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While the Northern Hemisphere’s meteorological fall ended at the flip of the calendar to December, the Southern Hemisphere has transitioned from late spring to early summer. The active weather pattern this past spring in Australia even pushed rainfall into record territory.

Several factors led to Australia’s wettest November on record. A change in Indian Ocean sea surface temperatures was a big factor leading into the spring season that helped foster rainfall. The mechanism used to define the fluctuation in Indian Ocean sea temperatures, the Indian Dipole, was negative early in the summer. This means Indian Ocean temperatures were cooler than average. When this occurs, westerly winds push warmer water towards the Australian coast and help enhance rainfall. As a matter of fact, a strong negative Indian Dipole contributed to a record yearly rainfall in 1974 and 2010.

Typically, a negative phase of the Indian Dipole coincides with La Nina or cooling of the equatorial Pacific. That’s exactly what transpired during the second half of the spring season; a developing negative phase of ENSO, La Nina, also pushed warm water closer to the Australian coast and helped drive showers and thunderstorms across the continent.

The outcome was Australia’s wettest November since records began in 1900 and the wettest spring since 2010. Last month’s active pattern produced 72 mm of rain, besting the previous record, set during the La Nina of November 1973, by more than 2 mm. The wettest day in November 2021 was on the 10th when 340.8 mm pelted Samuel Hill in Queensland.

Not to be outdone, November 2021 was Australia’s coolest since the La Nina of 1999 and the coolest spring since 2016.

La Nina typically brings more rain than normal to Australia during the summer and leads to an increased risk for tropical cyclone activity. Since the forecast trends support La Nina continuing through at least February, the risk for flooding is heightened this summer. Australia normally sees 9 to 11 tropical cyclones each year, with four cyclones making landfall.

While a negative Indian Dipole and a La Nina bring enhanced spring rainfall, a positive Indian Dipole and El Nino, or warming of the equatorial Pacific, contribute to dry weather and wildfires in Australia. A prime example was 1982 when a positive Indian Dipole and El Nino worked in tandem to produce the driest year on record for Australia. Drought triggered the devastating Ash Wednesday brush fire in March of 1982.

Source: Australian Government Bureau of Meteorology
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Story Image: A map of Australia is seen.

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Australia Breaks Through November Rainfall Record