The Indian Ocean has been a big driver in this dry spell (and has been now for some years) with warm water heavily favouring the western Indian Ocean. This results in shower and storm activity favouring areas away from Western Australia which means that when cold fronts sweep through there is very little moisture for it to drag across which is a primary producer of rain and storm activity across eastern Australia during spring.
To top it off, the weak “modoki El Nino” is seeing warm water favour the central Pacific. Conventionally there are three phases for the Pacific Ocean, and anything that does not fit into the category of a “El Nino” or “La Nina” is generally put into the “neutral” category. However there are many different sea surface temperature patterns that exist, and the central Pacific has been warmer which helps weaken the easterly trade winds slightly. In a normal year the impacts of this may not be significant – but when combined with a persistent negative Southern Annular Mode which promotes strong westerly winds over the continent due to an increase in frontal system, the effects become far more significant.
Essentially, since the easterly trades are somewhat weakened, and the westerly influence enhanced, it’s meant that westerly winds have dominated for much of spring. This gives us the following patterns:
- No moisture coming from the Indian Ocean
- Limited moisture coming from the Pacific Ocean
- Limited moisture coming from the Indonesia/Equatorial region (monsoon trough further north than it should be)
All of this has culminated into an incredibly dry period for not just November, but much of winter and spring (on top of already dry periods and years prior to this).
Some of the rainfall has been graphed to just demonstrate how well below average the rainfall has been (and in some cases, how much lower it has been compared to previous records!)