The heat is gone
Since December, the Madden Julian Oscillation (MJO) has passed through the Coral Sea twice on its circuit of the globe.
With each pass, the MJO has brought increased moisture, triggered by tropical instability, which is great news for Queensland and northern NSW surfers as those disturbances and storms are the source of their easterly swell - and this season they've received a lot.
Now, satellite observations of the Coral Sea and Western Pacific are showing up interesting sea surface temperatures and it's all linked to the recent tropical activity.
You see, atmospheric instability is only one of the precursors necessary for tropical storms of strength. Another is warm sea surface temperatures.
During La Niña years we see warm water piling up in the Western Pacific Ocean, filtering down into the Coral Sea. The warmer water provides more moisture and heat for the atmosphere to feed off, which, as mentioned, leads to increased convection (storms) and tropical cyclones when the setup is prime.
Once these depressions start to deepen, they feed off the surface sea temperatures, extracting energy as they do so.
The latent heat flux (transition) from ocean to atmosphere is the key driver for tropical cyclones and storms.
All things being equal, if energy is extracted from the sea surface, we see the water temperatures dropping. This is the reason why tropical cyclones need sea surface temperatures of 26.5 or more to maintain their strength. Any lower and there's not enough energy to continually power the cyclone, hence they break down once moving into cooler waters or over land (no water).
If we have a quick look at the sea surface temperature charts from just before the tropical activity kicked off (start of December) to now (February) one can see a fair bit of energy used up in the way of a drop in temperature across selected areas to our east. Marine heat waves are also shown, that being short periods of abnormally high temperatures.
This exchange of energy is best viewed on the sea surface temperature anomaly charts (difference of temperature compared to the long-term climate average). At the state of December, most of the Coral Sea and Western Pacific were 1-2 degrees warmer than normal. The equatorial upwelling that is the classic La Niña signal (cold water) can also be clearly seen along the equator.
Switching to now and we can see that there's been some cooling, most noticeably south of New Caledonia and in the immediate vicinity of Fiji and to its south.
The coldest areas appear to the south of Fiji and if we look at the recent evolution of tropical lows and cyclones over the past two months, that's the area in which they've strengthened and tracked.
Below is an overlay of the tropical storms on top of February's sea surface temperature anomaly chart and you can see the blue patches matching up with where the storms that formed between Vanuatu and Fiji tracked. That being south-southeast of Fiji. This included Tropical Cyclones Ana and Bina near Fiji and then Lucas which moved south across New Caledonia before turning extra-tropical, producing that blue patch in the central Coral Sea. Keen observers will notice the cooler temperatures around the Sydney region from the weekend's north-east upwelling event and also the upwelling off South Australia's South East from strong, persistent south-east winds.
We've ran similar articles in the past showing cyclone signatures of Pam (2015) and most impressive Yasi (2011) which is well worth a read. Yasi was such a significant, category 5 cyclone that by itself it dropped surface temperatures by some 4 degrees.
The MJO is forecast to remain across the Western Pacific Ocean over the coming one-two weeks which is positive for continued easterly swell energy, but follow the regional Forecaster Notes for more in depth analysis.