Eastern Boundary Considerations
Gerard McCarthy, Eleanor FrajkaWilliams, Aurélie Duchez and
David Smeed
National Oceanography Centre
The AMOC at 26ºN
Gulf Stream + Ekman + Upper Mid Ocean = AMOC
Rayner, D., et al. (2011),
Monitoring the Atlantic
Meridional Overturning
Circulation, Deep Sea Research II,
58, 1744–1753.
Gulf Stream, MOC, Ekman & Upper Mid-Ocean Transports
Interannual Variability in 09/10 shifted circulation from overturning to gyre (McCarthy et al., 2012, GRL)
Double dips in winters 09/10 and 10/11 described in Blaker et al., 2013, submitted to Climate Dynamics
AMOC timeseries and related data products are available from
Data from individual instruments are available from
Seasonal Cycle
Seas. Amp.
3 Sv
6 Sv
3 Sv
6.5 Sv
Large seasonal cycle in AMOC driven by upper Mid-Ocean transport seasonality
AMOC timeseries and related data products are available from
Basin-scale Seasonality
The Seasonal Cycle due to the Eastern Boundary
±2.6 Sv, SE=0.5 Sv
Chidichimo, M. P., T. Kanzow, S. A.
Cunningham, W. E. Johns, and J. Marotzke
(2010), The contribution of easternboundary density variations to the Atlantic
meridional overturning circulation at 26.5
N, Ocean Science, 6,
±1.8 Sv,SE=1.0 Sv
SD 3.1 Sv, Range 6.2 Sv
Kanzow, T., et al. (2010), Seasonal
variability of the Atlantic meridional
overturning circulation at 26.5°N, J. Clim.,
23(21), doi: 10.1175/2010JCLI3389.1171.
SD 3.5 Sv,
Range 7.0 Sv
• Largest seasonal influence
in mid-ocean transports
due to the east
• Recent variability hasn’t
changed this
Basinwide response to wind forcing
• Seasonal cycle coherent across the
basin in OFES and simple two layer
Seasonal cycle (Sv) in (top) OFES and
(bottom) two layer model
• Not solely an eastern boundary
phenomenom and not dependent
on location of RAPID moorings
Duchez et al. (2013) Density variations around the
Canary Islands and their influence in the AMOC at
26ºN, in prep
Zhao, J. and Johns, W. E. (2013) Wind driven
seasonal cycle of the Atlantic Meridional Overturning
Circulation, submitted
70ºW 60ºW
50ºW 40ºW 30ºW 20ºW
Forced Rossby wave response to wind stress curl
• Ocean response to wind stress curl forced Rossby wave model explains
the first baroclinic mode structure seen in observations
Modelled Mid-ocean transport anomaly
Observed Mid-ocean transport anomaly
Local Seasonality
Local Seasonality
• Not all of seasonal structure explained by first baroclinic mode structure
Observed Mid-ocean transport anomaly
Canarian Deep Poleward Undercurrent
Vélez-Belchi, P., et al. (2013), The
Canary deep poleward undercurrent. In
Machín, F., et al. (2009), Northward
Penetration of Antarctic Intermediate
Water off Northwest Africa, JPO, 39,
AAIW (MOW) flows
northward (southward)
in the Canarian deep
poleward undercurrent
(CDPU)during the
Autumn (Spring) forced
by seasonal layer
stretching (compression)
This deep poleward
undercurrent is unusual
and infrequently
Seasonal Reversals of Intermediate Water
Distinct seasonal
reversals of flow in the
Lanzarote Channel
Intermediate (1000 m)
correlate salinity
changes with current
High salinity Med.
Outflow water (MOW):
Southward flow in
Low salinity Antarctic
Int. Water (AAIW):
Northward flow in
Machín, F., et al. (2010), Seasonal Flow
Reversals of Intermediate Waters in the
Canary Current System East of the
Canary Islands, JPO, 40, 1902–1909
Evidence of CDPU in RAPID mooring data
Potential Temperature
Monthly Sal. Anom on 8 C
500 dbar
1000 dbar
2000 dbar
• Rapid data at Eastern Boundary shows clear salinity signal of seasonal oscillations
around 1000 dbar
• These are consistent with seasonal reversals of AAIW and MOW in the CDPU
Evidence of layer stretching
Large Scale Consequences
• Understanding the role of the Poleward Undercurrent is important to understand the
part of the seasonal cycle that is not explained by the Rossby Wave model
• It has important consequences for freshwater transport at 26 N as transport
fluctuations are associated with salinity changes
• Changes in freshwater flux near the eastern boundary of 0.1 Sv are not picked up by
Observed Mid-ocean transport anomaly
Integrated Freshwater flux at 26ºN
Courtesy of Elaine McDonagh
• MOC seasonal cycle is 6.7 Sv peak-to-peak
• UMO contributes the most pronounced seasonal cycle of 5.9 Sv
• Seasonal cycle in UMO is caused by heaving of the thermocline
forced by seasonal anomalies in the wind stress curl.
• Canarian deep poleward undercurrent plays a secondary role driven
by seasonal stretching of the intermediate layer
Contribution of the upper mid-ocean western and
eastern boundaries to the UMO seasonal cycle
Expectation that upwelling drives
stronger (weaker) thermocline
flow in the autumn (spring)
Reality is that seasonally
reversing intermediate flows
driven by water column
stretching (squeezing) are the
major factor
Chidichimo, M. P. et al.,(2010),
The contribution of easternboundary density variations to
the Atlantic meridional
overturning circulation at 26.5
N, Ocean Science, 6
The research leading to these results has
received funding from the European Union
7th Framework Programme (FP7 2007-2013),
under grant agreement n.308299

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