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 2007 forecasts

a. Summary of the forecasts issued

    On 23 April 2007, our forecasts suggested “below-normal activity for all the categories”.  A similar forecast was made on 25 June 2007 that we expected below-normal conditions for all categories.  Detailed numbers are summarized in Table 1, together with the observed numbers based on the warnings from JTWC and RSMC Tokyo (Table 2).

    Disagreements exist between the two warning centres on the intensity of some of the systems.  One system (06W) was classified by JTWC as having reached tropical storm intensity but not named by the RSMC.  On the other hand, RSMC Tokyo issued tropical storm warning for two systems (Haiyan and Podul) but no warning was given by JTWC.  Lekima was considered by JTWC as having reached typhoon intensity but not by RSMC Tokyo.

 b. Verification and discussion

    The TC activity in 2007 over the WNP was below-normal, with 23 (24) TCs reaching at least tropical storm intensity and 15 (14) TCs reaching typhoon intensity according to JTWC (RSMC-Tokyo) warnings (Table 2). Our forecasts for 2007 are therefore correct in all categories for both the April and June forecasts.  It is worth to note that it is the eighth year that the WNP has a below-normal TC activity since 1998 except for 2001 and 2004 (Fig. 1) and the inactive TC activity period seems to persist. 

    A moderate La Niña event has developed in 2007 and the historical records suggest that the TC activity over the WNP tends to be slightly below normal or below normal in a La Niña year (Table 3).  The TC activity in 2007 appears to be consistent with the atmospheric conditions associated with a La Niña event. 

    The summertime (June-September) 500-hPa geopotential height show positive anomalies over the subtropical WNP (Fig. 2), indicating the stronger than normal subtropical high.  The predictor related to the subtropical high suggested a stronger-than-normal subtropical high in the TC season and hence the below-normal TC activity, which were consistent with the observational results.

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

Table 1.  Forecasts of TC activity in 2007 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. 2007 summary of tropical cyclones over the western North Pacific. 

Table 3.  Number of TCs with at least tropical storms intensity and number of typhoons in La Niña years.

Fig. 1.  Annual number of tropical storms and typhoons in the western North Pacific between 1960 and 2007. The horizontal line indicates the climatological mean.

Fig. 2.  500-hPa geopotential height anomalies between June and September in 2007. Thick contours indicate geopotential heights (contour interval = 10 m) ³ 5860 m.