1. Introduction

    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).  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. Verification of the 2006 forecasts

a. Summary of the forecasts issued

    On 24 April 2006, our forecasts suggested “near-normal activity for all the categories”.  A similar forecast was made on 23 June 2006 that we expected near-normal conditions for all categories.  Detailed numbers are summarized in Table 1, together with the observed numbers based on the warnings from JTWC (individual TCs listed in Table 2).

 b. Verification and discussion

    The TC activity in 2006 over the WNP was slightly below-normal (Table 1), with 24 TCs reaching at least tropical storm intensity (the normal being 27) and 15 TCs reaching typhoon intensity (the normal being 17). Our forecasts from both April and June slightly over-estimated the TC activity, but with small errors, and are therefore considered to be acceptable.

     This below-normal activity follows that of 2005 (Fig. 1).  Although an El Niño developed in the latter part of 2006, it was not strong and apparently began too late in the season to have much effect.  While the 850-hPa zonal wind anomalies are westerly in the central and eastern equatorial Pacific, they are only weakly positive in the equatorial western Pacific and only west of 150^oE (Fig. 2).  As a result, other than a few high-latitude TCs, none of the TCs formed east of this longitude (Fig. 3), quite unlike the situation in a strong El Niño year (e.g. 1997, 2002 and 2004).  This happened despite an above-normal SST in the eastern part of the WNP (Fig. 4).  Thus, without the support of favourable dynamic conditions, a slight increase in SST cannot cause the formation of TCs in the WNP, consistent with results of previous studies (e.g. Chan and Liu 2004).  The below-normal activity is therefore likely a result of the favourable dynamic conditions not extending into the eastern part of the WNP.  As a result, TCs could only form west of 150^oE and hence the number of formations is less than normal.

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

Chan, J. C. L. and K. S. Liu, 2004:  Global warming and western North Pacific typhoon activity from an observational perspective.  J. Climate, 17, 4590-4602. Abstract

 Table 1.  Forecasts of TC activity in 2006 issued in April and June.

Table 2. 2006 summary of tropical cyclones over the western North Pacific. 

Fig. 1.  Annual number of tropical storms and typhoons in the western North Pacific between 1960 and 2006. The horizontal line indicates the climatological mean.

Fig. 2.  850-hPa zonal wind anomalies between June and Nov 2006 over the WNP.

Fig. 3. Tracks of tropical cyclones in 2006 based on JTWC warnings. Typhoon symbols indicate formation locations.

Fig. 4.  As in Fig. 2 except for SST.