This is an update of the forecast for the number of tropical cyclone making landfall in South China made in April 2010, using newest information from April and May.
Earlier in the year, various climate centres predicted that neutral El Niņo/Southern Oscillation (ENSO) conditions will prevail during the latter half of this year. However, the most current forecast models are suggesting that La Niņa conditions would very likely occur for the remainder of this year.
With this new information, the prediction made in June now calls for a below-normal number of landfalling TCs along the South China coast (NSC) for the main season (July to December). The predicted number is 3, which is slightly lower than the normal value of 4.
Table 1 shows the number of TCs making landfall on the South China coast in the main season after an El Niņo year (EN+1 year). It can be seen that the number tends to be normal to below normal (5 and 6 out of the 14 cases respectively). Specifically, in the three most recent cases in which the EN+1 year is a La Niņa year (1998, 2005, and 2007), NSC tends to be below normal. A study of the 500-hPa zonal wind pattern in the peak season (July, August, and September) reveals that the anomalous easterlies found over the South China Sea (SCS) during the earlier cases (1973, 1988, and 1995) (Fig. 1a) has become anomalous westerlies in the three recent cases (Fig. 1b). This could be the reason for the decreased NSC, as the flow has changed from steering TCs into the SCS to one that prohibits TCs from entering SCS.
Figure 2 shows the difference between the 500-hPa zonal wind pattern in 2010 and the average EN+1 year during the months of March, April, and May. It can be seen that anomalous westerlies are present over the SCS in the current year, and this anomaly has been continuing since early May (Fig. 3). If this situation continues, it is very possible that NSC would be below normal during the main season.
2. Korea and Japan (KJ)
The June prediction continues to suggest a slightly above-normal number of TCs affecting Korea and Japan (NKJ) in the main season. The predicted number is 4, slightly higher than the normal value of 3.
The results from Goh and Chan (2010) suggested no significant pattern for NKJ in an EN+1 year. In fact, of the 14 cases of EN+1, 6 show below normal NKJ, while 6 are normal. However, as in the case of NKJ, if EN+1 years that are La Niņa years are singled out, it can be seen that the earlier cases tend to be below normal, while the recent three cases tend to be normal or above normal (Table 1).
Figure 4 shows that the difference between the earlier and recent cases of the 500-hPa geopotential height is positive over much of the western North Pacific during the peak season, suggesting that the geopotential height anomaly has become lower. It can be inferred that in the recent cases, the flow over this area has at least become less northerly, which increases the chance of TCs being able to be steered towards Japan. In addition, the 500-hPa zonal wind pattern reveals that in the earlier cases, anomalous easterlies are present south of 25šN, steering TCs westwards (Fig. 1a), while in the recent cases, anomalous easterlies can be found over much of the western North Pacific (Fig. 1b). The combination of the height and zonal wind anomalies would suggest the presence of anomalous southeasterlies over the western North Pacific, steering TCs towards Japan and leading to increased NKJ in the recent cases
3. Summary of Predictions
The number of landfalling TCs along the South China coast and affecting the Korea-Japan region is shown in Table 2.
Goh, A. Z. C., and J. C. L. Chan, 2010: Variations and prediction of the annual number of tropical cyclones affecting Korea and Japan. Submitted to International Journal of Climatology.