This is an update of the predictions of the annual number of tropical cyclones (TCs) in the western North Pacific (WNP) for 2010 that we issued on April 26, 2010. These updates are made based on new information for the months of April and May 2010.
2. ENSO conditions in 2010
The El Niño event developed in 2009 has dissipated and the oceanic and atmospheric conditions in the Pacific are now in neutral status. The Niño3.4 and Niño4 indices in May are -0.03 and 0.39 respectively. At the same time, cooling of the central and east equatorial Pacific Ocean is observed and may continue into the summer. A summary of the various ENSO model forecasts from different climate centers suggests that most of them predict a cool condition in 4 to 7 months time (Table 1). Out of the 7 forecasts, 6 suggest the possible development of a La Niña event. It therefore appears that for the rest of 2010, development of La Niña conditions is possible. In other words, 2010 will likely be a La Niña year.
3. The predictions for 2010
For the total number of TCs, the prediction from the index of the western extent of the subtropical high gives decrease in TC number (from 30 to 29) compared with the April forecast (cf. Tables 2a and 2b). No change occurs using the other predictors and the final predicted number therefore decreases from 28 to 27.
For the number of tropical storms and typhoons, the predictions from the index of the western extent of the subtropical high and the index of the strength of the India-Burma trough give decreases in TC number (from 24 to 22 for the former and from 26 to 24 for the latter). No change occurs using other predictors. The final predicted number therefore decreases from 24 to 23.
For the number of typhoons, the predictions from the ENSO predictors give decreases in TC number, which is probably related to the recent cooling of the equatorial North Pacific. The Niño3.4 predictor gives a predicted number of 16 compared with 17 in the April forecast. A greater decrease, from 18 to 16, occurs using the Equatorial SOI. No change occurs using other predictors and the final predicted number therefore deceases from 16 to 15.
With these changes, it is expected that the overall TC activity and the number of tropical storms and typhoons are likely to be below-normal and the number of typhoons is likely to be slightly below-normal.
As a La Niña event could occur in 2010 as suggested in section 2, it is useful to discuss the TC activity during La Niña years. During the past five decades, the TC activity exhibited a significant interdecadal variation, with the active periods of 1960-76 and 1989-97 and the inactive periods of 1977-1988 and 1998-2009 (Fig. 1). The variations of the TC activity during La Niña years are quite different in the active and inactive periods (Table 3). In the active periods, the variations are quite large, with very active as well as very inactive TC seasons. Normally, the active TC seasons generally have the above-normal activity during the early season (Chan 2008). Indeed, the TC numbers between January and June for the three active seasons (1964, 1971 and 1974) are all above normal (with the numbers being five, eleven and eight respectively). However, the 2010 season has only two TCs occurring between January and June and therefore not likely to be an active season. Moreover, the La Niña years with moderate or strong El Niño events in the preceding years (1973, 1988, 1995, 1998 and 2007) are associated with normal or below-normal TC activity, which also suggests that the 2010 season (with a moderate El Niño event in 2009) is not likely to be an active season. On the other hand, the inactive TC period 1998-2009 will likely to continue into 2010, it is therefore more appropriate to discuss the TC activity during La Niña years occurring in the inactive periods. Of the six La Niña years, three are associated with below-normal number of tropical storms and typhoons and three are associated with normal number. Similar results are found for the number of typhoons. Therefore, the 2010 TC season will likely to be below-normal or slightly below-normal, which is consistent with our forecast.
The predictor related to the subtropical high (HWNP) continues to suggest a below-normal TC activity. The subtropical high over the WNP is stronger than normal in the months of April and May (Fig. 2). On the other hand, no tropical cyclone formed in the WNP during the months of April and May, which is probably related to the current atmospheric conditions.
Chan, J. C. L., 2008: A Simple Seasonal Forecast Update of Tropical Cyclone Activity. Weather Forecasting, 23, 1016-1021.
Fig. 1. Time series of the annual number of tropical storms and typhoons. Red circle and blue squares indicate the El Niño and La Niña years respectively. The green triangle indicated the predicted number in 2010. The thick horizontal line indicates the normal number of tropical storms and typhoons. The green vertical lines divide the years 1960-2009 into the active and inactive periods.
Fig. 2. 500-hPa geopotential height anomalies between April and May in 2010. Thick contour indicates the geopotential height (contour interval = 10 m) ³ 5860 m.