2014 Early Musings on the Vanuatu Weather Patterns

Andrew Roberts

With about one month to go until the start of the race, it's a good time to start looking at the prevailing patterns across the race course and how they evolve towards the end of June. Recent weather has included an active period of intensifying lows in the southern Tasman and off the eastern seaboard in late April/early May, followed by development of a strong blocking pattern with a near-stationary High in the Tasman that has persisted for some two weeks, directing warm northerly winds over Bass Strait and light variables across large tracts of the Tasman. A record run of temperatures above 20C in Melbourne was one of the outcomes of this blocking pattern.

Looking ahead, we can expect the blocking High to break up by late May with a return to a westerly pattern in Bass Strait with associated cold fronts blowing through. Late May/early June usually sees a cold snap event but often without follow through of the winter pattern. Rather, a period of the continental High dominating through June with cold, dry days often occurs, followed by a more sustained run of low pressure in Bass Strait by end of June, early July. This latter run often sees the proper onset of winter and anecdotally has been referred to in the Alps as the 'Wimbledon Dump' as it often heralds the first proper snowfalls co-inciding with timing of the tennis. This would also co-incide with the race start so will be interesting to see how the early winter pattern develops this year, and whether we are looking at a strong westerly flow in Bass Strait for the start or more benign conditions.

There is much talk of an el-nino establishing this winter, and with warm equatorial water in the eastern Pacific being boosted by more warm water being sent east by a series of recent westerly wind events from equatorial storms, it is looking likely. For the race, main implication would be lighter SE trade winds in the latter stages of the race, more like ~20 knots, rather than the ~25 knots in normal conditions, or ~30 knots in la nina periods. The position of the continental High leading up to the race will also be of interest. If it sits over the continent, this usually allows westerlies from southern ocean lows to push further up into the Tasman allowing good running conditions to prevail, or if it settles over the Tasman during the race as it currently has, numerous headaches instead for crews to negotiate the associated light and variable areas.