2005 Predictions of Seasonal Tropical Cyclone Activity over the Western North Pacific


1. Introduction

Issued on 27 April 2005

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, the intensity of the India-Burma trough.  Details can be found in Chan et al. (1998, 2001) and Liu and Chan (2003).


2. 2005 predictions

a. General discussion

            As pointed out in the verification of the 2004 forecasts, the number of TCs over the SCS has a significant decreasing trend in recent years (Fig. 1).  Our prediction scheme, which was first developed in 1997 (Chan et al. 1998) with an improvement in 2001 (Chan et al. 2001), however does not incorporate this trend and therefore overestimated the SCS TC activity during the last few years.  Therefore, we will not issue the forecasts for the SCS TC activity or the annual number of TCs making landfall along the South China coast until we can identify the possible reasons for this trend and make an improvement of the prediction scheme.

            Because an important determinant of the forecasts is the status of the ENSO condition, it is useful to have a discussion on the possible ENSO situation in 2005.  A weak El Niño event has developed in the summer of 2004 and a transition to ENSO-neutral conditions is observed during the last few months.  Out of the 12 ENSO model forecasts from different climate centres, 7 suggest neutral conditions throughout the summer and fall while some predictions forecast a re-development of such warm event.  Based on these results, it appears that 2005 will unlikely be a cold event year.  Rather, ENSO conditions tend to be neutral with the possibility of a re-development of a warm event.

 

b. The predictions

            All the predictors suggest slightly below-normal overall TC activity and the predictor related to the subtropical high forecasts activity much below normal (Table 1).  For the number of tropical storms and typhoons, most of the predictors forecast below-normal activity except for the ENSO predictor that suggests near-normal activity.  A similar difference between non-ENSO and ENSO predictors is also found for the number of typhoons, with the former (latter) prediction being much below (close to) normal.  Thus, it is expected that the overall TC activity over the WNP is likely to be slightly below normal.  A similar prediction can be made for the number of tropical storms and typhoons as well as the number of typhoons with a chance of the numbers being close to normal.  No above-normal activity should be expected in any of the categories.  The quantitative predictions are given in Table 1.

            As suggested last year, the possible error in the current predictions is given by an envelope of the possible errors, which are based on the predictions from individual predictors.  The smallest and largest numbers among the individual predictions are considered as the lower and upper bound of the final predictions.  A larger (smaller) difference between the lower and upper bound might then indicate lower (higher) predictability.  Based on this concept, we could see that for this year, predictions for the overall TC activity have the smallest spread and thus the highest predictability while the confidence of the predictions for the other parameters would be lower.

            Our forecasts are generally consistent with results from previous studies, which suggest that TC activity tends to be below normal in the year after an El Niño event (Chan 2000).  Indeed, of the 12 such years since 1960, 9 are associated with normal or below-normal TC activity.  The strongest signal in our predictions is the predictor related to the subtropical high, which suggests significant below-normal TC activity.  Such forecasts are partly based on the strong subtropical high observed between February and March in 2005 (Fig. 2).

            As discussed in Chan et al. (2001), we will provide an updated forecast sometime in June.

 

Summary of predictions

Entire western North Pacific

 

All TC

near normal to slightly below normal

Tropical storms and typhoons

below normal

Typhoons

near normal to slightly below normal


Table 1. Forecasts from various predictors and the weighted average of the forecasts.

 

Entire western North Pacific

All TC
HWNP HIB WP NINO4 Final forecast Normal
Prediction 25 30 29 29 28 31
Weight 0.70 0.69 0.59 0.72

Tropical storms and typhoons

HWNP HIB WP NINO3.4 Final forecast Normal
Prediction 21 23 25 28 24 27
Weight 0.66 0.63 0.72 0.72
Typhoons
HWNP HIB WP NINO3.4 ESOI Final forecast Normal
Prediction 13 13 13 19 18 15 17
Weight 0.60 0.66 0.61 0.74 0.60
 
HWNP Index of the westward extent of the subtropical high over the western North Pacific
HIB Index of the strength of the India-Burma trough (15o-20oN, 80o-120oE)
WP Primary mode of low-frequency variability over the North Pacific
NINO3.4 Sea surface temperature (SST) anomalies in the NINO3.4 region (5oS-5oN,
170o-120oW)
NINO4 Sea surface temperature (SST) anomalies in the NINO4 region (5oS-5oN, 160oE-150oW)
ESOI Equatorial Southern Oscillation Index (Equatorial SOI)
Equatorial Eastern Pacific SLP - Indonesia SLP (standardized anomalies)

 


Fig. 1.
 
Number of tropical cyclones over the South China Sea from 1965 to 2004. Red and blue dots indicate El Niño and La Niña years respectively.  The thick line indicates the 4th order polynomial fit.

 

Fig. 2.
 
500-hPa geopotential height anomalies between February and March in 2005. Thick contours indicate the geopotential height (contour interval = 10 m) ³5860 m.

References

Chan, J. C. L., 2000: Tropical cyclone activity over the western North Pacific associated with El Niño and La Niña Events. J. Climate, 13, 2960-2972. Abstract

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