Gabrielle Gives The Cold Shoulder
East Coast surfers would have at least witnessed, if not felt, the power and energy from Tropical Cyclone Gabrielle and its extra-tropical transition north of New Zealand.
A lot of energy was expelled to produce three days of large, powerful surf that fanned out across three states.
Where does all the energy come from to feed a significant storm like Gabrielle?
Looking at the sea surface temperature charts following the passage of Gabrielle, the answer is quite obvious.
A clear and significant cooling trend has occurred throughout the Coral Sea, dropping 3°C from before and after the transition south of New Caledonia.
The track, from just south of the Solomon Islands (where it developed into a tropical cyclone), is also apparent, as well as the loss of heat north of New Zealand as the extra-tropical system stalled earlier this week.
Tropical cyclones feed off the energy provided by warm sea temperatures directly underneath them, in an exchange called the air-sea latent heat flux.
Ocean surface water evaporates into the atmosphere, rising and then condensing which results in the release of heat. This further feeds convection, resulting in increasing wind speeds, increasing evaporation and further latent heat exchanges.
It's a classic positive feedback loop, occurring only when sea surface temperatures are above 26.5°C and when the mixed layer (depth of the warm sea temperatures) is of significant depth.
The stronger the tropical storm, the more energy it uses, and this results in a drop in the sea surface temperature, as shown in the images above. Localised upwelling also plays a small part to the cool signal following a tropical storm.
Phytoplankton blooms can also occur following the cooling and upwelling of deeper nutrient rich water, with one such bloom taking place off an atoll sitting half way between New Caledonia and Australia.
The cool water signal will likely persist for a few weeks before mixing back with the ambient sea water. In the short term, the cooler water suppresses any significant tropical developments in the area thanks to the lack of available heat energy.