Update May 28, 2015, with additional detail from Dr. McCarthy
Update May 29, 2015, with additional context from Bob Tisdale
The RAPID moorings being deployed. Credit: National Oceanography Centre
A new study, by scientists from the University of Southampton and National Oceanography Centre (NOC), implies that the global climate is on the verge of broad-scale change that could last for a number of decades. This new climatic phase could be half a degree cooler.
The change to the new set of climatic conditions is associated with a cooling of the Atlantic, and is likely to bring drier summers in Britain and Ireland, accelerated sea-level rise along the northeast coast of the United States, and drought in the developing countries of the Sahel region. Since this new climatic phase could be half a degree cooler, it may well offer a brief reprise from the rise of global temperatures, as well as resulting in fewer hurricanes hitting the United States.
The study, published in Nature, proves that ocean circulation is the link between weather and decadal scale climatic change. It is based on observational evidence of the link between ocean circulation and the decadal variability of sea surface temperatures in the Atlantic Ocean.
Lead author Dr Gerard McCarthy, from the NOC, said: “Sea-surface temperatures in the Atlantic vary between warm and cold over time-scales of many decades. These variations have been shown to influence temperature, rainfall, drought and even the frequency of hurricanes in many regions of the world. This decadal variability, called the Atlantic Multi-decadal Oscillation (AMO), is a notable feature of the Atlantic Ocean and the climate of the regions it influences.”
The strength of ocean currents has been measured by a network of sensors, called the RAPID array, which have been collecting data on the flow rate of the Atlantic meridonal overturning circulation (AMOC) for a decade.
Dr David Smeed, from the NOC and lead scientist of the RAPID project, adds: “The observations of AMOC from the RAPID array, over the past ten years, show that it is declining. As a result, we expect the AMO is moving to a negative phase, which will result in cooler surface waters. This is consistent with observations of temperature in the North Atlantic.”
Some additional detail from Dr. McCarthy:
Results from the RAPID array
Gerard McCarthy, David Smeed, Darren Rayner, Eleanor Frajka-Williams, Aurélie Duchez, Bill Johns, Molly Baringer, Chris Meinen, Adam Blaker, Stuart Cunningham and Harry Bryden
“The RAPID/MOCHA/WBTS mooring array at 26ºN in the Atlantic has been delivering twice daily estimates of the strength of the AMOC since 2004. A unique array, the observations have revolutionised our understanding of the variability of the AMOC on sub-annual, seasonal and, most recently, interannual timescales. An update to the AMOC timeseries has recently been produced. As well as extending the data, the timeseries to October 2012 contains several improvements to the calculation.
A dramatic low in the AMOC was observed in winter 2009/10, where the AMOC declined by 30%. This has been shown to have resulted in a sustained reduction in heat content of the North Atlantic. The 2009/10 dip in AMOC strength was followed by a second dramatic low in 2010/11. Historical analogues of double minima in successive winters have been identified in NEMO runs where they are associated with extreme negative values of the Arctic oscillation and have been linked with ocean re-emergence. Interestingly, there is also a link with surface air temperatures and, consequently, European wintertime conditions.
The latest update of the AMOC time series to October 2012 shows a continuing trend in the circulation at 26ºN switching from an overturning to a gyre circulation. This leads to weakened southward transport of lower North Atlantic Deep Water, the strength of which from 2004-2012 is weaker than in historical measurements. The IPCC report in 2007 reported that the AMOC was ‘very likely’ to weaken in the 21st century. Maintaining the sustained observations of the RAPID array is key to observing this climate metric.”
Rapid Project Webpage is here: http://www.rapid.ac.uk/rapidmoc/
Figure 1:Ten-day (colours) and three month low-pass (black) timeseries of Florida Straits transport (blue), Ekman transport (black), upper mid-ocean transport (magenta), and overturning transport (red) for the period 2nd April 2004 to mid- March 2014. Florida Straits transport is based on electromagnetic cable measurements; Ekman transport is based on ERA winds. The upper mid-ocean transport, based on the RAPID time series, is the vertical integral of the transport per unit depth down to the deepest northward velocity (~1100 m) on each day. Overturning transport is then the sum of the Florida Straits, Ekman, and upper mid-ocean transports and represents the maximum northward transport of upper-layer waters on each day. Positive transports correspond to northward flow.
Additional info here: http://www.livescience.com/50998-jet-stream-controls-atlantic-climate-cycles.html
Getting a reprieve from the dangers of global warming would be good news, but these facts were not well received by everyone last month at a conference in Vienna, as tweeted by Dr. McCarthy:
Bob Tisdale provides additional context on the AMO and on this paper, as well as critiques of some other papers here: https://bobtisdale.wordpress.com/2015/05/29/new-paper-confirms-the-drivers-of-and-processes-behind-the-atlantic-multidecadal-oscillation/
For more on this topic see: