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DID YOU KNOW?

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.


Super typhoon Haiyan - 2013

The question comes from our Facebook Page from the 2018, but is one that we have been asked a lot ever since Typhoon Haiyan in 2013.

"Hi OCC, I notice there has been heaps of cyclones in the North-West Pacific this year. Does a big season in the North-West Pacific Cyclone season mean we will have a big season in Australia (I'm asking particularly about the Coral Sea) "

The short answer is NO - quite the opposite could actually be true. A big season in the North-West Pacific particularly for typhoons that form east of the Philippines in July - October actually could decrease the cyclone risk of an Eastern Australian impact the following Summer/Autumn. This also applies the other way around (a weak/slow typhoon season particularly East of the Philippines generally means a better chance to see an active Queensland cyclone season). Want to know why? then read on


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For many years the effects of ENSO events (El Nino and La Nina) on Tropical Cyclone development have been studied, but it wasn't until last year (2019) that the topic the other way around was studied and guess what? A very strong relationship was observed between an active typhoon season RESULTING IN (not just caused by) a stronger El Nino. The interesting thing was that an active season in the NW Pacific east of the Philippines also decreased the effectiveness and strength of any La Nina events that may have been in the process of developing.


Typically negative (La Nina - brings more rain and clones to Australia) and positive (El Nino brings less rain and clones to Australia) events develop and mature in the Australian Spring/Early Summer. However a number of research meteorologists wanted to look at how these positive and negative events are impacted by typhoon development which also peaks in the Australian Spring.


What they found was that active typhoon season in the area bounded by (10°−20°N, 135°−170°E) resulted in: A - A weaker Walker Circulation - because it changes the low level wind flow pattern from easterly to westerly. B - A change the temperature profile of the Equatorial Pacific Ocean - the temperature differences between the West and East Pacific decrease in both horizontal and vertical planes through a complex interaction of oceanic Kelvin Waves and up-welling. Basically if the temperature differences between the West and East decrease, warmer water has to flow eastwards and this takes away the effectiveness of any developing La Nina and ADDS TO the intensity of an El Nino. C - The upper outflow created at the top of the typhoons descended stable air over the North-East Australian region/Coral Sea/Solomon Sea in the Spring/Summer period. The effects are well summarised in the diagram below.




"Schematic diagram of the modulation of running 3-month mean SST intensity for the Niño 3.4 region (5°N−5°S, 120°−170°W) by tropical cyclones (TCs) over the western North Pacific. Walker circulation (light blue circle) is weakened by the direct effect of asymmetrically anomalous westerlies within 0°−10°N (light blue thick arrows) related with TCs (TC symbol) at lower tropospheric levels and by the indirect effect of the Hadley-like circulation (red circle) over the tropical western Pacific. Red (blue) heavy arrows indicate updrafts (downdrafts). Moreover, TCs can shallow the thermocline (red dashed curve) in the tropical western Pacific (pink curve indicates the thermocline without TCs and blue solid line the climatological thermocline). Enhanced eastward-propagating equatorial Kelvin waves (red wavy arrow, pink wavy arrow indicates the Kelvin wave without TCs) carries warm water eastward, further deepening the thermocline in the tropical eastern Pacific, thereby reducing the gradient of the zonal thermocline in the equatorial Pacific Ocean. Both effects intensify (weaken) the El Niño (La Niña). Black arrows represent the changing direction of the thermocline" https://www.nature.com/articles/s41467-019-11720-w/figures/10




THE MILLION DOLLAR QUESTION - WHAT IS THE SEASON LIKE SO FAR IN 2020?

Well so far the season is very much below average and importantly it is ridiculously quiet in that area East of the Philippines. In fact this was the first year ever where we recorded zero typhoons in July. The most active typhoon month is September so we haven't got there yet, but early indications are that if the research holds true, a La Nina is on the way, likely to be enhanced by the lack of activity in this North-West Pacific region and with it should come a much increased chance of an active (over active) Eastern Australian Cyclone Season.

I WANT TO READ THE FULL SCIENTIFIC STUDY

No worries, here's the link it's great reading - https://www.nature.com/articles/s41467-019-11720-w



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That's great to hear, we have a variety of membership options if you'd like to support our work. Head to https://join.ozcyclonechasers.com.au for details

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