Ocean Eddies Impact Weather
July 8, 2013

Researchers Study Impact Of Ocean Eddies On Weather and Climate

redOrbit Staff & Wire Reports - Your Universe Online

Meteorologists have long been aware of the impact that large ocean currents have on the Earth's climate, but new research suggests smaller, swirling motions known as eddies can also have an impact on weather.

In research appearing in the journal Nature Geoscience, scientists from the Swiss Federal Institute of Technology (ETH Zurich) say these eddies typically arise due to the turbulence of ocean currents. A large number of these phenomena exist in each of the oceans at any time, with diameters of approximately 100 kilometers, according to the study authors.

Post-doctoral researcher Ivy Frenger and her colleagues analyzed satellite data in order to discover what impact these eddies had on the overlying atmosphere. They focused primarily in the Southern hemisphere, where eddies tend to be more common.

The ETH Zurich team was able to collect data on more than 600,000 transient eddies over the course of a decade. They compiled the data on those eddies and compared them to the corresponding overlying wind, cloud and precipitation data - information that had also been collected via satellite.

They discovered counter-clockwise rotating eddies (also known as anticyclonic eddies) typically caused a local increase in near-surface wind speed, cloud cover and rain probability. On the other hand, clockwise rotating eddies (also called cyclonic eddies) tended to cause reductions in near-surface wind speed, clouds and rainfall.

According to Frenger and her colleagues, surface water in anticyclonic eddies is warmer than in the surrounding waters, while the opposite is true for cyclonic eddies. These temperature variations are primarily due to whether the eddies originate from warmer or cooler waters, relative to their current position. The study authors determined wind speed increases of approximately five percent, cloud cover of three percent, and the probability of precipitation of eight percent for each degree Celsius that an eddy is warmer than its ambient water.

"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," the Institute said. "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," they added. "This study is the first examining such eddies systematically with regard to their impacts not only on wind and clouds but also on rainfall. Further, the ETH scientists inferred the mechanism of this phenomenon based on the spatial pattern of the local changes of the weather above the eddies."

The authors offer two possible hypotheses for this in their study. The first claims the anomalous sea surface temperature of the eddies leads to a change in the overlying atmospheric temperature, which in turn leads to changes in surface pressure and compensating air flow in the form of wind. If this hypothesis was true, scientists should be able to detect wind speed changes at the edge of the eddies.

However, the ETH team discovered the wind speed changes actually occur in the center of the eddies, suggesting a different mechanism is the cause - "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," they explained.

Thus far, Frenger and her co-authors have only looked at the impact of ocean eddies on weather. They have yet to examine whether or not the resulting atmospheric changes influence the ocean. This possibility is currently being examined through the use of computer simulations, officials at the Institute said in a statement.