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