Large changes in Atlantic Ocean circulation that have amplified abrupt changes in climate in the past — and parallel trends being observed today — are the subjects of a free, public lecture Monday, May 24, just weeks after UW and NASA scientists reported in the journal Science that the North Atlantic circulation system weakened considerably during the decade of the 1990s.
William Curry, senior scientist with the Woods Hole Oceanographic Institution, in Massachusetts, speaks at 7 p.m., in Kane Hall 120 on Climate Variability, Ocean Circulation and Changes in the Hydrological Cycle. Sponsored by the UW’s Program on Climate Change, Curry will describe the geological record that provides many lines of evidence for changes in the geometry and strength of Atlantic Ocean circulation linked to rapid and abrupt climate transitions in the past.
In a recent Science online article, a team reported that satellite measurements of sea surface height showed there has been a slowdown in the counterclockwise circulation of surface water in the subpolar region of the North Atlantic during the past decade. Whether this slowdown is part of a natural cycle or the result of other factors related to global warming is not known.
What was seen in the 1990s are surprisingly strong trends, “exceptional,” according to Peter Rhines, UW professor of oceanography and co-author of the Science article.
Computer models have shown that the slowing and speeding up of the sub-polar gyre, that is waters that travel near Labrador below the Arctic Circle before circulating southward, can influence the entire ocean circulation system, he said.
“The subpolar zone of the Earth is a key site for studying climate. It’s like Grand Central Station there, as many of the major ocean water masses pass through from the Arctic and from warmer latitudes.”
The subpolar gyre can take 20 years to complete its route. Warm water runs northward through the Gulf Stream, past Ireland, before it turns westward near Iceland and the tip of Greenland. This current loses heat to the atmosphere as it moves north. Westerly winds pick up that lost heat, creating warmer, milder European winters.
After frigid Labrador Sea winters, the water in the current becomes so cold, salty and dense, that it plunges beneath the surface, and heads slowly southward back to the equator.
This cycle is sensitive to wintertime storms and to buoyant fresh water from glacial melting and precipitation, all of which are experiencing great change.
While previous studies have proposed that winds have influenced the sub-polar gyre’s currents, this study found that heat exchanges from the ocean to the atmosphere may play a bigger role in the weakening current. The researchers calculated that Labrador Sea water in the core of the gyre warmed half a degree during the 1990s. This warming reduces the contrast with water from warmer southern latitudes — water that normally would be 10 to 25 degrees warmer. That contrast is part of the driving force for ocean circulation.
“The uncertain connection of this decline in the ocean circulation with global warming may be frustrating to people who want simple answers, but the climate system is very complex: its modes of oscillation are a sort of argument between atmosphere, land and ocean,” Rhines said.
The UW’s Program on Climate Change aims to provide the breadth, depth and duration of research that can provide solid guidance in such far-reaching issues. Along with sponsoring a major annual lecture, the program (see http://depts.washington.edu/uwpcc/) coordinates research and undergraduate and graduate teaching among units concerned with climate change.
Satellite data was used for the work — NASA researcher Sirpa Hakkinen was a paper co-author — along with deep-ocean measurements using buoys, ships and autonomous robot Seagliders developed by researchers from the UW’s School of Oceanography and Applied Physics Laboratory.
Using Seagliders in the Labrador Sea is providing a whole new way of seeing what’s there, Rhines said.