Equatorial Wind Burst Seeds Cyclones
Following a very early season tropical cyclone at the end of last month, we're now staring down the barrel at a second tropical cyclone forming in a similar location next week.
Incredibly, this will also coincide with a typhoon or two north of the equator. The potential tropical cyclone may form between Fiji and Solomon Islands, the typhoons north of Papua New Guinea.
If it happens, that'll be two tropical cyclones in just over a fortnight, and we're still in spring? What's happening...?
It's all linked to the now-fully coupled El Niño that's developed across the Pacific Ocean basin.
'Coupled' as in the atmosphere is now providing a positive feedback loop to the warmer than normal sea surface temperatures across the eastern Pacific, compared to a couple of months ago when we had the sea surface temperature signature of El Niño but no atmospheric response.
The positive feedback loop is the rising of air above the warm water signal in the eastern Pacific Ocean, flowing westward in the upper atmosphere towards Indonesia and then sinking while cooling. This rising and falling of air creates low pressure to the east, high to the west, and with air trying to find its way from high to low, we see westerly wind bursts blowing back across the Pacific Ocean, completing the feedback loop (know as the Walker Circulation shown below).
These westerly wind bursts and relaxation of the trades pile more heat up to the east, enhancing the atmospheric feedback and Walker Circulation in a classic positive feedback loop.
It's these westerly wind bursts (WWBs) that were the main driver for Severe Tropical Cyclone Lola and also the potential activity developing next week in the western Pacific Ocean.
The exact dynamics behind the formation of (WWBs) is still being studied, but what we do know is that they're associated with an active phase of the Madden Julian Oscillation (MJO) – that being a wave of tropical activity/instability that circles the globe.
The MJO is the trigger for tropical cyclones and interacts with El Niño in a positive way, as westerly winds feed into the back of the eastward tracking wave.
These westerly winds help create atmospheric rotation - the classic spinning formation - as they rub against easterly trade-winds both north and south of the equator. Add in excess moisture from the MJO signal and you have a prime environment for tropical cyclone formation and growth.
This looks to be the case over the coming week as a strong WWB forms north of Papua New Guinea and the Solomon Islands, laying the seed for the formation of tropical storms both north and south of the equator.
The below diagram shows the forecast winds across the equator for the coming fortnight, with red/brown being stronger than normal westerly winds (WWBs) and blue/purple being stronger than normal trades. It runs from west to east, with Africa left, Australia central, and America to the right.
The column of blue/purple is situated across the Indian Ocean and is associated with the strong positive Indian Ocean Dipole mode, ie stronger than normal trade-winds, while the bursts of red/brown are the WWBs. You can see one through the end of October, leading to the formation of Lola, and the next, stronger episode forecast over the coming fortnight.
Any tropical storm that does develop close to the equator helps strengthen the westerly wind bursts, feeding the El Niño positive feedback loop and strengthening the El Niño signal.
At this early stage it doesn't look like the tropical cyclone (if it forms) will produce anything like the swell seen last week when Lola went extra-tropical, but the Gold Coast and northern NSW are looking at a healthy run of easterly trade-swell in the lead up to development of the storm.
Keep an eye on the Forecaster Notes for updates on the developments to our north-east.