Seasonal forecasts of tropical cyclone numbers in the
Australian/southwest
contributed by Katrina A McDonnell and Neil J Holbrook
Department of Physical Geography,
Hakeem Shaik; Bureau of Meteorology,
Introduction
Poisson regression
model forecasts of the number of tropical cyclones expected to form during the
2005/06 tropical cyclone season (November-May) in the Australian/southwest
Pacific Ocean region (6°-20°S, 105°-170°E) (McDonnell et al. 2005) are
verified here, and forecasts for the upcoming 2006/07 season are
provided. The model uses the September lead (i.e., immediately prior to
the upcoming tropical cyclone season) saturated equivalent potential
temperature gradient (EPT) and September lead Southern Oscillation index (SOI)
as predictor variables to forecast the occurrences of tropical cyclone
formation (McDonnell and Holbrook 2004a,b). Figure 1 shows
the observed number of tropical cyclones formed and the cross-validated
hindcasts for the 1960/61-1992/93 tropical cyclone seasons. Model
forecasts for the previous 13 tropical cyclone seasons 1993/94-2005/06 together
with our forecasts for the upcoming 2006/07 season are also shown.
Forecast
verification of 2005/06 season tropical cyclone numbers
Using the SOI
predictor Poisson regression model, a total of 9 (8.6) tropical cyclones were
forecast to form in the Australian/southwest
Eleven tropical
cyclones formed within the Australian/southwest Pacific Ocean region during the
2005/06 season, four of which occurred within the
We note that all
11 tropical cyclones that formed in the Australian/southwest Pacific region
during the 2005/06 season occurred between January and April 2006. The
September 2005 SOI was +3.9 (weakly positive, but indicative of relatively
neutral conditions). However, the mean SOI for January-April 2006 was
+10.5 (suggesting La Nina like conditions - where La Nina would be expected to
correspond to a SOI > +10 - and an expectation for more tropical cyclones
than the long-term average). We further note that four of the 11 tropical
cyclones formed in the second half of March, with a further two occurring in
April. The monthly-mean SOIs during March and April 2006 were even larger
in magnitude at +13.8 and +15.2 respectively. The September SOI was
clearly insufficient in this case to predict the magnitude and persistence of
the La Nina-like climate anomaly during the January-April
period. Conversely, we found no clear pattern in the EPT observations
during the 2005/06 season to indicate why the EPT model was unsuccessful in
forecasting the larger number of tropical cyclones that occurred. Overall,
our tropical cyclone 'climate' forecasts using the SOI and EPT models provided
mixed results for last year's tropical cyclone season. While the SOI model
at least correctly forecast that there would be increased cyclone numbers
during the 2005/06 season [being informed by the weakly positive +3.9 September
2005 SOI], the non success of the EPT model suggests that the thermodynamic EPT
parameter, taken on its own, was inadequate as a climatic predictor for this
particular tropical cyclone case study season.
Forecasts of
2006/07 season tropical cyclone numbers
The
monthly-averaged SOI for September 2006 was -5.1 (around neutral). Using
the SOI model we forecast an average season total of 7-8 (7.7) tropical
cyclones to form in the Australian/southwest Pacific Ocean region during the
2006/07 season, including 2-3 (2.7) in the
We caution that
the forecasts reported here are experimental in nature. The reader is
advised that the methods and forecasts are subject to future change and
improvement.
References
McDonnell, K. A. and N. J. Holbrook, 2004a: A Poisson
regression model of tropical cyclogenesis for the Australian-southwest
McDonnell, K. A. and N. J. Holbrook, 2004b: A Poisson
regression model approach to predicting tropical cyclogenesis in the
Australian/southwest
McDonnell, K. A., N. J. Holbrook and H. Shaik, 2005:
Seasonal forecasts of tropical cyclone numbers formed in the
Australian/southwest
Figure 1. The
observed (solid line) and cross-validated hindcast (dashed line) seasonal
numbers of Australian/southwest Pacific Ocean tropical cyclones formed during
the period 1960/61-1992/93 using the (a) SOI, and (b) EPT predictor Poisson
regression models. Independent tropical cyclone genesis observations (o)
for the 1993/94-2005/06 seasons are also shown along with forecasts (x) for the
1993/94-2006/07 seasons using both models.
Table
1. Summary record of tropical cyclones that formed in the
Australian/southwest
|
Occurrence
Dates |
Formation
Latitude (o) |
Formation
Longitude (o) |
Maximum
wind speed (knots) |
|
|
Clare
|
7
-10 Jan |
14.2S |
123.0E |
39 (75) |
|
Daryl
|
19
- 23 Jan |
16.2S |
123.0E |
33 (65) |
|
Jim
|
28
- 1 Feb |
17.7S |
149.9E |
41 (80) |
|
Kate |
22 - 23 Feb |
10.8S |
144.5E |
26 (50) |
|
Emma
|
27
- 28 Feb |
18.9S |
114.7E |
23 (45) |
|
Larry
|
17
- 20 Mar |
16.6S |
157.6E |
52
(100) |
|
Wati
|
19
- 25 Mar |
15.9S |
163.2E |
44 (85)
|
|
Floyd
|
21
- 27 Mar |
11.9S |
116.3E |
54
(105) |
|
Glenda
|
27
- 31 Mar |
15.0S |
123.9E |
59
(115) |
|
Hubert
|
5
- 7 Apr |
17.0S |
116.3E |
28 (55) |
|
Monica
|
17
- 25 Apr |
12.4S |
150.0E |
70 (135) |
Table 2. Poisson regression coefficients (βj)
and standard errors (SE) estimated over the period 1960/61-2005/06, for both
the September lead SOI model and the September lead EPT model.
|
SOI
model βj |
SE |
EPT
model βj |
SE |
|
|
Intercept |
-4.58 |
0.07 |
-4.63 |
0.07 |
|
SOIsep |
0.17 |
0.06 |
- |
- |
|
EPTsep |
- |
- |
0.41 |
0.06 |
|
Month |
-0.52 |
0.07 |
-0.52 |
0.07 |
|
Cyclone
year |
0.07 |
0.05 |
0.11 |
0.06 |
|
Latitude |
-0.48 |
0.06 |
-0.72 |
0.06 |
|
Longitude |
-0.28 |
0.05 |
-0.32 |
0.06 |
