Download Gerard McCarthy, Darren Rayner, Ivan Haigh, Joel Hirschi

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
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
INTRODUCTION
Why we study the
AMOC:
• Impact on climate
• Evidence of major
changes in the past
• Projections of decline
with climate change
Atlantic Meridional Overturning Circulation (AMOC)
alt. Thermohaline Circulation, Great Ocean Conveyor Belt
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.
INTRODUCTION
Why we study the
AMOC:
• Impact on climate
• Evidence of major
changes in the past
• Projections of decline
with climate change
Atlantic Meridional Overturning Circulation (AMOC)
alt. Thermohaline Circulation, Great Ocean Conveyor Belt
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.
OUTLINE
Measuring the AMOC
and Heat Transport
Interannual Variability
Decadal Changes
Multi-Decadal—The Future
Measuring the AMOC
and Heat Transport
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.
Rayner, D., et al. (2011),
Monitoring the Atlantic
Meridional Overturning
Circulation, Deep Sea Research II
The AMOC Streamfunction
Transport per unit depth
red dots
x
TALL (t, z) = TGS (t, z) +TEk (t, z) +TWBW (t, z) +TINT (t, z) +TAABW ( z) +Tcomp ( t, z)
Internal Transport:
The AMOC:
j MAX ( t ) =
ò
0
hj MAX
TALL ( z, t )dz
McCarthy et al., 2014,
Measuring the Atlantic
Meridional Overturning
Circulation at 26N, Prog.
Oc. (accepted)
The AMOC
AMOC = 17.0±4.6 Sv
HEAT TRANSPORT
Net Heat Flux = 1.25 ± 0.36 PW (uncertainty 0.21 PW)
Johns, W. et al. (2011), Continuous, Arraybased 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)
•
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
HEAT TRANSPORT
•
Heat transported
north in GS is
recirculated by
mid-ocean and
overturning
circulation
•
90% is in the
overturning
INTERANNUAL VARIBILITY
Downturn in winter 2009/10
*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.
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 oceanatmosphere heat
exchange
Cunningham et al., (2013), AMOC
slowdown cooled the subtropical
ocean, GRL
also Bryden et al., 2014, Oc. Sci;
Sonnewald et al., 2013, Oc. Sci.
Double Dip: Winter 2010/11
• The SST pattern in winter 2010 pushed
the NAO into a negative state
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
Maidens et al. (2013) The Influence of Surface Forcings on
Prediction of the North Atlantic Oscillation Regime of Winter
2010-11. Monthly Weather Review
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
Smeed et al. (2014) Observed
decline of the Atlantic
Meridional Overturning
Circulation, Ocean Science
• Even excluding the extreme of 2009, this is significant at 90% level
• Downturn is concentrated in UMO i.e. geostrophic gyre return
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
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 level
Sub-polar
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
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
McCarthy et al., submitted, Sea level
shows ocean control of decadal
Atlantic climate variability
• Extension back in time supports AMV link
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?
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