Dipolar redistribution of summertime tropical cyclone genesisbetween the Philippine Sea and the northern South China Seaand its possible mechanisms

摘要

Recent observational records show that the dipole oscillation between the PhilippineSea (PS) and the northern South China Sea (nSCS) is a leading empirical orthogonalfunction (EOF) of summertime tropical cyclone (TC) genesis in the western North Pacific(WNP). This PS-nSCS oscillation is characterized by a distinguished decadal variabilityin addition to an interannual variability. Meanwhile, the typical El Nino—SouthernOscillation (ENSO)-related mode is found in the second EOF mode, which is predominantlyinterannual. With regard to the PS-nSCS oscillation, its interannual component appears tobe linked with the previous wintertime ENSO event, but the linkage is not so robust inthe sense that about half of its significant events are classified as the ENSO-related case,whereas its decadal component is coupled to a sea surface temperature anomaly (SSTA) inthe central Pacific with its equatorial core near the Nino-4 region, which is flanked by anopposite SSTA on both sides along the latitudinal belt. Interestingly, this SSTA patternresembles that related to the ENSO Modoki, and equivalently, the transitional phase ofENSO. The ENSO Modoki is regionally manifested by the anomalous zonal SST gradientbetween the equatorial western and central Pacific; thus the decadal modulation of thisanomalous zonal SST gradient can be regarded as an actual forcing that forms and modulatesthe decadal PS-nSCS oscillation. In addition, the decadal PS-nSCS oscillation is comparedwith the decadal basin-wide TC genesis variation in terms of the Pacific SSTA patternand the associated anomalous large-scale environments.