Ocean currents are a vital component of the global climate system because they move huge volumes of warm water towards the poles and cold water towards the equator. In the Atlantic basin, for example, warm surface water flows via the Gulf Stream north from the Caribbean diagonally across the ocean to Europe and then further north into the Arctic. This relatively salty water is then dramatically cooled. As part of a process known as thermohaline circulation, it sinks towards the bottom and starts back towards the south. At this point the water is referred to as North Atlantic Deep Water (NADW).
The
rate of formation of NADW, and hence of large
scale circulation in the Atlantic, is partially dependant on the salinity of
that cold polar water, because cold fresh water is not sufficiently dense to
sink towards the ocean bottom at the same rate as cold salty water.
The salinity of the north Atlantic water is the connection with
freshwater runoff from rivers.
Geologists from the US and Canada have established patterns of runoff from North America over the period from 20,000 to 8,000 years ago – a period which spans the last major deglaciation (Clark et al., 2001). As shown on the diagrams to the left, the significant factor in question is the extent to which glacial melt water flowed south into the Caribbean via the Mississippi channel, or east into the north Atlantic via the Hudson and St. Lawrence channels.
Clark et al. suggest that the extent of the Laurentide Ice Sheet (the major continental ice sheet of North America) was critical in controlling the flow of water. When the ice margin was situated south of the current Great Lakes (as shown on the left) the channels of the St. Lawrence and Hudson Rivers were cut off and most of the water flowed south into the Mississippi channel. This restricted the input of fresh water into the north Atlantic, which promoted formation of NADW, enhanced the thermohaline circulation and led to moderating temperatures. Over a period of approximately 1000 years the ensuing warming caused the ice sheet to recede (as shown on the right), restoring drainage through the Hudson and St. Lawrence channels into the north Atlantic, limiting the formation of NADW and triggering a cooling trend.
The authors’ point is that the cyclic nature of the last deglaciation can be attributed to this process. The ice started to recede around 15,000 years ago, there was a minor re-advance around 13,000 years ago, and another re-advance around 9,000 years ago. They suggest that a primary control over these recessions and re-advances was the direction of freshwater runoff from eastern North America, and that the drainage patterns were in turn controlled by the extent of the ice sheet.
Reference
Clark, P., Marshall, S, Clarke, G., Hostetler, S., Licciardi, J. and Teller, J., Freshwater forcing of abrupt climate change during the last glaciation, Science, V. 293, p. 283-286, July 2001
Steven Earle, 2001. Return to Earth Science News