1. National Oceanography Centre
RAPID/MOCHA/WBTS
10 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 Centre UK
Thank you. It gives me great pleasure to kick off this conference with a review of the results from the 26ºN AMOC monitoring array. This moored array is a trans-Atlantic collaboration between NERC in the UK, which funds the RAPID program, NSF, which funds the MOCHA heat transport array and NOAA which maintains the Florida Current timeseries under the WBTS program. I'd like to thank my co-authors and collaborators on both sides of the Atlantic many of whose work I will present here today.
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

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
Rahmstorf, S. and A. Ganopolski, Long-term global warming scenarios computed with an efficient coupled climate model. Climatic Change, : p
from Rahmstorf, S. and A. Ganopolski, Long-term global warming scenarios computed with an efficient coupled climate model. Climatic Change, : p
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
Rahmstorf, S. and A. Ganopolski, Long-term global warming scenarios computed with an efficient coupled climate model. Climatic Change, : p
from Rahmstorf, S. and A. Ganopolski, Long-term global warming scenarios computed with an efficient coupled climate model. Climatic Change, : p
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
Previous estimates of OHT transport have been derived from hydrographic sections. At 26.5°N these have given estimates between 1.1 to 1.4 PW with an uncertainty of about 0.3PW.
Variability of ocean heat transport is not well known but obviously is an important factor in long-term climate variability. So one goal of the RAPID array is to provide continuous estimates of the meridional heat flux.

7. 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 : First results from the 26°N RAPID-MOC array, J. Phys. Oceanog., 38(3),
Principle of mid-ocean array is to calculate the profile of geostrophic velocity from boundary dynamic height moorings. WB : moorings WB2, WBH2, moorings on either side of the MAR to account for any deep flows on either side of the ridge; WB a series of moorings crawling up the slope. AABW. In addition to dynamic ht moorings we have an array of bottom pressure recorders measuring.
Describe plot: Shows the annual mean meridional component of velocity measured by current meters for the period March 2004 to May 2005.
Antilles current, part of the subtropical circulation which remains outside of the Carribean. It is a subsurface intensified current.
Below about 900m the flow is southward. This is the inshore edge of the DWBC. However flow is weak in the inshore region because a submarine ridge to the north deflects the DWBC offshore.
Rayner, D., et al. (2011), Monitoring the Atlantic Meridional Overturning Circulation, Deep Sea Research II

8. The AMOC Streamfunction
Transport per unit depth
red dots x
LHS: Equation ; transport-per-unit-depth for each of the components
RHS : The MOC is taken as the maximum of the overturning stream function. It is the maximum northward transport shallower than 1025m.
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
Seasonal variability
Heat transport
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
Seasonal variability
Heat transport

12. INTERANNUAL VARIBILITY
Event Winter 2009/10
Heat content, budget, North Atlantic tripole
Double dip, reemergence, seasonal predictability

13. Downturn in winter 2009/10 18 month weakening of AMOC
Anomalously southward UMO: shift from overturning to gyre circulation
A 5.5 Sv downturn persisted for 1.25 years
*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
Let’s add a map
Cunningham et al., (2013), AMOC slowdown cooled the subtropical ocean, GRL
also Bryden et al., 2014, Oc. Sci; Sonnewald et al., 2013, Oc. Sci.

15. Double Dip: Winter 2010/11 Double Dip: Winter 2010/11
One-year pattern correlation function for North Atlantic SSTAs (5-65N, 80-10W) from March March 2011 (green), March 1969-March 1970 (blue), March 1978-March 1979 (red), and a 50-year ( ) average (black). Light (Dark) shading denotes the range of correlation within one (two) standard devi- ation(s) of the long-term mean signal.
Update with the Maidens reference
Winter of 2011, economic impact on UK. In the runup to Christmas estimated cost of 1.2 bn a day, culminating in a 13 bn loss.
: The first of these 2 winters saw snow in late December, around the New Year, in Eastern Scotland and England. Eastern Yorkshire saw a massive 16 inches! Mid February saw more snow, this time more to the West, with England and Wales seeing the most. Mid March saw more in the Pennines, and a TV mast fell down saw snow for Northern England, North Wales, and Scotland in mid November. Mid December saw snow for the North again. Mid February, most parts, and early March, snow in Wales and England, with the Midlands getting 12 inches.
the temperature remained at a low level all the time. and no matter which weather station you watch - from paris to stockholm on to warsaw. persistent snow cover. April and May were cold. Average temperature in spring in Hanover just 6.7 ° c. The December 1969 was in contrast to this winter also been very cold.
: Mid January, 6 foot drifts! A week later, and 4 inches fell. Mid February saw 4 inches also. Late January, heavy snow in Scotland, drifting, 28 inches falling in parts! Mid February (see above) was very snowy in the North East, East and South West. February 11th had 1 ft in Durham and Edinburgh. Feb th South West England, blizzard with huge drifts, sounds like my cup of tea! : The last really severe, snowy winter, for now anyway, and one my parents go on about! Late December falls of 6-7n inches in Southern Scotland and the North East started it off. It was very cold in parts. Mid February saw drifts of 6-7 feet on the East coast of England. Mid March had severe blizzards and drifting, in North Eastern England drifts reached a staggering 15 feet! Very snowy.
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 Monthly Weather Review
Maidens et al. (2013) The Influence of Surface Forcings on Prediction of the North Atlantic Oscillation Regime of Winter 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. Multi-Decadal: Will RAPID prove the link between the AMOC and the AMO?
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 (FP ), under grant agreement n
NACLIM