2006 Predictions of Seasonal Tropical Cyclone Activity over the Western North Pacific


1. Introduction

Issued on 24 April 2006

    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).

    As an important determinant is the status of the ENSO condition, it is useful to have a discussion on the possible ENSO situation in 2006.  The oceanic and atmospheric conditions briefly approached the La Niña threshold at the beginning of the year.  However, the ENSO conditions return to neutral with the recent warming of the central and eastern Pacific.  A summary of the various ENSO model forecasts from different climate centres suggests that most of them predict a neutral condition in 5 to 8 months time (Table. 1).  Out of the 12 forecasts, 10 suggest neutral conditions throughout the summer and fall while the others suggest a possible warm event.  Based on these results, it appears that 2006 will likely be a neutral year.


2. 2006 predictions

    Predictions of overall TC activity from the predictors all fall within the normal range (Table 2).  For the number of tropical storms and typhoons, most of the predictors forecast slightly below-normal to normal activity except for the ENSO predictor which suggests above-normal activity.  A weighted forecast from all the predictions therefore would suggest near-normal activity.  Similar forecast is obtained for the number of typhoons.  Thus, it is expected that the overall TC activity, the number of tropical storms and typhoons as well as the number of typhoons are likely to be near-normal.  The quantitative predictions are given in Table 2.

    While the predictors HIB and WP give near-normal TC activity, the predictor related to the subtropical high (HWNP) suggests a slightly below-normal TC activity.  Such forecasts are partly because of the stronger-than-normal subtropical high observed between February and March in 2006 (Fig. 1).

    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.  The confidence of the predictions for the other parameters is lower, which may be related to the high uncertainty in the ENSO condition in 2006.

    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

Tropical storms and typhoons

near normal

Typhoons

near normal


Table 1. Summary of model forecasts extracted from the Australian Bureau of Meteorology homepage.

 

GROUP

5 MONTHS
(Aug 2006)

8 MONTHS
(Nov 2006)

POAMA
(run at Bureau of Met)

Neutral

Warm

CPC

Neutral

Neutral

ECMWF

Neutral

Not Available

UKMO

Neutral

Not Available

LDEO

Neutral

Neutral

NCEP

Neutral

Neutral

NOAA LINEAR INVERSE

Neutral

Neutral

SCRIPPS/MPI

Neutral

Neutral

NSIPP/NASA

Warm

Warm

JMA

Neutral

Not Available

SSES (Ohio)

Neutral

Not Available

CLIPER

Neutral

Neutral

 

Table 2. Forecasts from various predictors and the weighted average of the forecasts.

 

Entire western North Pacific

All TC
HWNP HIB NINO4   Final forecast Normal
Prediction 31 31 32   32 31
Weight 0.66 0.68 0.70  

Tropical storms and typhoons

HWNP HIB WP NINO3.4 Final forecast Normal
Prediction 25 26 26 32 27 27
Weight 0.71 0.65 0.62 0.68
Typhoons
HWNP HIB NINO3.4 ESOI Final forecast Normal
Prediction 15 15 19 17 17 17
Weight 0.64 0.58 0.73 0.67
 
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.
 
500-hPa geopotential height anomalies between February and March in 2006. Thick contours indicate the geopotential height (contour interval = 10 m) ³5860 m.

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