La Niña Impacts for this Australian Summer
La Nina – the wet phase of the El Nino Southern Oscillation (ENSO) – has been declared for a 3rd year in a row. What has La Nina delivered in the 2 years, and what does this tell us about the upcoming summer?
Since La Nina developed in 2020 there have been many waves of flooding in eastern Australia due to heavy rainfall; Sydney and Brisbane have dominated headlines, though there have also been periods of major flooding in the Hunter and Latrobe Valleys. Let’s look at warm season rainfall across the last two years to understand how rainfall played out during the last two La Nina events.
- November 2021 - April 2022 saw La Nina drive above average rainfall across a large swath of central and eastern Australia; see left image below, where dark blue colours denote above average to record rainfall.
- The period November 2020 - April 2021, also La Nina, brought rainfall to NSW, but below average rains to parts of central Qld; see right image below, where red colours indicate below average rainfall for those months.
Over the last couple of months, another climate driver, called the Indian Ocean Dipole (IOD), has been at play and is helping to bring increased rainfall to southern and southeastern Australia. A negative IOD event is associated with warm waters off the northwest coast of Australia. These warm waters allow more moisture to be evaporated into the atmosphere so that when fronts or lows pass across the continent they have more moisture and rainfall associated with them.
The negative IOD has helped to bring increased rainfall and subsequent flooding to inland parts of NSW, Qld and northern Vic. The negative IOD is likely to remain in place until late spring (when rainfall associated with the Australia Monsoon will re-set sea surface temperatures). Catchments across much of NSW, SEQ and Vic remain wetter than average, including the Hunter region.
Looking at the months ahead, we know La Nina is associated with an increased risk of above average rainfall across eastern Australia. This is evident in the image below which depicts the overall rainfall expectation during a La Nina summer, and is created by averaging the rainfall over the past 13 events. It shows that roughly 6 to 7 La Nina summers are wetter than average in Qld and NSW and Vic; but as we have already noted, no two La Nina events are the same. With wet or wetter than average catchments across much of eastern Australia, this signal for increased rainfall translates into an increased risk of flooding, too.
Contact MetraWeather for a detailed 6 month rainfall forecast.
