The BIG One-Off NE NSW Climate Drivers Post

We talk a lot about the climate drivers for our region, so I thought it would be useful to put a post together that looks in more detail @ the key climate influencers for our region. These drivers don’t impact on the day to day weather in a direct way – but do impact across a longer time period. It’s worth thinking of them like rolling a dice – whilst you can always have a chance of throwing a 1, 2, 3, 4, 5, or 6 on a day to day level, these drivers weight the dice, so they increase the likelihood of different numbers coming up. I’ll keep this post fairly general and then post from time to time, referencing this post, as we look at where the climate drivers are currently sitting and what impact we’re likely to see across our region.

Pacific Ocean – El Nino and La Nina

One of the key influencers across Australia (and in fact around the world). Under La Nina conditions (warmer water in the West and cooler in the East) we’ll usually see more rain and cooler temperatures as the trade winds bring warm, wetter rising air across our region):

Under El Nino conditions (cooler water in the West and warmer in the East) we’ll usually see drier and hotter conditions as the winds descend across our region:

The impact is magnified across the inland, but we certainly see some impacts along the coast as well. The strongest influence across our region is usually in Summer and Autumn. To understand more about how the different phases of the Pacific impact our weather (and why) then check out this great explainer from the BoM:

Indian Ocean – The Indian Ocean Dipole (IOD)

On the other side of Australia sits the Indian Ocean. You’d think it would be too far away to have much impact across SE Aus, but in Winter and Spring it is actually one of the key influencers of our weather. Again the influence is magnified west of the Divide, but certainly unfluences us as well.

In a postive phase of the IOD we’ll see much drier weather across SE Aus as the air descends to the north west of us before swinging across our region – descending air is usually warmer and much drier than air that rises over the ocean:

The flip side is the negative IOD. During the negative phase the air rises to the North West of Australia, drawing up warm, wet air – that then usually moves across the country ahead of changes, bringing the famous North West Cloudbands:

The strongest influence from the Indian Ocean is usually in Winter and Spring. Again the BoM have produced a great short explainer on this topic – well worth taking a couple of minutes to read:

Southern Annular Mode (SAM, or AAO)

The last key influencer is the one that has a really key role to play across our region. A negative SAM will bring drier than usual weather across our region as the Westerly influence moves further North. It is most pronounced in Winter and Spring but you’ll also see it have an influence through summer as well:

…And conversely a positive phase will usually bring wetter than usual conditions as the Westerly influence moves further South, allowing onshore winds to develop across the East coast:

Once again our fantastic BoM have produced a short explainer on the SAM:

West of the Divide all three influencers have a similar impact, but east of the Divide it is the SAM that has by far the strongest fire / drought weather influence in Spring, while the Pacific has the largest influence in Summer and Autumn:

The strength of the relationship between climate drivers and fire weather in spring. Purple squares show the strength of the relationship. Larger squares indicate a stronger relationship. 

….so the SAM is a key one for us to watch – and is the reason why it gets mentioned so much during Winter and Spring. All three influence our weather to some degree – and at different times of the year each will play a lesser or greater role.

I’ll post more about the current state of each ocean and the likely impacts for our region from time time, but hope the information here serves as a good starting point. Please feel free to comment with questions.

Images: Thanks to the BoM and The Conversation. Videos, thanks to BoM.

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