How Cold, Deep Ocean Waters Influence Sea Surface Temperatures
July 26, 2013

Understanding The Mechanisms Behind The Pacific Equatorial Cold Water Region

Brett Smith for - Your Universe Online

Newly published research from Oregon State University has revealed new details on the forces behind El Nino and could lead to more refined predictions for global warming.

According to their report in the journal Nature, OSU scientists have used new measurements of ocean mixing to record the first multi-year data set that will allow for a comprehensive analysis of how cold, deep ocean waters can influence sea surface temperatures on a long-term basis.

Because enormous swaths of ocean can influence atmospheric temperatures, understanding how ocean currents and deep water upwellings function can lead to a more informed analysis of the global climate.

Previous research has used data collected with shipboard instruments that were limited in scope. In the latest study, the oceanographers used small, battery-powered instruments that were deployed on deep-sea equatorial moorings to gather information.

According to study co-author Jim Moum, mixing occurs in the deep sea ocean much like cold cream does when poured into a cup of hot coffee - albeit on a much grander scale.

"Mixing is obviously important in our normal lives, from the kitchen to the dispersal of pollutants in the atmosphere, reducing them to levels that are barely tolerable," said Moum, a professor of physical oceanography at OSU.

In the new study, Moum and colleagues describe the mechanisms behind the equatorial Pacific cold tongue, a broad swatch of ocean about the size of the continental United States. The surface temperatures on the tongue are significantly cooler than in surrounding waters.

In temperate sections of ocean, the atmosphere goes through the seasonal cycles of heating and cooling. This cycle also causes a cycle in sea surface temperatures. In equatorial regions, the atmosphere heats the surface of the ocean year-round. However, oceanographers have detected seasonal sea temperature cycle in this region as well - suspecting mixing may be responsible.

Starting in 2005, the study researchers have been documenting temperatures at various depths in the Pacific cold tongue on an annual basis.

"This is a very important area scientifically, but it's also quite remote," Moum said. "From a ship it's impossible to get the kinds of record lengths needed to resolve seasonal cycles, let alone processes with longer-term cycles like El Nino and La Nina. But for the first time in 2005, we were able to deploy instrumentation to measure mixing on a NOAA mooring and monitor the processes on a year-round basis."

The data showed mixing is responsible for sea surface cooling in the cold tongue and the scale of this mixing is affected by equatorial currents that flow from east to west at the surface, and from west to east in waters 330 feet beneath the surface and deeper.

"There is a hint - although it is too early to tell - that increased mixing may lead, or have a correlation to the development of La Nina," Moum said. "Conversely, less mixing may be associated with El Nino. But we only have a six-year record - we'll need 25 years or more to reach any conclusions on this question."