At the end of the last Ice Age --11.5 to 13 thousand years ago -- the north Atlantic deep water circulation system that drives the Gulf Stream may have shut down because of melting glaciers that added freshwater into the north Atlantic Ocean over several hundred years, confirm researchers funded by the National Science Foundation (NSF)'s paleoclimate program.
"For the first time, we have shown that realistic additions of glacial meltwater into the north Atlantic would have shut down north Atlantic deep water production over a period of a few hundred years, if the initial ocean circulation was somewhat weaker than that of today," said David Rind, lead author of the study and a senior climate researcher at the NASA Goddard Institute for Space Studies. The study appears in the current issue of Journal of Geophysical Research - Atmospheres.
While the study finds that freshwater input could slow the Gulf Stream, it would not stop it entirely. That's because the stream is partially pushed by winds. As a result, the model shows the reduced Gulf Stream would transport only about half as much heat northward, thereby cooling western Europe.
"This discovery illustrates the importance of a systems approach to research-studying the interactions among land, ocean, and atmospheric processes-to understand the complex behavior of Earth's climate," said James Yoder, director of NSF's division of ocean sciences. "But we still have much to learn before we can explain the rapid climate changes that have apparently occurred over the past few thousand years, and what those changes can tell us about what may happen in the future."
The computer model simulations of ocean and atmosphere processes used in this study imply that a similar phenomenon has the potential to occur in the future due to freshwater additions from increased rain and snow caused by global climate change.
When Rind and his colleagues entered into their model realistic estimates of freshwater from melting glaciers, they found the north Atlantic circulation stopped completely after some 300 years. When the model was adjusted to make the circulation weaker than it is today, cessation of the Gulf Stream took only 150-200 years, matching current estimates based on paleo-climate records.
When the Gulf Stream moves warm surface water from the equator north through the Atlantic, the water cools, gets saltier due to evaporation and becomes very dense. By the time it approaches the coast of Newfoundland, or further northeast in the Norwegian Sea, it becomes dense enough to sink. This process is called overturning. The dense water then slowly travels through the deep water southward into the Southern Hemisphere, with the return flow to the north occurring at the surface.
But when freshwater gets mixed with the salty water in the north Atlantic, it makes the water less dense and slows the overturning process and the ocean circulation.
Note: For related graphic, see http://www.nsf.gov/od/lpa/news/press/01/pr0192.htm