Updated 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 04 July 2011

This is an update of the predictions of the annual number of tropical cyclones (TCs) in the western North Pacific (WNP) and the number of TCs making landfall in South China and the Korea and Japan region (KJ)  for 2011 that we issued on 09 May 2011.  These updates are made based on new information for the months of April and May 2011.


 2.  ENSO conditions in 2011

The La Niña event developed in 2010 has dissipated and the oceanic and atmospheric conditions in the Pacific are now in neutral status.  The Niño3.4 and Niño4 indices in May are -0.45 and -0.53 respectively.  The warming of the central and east equatorial Pacific Ocean is expected to continue into the summer.  A summary of the various ENSO model forecasts from different climate centres suggests that most of them predict a neutral condition in 4 to 6 months time (Table 1).  In other words, 2011 will most likely be an ENSO-neutral year.


 3.   Predictions for the WNP

No significant change is found on the predicted total number of TCs (Table 2).  All the predictors give numbers similar to those in the April forecast and therefore the final predicted number is 31.

For the number of tropical storms and typhoons, the prediction from the index of the strength of the India-Burma trough gives a decrease in TC number (from 28 to 27).  No change occurs using other predictors.  With this small change, the final predicted number is therefore still 27 (Fig. 1).

For the number of typhoons, the prediction from the west Pacific index gives a decrease in TC number (from 18 to 17).  A greater decrease, from 19 to 17, occurs using the index of the strength of the India-Burma trough.  No change occurs using the ENSO predictors and the final predicted number therefore deceases from 16 to 15. 

With these changes, it is expected that the overall TC activity and the number of tropical storms and typhoons are likely to be near-normal and the number of typhoons is likely to be slightly below normal.

As 2011 is a year with a moderate La Niña event in the preceding year and will most likely be an ENSO-neutral year as suggested in section 2, it is useful to discuss the TC activity in a year after a La Niña event and becoming neutral.  During the past five decades, the TC activity exhibited a significant interdecadal variation, with the active periods of 1960-76 and 1989-97 and the inactive periods of 1977-1988 and 1998-2010 (Fig. 1).  The variations of the TC activity are quite different in the active and inactive periods (Table 3) Since the inactive TC period 1998–2010 will likely to continue into 2011, it is more appropriate to discuss the TC activity in the inactive periods.  All the 3 years are associated with near-normal number of tropical storms and typhoons (Table 3), which is consistent with our forecast.  However, the variations are quite large for the number of typhoons, with below-normal, normal as well as above-normal TC numbers Our prediction for the number of typhoons therefore has the larger spread (see Table 2) and thus the lower predictability.


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

Considering the newest information, the predicted number of landfalling tropical cyclones along the South China coast in the main season (July to December) is 7, which is above the normal value of 4 (Table 4). 

As mentioned in the April forecast, TC activity in the South China Sea (Chan 2000) and chances of TCs making landfall along the South China coast (Liu and Chan 2003) is enhanced in the year after a La Niña event. Since the current La Niña episode, albeit weakening, is still continuing, its effect might be extended into the latter part of the TC season.


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

The June prediction suggests an above-normal number of TCs affecting Korea and Japan in the main season (July to December). The predicted number is 7, higher than the normal value of 3 (Table 4).  

Referring to the April forecast, an increased number of TCs tends to affect KJ in the year after a La Niña event. This could be due to the enhanced TC activity in the whole Western North Pacific basin in such years, as suggested by Chan (2000).


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. 

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.


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

 

MODEL /
GROUP

1-3 MONTHS
(Jun to Aug)

4-6 MONTHS
(Sep to Nov)

POAMA
(run at Bureau of Met)

Neutral

Neutral / warm

System 3
ECMWF

Neutral

Neutral

GloSea
UK Met Office

Neutral 

Neutral

CFS
NCEP

Neutral

Neutral

CGCMv1
GMAO/NASA

Warm

warm / neutral

BCC CGCM
BCC/CMA

Neutral / warm

Neutral / warm

JMA-CGCM02
Japan Met. Agency

Neutral

Neutral


 


Table 2. Forecasts from various predictors and the weighted average of the forecasts issued in (a) April and (b) June.

(a) April Forecast

 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

 

 

 

(b) June Forecast

 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

27

27

27

 

27

27

 

Weight

0.71

0.59

0.66

 

 

 

 

Typhoons

 

HIB

WP

NINO3.4

ESOI

 

Final forecast

Normal

Prediction

17

17

14

15

 

15

17

Weight

0.59

0.61

0.77

0.66

 

 

 

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.

Number of tropical storms and typhoons and number of typhoons during the years after La Niña events and becoming neutral.  Green and blue shadings indicate the above-normal and below-normal TC activity respectively.

 

 

 

Number of tropical storms and typhoons

Number of typhoons

Active period

1989

31

21

1996

33

21

Inactive period

1985

25

17

2001

29

20

2008

27

12

 

 Table 4.

Summary of all forecasts.

 

Entire western North Pacific

 Forecast

Normal

All TC

31 (normal)

31

Tropical storms and typhoons

27 (normal)

27

Typhoons

15 (slightly below normal)

17

No. of landfalling TCs over South China

Main season (July to December)

7 (above normal)

4

No. of TCs affecting Korea and Japan

Main season (July to December)

7 (above normal)

3

 


Fig. 1.
 

Time series of the annual number of tropical storms and typhoons.  Red circle and blue squares indicate the El Niño and La Niña years respectively.  The green triangle indicated the predicted number in 2011.  The thick horizontal line indicates the normal number of tropical storms and typhoons.  The green vertical lines divide the years 1960-2010 into the active and inactive periods.