Ocean currents have a big impact on weather and climate. Without the Gulf¹ Stream, the climate of Northern and Western Europe would be cooler. Scientists at ETH Zurich now uncovered that also relatively² small swirling³ motions in the ocean, so called eddies⁴, impact weather. A large number of such eddies exists in all oceans at any time, featuring diameters of about one hundred kilometers. Eddies arise because ocean currents are generally turbulent（骚乱的）, affected⁵ for instance by the topography（地势） of the ocean bottom, explains Ivy⁶ Frenger, a postdoc in the group of ETH-professor Nicolas Gruber at the Institute of Biogeochemistry and Pollutant⁷ Dynamics⁸. "An analogy to this topographic effect are the swirls⁹ that occur downstream of a rock in a creek," says Frenger. In the ocean, eddies can be carried along by large-scale currents over vast distances, and also move around independently.

 

 

The ETH scientists analysed comprehensive satellite data to determine the impact of these eddies on the overlying atmosphere. Their focus is the Southern hemisphere where such eddies are especially frequent. They detected the eddies based on precise measurements of sea surface topography. "Eddies appear as bumps or dips on the sea surface as the density¹⁰ of water within the eddies differs from that of the surrounding ambient（周围的） water," explains Frenger.

 

The scientists investigated data collected over nearly a decade allowing them to extract information for more than 600'000 transient eddies. They compiled these eddy¹¹-data, and compared them to the corresponding overlying wind, cloud and precipitation data which had been retrieved¹² by means of satellites as well. The scientists found that so-called anticyclonic (meaning they rotate counter clockwise in the southern hemisphere) eddies cause on average a local increase of near-surface wind speed, cloud cover and rain probability. In contrast, the clockwise rotating (so-called cyclonic¹³) eddies reduce near-surface wind speed, clouds and rainfall.

 

 

Surface water in anticyclonic eddies is warmer than in their surroundings, for cyclonic eddies it is the opposite. These temperature differences mainly reflect the origin of the eddies, meaning they originate from either warmer or cooler waters relative to their current position. Frenger and colleagues computed¹⁴ that wind speed increases by roughly 5 percent, cloud cover by 3 percent and rain probability by 8 percent for each degree Celsius¹⁵ that an eddy is warmer than its ambient water.

 

According to Frenger, the number of warm and cold eddies is similar in most of the ocean, so that their opposite signals in the atmosphere tend to neutralize¹⁶ themselves, likely leading to only a small change on average. However, the oceanic eddies increase atmospheric¹⁷ variability and hence may influence extreme events. If a storm blows over such an eddy, peaks in the wind speed may be diminished or amplified¹⁸ depending on the sense of rotation¹⁹ of the underlying²⁰ eddy. Possibly, eddies may also influence the intensity²¹ or course of such a storm. "It is important to know the variability caused by ocean eddies and account for it in weather and climate models," concludes Frenger. In addition, in areas where either warm or cold eddies dominate, they may also have larger-scale effects.

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