The El Nino patterns in the Pacific Ocean have continued to weaken in the past week and while the index used by the Australian Bureau of Meteorology keeps the Pacific Ocean in an El Nino, the sea surface temperature patterns are closer to neutral because both the eastern and western Pacific Ocean are about 2-3 degrees above average. It has been warmer SSTs over the western Pacific that has fuelled rain events in past months in eastern and north east Australia.
In the sub-surface of the Pacific Ocean, a strong cold pool of water is present in the central Pacific and is tracking eastwards. It will upwell across the eastern Pacific in the coming months and completely erode the warm water anomalies there now and possibly leading to a La Nina pattern developing, especially as there is no indication of the water cooling over the western Pacific. Such a pattern could occur as early as late winter / early spring over the region.
La Nina events often correlate more strongly to east coast rainfall than El Nino events do negatively because the main driving factor for the shift in average rainfall patterns sits closer to Australia during a La Nina. As noted last month, the northern hemisphere models are strongly favouring a La Nina while the Australian models are holding the Pacific Ocean in a neutral state until August, after which a weak La Nina develops.
The Indian Ocean is in a neutral phase with a neutral to warm phase favoured into the remainder of autumn and winter now, so this means that the Indian Ocean is unlikely to be conducive to either wetter or drier patterns across Australia for much of the year. Given the potential rapid onset of the La Nina (thanks to the warm SSTs over the western Pacific), a spring peak in the La Nina is possible.
Based on this, it suggests a wetter than average April, May and June period is possible (simply due to the warm ocean temperatures which tend to be favourable for increased rainfall during this time). Above average rainfall may then be a trend looking further into late winter/early spring but both August and September are often fairly dry months in Queensland, NSW and northern Victoria, so it only takes a small increase in rainfall to result in above average rainfall to develop. A key factor to this will be the above average SSTs off the NSW coastline being maintained.
Expected increased levels of moisture may mean the potential for warmer than average minimum temperatures to occur with a slightly decreased frost risk. Above average maximums are still likely to occur through autumn but may trend closer to average for winter.
