In recent months, this column has noted that sea surface temperatures in the western tropical Pacific Ocean are uncharacteristically warm for an El Nino, which has likely contributed to a slight moderation of the current El Nino pattern and the reason behind the Bureau of Meteorology's reluctance to declare an El Nino earlier.
However, in recent weeks, there has been some cooling across the western Pacific Ocean with broader areas of cool water surface anomalies occurring across the Coral Sea, although the western Equatorial Pacific remains warmer than expected for an El Nino.
Exactly how this will impact on eastern Australian rainfall patterns is a little unclear (such a setup has not occurred before) but it is likely to moderate the normal effects of an El Nino a little.
There has been little change in the sub-surface ocean temperatures for the past couple of months with a broad region of very warm (+3 to +6C) water sitting here and upwelling to the surface across the eastern Pacific Ocean. This source of warm water means that the current El Nino may even increase in strength in the coming couple of months and help the El Nino patterns to persist into 2024.
In the Indian Ocean, waters have continued to cool across the eastern areas which is enforcing a positive Indian Ocean Dipole. Positive IOD events traditionally result in limited moisture coming across from the eastern Indian Ocean into Australia and is particularly prominent in spring rainfall.
The last time an El Nino and positive IOD occurred at the same time was 2019, which was an extremely dry year across much of eastern Australia. 2019 had the added complication of a sudden stratospheric warming event in spring that contributed to a more dominant westerly pattern than usual which amplified the impacts of the positive IOD and El Nino. This isn't present for 2023 so while broadly drier patterns are expected, conditions are not expected to be as dry as 2019.
The recent major rain event across Victoria and parts of NSW that brought widespread rainfall across the region might appear as an unusual event given the current climatic patterns but such occasional "one off" events are likely for the remainder of 2023 even though overall rainfall in the eastern half of the country will remain below average.
El Nino patterns can also result in a more negative Southern Annular Mode phase which can bring an increase in cold frontal activity over eastern Australia. This pattern often drags down hot north-west winds in a pre-frontal pattern increasing bushfire potential in spring rather than summer for eastern NSW.
