Seasonal Climate Outlook Message for February to May 2010

The bottom line

Ex-Tropical Cyclone Olga brings rain Dave McRae, Qld Climate Change Centre of Excellence, 01/02/10.

Tropical Cyclone Olga is the first cyclone for some time to travel south through inland Queensland. It will be interesting to see if it delivers high rainfall totals but preferably without significant wind damage to buildings and agricultural crops.

Cyclones are low pressure systems that form over warm tropical waters and have gale force winds (sustained winds of 63 km/h or greater and gusts in excess of 90 km/h) near the centre. For a cyclone to develop it needs a pre-existing low pressure system or disturbance (trough or surface level low), warm ocean temperatures (greater than 26.5 degrees C), a moist unstable atmosphere (such as suitable for strong thunderstorm development), be at least 500 km away from the equator (to allow for coriolis deflection or spin to develop) and have little change in wind with height (only weak vertical wind shear present to allow for development of the cyclone centre).

TC Larry (March 2006) was the last significant cyclone to impact on Queensland. It crossed the coast near Innisfail in north Queensland as a category 4 cyclone and caused much devastation in that region. A maximum wind gust of 294km/hr was recorded at the Bellenden Ker Tower (CSIRO weather observation site, elevation of 1450m) during TC Larry.

Other cyclones to impact on Queensland recently include TC Charlotte during January 2009 (category 1), TC Ellie during February 2009 (category 1) and TC Hamish during March 2009 reached category 5 although it did not cross the coastline.

For a cyclone to be rated as a category 4 it should have wind gusts between 225 to 280km/hr, an average maximum wind speed between 160 to 200km/hr and would usually have an approximate central pressure of 955 to 930hPa. TC Tracey was category 4 (Darwin 1974).

For a cyclone to be rated as a category 5 it should have wind gusts in excess of 280km/hr, an average maximum wind speed in excess of 200km/hr and would usually have an approximate central pressure of less than 930hPa. A category 5 cyclone would be extremely dangerous and cause widespread destruction.

For more information on cyclones and warnings, go to the Bureau of Meteorology tropical cyclone warning centre at www.bom.gov.au/weather/cyclone/

Interestingly a key time to watch for the potential development of cyclones is after summer passages of the MJO. Based on its current timing it is looking like the next MJO should cross northern Australia during the second half of February.

The 30day average of the SOI as of the 1st February was minus 11.1. For more information go the www.longpaddock.qld.gov.au