2011 Predictions of
(1) Seasonal Tropical Cyclone Activity over the Western North Pacific,

(2)
Number of Tropical Cyclones Making Landfall in South China, and
(3)
Number of Tropical Cyclones Affecting Korea and Japan


1. Introduction

Issued on 09 May 2011

Real-time predictions of the annual number of tropical cyclones (TCs) affecting the western North Pacific (WNP) and the South China Sea (SCS) were first issued in 2000 by the Laboratory for Atmospheric Research (LAR) at City University of Hong Kong (CityU) and annually thereafter until 2008 when such predictions were issued by the Guy Carpenter Asia-Pacific Climate Impact Centre, also at CityU.  Verifications of the predictions for the past ten years have shown that the predictions are mostly correct within the error bars.  We also began to predict the number of TCs making landfall in South China (SC) and the Korea and Japan region (KJ) in 2009 and 2010 respectively. 

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), Liu and Chan (2003), and Goh and Chan (2010a, b).


2. ENSO conditions in 2011

As an important determinant is the status of the ENSO condition, it is useful to have a discussion on the possible ENSO situation in 2011.  A moderate La Niña event that developed in the summer of 2010 is showing signs of weakening in recent months.  In March, SSTs remain colder than normal in the central and eastern equatorial Pacific Ocean. The Niño3.4 and Niño4 index in March are -1.00 and -0.78 respectively.  A summary of the various ENSO model forecasts from different climate centres suggests that La Niña is likely to continue through the spring but a transition to ENSO-neutral condition is expected during the summer (Table 1).  Based on these results, it appears that 2011 will likely be an ENSO-neutral year.


3. Predictions for the WNP

         All the predictors (HIB and NINO4 index) suggest a normal overall TC activity (Table 2).  The final forecast is therefore for a normal overall TC activity (31 tropical cyclones).  It should be noted that the index of the westward extent of the subtropical high over the western North Pacific (HWNP) is not included in the forecast this year because the subtropical high during the recent months is too weak (Fig. 2) and the monthly values of this predictor in these months are not defined.

For the number of tropical storms and typhoons, all the predictors also consistently forecast a normal activity (ranging from 27 to 28) and therefore a normal TC activity (27 tropical storms and typhoons) is expected for this category (Table 2 and Fig. 1).

A slightly difference between non-ENSO and ENSO predictors is found for the number of typhoons.  The ENSO predictors (NINO3.4 index and equatorial SOI index) suggest a slightly below-normal TC activity (predicted numbers being 14 and 15 respectively) while the other predictors forecast a slightly above-normal TC activity, with predicted numbers ranging from 18 to 19.  Therefore, the final forecast is 16 typhoons, which is near the normal number.

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 normal.  The quantitative predictions are given in Table 2.

It should be noted, however, that the TC activity over the WNP appears to enter an inactive period since 1998 (Fig. 1).  In the last 13 years, only two years (2001 and 2004) have the above-normal TC activity.  The activity in most of the other years is below normal.  Indeed, the 2010 TC season has only 14 tropical cyclones with at least tropical storm intensity, which is the lowest since the reliable record beginning in 1960.  Therefore, it is possible that because this downward trend was not included in the prediction scheme, the predictions this year could suffer from the same problem as in the last few years in that they are all over-predictions. Nevertheless, as we do not have a new scheme that incorporates this downward trend, we will need to assume that the same forcings would apply for this year and issue the predictions as given by the regression equations. Work is currently underway to identify the possible factors for this downward trend. If factors can be identified, we could include them in the predictions for next 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 may be 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, prediction for the overall TC activity and the number of tropical storms and typhoons has the smallest spread and thus the highest predictability.

As in previous years, we will provide an updated forecast sometime in June.


4. Predictions for the number of landfalling TCs over South China

As discussed in section 2, the ENSO will be returning to near-neutral conditions as the tropical cyclone (TC) season progresses.  Thus, the current year, a year after a La Niña event (LN+1 year), is very likely to be an ENSO-neutral year. 

Table 3 is a summary for this year’s predicted number of TCs making landfall on the coast of Southern China.  The predicted number for the early season (May to August) is 5, which is above the normal value of 3.  On the other hand, the forecast for the late season (September to December) is 2, which is equal to the normal value. This gives a total number of 7 TCs forecasted to be making landfall along the South China coast over the whole season, which is above the normal value of 5. 

