Explaining the East Coast's cruel summer
Readers of Bill Finnegan's Barbarian Days may recall that in late 1978 Bill and his travelling partner landed in Queensland to abject flatness. "Not to worry," said the cheery locals. "The waves will appear straight after Christmas."
And lo and behold they did.
"The first cyclone swell hit right on Boxing Day," says Bill. "Kirra woke up."
As fanciful as it sounds, that is roughly the way the pattern in Queensland works: around Christmas the spring northerlies finally give way to more a productive trade swell pattern. This year, however, the northerlies continued right through January, as did the small waves. The southern half of the East Coast fared little better, though a couple of mid-range south swells brough some reprieve.
This week the drought finally broke for the whole East Coast. The quality has been marginal but that barely mattered after such a lacklustre season.
The enduring flat spell has had surfers wondering at the cause: Why has it been such a poor summer? Why has there been so few typical trade swells?
After a little bit of research it seems that one of the main contributors was something known as the Southern Annular Mode (SAM), or Antarctica Oscillation. The SAM is basically an index showing how far north or south the westerly storm track is from normal.
When SAM is in the negative the polar westerlies shift further north, closer to Australia, and the opposite is true when it's in positive - the westerlies move south toward the polar ice shelf.
Southern Annular Mode Index (October through February)
Over the last couple of months we've seen a negative SAM event, especially through January, and with this the westerly storm track, although weak, has been sitting closer towards Australia. However, not only does the storm track move north but so do the sub-tropical highs that sit just north of it.
These sub-tropical highs are the main source of trade-swell on the East Coast and usually sit across the central to southern Tasman Sea, with tropical instability further north setting up elongated, broad and sustained trade-flows.
What appears to have happened is the sub-tropical highs have shifted north and out of Queensland's swell window. Subsequently the trades have been too far north to generate any meaningful swell energy, while also directing less favourable northerly winds down the East Coast. This is a double whammy with little to no swell as well as poor winds that severely limit surfing options.
One more flow on affect with the sub-tropical ridge being pushed further north is that it also shifts the monsoon trough further north, with it having less of an influence in our swell windows. There's been less opportunity for tropical instability to spawn any swell in the Coral Sea.
The change in storm track has created winners and losers. While the East Coast has been a 'loser' the south and west facing coastlines of South Australia have 'won'. Late December and January saw one of the best runs of westerly swell in recent memory across the Mid Coast, with surfable days and favourable winds extending for nearly four weeks.
At present the SAM index is returning to near neutral, taking with it the Mid Coast swells but allowing the East Coast to finally start receiving surf.
A final note, with the storm track sitting further north than normal in summer, New Zealand has experienced one of its coldest summers with relentless cold fronts moving across the region.