Welcome to 'Did You Know'. A daily OCC segment where we tackle some of the easy and not so easy questions we come across on our social networks. Tonight's topic comes from a subscriber.
Did You Know: Question comes from 'Erita'
"I have read that these strong trade winds help cause a La Nina. Does the La Nina then at some time influence these trade winds? Do we get strong winds right up to the first early storms? Any more info on the correlation between the 2 would be great!"
There is a clear direct relationship between the increase in trade winds and the development of a La Nina. These two then create a positive feedback loop to reinforce each other.
Want to know more? well read on
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I'm going to assume everyone reading this has knowledge of the Walker Circulation and ENSO phases that the video shows above because that is whole other bunch of 'Did You Know's' that I wasn't planning on doing. If you don't know basics about El Nino, Neutral and La Nina patterns in the Pacific and how they affect us in Australia, the Bureau Of Met have a great page on it here. The image at the top of the page basically sums up what is approximately happening at the moment as we move from a neutral to La Nina pattern. As many of our North QLD fisherman and women know, it has been a windy Winter and Spring is starting even windier.
If you recall from previous 'Did You Know's' we mentioned the three key rules in Meteorology 1 - Temperature Differences Drive Pressure Differences 2 - Pressure Differences Drive Winds 3 - Winds Drive Weather
With the development of a La Nina the thermal gradient between the East and West Pacific increases. Let me deal with some absolute numbers for you here. In December 1997 (a very strong El Nino year) the temperature of the Eastern Equatorial Pacific Ocean was around 27 degrees celsius, in the Central Pacific it was 29 degrees and north of Australia it was 28.5 degrees. There is a very weak thermal gradient there and in fact the thermal gradient reverses between waters north of Australia and the date line. If we KNOW temperature drives pressure and pressure drives winds we know that the Walker circulation will be weak and we know that the trade winds will be very weak or non-existent. Let's compare that to right now as of September 4 2020. The temperature in the East Pacific is 24 degrees, the Central Pacific is 28 degrees and the West Pacific is 31 degrees. There is a clear gradient there as we head west the water gets warmer.
Warmer waters mean the air in contact with the water is warmer, it means that air becomes less dense and so it rises, expands, cools. condenses etc etc and wallah just like magic we have a ton of rain and cyclones in the West Pacific a little while later. This rising air then, for a little while at least reinforces more air to come and replace it. Thus creating a stronger airflow pattern from East to West thus increasing our trade winds. That increase in trade winds allows air to rise at a greater rate when it eventually collides with landmasses (like the East Queensland ranges or the massive mountain ranges of PNG) or other air (for example the collision zone along the Monsoon trough ITCZ becomes much stronger and more active). This creates a positive feedback loop, the stronger the La Nina, the stronger the trade winds become for a little while anyway.
Now... here's the kicker. Eventually there is so much weather going on in the Western Pacific that we see the constant cloudiness decrease the amount of solar radiation on the water, frequent strong weather systems like Tropical Cyclones upwell cooler water from beneath the surface and this then decreases the thermal gradient in the waters between America and Asia/Australia. The LOWER pressure region in the West Pacific is no longer as pronounced and deep, because the air has become less buoyant and because of that, the winds no longer need to blow as strongly and that further decreases the thermal gradient so the La Nina breaks down. All of this takes many month to happen. Sometimes if the atmosphere doesn't react as quickly or strongly to these thermal gradients La Nina's can last for a few years. You will sometimes see the BoM mention that the oceans and the atmosphere are not coupling (or the word sometimes used is reinforcing) that means they are not working together and are out of sync. Therefore, not all La Nina's and El Nino's get the same response out of the ocean and atmosphere. In an ideal world where we understand everything about our atmosphere and our oceans that wouldn't happen, but, the fact that it does happen means we have missed something and there's so much we can't model and understand yet so chaos prevails. To quote one of my favourite movies Twister - I'm a big believer in this saying about the weather - "Things go wrong. You can't explain it, you can't predict it." but the struggle and ultimately the reward is that we keep trying to and that we make little key discoveries along the way. By the way, that last part of your question is that those stronger trade winds last much longer than the first storms of the season. But as heat builds right across the country it decreases the temperature gradients between Southern and Northern Australia and less dense LOWER pressure air begins to dominate and HIGH pressure systems weaken (because remember TEMPERATURE DIFFERENCES DRIVE PRESSURE DIFFERENCES and the rate of heating for the tropics is not as strong as it is for the sub tropics so we begin to equalise temperature differences across the country) we will then begin to see the trade wind strength affecting the coastal regions of Queensland decrease (because remember PRESSURE DIFFERENCES DRIVE WINDS ) as we head into November and December so if you're a keen fisherman I'm sure you'll be looking forward to that time of the year (at least until the cyclones come anyway). Thanks Erita for your question.
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