Discussion: Last week the Bureau of Meteorology/Australia announced expectations of a negative Indian Ocean Dipole (-IOD) developing for the Australian winter (Fig. 1). –IOD occurs when the water surface northwest of Australia and into the eastern equatorial Indian Ocean is warmer than normal while the western equatorial Indian Ocean is cooler than normal. The ocean surface temperature differential inspires convective currents northwest of Australia and subsidence across the western tropical Indian Ocean (Fig. 2).
The –IOD has profound influence on climate in the affected regions. Announcement of a –IOD pattern is of particularly important to Australia where a harsh drought has affected southeastern parts of the continent through the just-ended summer season (Fig. 3). Typically, a –IOD pattern produces wetter than normal climate across Western Australia and south-to-southeast sections. There is now hope that the southeastern Australia drought will fade during the winter season.
The prevailing jet stream pattern carrying storms across Australia during winter will have increased moisture to entrain from the warm ocean west and northwest of Australia. Storms rinse out that evaporated moisture downstream across Australia. However, where subsidence is occurring in the western Indian Ocean high pressure increases and drought affecting eastern Africa is likely to redevelop and strengthen.
The subsidence phase of the –IOD regime also defeats tropical waves from gaining strength moving across tropical Africa which lowers the risk of strong tropical waves entering the tropical North Atlantic to potentially develop into tropical cyclones during summer.
Fig. 1: The Bureau of Meteorology/Australia forecasts negative phase of the Indian Ocean Dipole for winter ahead.
Fig. 2: Negative phase Indian Ocean Dipole favors a wet winter climate across western and southern Australia.
Fig. 3: Bureau of Meteorology/Australia identifies extreme drought locations in southeast portions of Australia.