by Staff Writers
Zurich, Switzerland (SPX) Jul 10, 2013
Ocean currents have a big impact on weather and climate. Without the Gulf Stream, the climate of Northern and Western Europe would be cooler. Scientists at ETH Zurich now uncovered that also relatively small swirling motions in the ocean, so called eddies, 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, affected for instance by the topography of the ocean bottom, explains Ivy Frenger, a postdoc in the group of ETH-professor Nicolas Gruber at the Institute of Biogeochemistry and Pollutant Dynamics.
"An analogy to this topographic effect are the swirls 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.
Precise satellite measurements
The scientists investigated data collected over nearly a decade allowing them to extract information for more than 600'000 transient eddies. They compiled these eddy-data, and compared them to the corresponding overlying wind, cloud and precipitation data which had been retrieved 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 cyclonic) eddies reduce near-surface wind speed, clouds and rainfall.
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 neutralize themselves, likely leading to only a small change on average. However, the oceanic eddies increase atmospheric variability and hence may influence extreme events.
If a storm blows over such an eddy, peaks in the wind speed may be diminished or amplified depending on the sense of rotation of the underlying eddy. Possibly, eddies may also influence the intensity 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.
Indications for the mechanism
Two main hypotheses have been discussed in the literature: the first argues that the anomalous sea surface temperatures of the eddies cause a change in the overlying temperature of the atmosphere, which in turn results in changes in surface pressure. This leads to a compensating air flow, more specifically wind. If this hypothesis was true, one would expect wind speed changes at the edge of eddies.
However, the data evaluated by the ETH scientists reveal that the wind speed changes not at the edge of eddies, but rather at the centre.
This points to another mechanism to be dominant, one where the anomalous ocean surface temperature modifies primarily the level of turbulence in the overlying atmosphere: the warmer the eddy, the greater the disturbance in the atmosphere above and the greater the altitude to which the eddy affects the lower atmosphere, which subsequently may change wind, clouds and rain.
In this project, the scientists so far only examined the impact of ocean eddies on weather, neglecting the possibility that the resulting changes in the atmosphere influence the ocean, leading to a fully coupled atmosphere ocean system at scales of 100 kilometres and less. In an on-going study, the researchers are investigating this effect with computer simulations.
Frenger I, Gruber N, Knutti R, Munnich M: Imprint of Southern Ocean eddies on winds, clouds and rainfall. Nature Geoscience, 2013, Advance Online Publication, doi: 10.38/ngeo1863
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