This is an update of the predictions of the annual number of tropical cyclones (TCs) in the western North Pacific (WNP) for 2008 that we issued on 18 April 2008. These updates are made based on new information for the months of April and May 2008.
2. ENSO conditions in 2008
The 2007 La Niña event continues to weaken and the consensus is that the ENSO condition is currently in a neutral status. In May, SSTs are slightly colder than normal in the central and east-central equatorial Pacific but become warmer than normal in the far eastern equatorial Pacific. The Niño3.4 index in May is -0.59. Most model forecasts of ENSO from different climate centres call for a neutral condition in 2 to 7 months time (see summary in Table. 1). Out of the 6 forecasts, 4 suggest that the ENSO will return to its neutral condition during the summer and fall and 2 suggest a transition to a weak El Niño event. These predictions suggest that the La Niña event developed in 2007 is not likely to persist for the rest of the 2008 season. In other words, 2008 will most likely be an ENSO-neutral year although a transition to an El Niño event cannot be ruled out.
An analysis of the TC activity in a year after a La Niña event and becoming neutral or changing to warm ENSO conditions (see Table 3) suggests that the TC season in 2008 tends to be normal or above-normal, which is consistent with our forecast (Table 2).
3. Predictions for 2008
No significant change is found on the predicted total number of TCs. All the predictors give numbers similar to those in the April forecast and therefore the final predicted number is 33.
For the number of tropical storms and typhoons, the prediction from the west Pacific index gives an increase in TC number (from 28 to 29) but the Niño3.4 predictor gives a predicted number of 30 compared with 31 in the April forecast (cf. Tables 2a and 2b). No change occurs using the index of the strength of the subtropical high and the India-Burma trough. The final predicted number therefore is still 30.
A similar situation is found for the predicted number of typhoons. The west Pacific index gives an increase in TC number but the Niño3.4 predictor gives a decrease and no change occurs using other index. The final predicted number therefore is still 19.
With these changes, the average predictions from all the potential predictors therefore call for a slightly above-normal to above-normal year for all TC categories for the entire WNP (See Table 2b).
The predictor related to the subtropical high (HWNP) continues to suggest an above-normal TC activity, which is probably related to the weaker-than-normal subtropical high over the western part of the WNP (Fig. 1).
Summary of predictions
(a) April Forecast
(b) June Forecast