2011/12 Predictions of Seasonal Tropical Cyclone Activity in the Australian Region


1. Introduction

 

Issued on 01 Dec 2011

Since the 2009/10 season, the Guy Carpenter Asia-Pacific Climate Impact Centre (GCACIC) at City University of Hong Kong has been issuing real-time predictions of the annual number of tropical cyclones (TCs) affecting the Australian region (90E-160E, 40S-0N) and its sub-regions (eastern Australian region, 135E-160E, 40S-0N and western Australian region, 90E-135E, 40S-0N).  Hindcasts for the period of 1970-2008 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.  The most prominent ones include the proxies for El Nio/Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD).  These should be considered to be experimental forecasts and verifications will be made after each season.


2.  ENSO and IOD conditions in 2011/12

As an important determinant is the status of the ENSO condition, it is useful to have a discussion on the possible ENSO situation in 2011/12.  A weak La Nia event has developed in the fall of 2011.  SSTs remain cooler than normal in the central and eastern equatorial Pacific Ocean in October.  The Nio3.4 and Nio4 indices in October are -0.97 and -0.72 respectively.  A summary of the various ENSO model forecasts from different climate centres suggests that cold conditions may persist in the next 4 to 6 months.  Based on observations and model forecasts, the La Nia event is expected to continue into the Southern Hemisphere summer.

A weak positive IOD event has developed, as suggested by the positive values of the Dipole Mode Index (DMI) in the last few months.  Thus, ENSO and IOD will likely be the dominant factors affecting the TC activity in the Australian region during the 2011/12 season.


3.   The Predictions for 2011/12

For the entire Australian region, all the ENSO predictors (NINO4 index, trade wind index and OLR index) consistently forecast a near-normal to above-normal activity (ranging from 14 to 18) (Table 1).  However, the IOD predictor suggests a below-normal TC activity (predicted number being 11), which is related to the observed weak positive IOD event in September and October.  The final forecast is therefore for a near-normal TC activity (15 tropical cyclones) for this region.

A similar forecast is obtained for the western Australian region. The ENSO predictors suggest an above-normal TC activity, with the predicted numbers ranging from 11 to 13, but the IOD predictor gives a below-normal TC activity (predicted number being 7).  Therefore, the final forecast is 10 tropical cyclones, which is near the normal number.

For the eastern Australian region, the trade wind index and the IOD predictor suggest a near-normal TC activity (predicted number being 5) while the NINO4 index gives an above-normal TC activity (predicted number being 8). The final forecast is 6 tropical cyclones affecting this region, which is near the normal number.

It should be noted that the sum of the TC numbers in the western and eastern Australian regions may not be equal to the TC number in the entire Australian region because some TCs may move through both the western and eastern Australian regions.

Thus, it is expected that the TC activity in the entire Australian region, the western Australian region and the eastern Australian region is likely to be near-normal.  The quantitative predictions are given in Table 1.  

As a La Nia event has developed in 2011 (see section 2), it is useful to discuss the TC activity during previous La Nia years.  Out of the 13 TC seasons associated with La Nia, 7 are associated with above-normal TC activity (TC number ≥ 16) and 5 are associated with normal TC activity (TC number between 12 and 15) in the entire Australian region (Table 2).  Similar results are obtained for the western Australian region.  Thus, the annual number of tropical cyclones tends to be normal or above-normal in the entire and western Australian regions For the eastern Australian region, the enhancement of TC activity is less significant.  The TC activity in the Australian region shows a decline in the recent decade (Fig. 1).  In the last 12 years, all the TC seasons had near-normal or below-normal TC activity, even for the four TC seasons associated with La Nia events (1999/2000, 2000/01, 2007/08 and 2010/11). Thus, the 2011/12 season (associated with a weak La Nia event) is expected to be not as active as other TC seasons associated with La Nia events, which are consistent with our forecasts.


Summary of predictions

 

Annual number of tropical cyclones
(with at least tropical depression intensity)

Forecast

Normal

Entire Australian region
(90E-160E, 40S-0N)

15 (near-normal)

12-15

Western Australian region
(90E-135E, 40S-0N)
10 (near-normal) 9-10

Eastern Australian region
(135E-160E, 40S-0N)

6 (near-normal)

5-6

 


Reference: 

Liu, K. S. and J. C. L. Chan, 2010: Interannual variation of Southern Hemisphere tropical cyclone activity and seasonal forecast of tropical cyclone number in the Australian region. Int'l J. Climatology, DOI: 10.1002/joc.2259


Table 1. Forecasts from various predictors and the weighted average of the forecasts.

 

Annual number of tropical cyclones

Entire Australian region
NINO4 TW DMI OLR Final forecast Normal
Prediction 18 14 11 16 15 12-15
Weight 0.82 0.81 0.74 0.70

Western Australian region

NINO4 TW DMI OLR Final forecast Normal
Prediction 13 11 7 11 10 9-10
Weight 0.56 0.66 0.53 0.74

Eastern Australian region

NINO4 TW DMI   Final forecast Normal
Prediction 8 5 5   6 5-6
Weight 0.61 0.68 0.60  
NINO4 Sea surface temperature (SST) anomalies in the NINO4 region (5oS-5oN, 160oE-150oW)
TW Trade wind index: mean 850-hPa zonal wind anomaly index over the West Pacific (5oS-5oN, 135oE-180oE)
OLR Outgoing long wave radiation (OLR) index near equator (160E-160W)
DMI Dipole mode index: difference in SST anomaly between tropical western Indian Ocean (60E-80E, 10S-10N) and tropical south-eastern Indian Ocean (90E-110E, 10S-0)

 


Table 2.

Annual number of tropical cyclones in the entire, western and eastern Australian regions in a La Nia year.  Green and blue shadings indicate the above-normal and below-normal TC activity respectively.

 

TC season with
La Nia event

Entire
Australian region
(90o-160oE)

Western
Australian region
(90o-135oE)

Eastern
Australian region
(135
o-160o
E)

1970/1971

14

9

7

1971/1972

16

6

11

1973/1974

20

11

9

1974/1975

18

14

5

1975/1976

19

11

10

1984/1985

19

12

10

1988/1989

15

10

5

1995/1996

16

13

6

1998/1999

22

16

6

1999/2000

14

12

3

2000/2001

11

9

5

2007/2008

14

11

4

2010/2011

13

9

4

 



Fig. 1.  Annual number of tropical cyclones in the entire Australian region between 1970 and 2011.  The year 1970 denotes the TC season spanning from July 1969 to June 1970.  The horizontal line indicates the climatological mean.  Red circle and blue squares indicate the El Ni
o and La Nia years respectively.