February 11th, 2010

Guy Carpenter Asia-Pacific Climate Impact Center Reviews 2009 Tropical Cyclone Activity in Western North Pacific and the Success of its Forecasts

Posted at 11:00 AM ET

The Guy Carpenter Asia-Pacific Climate Impact Center (GCACIC), a joint initiative of Guy Carpenter & Company, LLC, the leading global risk and reinsurance specialist, and the City University of Hong Kong, has published a report that discusses tropical cyclone activity in the Western North Pacific (WNP) in 2009 and the success of the Center’s earlier predictions for the typhoon season.

 

According to the GCACIC, 2009 tropical cyclone activity in the WNP was slightly below-average, continuing the inactive tropical cyclone period that started in 1998 (see Figure 1). The Joint Typhoon Warning Center (JTWC) said a total of 25 tropical storms developed in 2009, of which there were 14 typhoons and 7 major typhoons . This compares to the long-term average of 27 tropical storms and 17 typhoons.

Figure 1: Annual Number of Tropical Storms and Typhoons in WNP between 1960 and 2009 (the horizontal line indicates the climatological mean and the clear circles, red circles and blue squares indicate neutral, El Niño and La Niña years, respectively)

verification1

The City University of Hong Kong has been issuing pre-season forecasts of tropical cyclone activity in the WNP basin for the last ten years, and for the first time in 2009, the Center also provided predictions for tropical cyclones making landfall in South China. The predictions were based on statistical analysis of atmospheric and oceanic conditions up to March 2009.

Western North Pacific Forecast

Forecasts for the 2009 WNP season were released by the GCACIC in April and June and “near-normal activity” was predicted. The forecasts are summarised and verified against observed tropical cyclone development in Table 1.

Table 1: 2009 GCACIC WNP Basin Forecasts and Observed Storm Development

 

 

Tropical Storms

Typhoons

Major Typhoons

Long-term Average (1960-2008)

 

27

17

-

Observed Storm Development in 2009, according to JTWC

 

25

14

7

GCACIC Forecasts

April 2009

27

18

-

June 2009

27

19

-

Although the GCACIC forecasts slightly over-estimated the number of tropical storms that developed in 2009, the variance was small and fell within forecast error ranges. However, the GCACIC significantly over-predicted the number of typhoons that ultimately developed, and the Center states much of this can be attributed to the formation of an El Niño event in the summer of 2009.

El Niño years have historically seen elevated typhoon activity as tropical cyclones tend to develop further east than normal, extending the life span of the storms and giving a higher probability of attaining typhoon status. Therefore, when the GCACIC foresaw the El Niño event, the Center predicted higher than average typhoon activity. However, this eastwards shift did not occur in 2009 and the GCACIC consequently predicted a higher number of typhoons than actually transpired. Predictors related to subtropical highs also played a part.

South China Landfall Forecasts

For the first time in 2009, the GCACIC released landfall forecasts for South China and the predictions called for a slightly above-normal number of tropical cyclones making landfall in the area early in the season (May to August), a below-average number in the late season (September to December), and a slightly below-normal number overall. The forecast issued in June also suggested the number of landfalling tropical cyclones would be slightly below-normal for the main season (see Table 2 for full details).

Table 2: 2009 GCACIC South China Landfall Forecasts

 

GCACIC Forecasts

Observed

Normal

 

April

June

JTWC

RSMC

 

Early Season (May to August)

4

-

4

-

3

Late Season (September to December)

0

-

3

-

2

Main Season (July to December)

-

3

6

-

4

Whole Season (May to December)

4

-

7

-

5

The actual number of tropical cyclones that made landfall along the coast of South China was 4 in the early season, 3 in the late season, 6 in the main season and 7 in the whole season. The GCACIC forecast for the early season was accurate but the predictions for the late and main seasons under-estimated the number of landfalling tropical cyclones, which inevitably meant the GCACIC landfall forecast for the whole season was also on the low side.

This low estimate again appears to be related the El Niño event. Historically, El Niño periods tend to see below-average landfalling tropical cyclones in South China. However, as the atmospheric patterns associated with the 2009’s El Niño event were not typical, the late season in South China defied the historical trend and saw three more landfalling tropical cyclones.

The forecast verifications are fully discussed in a GCACIC briefing, and this can be downloaded by clicking here.

GCACIC plans to issue its first 2010 forecast for the WNP basin in April.

1 The Tokyo Regional Specialised Meteorological Center (RSMC) only recorded 22 tropical storms and 13 typhoons

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