Color-dependent cyclone activity

Under the hot summer sun, the ocean’s surface waters become warmer than the atmosphere above them. As the heat is transferred to the atmosphere, it can strengthen low-pressure disturbances and drive the characteristic weather phenomena known in the Atlantic region as hurricanes and in the Pacific as typhoons or tropical cyclones (see the Quick Study on hurricane formation by Kerry Emanuel in Physics Today, August 2006, page 74 ). A new model from a research collaboration led by Anand Gnanadesikan at the National Oceanic and Atmospheric Administration’s Geophysical Fluid Dynamics Laboratory in Princeton, New Jersey, shows how strongly correlated the sea surface temperature (SST) is to the ocean’s color. The image depicts average concentrations (in mg/m³) of chlorophyll—the green pigment in phytoplankton—from 1997 to 2000 in the Pacific Ocean, where more than half of the reported typhoon-force winds (greater than 32 m/s) occur. Considering an extreme scenario, the researchers set the concentration of chlorophyll to zero and then studied the evolution of cyclones in the North Pacific Ocean. Without chlorophyll to absorb much of the solar radiation, SSTs drop. Air over the colder water sinks, drying the atmosphere and increasing wind shear, which quenches typhoon formation. Although typhoon frequency increased by 20% along the equator in that scenario, the simulation predicted an overall drop in the region—up to 70% for areas beyond 15 degrees north of the equator—and a decrease in frequency of the most intense cyclones. (A. Gnanadesikan et al. , Geophys. Res. Lett. , in press.)