The sensitivity of tropical cyclone activity off the Australian northwest coast to Indian Ocean temperature anomalies

The sensitivity of tropical cyclone activity off the Australian northwest coast

to Indian Ocean temperature anomalies

¹ Center for Environmental Modeling and Prediction, University of New South Wales,

Sydney, Australia 2052 Email: L.Leslie@unsw.edu.au

² Bureau of Meteorology, P.O. Box 413, Darlinghurst, Sydney, Australia 1300

Email: M.Speer@bom.gov.au

³ Bureau of Meteorology, P.O. 1289K, Melbourne, Australia 3000

Email: T.Skinner@bom.gov.au

ABSTRACT

The role of east Indian Ocean temperatures on tropical cyclone activity is becoming increasingly recognised in terms of its consequences for northwest Australia, including water resources and their management. In the 1998-99 tropical cyclone season four tropical cyclones (TCs) crossed the West Australi an coastline over a two month period. This was a period of large SST anomalies over the east Indian Ocean. The four storms were 'Billy' (December 1998), 'Thelma' (December 1998), 'Vance' (March 1999), and 'Gwenda' (April 1999). 'Thelma' was the first category 5 (most intense) known to have developed in the Timor Sea just north of Australia. Estimated peak gusts reached 320 km/hr as it crossed the northwest coast of Australia. 'Vance' devastated the town of Exmouth as it crossed the coast as a category 4 TC. 'Gwenda' reached category 4 before weakening and hitting the coast. 'Billy' crossed the coast after weakening from a category 2 earlier in its development.

At the same time the Indian Ocean dipole was strongly developed. The Indian Ocean dipole refers to a sea surface temperature pattern that features positive anomalies in the northeast (over Indonesia and off the northwest coast of Australia) and negative anomalies in the southwest Indian Ocean. This pattern persisted throughout the 1998/99 tropical cyclone season.

In this study, the sensitivity of TC motion and intensity to the dipole is investigated using the adjoint of the UNSW HIRES numerical weather prediction (NWP) model for the four tropical cyclones. Also the role of the positive anomalies themselves off the northwest Australian coast on the sensitivity of TC motion and intensity is investigated.

It has been recognised for some time now that NWP model sensitivity studies can be carried out using the adjoint of the NWP model. Adjoint methods can be used to identify regions and variables that have the largest impact on the model forecasts. In such cases, small perturbations grow rapidly and quantitative estimates of this sensitivity can be made. In the case studied here, we examine the effects of SSTs in a particular region, the east Indian ocean, using the adjoint method. Expressed simply, gradients in the so-called cost function or scalar forecast aspect, J, are calculated. J is a function of the model variables, X . The initial and forecast values of X are defined as X₀ and X_{f .} The (non-linear) response of J to changes in X₀ are given by,

dJ=dX₀ . ( dJ/dX₀ ) , where . is the dot product,

and dX₀ is the perturbed initial state (in this case containing small changes in the SST field).

Results obtained using the adjoint method show that the higher SSTs are more important than the dipole and that they have a significant impact in increasing cyclogenesis, with more intense TCs. Finally, the TC track forecasts are also very sensitive to areas of high SST anomalies.