Effects of tropical North Atlantic SST on tropical cyclone genesis in the western North Pacific

The tropical cyclone genesis number (TCGN) in July-October (JASO) over the western North Pacific (WNP) exhibits a robust interannual variation. The longitudinal distribution of the dominant interannual TCGN mode is examined through the diagnosis of NCEP reanalysis dataset during 1970-2009. It shows a longitudinally tripole pattern with a high in the eastern WNP and South China Sea (SCS) and a low in the western WNP. To understand the cause of the longitudinal distribution, we examine the contributions of environmental parameters associated with the TC genesis potential index (GPI). It is found that relative humidity is the most important factor responsible for TC distribution in the SCS and western WNP, while vertical shear is crucial in determining TC activity in eastern WNP.

A simultaneous correlation analysis shows that the WNP TCGN in JASO is significantly negatively correlated with SST anomalies (SSTA) in the tropical North Atlantic (TNA). The longitudinal distribution of TC genesis frequency regressed onto TNA SSTA resembles that regressed to the WNP TCGN series. The spatial patters of regressed environmental variables associated with TNA SSTA also resemble those derived based on TCGN in the WNP, that is, an increase (decrease) of relative humidity in the SCS (western WNP) and a weakening of vertical shear in the eastern WNP are all associated with cold SSTA in the TNA.

A further diagnosis shows that the cold SSTA in the TNA induces a negative heating in situ. In response to this negative heating, low-level anomalous easterlies (westerlies) are induced. The anomalous low-level winds converge into the central equatorial Pacific and induce a positive heating anomaly in situ. The response to this anomalous heating strengthens low-level anomalous westerlies and upper-level anomalous easterlies to the west, forcing a low-level cyclonic vorticity anomaly and a negative vertical shear anomaly in the eastern WNP. Meanwhile, the TNA heating anomaly forces a tropical-belt tropospheric cooling. This tropospheric cooling is responsible for the increase of environmental mid-level relative humidity in the SCS. The decrease of specific humidity due to mid-level dry air meridional advection, on the other hand, is responsible for the decrease of relative humidity in the western WNP.