The current prediction seems to agree with previous studies.  For example, Chan (2000) pointed out that TC activity is enhanced in the South China Sea in a LN+1 year, especially between months of May and July. At the same time, Liu and Chan (2003) also found that in such years, the chance of TCs landfalling along the South China coast increases significantly.   

At the same time, as ENSO is forecasted to return to neutral in the latter part of the year, it is likely that the effect of ENSO on landfalling TCs during the late season might not be very prominent, thus explaining a normal number being forecasted. 

As in the last two years, an updated prediction for the late season will be issued in June.


5. Predictions for the number of TCs affecting Korea and Japan

The number of TCs predicted to affect Korea and Japan (KJ) is 6 for the whole TC season, which is above the normal value of 4 (Table 3). Chan (2000) discovered that in LN+1 years, TC activity in the entire western North Pacific basin is higher than normal. Specifically, an anomalous low is seen at the 500-hPa level to the south of Japan during the peak season of these years (July to September) (Fig. 3). The associated southeasterly flow could help steer TCs towards KJ.

Similar to typical LN+1 years, the 500-hPa geopotential height has mostly seen negative anomaly in 2011 over the western North Pacific south of Japan (between 120ºE and 140ºE) (Fig. 4). If this situation continues into the peak season, it is very possible that the number of TCs making landfall over KJ would be above normal, which is as predicted.

As in last year, an updated forecast for KJ will be issued in June.


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

 

MODEL /
GROUP

1-3 MONTHS
(May-Jul)

4-7 MONTHS
(Aug-Oct)

POAMA
Australian Bureau of Meteorology

Neutral

Neutral / Warm

System 3
ECMWF (EU)

Neutral

Neutral

GloSea *
UK Met Office  

Neutral

Neutral

CFS
NCEP (US)

Neutral

Neutral

CGCMv1
NASA Goddard GMAO (US)

Neutral / Warm

Neutral

JMA-CGCM02
Japan Met. Agency

Neutral / Warm

Warm / Neutral

KMA-SNU
Korean Met. Administration
Neutral Neutral

 


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

 

Entire western North Pacific

All TC
HIB NINO4     Final forecast Normal
Prediction 31 31     31 31
Weight 0.68 0.73    

Tropical storms and typhoons

HIB WP NINO3.4 Final forecast Normal
Prediction 28 27 27 27 27
Weight 0.67 0.58 0.66
Typhoons
HIB WP NINO3.4 ESOI Final forecast Normal
Prediction 19 18 14 15 16 17
Weight 0.57 0.59 0.77 0.66
 
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)

 


Table 3. Summary of all forecasts.

 

 

Forecast

Normal

Entire western North Pacific

All TC

31 (normal)

31

Tropical storms and typhoons

27 (normal)

27

Typhoons

16 (normal)

17

No. of landfalling TCs over South China

Early Season (May to August)

5 (above normal)

3

Late Season (September to December) 2 (normal) 2

Whole Season

6 (above normal)

5

No. of TCs affecting Korea and Japan

Whole Season

6 (above normal)

4

 


Fig. 1.
 

Time series of the annual number of tropical storms and typhoon.  Red circle and blue squares indicate the El Niño and La Niña years respectively.  The thick horizontal line indicates the normal number of tropical storms and typhoons.  The green triangle indicated the predicted number in 2011.

 


Fig. 2.
 

500-hPa geopotential height anomalies between February and March in 2011. Thick contours indicate the geopotential height (contour interval = 10 m) ³ 5860 m.



Fig. 3.
 

Anomaly of 500-hPa geopotential height in LN+1 years during the peak season (July to September).


Fig. 4.
 

Time-longitude section plot of 500-hPa geopotential height anomalies from January 2010 to April 2011 over the western north Pacific. (Image provided by the NOAA/ESRL Physical Sciences Division, Boulder Colorado from their Web site at http://www.esrl.noaa.gov/psd/.)

 


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.

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.

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.

Goh, A. Z. C., and J. C. L. Chan, 2010a: An Improved Statistical Scheme for the Prediction of Tropical Cyclones Making Landfall in South China. Weather and Forecasting, 25, 587-593.

Goh, A. Z. C., and J. C. L. Chan, 2010b: Variations and Prediction of the Annual Number of Tropical Cyclone Affecting Korea and Japan. International Journal of Climatology, DOI: 10.1002/joc.2258.

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.