Laboratory for Atmospheric Research, Department of Physics and Materials Science, City University of Hong Kong

2001 Forecasts of number of tropical cyclones (a) over the Western North Pacific (b) over the South China Sea (c) making landfall along the South China coast Johnny C. L. Chan and Liu Kin Sik

Issued on 10 May 2001

1. Background

        Following the success of the forecasts of tropical cyclone activity over the western North Pacific (WNP) and the South China Sea (SCS) in 2000, we are now issuing the forecasts for such activities for 2001.  The details of the modified procedure can be found in Chan et al. (2001) [to appear in Weather and Forecasting and can be downloaded from here (document in MSWord format)].  As mentioned in the paper, we will issue an update in June as well. 

        In addition, with the sponsorship from the Risk Prediction Initiative, we have developed a similar technique for the prediction of the number of tropical cyclones making landfall along the South China coast (Fig. 1).  Forecasts for 2001 are also issued.  Note that since this is the first time such landfall forecasts are made, future refinements may be necessary.  A paper on the details of this technique and the predictors employed is currently under preparation.

Fig. 1. Coastline of South China and Hainan (blue line).

2. Forecasts

        Forecasts (Table 1) over the western North Pacific call for a close-to-normal TC activity in 2001.  Most of the individual predictors give similar forecasts except for those related to ENSO, which predict above-normal activity.  At the time of issuance of this forecast, ENSO conditions over the equatorial Pacific do not indicate an imminent onset of a warm event.  No consensus appears to arise from the various ENSO forecasts, with about half predicting neutral and the other half predicting a warm event.  Even if a warm event does occur, it is likely that the onset will take place in the summer, which will have less effect on TC activity over the western North Pacific. 

        However, the prediction of TC activity over the South China Sea generally shows below-normal activity, which is mainly contributed by ENSO predictors.  The less active SCS therefore results in the below-normal number of TCs that are likely to make landfall along the South China coast.

         As mentioned in Chan et al. (2001), the ENSO signal can sometimes appear later in the year.  Therefore, an update will be issued in June based on the latest ENSO information.

Table 1. Predictions of the number of tropical cyclones over the western North Pacific and the South China Sea in 2001.

List of predictors used in the tropical cyclone activity prediction scheme

Below are the predictors used in the various prediction schemes. Note however that not all the predictors are used in every scheme.

1. Parameters related to El Nino phenomenon

2. Parameters related to the large-scale circulation

Predictor	Description
Index of the area of the subtropical high over the western North Pacific	Area enclosed by the 5880 gpm contour (characteristic height of the subtropical high) on the 500-hPa monthly mean chart within the area (10-50oN, 110oE-115oW)
Index of the northern extent of the subtropical high over the South China Sea	Mean latitude of the northern flank of the 5880 gpm contour (characteristic height of the subtropical high) on the 500-hPa monthly mean chart between 110-120oE
Index of the strength of the subtropical high over Tibet	Average 500-hPa geopotential height minus 5000 gpm within the area (25-35oN, 80-100oE)
Index of the strength of the India-Burma trough	Average 500-hPa geopotential height minus 5800 gpm within the area (15-20oN, 80-100oE)
Index of the area of the polar vortex in the Pacific sector	Area enclosed by the characteristic geopotential height contour of the polar vortex (which varies from 5480 m in January to5720 m in July and August) on the 500-hPa monthly mean chart within the longitude band 150oE-120oW.
Zonal index over Asia	Defined as / where is the longitudinal average within the longitude band (60-150oE) of the 500-hPa monthly mean geopotential height within each latitude band (10o latitude) and is the latitude (from 45 to 65oN)
Index of the frequency of cold-air intrusion into China during September-December and January-May	An occurrence of a cold air intrusion is defined as when 8 out of 15 stations over China (evenly spread around from north to south) have a temperature drop of ³ 5oC within the same three days
[Data are obtained from the National Climate Center of China]

References

Barnston, A. G. and R. E. Livezey, 1987: Classification, seasonality and persistence of low-frequency atmospheric circulation patterns.  Mon. Wea. Rev., 115, 1083-1126.

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. Wea. Forecasting, 13, 997-1004.
Abstract

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

Chan J. C. L. and J. J. Xu, 2000: Physical mechanisms responsible for the transition from a warm to a cold state of the El Nio Southern Oscillation. J. Climate, 13, 2056-2071. 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. Wea. Forecasting, 16, 491-498. Abstract

Xu J. J. and J. C. L. Chan, 2001: The role of the Asian-Australian monsoon system in the onset time of El Nio events.. J. Climate, 14, 418 - 433. Abstract