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) and the South China Sea (SCS).  From 2001 onwards, LAR also issued predictions on the annual number of TCs making landfall along the South China coast.  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 2005 forecasts

a. Summary of the forecasts issued

    On 27 April 2005, our forecasts suggested that “no above-normal activity should be expected in any of the categories”, with below-normal number of tropical storms and typhoons and near-normal to slightly-below-normal conditions for the other categories.  A similar forecast was made on 24 June 2005 that we expected near-normal to slightly-below-normal conditions for all categories.  Detailed numbers are summarized in Table 1, together with the observed numbers based on the warnings from JTWC (Table 2).

 b. Verification and discussion

    It can be seen from Table 1 that our forecasts from both April and June were correct, especially for the number of tropical storms and typhoons as well as the number of typhoons.  There was only one tropical depression (20W), and thus the predicted total number of tropical cyclones is slightly more than the observed, although the near- to below-normal condition was correctly predicted. 

    Although 2005 was considered by almost everyone as an ENSO-neutral year, the TC distribution in the WNP shows some interesting features, with quite a number of TCs forming at rather high latitudes (at or north of 20^oN) but no low-latitude (at or around 15^oN) TCs east of 155^oE (Fig. 1).  The formation locations apparently were split, with no formation within the 15-20^oN band.  This pattern of formation may be related to the 850-hPa flow pattern (Fig. 2) that had strong easterly anomalies over the tropical western North Pacific east of 150^oE as well as around 25^oN.  The former made it difficult for TCs to form further east and the latter perhaps partly responsible for the formation within the 20-25^oN band.

    The SCS continued to have fewer than the climatological number of TCs, again mainly as a result of few TCs entering the SCS from east of the Philippines (see a more detailed discussion of this in the 2004 verification).

    It should also be noted that this is the first in the last 50 years that the number of TCs in the WNP is less than that in the Atlantic.  If global warming would indeed lead to more tropical cyclones and/or more intense ones, one would expect more TCs and/or more intense TCs in all TC basins.  This is, however, not observed even though the SST over the WNP was anomalously warm in 2005 (Fig. 3).  This is consistent with the conclusion of Chan and Liu (2004) that TC activity in the WNP is not controlled by local SST.  It should also be noted that the number of tropical storms and typhoons in the last few years have been oscillating around the climatological mean, with no trend observed (Fig. 4).  A similar situation can be seen in the number of Cat 4/5 typhoons (Fig. 5).

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 2005 issued in April and June.

Table 2. 2005 summary of tropical cyclones over the western North Pacific. 

Fig. 1. Tracks of tropical cyclones in 2005 based on JTWC warnings. Typhoon symbols indicate formation locations.

Fig. 2.  850-hPa zonal wind anomalies between June and Nov 2005 over the WNP.

Fig. 3.  As in Fig. 2 except for SST.

Fig. 4.  Annual number of tropical storms and typhoons in the western North Pacific between 1960 and 2005. The horizontal line indicates the climatological mean.

Fig. 5.  Annual standardized variations of PDI, the potential destruction index, NCat45 the number of Category 4 and 5 typhoons according to the Saffir-Simpson scale, and SSTA the sea-surface temperature anomalies within the area 5-30^oN, 120^oE-180 averaged between May and November for the period 1960 to 2004.