rossby
Image: TOPEX/POSSEIDON

Just as fish food settles in the bottom of the tank, nutrients in the sea sink. To feed the microscopic plants at the bottom of the marine food chain, though, the nutrients must regularly rise; only within the top 50 to 150 meters of water do these plants get enough sunlight to grow. Scientists have known that wind and deep water wells do some of the work, stirring the water and drawing up the food. But those mechanisms can't account for the amount of nutrients that must surface. Today a paper in Nature describes another source of help: Rossby waves.

These waves are generated by the Earth's rotation, and thus slush slowly westward around the planet. Although they are only a few centimeters deep, they stretch several thousand kilometers wide. And Mete Uz of the University of Rhode Island and his colleagues believe that by compressing the ocean's upper layers, these waves help suck nutrients up from the depths.

Uz's group analyzed satellite data to follow Rossby waves crawling through the oceans. They identified them as large, shallow depressions that lingered after adjustments made for tides and weather conditions. These depressions, they discovered, corresponded with increases in chlorophyll at the sea surface. Photosynthesis rose when the front of the waves hit, and then diminished. In all, they estimate that Rossby waves may explain a tenth of the changes in sea-surface photosynthesis.