April rainfall in the SWLD was the eighth driest on record. The last three months, February to April was very much above average, the Bureau outlook saw probabilities close to 50% for this period and DAFWA outlook was indicating above median for the Great Southern but below average to average for elsewhere, so offered little guidance (See February edition of the Seasonal Climate outlook). The latest soil water map (2 May) indicates high soil moisture for southern region, but low levels in the north and southwest corner after receiving little to no rainfall in April. Check DAFWA rainfall to date tool and DAFWA soil water tool for rainfall totals and modelled stored soil moisture estimates.
In April, the SWLD maximum temperatures were above average to average whilst minimum temperatures were below average to average.
The El Niño-Southern Oscillation (ENSO) remains neutral but tropical Pacific Ocean waters have been steadily warming since January. Five out of eight international climate models indicate an El Niño is likely to develop by September. The Bureau’s El Niño watch continues and the odds of an El Niño developing in 2017 are double the normal chance. An El Niño typically biases Australia's climate towards a drier than average winter-spring (for eastern Australia), and warmer daytime temperatures in the south.
The Indian Ocean Dipole (IOD) remains neutral. Four out of six international climate models indicate the development of a positive IOD is possible by July, which means a greater likelihood of reduced rainfall for large parts of the SWLD in winter and spring. Positive IOD also typically means above normal maximum and minimum temperatures for the SWLD. In the past, a positive IOD combined with an El Niño has generally meant reduced rainfall for winter and spring for SWWA. See the Bureau of Meteorology’s IOD and Pacific Ocean interaction for details.
Indian Ocean sea surface temperatures (SSTs) north and west of Australia remain cooler than normal. This means that heavy rainfall events from cold fronts are less likely as there is a weaker north/south temperature gradient, which suppresses frontal development, and less likelihood of tropical moisture from the north interacting with cold fronts. For a summary of Pacific and Indian Ocean outlooks, see the Climate Model Summary produced by the Bureau.
The Southern Annular Mode (SAM), also known as the Antarctic Oscillation (AAO), describes the north–south movement of the westerly wind belt that circles Antarctica, dominating the middle to higher latitudes of the southern hemisphere. SAM is currently positive and is expected to remain so through May. A positive SAM in autumn and winter can mean drier conditions for the south-west corner though has a limited impact on the wheatbelt.
In April the atmospheric pressure was higher than normal over the southwest. The mean sea level pressure is forecast to remain higher in the coming months, meaning fewer cold fronts are likely to cross the coast.
The table below gives a summary of past month and three month southwest Western Australia (SWWA) climate conditions, and can be used as an indication of what is likely to occur in the near future, if climate conditions follow the current pattern.
|Climate indicator||Past month||Past three months|
|SWWA rainfall||Below average||Above average|
|SWWA mean temperature||Average||Average to below average|
|SWWA atmospheric pressure||High||High|
|Indian Ocean sea surface temperature||Cooler||Cooler|
|El Nino/Southern Oscillation (ENSO)||Neutral||Neutral|
|Indian Ocean Dipole (IOD)||Neutral||Neutral|
|Southern Annular Mode (SAM)||Positive||Positive|