On the intensification and recurvature of tropical cyclone Tracy (1974)

Abstract

On Christmas Eve, 1974, tropical cyclone Tracy (hereafter Tracy) made landfall at Darwin, Australia, with tragic loss of life and property. The current study uses objective analyses from the NCEP and ERA40 re-analysis projects, and the Bureau of Meteorology’s operational Tropical Cyclone Limited Area Prediction System, TC-LAPS, to investigate (a) changes in the tropical circulation, and (b) changes in Tracy’s motion and intensity, during the days prior to landfall. Within the limitations of the observational network available in 1974, and based on observational and model diagnostics, we postulate the following scenario for the intensification and recurvature. Landfall at Darwin coincided with the onset of the Australian monsoon. As the monsoon westerlies developed over Indonesia, the storm changed its direction of motion under the influence of the developing monsoon flow. Intensification commenced just prior to recurvature and was associated with an upper tropospheric flow transition. During this time, rapid and large scale flow changes: (i) provided a favourable low level environment of a developing monsoon trough, (ii) resulted in an equatorward movement of the upper ridge and a reduction in environmental wind shear, (iii) provided a connection between the storm outflow and the midlatitude westerlies to form an extended outflow channel for intensification, and (iv) directly influenced the development of Tracy’s upper vortex via downstream development of a weak environmental trough, which overlay the low-level circulation.