December rainfall in the SWLD was above average along the coast, near average in the central wheatbelt and below above in the eastern wheatbelt. December maximum and minimum temperatures were near average or above average.
Pacific Ocean climate patterns indicate a weak La Niña persists in the tropical Pacific. The event is expected to be short-lived, and end in the southern autumn of 2018. Past late developing La Niñas have had little influence on south-west Western Australia (SWWA) in summer.
The Indian Ocean Dipole (IOD) is neutral. IOD events are typically unable to form between December and April due to the influence of the monsoon trough over the tropical Indian Ocean. See the Bureau of Meteorology’s IOD and Pacific Ocean interaction for details.
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 positive until the end of January. In a positive SAM event, the belt of strong westerly winds contracts towards Antarctica. In spring and summer, a strong positive SAM can mean that southern Australia is influenced by the northern half of high pressure systems and hence there are more easterly winds bringing moist air from the Tasman Sea to south-east Australia. This increased moisture can turn to rain as the winds hit the coast and the Great Dividing Range. However, a positive SAM generally has little influence on spring/summer rainfall in the SWLD.
The table below gives a summary of past month and three month 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||Above average to average||Generally average|
|SWWA mean temperature||Near average||Above average|
|SWWA atmospheric pressure||Near normal||Near normal|
|Indian Ocean sea surface temperature||Cooler||Cooler|
|El Niño/Southern Oscillation (ENSO)||La Niña||La Niña|
|Indian Ocean Dipole (IOD)||Neutral||Neutral|
|Southern Annular Mode (SAM)||Positive||Positive|