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10 years of AMOC measurements from the RAPID program and a view to the future. RAPID/MOCHA/WBTS. Gerard McCarthy, Darren Rayner , Ivan Haigh , Joel Hirschi and David Smeed National Oceanography Centre UK. - PowerPoint PPT Presentation
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RAPID/MOCHA/WBTS10 years of AMOC measurements from the
RAPID program and a view to the future
Gerard McCarthy, Darren Rayner, Ivan Haigh, Joel Hirschi and David
Smeed
National Oceanography CentreUK
with thanks to: Molly Baringer, Adam Blaker, Harry Bryden, Julie Collins, Stuart Cunningham,
Aurélie Duchez, Eleanor Frajka-Williams, Joel Hirschi, Bill Johns, Chris Meinen, Ben Moat,
and the technicians and crew
INTRODUCTION
INTRODUCTIONWhy we study the AMOC:• Impact on climate• Evidence of major
changes in the past• Projections of decline
with climate change
from Rahmstorf, S. and A. Ganopolski, Long-term global warming scenarios computed with an efficient coupled climate model. Climatic Change, 1999. 43: p. 353-367.
Atlantic Meridional Overturning Circulation (AMOC)alt. Thermohaline Circulation, Great Ocean Conveyor Belt
INTRODUCTIONWhy we study the AMOC:• Impact on climate• Evidence of major
changes in the past• Projections of decline
with climate change
from Rahmstorf, S. and A. Ganopolski, Long-term global warming scenarios computed with an efficient coupled climate model. Climatic Change, 1999. 43: p. 353-367.
Atlantic Meridional Overturning Circulation (AMOC)alt. Thermohaline Circulation, Great Ocean Conveyor Belt
OUTLINE
Multi-Decadal—The Future
Measuring the AMOC and Heat Transport
Interannual Variability
Decadal Changes
Measuring the AMOC and Heat Transport
Rayner, D., et al. (2011), Monitoring the Atlantic Meridional Overturning Circulation, Deep Sea Research II
Boundary Currents and the mid-ocean Dynamic Height and Bottom Pressure Array
Johns, W. E., L. M. Beal, M. O. Baringer, J. Molina, D. Rayner, S. A. Cunningham, and T. O. Kanzow (2008), Variability of shallow and deep western boundary currents off the Bahamas during 2004-2005: First results from the 26°N RAPID-MOC array, J. Phys. Oceanog., 38(3), 605-623.
The AMOC Streamfunctionred dots
Internal Transport:
The AMOC:
Transport per unit depth
x
McCarthy et al., 2014, Measuring the Atlantic Meridional Overturning Circulation at 26N, Prog. Oc. (accepted)
The AMOC
AMOC = 17.0±4.6 Sv
HEAT TRANSPORTNet Heat Flux = 1.25 ± 0.36 PW (uncertainty 0.21 PW)
Johns, W. et al. (2011), Continuous, Array-based Estimates of Atlantic Heat Transport at 26.5°N, J. Clim., 24, pp. 2429–2449.
• Overall MHT of 1.3 PW similar to hydrographic estimates
• Seasonal variability is in the mid-ocean heat transport
• 47% variance in Ekman
updates in McCarthy et al. (2014), Measuring the Atlantic Meridional Overturning Circulation at 26N, Prog. Oc. (accepted)
Mid-Ocean heat transports now incorporate Argo to include the ‘eddy’ heat transport
HEAT TRANSPORT
• Heat transported north in GS is recirculated by mid-ocean and overturning circulation
• 90% is in the overturning
INTERANNUAL VARIBILITY
McCarthy, G., et al. (2012), Observed Interannual Variability of the Atlantic Meridional Overturning Circulation at 26.5N, Geo. Res. Lett.
*Seasonal cycle was removed, and data smoothed with 180-day filter
• 18 month weakening of AMOC
• Anomalously southward UMO: shift from overturning to gyre circulation
Downturn in winter 2009/10
Cunningham et al., (2013), AMOC slowdown cooled the subtropical ocean, GRL
Implications for Heat Content
also Bryden et al., 2014, Oc. Sci; Sonnewald et al., 2013, Oc. Sci.
