Seasonal prediction of typhoon activity in the Northwest Pacific basin

Typhoons rank as the most expensive and deadly natural catastrophe affecting much of southeast Asia. The 2004 typhoon season was the third most active since 1965 with insured losses across southeast Asia exceeding US$ 6bn. The dominant factor causing interannual variability in intense typhoon numbers is ENSO - the strongest interannual climate signal on the planet. The seasonal predictability of intense typhoon numbers and the northwest Pacific Accumulated Cyclone Energy (ACE) index, defined as the sum of the squares of the 6-hourly mean windspeeds (in kts) of all storms while they are at least tropical storm intensity, is investigated using the August/September Niño 3.75 (defined as the region 5^oS-5^oN, 180^o-140^oW) sea surface temperature anomaly. The physical link between sea surface temperature anomalies in the Niño 3.75 region and the ACE index is shown to occur through these anomalies being associated with low level wind anomalies which in turn cause enhanced cyclonic vorticity over the NW Pacific basin where intense typhoons form. The stability and predictability of all three predictors is investigated, and the cross-validated hindcast skill for the ACE index and intense typhoon numbers over the period 1965-2004 is examined. Positive skill to 95% confidence exists from early May. Hindcast correlation skill of at least r=0.5 is obtained from early May onwards.