Last month, the Pacific Ocean moved into a neutral phase with the El Nino dissipated. The eastern Pacific has continued to cool but remains a little above average apart from a growing pool of cool water close to South America. However, the sea surface temperatures around the western Pacific remain warm and above average and will continue to play a significant role in contributing to rainfall systems across eastern Australia.
The sub-surface equatorial Pacific shows a very large pool of cold water lying 100-200m below the surface. This water is spreading eastwards where it's likely to upwell and continue the cooling trend in the eastern Pacific Ocean. Consequently, the cool water near South America will continue to grow over the coming months with a general trend towards a cool neutral and then to La Nina pattern. About half of all models looking at the ocean temperature patterns indicate a La Nina may be slightly more favoured by early spring but it is by no means a certainty yet.
However, if the past summer has taught us one thing, there is a lot more to the climatic patterns than whether the Pacific Ocean is sitting in an El Nino or La Nina phase. The Indian Ocean should also be watched as we head into winter with a growing pool of cool water off the northwestern WA coastline. This is consistent with recent modelling which supports a higher chance of a positive Indian Ocean Dipole. A positive IOD often results in a lower chance of rainfall and northwest cloud bands across the country in winter and can be a more significant driver of winter rainfall than the Pacific, especially in southern NSW and Victoria.
However, the developing situation is unusual and complex because the western Pacific is expected to remain warm, which supports increased rainfall. If a positive IOD does develop, then expect this to potentially peak earlier than normal, which means spring rainfall may not be as significantly impacted by this and more influenced by the Pacific Ocean situation.
Considering all factors, it is likely that May and June will experience above average rainfall, with a bias towards eastern NSW and south east and central eastern Queensland, where moisture from the western Pacific can more readily reach. Moving into July and August, it is possible that rainfall will trend closer to average or even a little below average across these areas but this prognosis depends on the continued development of a positive IOD event in the Indian Ocean. The warm SSTs across the western Pacific will also continue to maintain an elevated risk of lows off the coastline such as east coast lows.
