Cold Shoulder to Ice Age Scenario
Friday, September 2, 2011 3:27:29 PM
For years it was thought that the primary source of the Denmark Strait Overflow was a current adjacent to Greenland known as the East Greenland Current (EGC). (The Denmark Strait is the strait between Greenland and Iceland). This view was recently called into question by two oceanographers from Iceland who discovered a deep current flowing southward along the continental slope of Iceland. They named the current the North Icelandic Jet (NIJ) and hypothesized that it formed a significant part of the overflow water.
The existence of the North Atlantic jet has now been confirmed. It is narrow, often only about 20 km, and was found at a depth of 650 m. The North Atlantic Jet contributes to a key component of the Atlantic Meridional Overturning Circulation (AMOC), critically important for regulating Earth's climate. As part of the planet's reciprocal relationship between ocean circulation and climate, the AMOC transports warm surface water to high latitudes where the water warms the air, then cools, sinks and returns toward the equator as a deep flow. The NIJ apparently takes care of half of the Gulf Stream’s return stream through the Denmark Strait.
In a new study published online on 21 August 2011 in the Nature Geoscience it is in fact estimated “that the jet supplies about half of the total overflow transport, and [the authors] infer that it is the primary source of the densest overflow water. Simulations with an ocean general circulation model suggest that the import of warm, salty water from the North Icelandic Irminger Current and water-mass transformation in the interior Iceland Sea are critical to the formation of the jet. [The authors] surmise that the timescale for the renewal of the deepest water in the meridional overturning cell, and its sensitivity to changes in climate, could be different than presently envisaged.”
The East Greenland current is vulnerable for the (present) melting of Greenlandic ice, and this has given rise to fears of “new ice age” scenarios. As the newly discovered North Atlantic jet apparently contributes far more to the deeper parts of the return stream than does the East Greenland current, the Gulf Stream system may be far less sensitive to climate change than previously thought.
In other words: If a large fraction of the Denmark Strait overflow water comes from the NIJ, it is necessary to re-think how quickly the warm-to-cold conversion of the AMOC occurs, as well as how this process might be altered under a warming climate.
See also my post on North Atlantic Circulation