In clear, blue ocean water, sunlight – and hence solar heat – can penetrate to a depth of 100 metres, where it is carried away by deep currents. In murkier waters colonised by phytoplankton, all of the heat is contained within the top five to ten metres. These warmer waters are more likely to spawn cyclones.

By creating a model of the North Pacific subtropical gyre, a region of circulating currents over which cyclones often form, a team of researchers from the US National Oceanic and Atmospheric Administration was able to quantify this effect. In a paper published in Geophysical Research Letters, they reported that reducing the amount of chlorophyll in the gyre’s water to half of current levels would result in a 35 per cent reduction in the number of tropical cyclone days, while a completely clear gyre would lead to a 70 per cent reduction.

The study’s results suggest that satellite monitoring of ocean colour could eventually be used to help predict the formation of cyclones.

October 2010