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The Australian Rainfall Problem – Why Drought Will Intensify

Discussion: During the past week a welcome surprise was observed in southeast Queensland and northeast New South Wales as 1-2 in. of rain was reported (Fig. 1). Near 1 in. of rain occurred in far southwest Western Australia and the south coast of South Australia. However, the rainfall deficits since April 1 remain severe away from the southeast and far west coast of the continent (Fig. 2).

The explanation for the dry climate is rooted primarily in the presence of warmer-than-normal sea surface temperature anomalies (SSTA) in the central tropical Pacific and central subtropical South Pacific (Fig. 3). In these regions the tendency for convective rainfall persists leaving Australia in the subsidence region west of the heavy rainfall tendency. Not helping is the borderline positive phase of the Indian Ocean Dipole whereas SSTA west and northwest of Australia are cooler than normal. The cooler0than-normal source region for Australian storm track moisture is limiting rainfall.

The ECMWF SEP/OCT/NOV 2018 global rainfall anomaly forecast identifies the Australian rainfall problem continuing. The heavy rains follow the warm SSTA pattern in the central/east-central Pacific tropics and subtropics leaving Australia to the west in the dry subsidence zone (Fig. 4). There is risk of this set-up continuing through Australian summer which would cause acceleration of anomalous heat (Fig. 5) further worsening the drought condition.

Fig. 1: During the past week beneficial (1-2 in.) rainfall has occurred in parts of northeast New South Wales and southeast Queensland.

Fig. 2: Rainfall deficits since April 1 identify the cause of the profound Australian drought.

Fig. 3: The SSTA regime across the tropical/subtropical Pacific Ocean helps to explain the dry climate pattern affecting Australia.

Fig. 4: The ECMWF global anomalous rainfall outlook indicates Australia is drier than normal.

Fig. 5: Current temperature probability forecast for Australia during DEC/JAN/FEB 2018-19 by the International Research Institute for Climate and Society.