La Niña Hiding In The Depths
Is it too early to be talking about La Niña?
It’s probably the last thing those suffering relentless onshore winds and poor surf in Victoria and South Australia want to hear, while it’s probably no surprise to the recently flooded areas on the East Coast right now.
As discussed last year, not all El Niño’s are hot and dry, and the current event was a perfect example of this. Most El Niños are though, with the warm water signature in the central and eastern Pacific Ocean flanked by cold water anomalies which hug the Australian region, leading to a moisture deficit in the atmosphere.
This wasn’t the case for our most recent El Niño, with significant pools of warmer than normal water developing in the western Pacific Ocean, Coral Sea, Tasman Sea and directly off the northwest of Australia. The Southern Oscillation Index also shows the lack of correlation between the Pacific and our country with it only hitting the -7 El Niño threshold during spring before weakening back to near neutral into the end of the year.
The heating has continued during the summer with the country now almost fully surrounded by warmer than normal water, apart from cold upwelling along the South Australian coast and off the Pilbara in Western Australia.
This warm water is loading the atmosphere with moisture, which is brought ashore from the east by the southward moving sub-tropical high. All through December we’ve seen the sub-tropical high further south than normal, known as a positive Southern Annular Mode, and this has exacerbated things bringing torrential downpours to the east while at the same time bringing relentless south-east winds to Victoria and South Australia.
But what does this have to do with La Niña?
Firstly, it’s worth noting that La Niña’s very often follow strong El Niño years.
The reason lies in the oceanic dynamics that feed and strengthen El Niño events, that being strong westerly wind bursts in the western Pacific Ocean in response to rising air above the warm pool to the east.
During the summer peak of El Niño these westerly winds actually start to shift southward away from the equator, towards the developing South Pacific Convergence Zone. This is the zone where the south-east trades meet the easterly trades, with it shifting south during the peak of summer and early autumn.
This means the driver for the pile up of warm water (westerly wind bursts) are shifted away from the equator, allowing easterly trade-winds to kick back in.
With all that warm water piled up to the east, the sea level is higher than surrounding waters and this creates a transport of the warm water away from the equator, to the north, south and even west as it tries to balance itself back out again.
All things being equal the flow of warm water away from the equator induces the upwelling of cooler, sub-surface water and the rapid breakdown of the El Niño. If the cold pool is significant enough we see a quick transition from an El Niño state to La Niña the following year.
Will we see this play out this year? The evidence is already hiding at depth below the Pacific Ocean. A significant pool of cooler than normal sub-surface water is developing in the western Pacific Ocean, with it forecast to travel east over the coming months, surfacing this coming winter (diagrams below).
The first diagram can be pictured as a cross-section below the Pacific Ocean, running from west to east. The ocean surface is at 0m, going down to 300m deep. Africa is the grey shade far left, Australia just to the left of 120E, and South America to the right.
The long range wind anomaly charts across the equator also show a halt to the westerly wind bursts and intensification of easterly trade-winds over the coming month, spelling the end of the current El Niño.
With the western Pacific also being primed with much warmer than normal water than usually seen through a typical El Niño, this further bolsters the confidence of a quick swing to the La Niña state during the coming winter.
As always we’ll continue to monitor developments over the coming week and months and provide running updates, along with more detail on how each regions surf season looks to evolve.