2007 Predictions of Seasonal Tropical Cyclone Activity over the Western North Pacific

Since 2000, the Laboratory for Atmospheric Research (LAR) at City University of Hong Kong has been issuing real-time predictions of the annual number of tropical cyclones (TCs) affecting the western North Pacific (WNP) and the South China Sea (SCS).  Verifications of the predictions have shown that the predictions are mostly correct within the error bars.   

These are all statistical predictions with predictors drawn from a large group of indices that represent the atmospheric and oceanographic conditions in the previous year up to the spring of the current year.  The most prominent ones include the proxies for El Niño/Southern Oscillation (ENSO), the extent of the subtropical ridge, and the intensity of the India-Burma trough.  Details can be found in Chan et al. (1998, 2001) and Liu and Chan (2003).

2. ENSO conditions in 2007

As an important determinant is the status of the ENSO condition, it is useful to have a discussion on the possible ENSO situation in 2007.  After a weak El Niño event in 2006, the ENSO condition is currently in a neutral status.  In March, SST anomalies remain near-zero in the central equatorial Pacific but below normal in the eastern equatorial Pacific.  A summary of the various ENSO model forecasts from different climate centres suggests that most of them predict a neutral condition in 5 to 8 months time (Table. 1).  Out of the 12 forecasts, 9 suggest neutral conditions throughout the summer and fall while the others suggest a possible cold event.  Based on these results, it appears that 2007 will likely be either an ENSO-neutral or a La Niña year.

3. 2007 predictions  

All the predictors suggest slightly below-normal overall TC activity (Table 2).  For the number of tropical storms and typhoons, the ENSO predictor (NINO3.4 index) and the subtropical high predictor (HWNP) forecast below-normal activity and the West Pacific Index suggests near-normal activity.  A below-normal activity is therefore expected.  Similar forecast is obtained for the number of typhoons.  Thus, it is expected that the overall TC activity, the number of tropical storms and typhoons as well as the number of typhoons are likely to be below-normal.  The quantitative predictions are given in Table 2.  It should be noted that the index of the strength of the India-Burma trough is not included in the forecast this year because this predictor give above-normal TC activity, which is not consistent with other predictors.  The ENSO and subtropical high predictors tends to the dominant factors affecting the TC activity this year.

As a La Niña event could occur in 2007 as suggested in section 2, it is useful to discuss the TC activity during La Niña years.  Out of the 12 La Niña years, 5 are associated with below-normal TC activity and the TC activity is normal in 4 years (Table 3).  The three years (1964, 71 and 74) with above-normal TC activity are found in the TC active period 1960–74 (Fig. 1).  Since the inactive TC period 1998–2006 will likely to continue into 2007, an above-normal TC season is not expected in 2007.  Therefore, the 2007 TC season will likely to be normal or below-normal, which is consistent with our forecast.

The predictor related to the subtropical high (HWNP) suggests a below-normal TC activity.  Such forecasts are partly based on the stronger-than-normal subtropical high observed between February and March in 2007 (Fig. 2).

The possible error in the current predictions is given by an envelope of the possible errors, which are based on the predictions from individual predictors.  The smallest and largest numbers among the individual predictions are considered as the lower and upper bound of the final predictions.  A larger (smaller) difference between the lower and upper bound might then indicate lower (higher) predictability.  Based on this concept, we could see that for this year, prediction for the number of tropical storms and typhoons has the smallest spread and thus the highest predictability.

As discussed in Chan et al. (2001), we will provide an updated forecast sometime in June.

Summary of predictions

References

Chan, J. C. L., J. E. Shi and C. M. Lam, 1998: Seasonal forecasting of tropical cyclone activity over the western North Pacific and the  South China Sea. Weather Forecasting, 13, 997-1004. Abstract

Chan, J. C. L., J. E. Shi and K. S. Liu, 2001: Improvements in the seasonal forecasting of tropical cyclone activity over the western North Pacific. Weather Forecasting, 16, 491-498. Abstract

Liu, K. S. and J. C. L. Chan, 2003: Climatological characteristics and seasonal forecasting of tropical cyclones making landfall along the South China coast. Monthly Weather Review, 131, 1650-1662. Abstract