First, it would be useful to discuss the El Niño/Southern Oscillation (ENSO) condition as it contributes to a certain extent the frequency of landfalling tropical cyclones (TCs) along the South China coast (Goh and Chan 2010a). The El Niño event that began in mid to late 2009 and continued through the winter has shown signs of weakening. Indeed, forecast models from various climate centres have suggested that the ENSO condition is likely to return to near neutral as the Northern Hemisphere TC season progresses. Thus, the current year, which is a year after an El Niño event (EN+1 year), is very likely to be an ENSO-neutral year.
Our prediction scheme of the number of landfalling TCs along the South China coast is a statistical one based on both the atmospheric conditions and the ENSO and Pacific Decadal Oscillation (PDO) indices prior to the season (Goh and Chan 2010a). The predicted number for the early season (May to August) is 4, which is slightly above the normal value of 3. On the other hand, the forecast for the late season (September to December) is 2, which is equal to the normal value. This gives a total number of 6 TCs forecasted to be making landfall along the South China coast over the whole season, which is also slightly above the normal value of 5.
Liu and Chan (2003) found no significant patterns in the number of TCs making landfall on the South China coast in an EN+1 year. However, the authors also revealed that during the months of May and June in such years, the number of landfalling TCs tends to be below normal, which seems to disagree with the current forecast. But as Table 1 suggests, in the past 13 EN+1 years, 5 had an above-normal number of landfalling TCs. Furthermore, of the four ENSO-neutral years that are also EN+1 years, three had above-normal number of early landfalling TCs, and none below normal. One of the possible reasons for this year’s above-normal prediction could be the anomalously high geopotential height over the western north Pacific between January and March of this year compared to the average EN+1 year (Fig. 1), and it is worth-noting that this anomalous high has been present since early February 2010 (Fig. 2). If this anomalous high continues to persist into the early TC season, it would be much easier for TCs formed over the western north Pacific to be steered into the South China Sea, increasing the chances of them making landfall along the South China coast.
As the ENSO condition is predicted to return to neutral in the latter part of the year, it is likely that the effect of ENSO on landfalling TCs during the late season might not be very prominent, which is consistent with the normal number being predicted.
As discussed in Goh and Chan (2009a), an updated prediction for the late season will be issued in June.
2. ENSO conditions in 2009
The oceanic and atmospheric conditions in the Pacific are now in neutral status. The Niño3.4 and Niño4 indices in May are 0.27 and 0.32 respectively. At the same time, warming of the central and east equatorial Pacific Ocean observed and may continue into the summer. A summary of the various ENSO model forecasts from different climate centres suggests that most of them predict a warm condition in 5 to 7 months time (Table. 1). Out of the 7 forecasts, 5 suggest the possible development of an El Niño event. It therefore appears that for the rest of 2009, a development of El Niño conditions is possible. In other words, 2009 will likely be an El Niño year.
3. Predictions for the WNP
For the total number of TCs, the prediction from the index of India-Burma trough gives an decrease in TC number (from 30 to 28) compared with 30 in the April forecast (cf. Tables 2a and 2b). No change occurs using the other predictors and the final predicted number therefore decreases from 31 to 30.
No significant change is found on the predicted number of tropical storms and typhoons (Table 2). All the predictors give numbers similar to those in the April forecast and therefore the final predicted number is still 27.
For the number of typhoons, the prediction from the Niño3.4 predictor gives an increase in TC number (from 20 to 22). This is probably related to the recent warming of the equatorial North Pacific. An increase (from 17 to 18) is also found based on the index of India-Burma trough and no change occurs using other predictors. The final predicted number therefore increases from 18 to 19.
As an El Niño event could occur in 2009 as suggested in section 2, it is useful to discuss the TC activity during El Niño 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 1998-2008 (Fig. 1). The variations of the TC activity during El Niño years are quite different in the active and inactive periods (Table 3). Since the inactive TC period 1998–2008 will likely to continue into 2009, it is more appropriate to discuss the TC activity during El Niño years occurring in the inactive periods. The number of tropical storms and typhoons tends to be normal or below-normal except for 2004 (Table 3). However, the number of typhoons tends to be normal or above-normal except for 2006 (Table 3). Therefore, the 2009 TC season will likely to be normal, with the possibility of a slightly below-normal number of tropical storms and typhoons and a slightly above-normal number of typhoons, which is consistent with our forecast.
The predictor related to the subtropical high (HWNP) continues to suggest a near-normal to slightly below-normal TC activity, which is probably related to the stronger-than-normal subtropical high over the eastern part of the WNP (Fig. 2).
4. Predictions for the number of landfalling TCs over South China
The number of TCs making landfall on the coast of Southern China between July and December is forecast to be 3, slightly below the normal number of 4. The ENSO forecast calls for El Niño conditions in the latter part of this year, which, as suggested in section 2, would cause fewer TCs to form or be steered into the SCS, especially during the late season. This would explain the slightly below-normal forecast for July to December landfalling TCs.
Summary of predictions
With these changes, it is expected that the overall TC activity and the number of tropical storms and typhoons are likely to be near-normal but the number of typhoons is likely to be slightly above-normal (See Table 4). The number of TCs making landfall on the coast of Southern China between July and December tends to be slightly below-normal.
(a) April Forecast
(b) June Forecast