October Tropical Cyclone Consistent With El Nino
Latest Update: The below mentioned depression has today again increased its likelihood of forming into a tropical cyclone.
As mentioned in Monday's and Wednesday's forecast notes an area of increased convection and instability drifting from near the Equator/Dateline region towards the Solomon Islands may this weekend deepen into a tropical cyclone.
Straight off the bat there are some points we need to make.
This tropical cyclone, if it forms, is unlikely to be a direct swell source for the East Coast (though we may see some small, marginal tradewind swell late in the piece depending on the positioning of a large high moving through the Bight).
Secondly, the media have already got into a froth about the prospect of an October cyclone in the Coral Sea, however this system is expected to occupy the South Pacific corridor and is unlikely to reach the Coral Sea. Edit: a small possibility still remains it may just make the Coral Sea, we'll stay up to date with the track and update below the line.
The reason for making this correction is that tropical cyclones in the Coral Sea overwhelmingly form along, or at the terminus, of the Northern Australian Monsoon Trough. Early season cyclones are incredibly rare in this region because the Northern Australian Monsoon is still a long way from forming. It usually begins in December.
South Pacific cyclones are less specific and can form in broad areas of westerly winds south of the Equator as well as localised troughs in the South Pacific Convergence Zone. There are less stringent parameters needed to be met for early season South Pacific cyclones.
With different mechanisms for formation it's not completely surprising that phase changes in the El Niño/Southern Oscillation cycle would have different effects on probability of Coral Sea and South Pacific cyclone formation.
We're currently moving into what appears to be a moderate-strong El Niño. Importantly, El Niño's of this magnitude typically suppress cloudiness and convection in the Northern Australian Monsoon and subsequently lead to fewer Coral Sea cyclones.
In contrast, El Niño's in the South Pacific region can have the opposite effect. The South Pacific Convergence Zone can shift northwards, elevating probability in the tropical South Pacific. Tellingly, the latest NIWA (National Institute of Water and Atmospheric Research) cyclone outlook for the South Pacific this summer finds an elevated risk of tropical cyclone formation across most of the region, particularly Vanuatu, Fiji, and the Cook Islands.
A recent study by the NZ Met service found the maximum number of cyclones (16) occurred in the strong El Niño period of 1982-83 including TC Joti in October which formed in a similar area as the current system. Most of the cyclones during that strong El Nino year, and also in the 1986-87 El Niño, formed in the Fiji-Vanuatu-New Caledonia corridor.
Given this information, we shouldn't be too surprised to see a potential tropical cyclone form near the Solomons in October. It's in keeping with our developing El Niño.
Semantics out of the way, what are the implications for East Coast surf this summer?
Whilst we know cyclones in general can be fickle swell producers, South Pacific cyclones can be excellent swell generators. Westward movement is observed in a high number of these systems, especially during the intensification phase. If this occurs in the swell window, high quality groundswells can result. So even if this October surprise turns out to be a dud, there's reason to believe we'll see a number of high quality easterly groundswells from South Pacific cyclones this summer,
// STEVE SHEARER