• The downturn in the AMOC substantially cooled the subtropical Atlantic
• The divergence in ocean heat transport played a much larger role than ocean-atmosphere heat exchange
Double Dip: Winter 2010/11
• The SST pattern in winter 2010 pushed the NAO into a negative state
Double Dip: Winter 2010/11
Maidens et al. (2013) The Influence of Surface Forcings on Prediction of the North Atlantic Oscillation Regime of Winter 2010-11. Monthly Weather Review
Buchan et al. (2013), North Atlantic SST anomalies and the cold north European weather events of winter 2009/10 and December 2010. Monthly Weather Review
• Evidence that this second negative is predictable due to correct initialisation of Atlantic SST
Decadal Changes
Evidence of a decline
• IPCC predicts an AMOC downturn of 0.5 Sv per decade
• We see a decline of 0.6 Sv per year
• Even excluding the extreme of 2009, this is significant at 90% level
• Downturn is concentrated in UMO i.e. geostrophic gyre return
Smeed et al. (2014) Observed decline of the Atlantic Meridional Overturning Circulation, Ocean Science
Evidence of a decline
Black: EN3, Red: Smith & Murphy
Robson et al., 2014, Atlantic overturning in decline? Nature Geoscience
• Density changes in the Labrador Sea support a declining AMOC
• and indicate continuing decline
Trend or Oscillation?Smeed et al. (2014) Ocean Science
• The Atlantic is a region of large multi-decadal variability e.g. sea-surface temperatures
• The rapid decline we observe is larger than the long slow decline predicted by the IPCC
Multi-Decadal—The Future
AMV and Ocean Circulation• The Atlantic is a place of large multi-decadal
variability esp. the Atlantic Multi-decadal Variability of SSTs (AMV)
• The AMO has a range of important climate impacts (left: from Zhang and Delworth, 2007, GRL)
• It is widely hypothesised that the AMOC controls the phases of the AMV through control of ocean heat content e.g. Delworth and Mann, 2000, Clim. Dyn.
• … but there are no direct observational records of sufficient length to prove this
AMV and Ocean Circulation• RAPID will eventually provide a timeseries of overturning circulation to prove an
AMOC-AMV link
• For now, we need proxies. Here we use sea-level along the US east coast
McCarthy et al., submitted, Sea level shows ocean control of decadal Atlantic climate variability
AMV and Ocean Circulation• RAPID will eventually provide a timeseries of overturning circulation to prove an
AMOC-AMV link
• For now, we need proxies. Here we use sea-level along the US east coast
McCarthy et al., submitted, Sea level shows ocean control of decadal Atlantic climate variability
Northern sea levelSub-polar
Southern sea levelSubtropical
• Difference in sea level (south – north) is a measure of the circulation between the subtropical and subpolar gyres: in the Gulf Stream extension
AMV and Ocean Circulation• RAPID will eventually provide a timeseries of
overturning circulation to prove any AMOC-AMV link
• For now, we need proxies. Here we use sea-level along the US east coast
• The accumulation of the circulation proxy leads the changes in heat content
McCarthy et al., submitted, Sea level shows ocean control of decadal Atlantic climate variability
AMV and Ocean Circulation• RAPID will eventually provide a timeseries of
overturning circulation to prove any AMOC-AMV link
• For now, we need proxies. Here we use sea-level along the US east coast
• The accumulation of the circulation proxy leads the changes in heat content
• Extension back in time supports AMV linkMcCarthy et al., submitted, Sea level shows ocean control of decadal Atlantic climate variability
CONCLUSIONS
Multi-Decadal: Will RAPID prove the link between the AMOC and the AMO?
Interannual Variability: Unexpected (larger than seen in climate models) interannual drops in AMOC. Linked with North Atlantic cooling and NAO variability
Decadal Changes: Rapid decline in strength of circulation over the 10 years of observations (0.5 Sv per year)
End
•The research leading to these results has received funding from the European Union 7th Framework Programme (FP7 2007-2013), under grant agreement n.308299•NACLIM www.naclim.eu