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 (90°E-160°E, 40°S-0°N) and its sub-regions (eastern Australian region, 135°E-160°E, 40°S-0°N and western Australian region, 90°E-135°E, 40°S-0°N). 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 Niño/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 Niña event has developed in the fall of 2011. SSTs remain cooler than normal in the central and eastern equatorial Pacific Ocean in October. The Niño3.4 and Niño4 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 Niña 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 Niña event has developed in 2011 (see section 2), it is useful to discuss the TC activity during previous La Niña years. Out of the 13 TC seasons associated with La Niña, 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 Niña events (1999/2000, 2000/01, 2007/08 and 2010/11). Thus, the 2011/12 season (associated with a weak La Niña event) is expected to be not as active as other TC seasons associated with La Niña events, which are consistent with our forecasts.
Summary of predictions
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