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Gulf Stream and Atlantic Heat Conveyor

NOCS

WHAT IS ALREADY HAPPENINGWHAT COULD HAPPEN
  • The Atlantic Heat Conveyor (within which the Gulf Stream plays a role) helps to maintain relatively mild temperatures in north-west Europe. Some observations suggest that the Atlantic Heat Conveyor has reduced in strength by up to 30% since the early 1990s. More data are needed to distinguish this trend from natural variability, which has recently been shown to be large on a day-to-day basis.
  • It is very likely that the Atlantic Heat Conveyor will slow during this century, but not sufficiently to completely offset warming across the UK.
  • There is considered to be less than a 10% chance of a collapse of the Atlantic Heat Conveyor this century.

The meridional overturning circulation (MOC)The present oceanic circulation system within the Atlantic Ocean, of which the Gulf Stream is a major part, characterized by the net northwards transport of heat and water at the surface, and a cooler return southwards flow at depth. is part of a global ocean circulation that redistributes heat from Equatorial to Polar Regions. In the Atlantic the MOC carries heat northward which is released to the atmosphere and maintains UK temperatures between 3-5°C higher than elsewhere at similar latitudes. However, the present strength and structure of the MOC may not continue: climate models suggest that increasing atmospheric greenhouse gas could lead to an abrupt rearrangement of the MOC and climate models and paleoclimateReferring to the historical climate prior to the commencement of direct observations of the atmosphere and oceans using instruments (the so-called 'instrumental period'). records indicate that the MOC has undergone large and rapid changes in the past 20,000 years. The Fourth assessment report of the Intergovernmental Panel on Climate Change suggests that there is less than 10% chance of abrupt changes during the 21st Century, but that there is greater than 90% chance that the MOC will slow by 25%. Recent observational evidence suggests that since the early 1990s the MOC has slowed by up to 30%, with a significant adjustment of ocean water masses. The interpretation of a 30% slowing is controversial because of our lack of understanding of the natural variability of the MOC, and possibly conflicting evidence of warmer North Atlantic sea surface temperatures. There is a broad scientific consensus that continuous observations of the strength and structure of the MOC are required. A UK/US consortium installed a continuous monitoring system in 2004. First results show that the array provides continuous observations of MOC strength and that the error of the annual mean strength is 8%. If the circulation passes through a bifurcation or if the overturning reduces by 25% as coupled climate models suggest it might under increasing CO2 concentrations, we should be able to identify the change relative to the 2004-05 average.

What is already happening - Low Large amount of disagreement between studies and there are limitations to models and observations. There is a high level of confidence that abrupt changes in the annual mean MOCThe Atlantic Heat Conveyor or Atlantic Meridional Overturning Circulation (MOC). The present oceanic circulation system within the Atlantic Ocean, of which the Gulf Stream is a major part, characterized by the net northwards transport of heat and water at the surface, and a cooler return southwards flow at depth. larger than 8% will be observed by the Rapid monitoring system.

What could happen in the future - Low

See review.

Not stated.

Stuart Cunningham
National Oceanography Centre, Southampton, European Way, Southampton SO14 3ZH