Verification of Forecasts of Tropical Cyclone Activity over the Western North Pacific and
Number of Tropical Cyclones Making Landfall in South China in 2009

1. Introduction

Since 2000, 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).  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).

2. Verification of the 2009 forecasts

a. Summary of the forecasts issued

1) TC activity over the WNP

Our April forecasts made on 26 April 2010 suggested “below-normal activity for all the categories”.  The June forecasts (issued on 24 June 2010) gave a similar forecast.  Detailed numbers are summarized in Table 1, together with the observed numbers based on the warnings from JTWC and the Tokyo Regional Specialised Meteorological Center (RSMC) (Table 2).

            Disagreements occurred among the warning centres on the intensity of some of the systems.  Meranti was considered by JTWC as having reached typhoon intensity but not by RSMC Tokyo.

2) TC landfall in South China

b. Verification and discussion

1) TC activity over the WNP

The TC activity over the WNP in 2010 was exceptionally low.  Based on the JTWC warnings, the number of TCs with at least tropical storm intensity is only 14 (Table 2), which is the lowest since the records began in 1960 (Fig. 1).  It is 13 less than the normal number (the normal being 27).  The typhoon activity also broke the record, with only 8 typhoons which is 9 less than the normal number (the normal being 17).  Our forecasts from both April and June correctly predicted the below-normal TC activity.  However, the predicted TC numbers are higher than the observed numbers, the possible reasons of which are discussed below.

A strong La Niña event developed in the summer of 2010 and the mean Jun-Nov Niño3.4 index is -1.21.  The changes in atmospheric circulation associated with the La Niña event should be the dominant factor affecting the TC activity.  Previous studies suggest that in a La Niña year, easterly anomalies are generally found over the tropical WNP, resulting in the weakening of the monsoon trough and hence a lower TC activity (Wang and Chan 2002) (Table 3).  This is a main reason for our forecast of the below-normal TC activity.  In the 2010 TC season, strong easterly anomalies are found over the entire tropical WNP, with the maximum amplitude between 130^oE and 160^oE (Fig. 2).  It should be noted that the anomalies are even stronger than those found in previous La Niña years.  As a result, the monsoon trough is much weaker than normal and the atmospheric conditions are therefore not favourable for TC genesis and development.  The mean genesis location shifted westward and only one TC formed over the tropical WNP east of 150^oE, which is the typical pattern associated with a La Niña event.  At the same time, the June-November 500-hPa geopotential height shows positive anomalies over the subtropical WNP, indicating the stronger than normal subtropical high (Fig. 3).  Thus, all atmospheric conditions are not favourable for TC genesis, which is likely the reason for the record-breaking low TC activity.  Our statistical model is not capable of predicting the extreme TC numbers.

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.  The inactive TC period 1998–2009 appeared to continue into 2010.  The number of tropical storms and typhoons is below the climatological mean in the 2010 TC season, which is the 11th out of the last 13 years since 1998 with a below-normal TC activity (Fig. 1).

2) TC landfall in South China 

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

Goh, A. Z. C., and J. C. L. Chan, 2009a: An improved statistical scheme for the prediction of tropical cyclones making landfall in South China. Submitted to Weather and Forecasting.

Goh, A. Z. C., and J. C. L. Chan, 2009b: Interannual and interdecadal variations of tropical cyclone activity in the South China Sea. International Journal of Climatology, DOI: 10.1002/joc.1943.

Wang, B. and Chan, J. C. L., 2002: How strong ENSO events affect tropical storm activity over the western North Pacific. J. Climate, 15, 1643-1658.

Table 1.  Forecasts of TC activity in 2009 issued in April and June.  The observed activity based on both the JTWC and RSMC-Tokyo warnings and the normal values are also shown.

Table 2. 2009 summary of tropical cyclones over the western North Pacific and tropical cyclones making landfall in South China.

Table 3.  Number of tropical storms and typhoons and number of typhoons in an El Niño year.  Red and blue shadings indicate the above-normal and below-normal TC activity respectively.

Fig. 1.  Annual number of tropical storms and typhoons between 1960 and 2009. The horizontal line indicates the climatological mean.  Red circle and blue squares indicate the El Niño and La Niña years respectively. 

Fig. 2.  850-hPa wind anomalies (vector) between June and November in 2009.  Shadings indicate the zonal wind (interval = 0.5 m s^-1).

Fig. 3. 500-hPa geopotential height anomalies between June and November in 2009. Thick contour indicates the geopotential height (contour interval = 10 m) ³ 5860 m.

Fig. 4. Differences in the late season (September-December) atmospheric fields between 2009 and other El Niño years. (a) 850-hPa geopotential height (contour interval = 2 m), (b) 200-850 hPa vertical wind shear (contour interval = 1 m s^-1), (c) MSE (contour interval = 3 x 10⁶ W m^-2) and (d) 850-hPa relative vorticity (contour interval = 2 x 10^-6 s^-1).