Tropical Cyclone Owen – An Early Tropical Cyclone

Early season tropical cyclones are rare in the Coral Sea but they do happen.  Perhaps the best example was earlier this year with Tropical Cyclone Liua (September 26-28).  Liua technically developed in the Fiji area of responsibility and was immediately downgraded by the Australian Bureau of Meteorology when Liua moved into Australia’s area of responsibility.  This means that newly formed Tropical Cyclone Owen is officially the first tropical cyclone of the Coral Sea (in Australia’s area of responsibility).  It also makes it the earliest tropical cyclone to occur in the Coral Sea (again in Australia’s area of responsibility) since Tropical Cyclone Guba formed on November 14, 2007 (based from the BoM database).  Guba became unusually intense for an early season tropical cyclone reaching category 3 strength.

Contrary to popular belief, the sea surface temperatures in the northern Coral Sea are warm enough for tropical cyclones nearly all year round!  However tropical cyclones require very specific conditions to occur, and mid-latitude systems often disrupt these conditions.  During the warmer months, the influence of these mid latitude systems contract southwards which allow conditions to become more favourable for tropical cyclones.  Sea surface temperatures also become warmer and these warmer sea surface temperatures extend southwards (which explains why most early season tropical cyclones are confined to the northern Coral Sea.  Unlike northern areas, the southern Coral Sea while warm enough to sustain tropical cyclones in late summer and early autumn, is often too cool at other times of the year).

In recent years there has been a lull in early season tropical cyclones in the Coral Sea but TC Freda did move into the Australian area of responsibility in December 2012.  Perhaps one of the most well known early season Coral Sea tropical cyclones was the somewhat ironically named Tropical Cyclone Joy in 1990.  TC Joy brought anything but Joy during the Christmas of 1990 as it sat close to Cairns as a destructive category 4 system.   Fortunately, Joy quickly weakened and eventually crossed the coastline near Townsville as a weak category 1 system.

For the last few months, sea surface temperatures in the Coral Sea have largely been above average but a recent analysis shows that temperatures have cooled (relative to their averages) slightly over the Coral Sea.  One of the contributing factors to the development of Tropical Cyclone Owen is likely to be due to the unusual shift of systems across the country.  During November and December, a typical pattern is for a heat trough to establish itself through inland Queensland/NSW generating showers and storms here.  Instead, we’ve seen the Queensland/NSW trough sit largely offshore for the last 2 weeks (hence the Queensland heatwave in November).  The increase in convergence is thanks to the offshore trough converging with the trade easterlies (that should normally lie over eastern Australia).  These unusual patterns also explain the rare lack of storms for this time of year over NSW and Queensland.  Typically the high pressure system (that’s been sitting 1000km offshore), should be sitting much closer to the east Australian coastline.

Other factors have also had to come together to help generate this early tropical cyclone, but the unusual weather patterns of late are a likely contributor to this system.

Forecasting where tropical cyclones will move in the Coral Sea basin is always extremely difficult but most likely Owen will be steered by a strengthening ridge along the Queensland coast later this week and will move closer to the Queensland coastline though currently forecast modelling keeps the system at the lower end of the tropical cyclone intensity scale during this time.