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

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. We also began in 2009 to predict the number of TCs making landfall in South China.

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 Goh and Chan (2009a).

2. Verification of the 2009 forecasts

a. Summary of the forecasts issued

1) TC activity over the WNP

Our April forecasts (issued on 20 April 2009) suggested “near-normal activity for all the categories”.  The June forecasts made on 18 June 2009 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.  Two systems (18W and 25W) were classified by the JTWC as having reached tropical storm intensity but were not named by the RSMC.  Linfa was considered by JTWC as having reached typhoon intensity but not by RSMC Tokyo.  In addition, two TCs (02C and Maka) formed over the Central Pacific and moved into the WNP.

2) TC landfall in South China

The April forecast called for a slightly above normal number of TCs making landfall along the South China coast in the early season (May to August), a below-normal number in the late season (September to December), and a slightly below normal number overall. The forecast issued in June also suggested the number of landfalling tropical cyclones (TCs) to be slightly below normal for the main season (July to December). Table 1 shows a summary of these forecasts, along with the observed number in 2009 and the normal values.

 b. Verification and discussion

1) TC activity over the WNP

Based on the JTWC warnings, TC activity in 2009 over the WNP was slightly below-normal (Fig. 1), with 25 TCs reaching at least tropical storm intensity and 14 TCs reaching typhoon intensity (Table 2).  The number of tropical storms and typhoons is 2 less than the normal number (the normal being 27) while the number of typhoons is 3 less than the normal number (the normal being 17).  Our forecasts from both April and June slightly over-estimated the number of TCs and the number of TCs with at least tropical storm intensity, but with small errors, and are therefore considered to be acceptable.  However, our forecasts over-estimated the number of typhoons, the possible reasons of which are discussed below.

An El Niño event developed in the summer of 2009 and the mean Jun-Nov Niño3.4 index is 0.97.  Normally, in an El Niño year, westerly anomalies are generally found over the eastern part of the tropical WNP, resulting in an eastward extension of the monsoon trough and hence a higher TC activity in this region (Wang and Chan 2002).  TCs generally have the longer life span and therefore more chance to attain typhoon intensity (Table 3).  This is a main reason for our forecast of the slightly above-normal number of typhoons.  However, in the 2009 TC season, although the sea-surface temperatures in the central equatorial Pacific are above normal, easterly anomalies are found at the low-levels (850-hPa) over the eastern part of the tropical WNP, with the maximum amplitude near 170^oE (Fig. 2) and therefore no eastward extension of the monsoon trough was observed.  As a result, no significant eastward shift in the mean genesis location was found and only a few storms formed over the tropical WNP east of 160^oE, which is quite different from a TC season associated with a typical El Niño event.  These partly explain the slightly below-normal TC activity especially for the number of typhoons. On the other hand, the June-November 500-hPa geopotential height shows positive anomalies over the subtropical WNP, indicating a stronger than normal subtropical high (Fig. 3). Such atmospheric conditions are therefore less favourable for TC genesis and this partly explains the slightly below-normal TC activity.  Indeed, the predictor related to the subtropical high gave the better predictions (predicted number of TCs with at least tropical storm intensity being 27 and number of typhoons being 16) than the ENSO predictors. 

It should also be noted that 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.  The inactive TC period 1998–2008 appears to continue into 2009.  The number of tropical storms and typhoons is below the climatological mean in the 2009 TC season, which is the 10th out of the last 12 years since 1998 with a below-normal TC activity (Fig. 1).

2) TC landfall in South China 

The number of TCs making landfall along the coast of South China was 4 in the early season, 3 in the late season, 6 in the main season, and 7 in the whole season (Table 1). The prediction for the early season was perfect. However, the predictions for the late and main seasons, even if TC Parma, which had taken on a highly unusual track of looping around the northern tip of the Philippines, were not considered, had underestimated the number of landfalling TCs, which also led to an underestimation of the predicted value for the whole season.

Since the error is most prominent in the late season, it is worth studying the atmospheric patterns during that period. In our forecasts issued in April and June, the year 2009 was expected to be an El Niño year, which according to Goh and Chan (2009b), is supposed to see below-normal number of TCs making landfall in South China. However, as mentioned above, the atmospheric patterns are not typical of an El Niño year. In addition to the 850-hPa flow pattern found to be different from that in a typical El Niño year, the formation factors of 850-hPa geopotential height (Fig. 4a) and 200-850 vertical wind shear (Fig. 4b) both show negative anomaly relative to the average during El Niño years in the South China Sea and around the Philippines, while both 1000-500-hPa moist static energy (Fig. 4c) and 850-hPa vorticity (Fig. 4d) show positive anomaly. These factors are included in the current prediction scheme (Goh and Chan 2009a), and are conditions favourable for the formation of TCs, and thus lead to more TCs making landfall along the coast of South China. Therefore the underestimation of this year’s late season forecast appears to be related to the atypical situation of the atmospheric patterns compared to those during typical El Niño years.

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.

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