ than another index defined with the maximum value of convergence. Correlation analysis between the index of ITCZ and the maximum cloud cover in the tropics showed that there were close relations between the ITCZ determined by the index and the maximum tropical cloud belt. The strength index of an ITCZ was defined as the zonal wind speed difference at latitudes south and north of the ITCZ. It was found that there are close relations between the ITCZ intensity and typhoon occurrence in the South China Sea [10°N–20°N, 100°E–120°E] and regions east of the Philippines and near the Mariana Islands [5°N–20°N, 127.5°E–150°E].