Assuming 16 cm standard deviation

The final result – 5 of these records were noisy Halifax Grand Banks Line W 4100 m 2700 m 3250 m 2250 m 1800 m 3650 m

OCCAM: First EOF of bottom pressure over this region

OCCAM: correlation of bottom pressure with subpolar overturning

MICOM: correlation of bottom pressure with subpolar overturning

A good rule of thumb: Assume a layer thickness of 1000 m, and a midlatitude f = Then 1 cm of sea level or 1 mbar of pressure represents 1 Sv of transport

Dynamics: The geostrophic calculation at 42N Conclusion: Knowledge of western boundary pressure variations are sufficient to monitor to interannual variability of the MT at 42N Upper layer transport RMS error: 0.28Sv 93% of variance captured Lower layer transport RMS error: 0.31Sv Actual Inferred from western boundary pressure Bingham and Hughes, JGR 2008

OCCAM MOC and Sea Level In OCCAM, interannual sea level and MOC covary, as expected 2 cm/Sv

GECCO MOC and Sea Level The same in GECCO 2 cm/Sv

sverdrups Standard deviation 1.25 Sv This is the overturning variation which would be implied by the tide gauge data

The message: Pressure differences give integrals of transport Bottom pressure variability is much smaller than mid-ocean pressure variability Only integrals all the way across the basin are meaningful – other integrals are dominated by eddies/meanders/Rossby waves These pressure signals are spatially coherent, so they relate to something meaningful for the large-scale ocean circulation, but this is an Eulerian measure of the MOC.

Meridional transport anomaly between 100m and 1000m depth OCCAM HadCM3 Bingham et al, GRL 2007

MICOM simulations Fine resolution Coarse resolution Annually-repeating forcing

Origin of meridional differences: Key latitudes

1000 years of HadCM3 overturning circulation

Origin of meridional differences: Evolution of boundary density P1 P2 P3 P1 P2 P3 Anomalous density along the 1000m isobath Advection Convection + advection + waves 50N 42N Advection + waves advection 0.9cms -1 wave: 1.8ms -1 Seasonal cooling events associated with NAO are integrated to give low frequency mode clear at 50N 50N signal advected to lower latitudes, and degraded along the way

ρfw z = ρu.  (f + ζ) +  × τ z and the bottom boundary condition: ρfw b = - J(p b,H) = - ρfu b.  H Why should bottom pressure not be dominated by eddies too? It comes down to the (steady) vorticity balance:

ρfw z = ρu.  (f + ζ) +  × τ z ρfw = ρh(βv + u 2 /L 2 ) +  × τ = 2× ρfw b = - ρfu b.  H = - J(p b,H) =0.01  H Scalings in SI, with u=10 cm/s, h=1000m, L=100 km, β=2× m -1 s -1 So would need  H = or 10m/100km But actual continental slope  H is between &

Standard deviation of Sea level (18 years of 5-day means) Bottom pressure in OCCAM Bingham and Hughes, GRL 2008

Admittance (BP/SL) Shallow Deep Deep (spatial smoothing) Mid-latitudes High latitudes Eddying regions Quiet regions

Time series of bottom pressure from 3 instruments, 300km apart, in a triangle around Tristan da Cunha island (S Atlantic) Hughes and Smithson, GRL mbar RMS

Altimetry: sea level signals 5 degrees east of continental slope

Altimetry: sea level signals on continental slope

5 deg east of continental slope Continental slope

RAPID WAVE array 26N array, with thanks to Torsten Kanzow et al

Bottom pressure (mbar) at three of the WAVE array positions and at the Western and Eastern end at 26.5ºN

Standard deviation of bottom pressure records 3 to 100 day periods, mbar

Correlations between BPRs: 3 to 100 day periods

Near-five-day bottom pressure (mbar) at three of the WAVE array positions and at the Western and Eastern end at 26º N

5-day waves Arctic Southern Ocean Atlantic Pacific Indian Spectra of basin-averaged sea level and bottom pressure 5 days

8-100 day bottom pressure (mbar) at three of the WAVE array positions and at the Western and Eastern end at 26º N

EOFs of BPR data

Start with 2 mbar standard deviation Subtract common signal, explaining (at least) 60% of variance Leaves 2 x sqrt(0.4) = 1.26 mbar A factor of 13 smaller than Wunsch’s assumed 16 mbar

Continental slope 5 deg east of continental slope Continental slope

5 deg east of continental slope Continental slope

geostrophy hydrostatic balance

MOC measurement Bottom geostrophic current Bottom density 1 Sv over 1 km depth range requires accuracy of about 1 mbar Accuracy needed for current depends on how steep the slope is: more gentle slopes need greater accuracy. The steeper part of Section B requires about 4cm/s accuracy for averaged bottom current, to give 1 Sv for MOC. Gentler slopes require about 1 cm/s

BPR measurements still needed initially to test integrity of the system. Longer term, only current and density needed for monitoring. With much thanks to Bedford Institute of Oceanography

Summary MOC changes have both advective and wave-like causes. Advection is slow, highly eddy dependent, difficult to monitor. Bottom pressure ‘filters out’ eddy effects (in most places). The wave propagation speed is much faster than advection, resulting in much more spatial coherence than eddies would suggest, although subtropical and subpolar regions remain independent to decadal periods or longer. Despite the importance of eddies, Eulerian measures of the MOC (integrated at constant level) are possible with accuracy of better than 1 Sv. This can be done, for interannual variations, with only western boundary bottom pressure.

Leading EOFs of interannual sea-surface height and bottom pressure SSHBP SSH BP

Leading EOFs of interannual sea-surface height AltimetryOCCAM Altimetry OCCAM

Bottom Pressure