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RAPID/MOCHA/WBTS

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.

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RAPID/MOCHA/WBTS

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  1. 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 with thanks to: MollyBaringer, Adam Blaker,Harry Bryden, Julie Collins, Stuart Cunningham, AurélieDuchez, Eleanor Frajka-Williams, JoelHirschi, BillJohns, ChrisMeinen, Ben Moat, and the technicians and crew

  2. INTRODUCTION

  3. INTRODUCTION • Why 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

  4. INTRODUCTION • Why 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

  5. OUTLINE Measuring the AMOC and Heat Transport Interannual Variability Decadal Changes Multi-Decadal—The Future

  6. Measuring the AMOC and Heat Transport

  7. Boundary Currents and the mid-ocean Dynamic Height and Bottom Pressure Array Rayner, D., et al. (2011), Monitoring the Atlantic Meridional Overturning Circulation, Deep Sea Research II 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.

  8. The AMOC Streamfunction Transport per unit depth red dots x Internal Transport: McCarthy et al., 2014, Measuring the Atlantic Meridional Overturning Circulation at 26N, Prog. Oc. (accepted) The AMOC:

  9. The AMOC AMOC = 17.0±4.6 Sv

  10. HEAT TRANSPORT Net Heat Flux = 1.25 ± 0.36 PW (uncertainty 0.21 PW) • Overall MHT of 1.3 PW similar to hydrographic estimates • Seasonal variability is in the mid-ocean heat transport • 47% variance in Ekman Mid-Ocean heat transports now incorporate Argo to include the ‘eddy’ heat transport Johns, W. et al. (2011), Continuous, Array-based Estimates of Atlantic Heat Transport at 26.5°N, J. Clim., 24, pp. 2429–2449. updates in McCarthy et al. (2014), Measuring the Atlantic Meridional Overturning Circulation at 26N, Prog. Oc. (accepted)

  11. HEAT TRANSPORT • Heat transported north in GS is recirculated by mid-ocean and overturning circulation • 90% is in the overturning

  12. INTERANNUAL VARIBILITY

  13. Downturn in winter 2009/10 18 month weakening of AMOC Anomalously southward UMO: shift from overturning to gyre circulation *Seasonal cycle was removed, and data smoothed with 180-day filter McCarthy, G., et al. (2012), Observed Interannual Variability of the Atlantic Meridional Overturning Circulation at 26.5N, Geo. Res. Lett.

  14. Implications for Heat Content • 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 Cunninghamet al., (2013), AMOC slowdown cooled the subtropical ocean, GRL alsoBryden et al., 2014, Oc. Sci; Sonnewald et al., 2013, Oc. Sci.

  15. Double Dip: Winter 2010/11 Double Dip: Winter 2010/11 • The SST pattern in winter 2010 pushed the NAO into a negative state • Evidence that this second negative is predictable due to correct initialisation of Atlantic SST • 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 • Maidens et al. (2013) The Influence of Surface Forcings on Prediction of the North Atlantic Oscillation Regime of Winter 2010-11. Monthly Weather Review

  16. Decadal Changes

  17. 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

  18. Evidence of a decline Black: EN3, Red: Smith & Murphy • Density changes in the Labrador Sea support a declining AMOC • and indicate continuing decline Robson et al., 2014, Atlantic overturning in decline? Nature Geoscience

  19. 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

  20. Multi-Decadal—The Future

  21. 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

  22. 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

  23. 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 Northern sea level Sub-polar McCarthy et al., submitted, Sea level shows ocean control of decadal Atlantic climate variability Southern sea level Subtropical • Difference in sea level (south – north) is a measure of the circulation between the subtropical and subpolar gyres: in the Gulf Stream extension

  24. 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

  25. 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 link McCarthy et al., submitted, Sea level shows ocean control of decadal Atlantic climate variability

  26. CONCLUSIONS 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) Multi-Decadal: Will RAPID prove the link between the AMOC and the AMO?

  27. End

  28. 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

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