Measuring the South Atlantic MOC – in the OCCAM ocean model

Povl Abrahamsen Joel Hirschi

Emily Shuckburgh Elaine McDonagh

Mike Meredith Bob MarshBritish Antarctic Survey National Oceanography Centre, Southampton

Outline

• Motivation

• Model introduction

• Characteristics of the modelled circulation

• Measuring the MOC from thermal wind – preliminary results

• Ongoing measurements in the Southern Ocean – measuring the lower limb of

the overturning circulation

Motivation

• To investigate the feasibility of monitoring the MOC in the South Atlantic in an efficient and cost-effective way, using output from eddy-permitting and eddy-resolving ocean models.

OCCAM model(Coward and de Cuevas 2005)

• Z-level coordinates; based on Bryan-Semtner-Cox, with free surface• Global, with rotated grid in North Atlantic and Arctic• 66 depth levels• Run at 1/4° and 1/12° resolution• Forced with NCEP reanalysis data, from 1985 to end of 2004/6• Surface restoration of salinity to Levitus climatology (time scale of

30 days) through added freshwater flux; should also conserve volume

• 5-day output used for calculations shown here• More detailed information – and output fields – are available from

http://www.noc.soton.ac.uk/JRD/OCCAM/

15° S25° S35° S

Model sectionsaverage meridional speed (cm/s)

1/4-degree 1/12-degree

Can we just measure the currents on the western boundary?

• Plots show correlation between currents integrated from western boundary to indicated longitude, from the surface to depth of maximum overturning (in this case, around 1300 m) – with the current integrated across the entire domain – in red.

• Green lines are from eastern boundary• Black line is correlation between eastern and western currents.

• Appears to work well at 15°S• Can get most of the variability without

extending far from the shelf break – in the west.

• Can’t get much at all on the eastern boundary

Can we just measure the currents on the western boundary?

• More problematic at 25°S• Lower correlations – and drops off after a

certain point.• Strong anticorrelation between currents in

east and west – indicating gyre.

• Plots show correlation between currents integrated from western boundary to indicated longitude, from the surface to depth of maximum overturning (in this case, around 1300 m) – with the current integrated across the entire domain – in red.

• Green lines are from eastern boundary• Black line is correlation between eastern and western currents.

• Plots show correlation between currents integrated from western boundary to indicated longitude, from the surface to depth of maximum overturning (in this case, around 1300 m) – with the current integrated across the entire domain – in red.

• Green lines are from eastern boundary• Black line is correlation between eastern and western currents.

Can we just measure the currents on the western boundary?

• Not too bad at 35°S – until we get to the Mid-Atlantic Ridge.

• But if we try to measure the eastern and western parts separately, and they are strongly anti-correlated, with uncorrelated errors, the errors could end up drowning out the actual signal we are after!

Calculating the MOC from thermal wind• Decompose meridional currents into Ekman and geostrophic

components:

(between two density profiles spaced x apart)

• Integrated to provide transport – and mean subtracted uniformly across section to give zero net flow through section.

• Barotropic flow is not included; in an ideal square basin this would not matter, but with sloping bathymetry, there can be a baroclinic residual (external mode)

∫∗−=L

xek dxfA

v0

1τ

ρ ∫−

∗ −

−=z

H

EWg dzfx

gzv )()( ρρ

ρ

Differences in bathymetry

• Test

In an ideal world…

Runs using all density points• All grid cells are used to calculate maximum overturning.• Boundary points are also used (ignoring internal bathymetry)

Runs using subsets of densities• Success doesn’t only depend on number of moorings

– but also on placing moorings correctly!

How can we improve on these results?

• Calculate the external mode/baroclinic-barotropic compensation from bottom pressure (see Bryden et al. 2009)

• Include bottom currents to add a reference to calculated geostrophic currents.

• Overall, results seem promising – density moorings can be used as long as the boundary currents are resolved.

How are we currently measuring the lower limb of the MOC?A23 section:•Measured in 1995, 1999, and 2005

(Meredith et al. 2008)•Repeated in 2010 – with chemistry•Repeats planned annually from 2012•CLIVAR repeat hydro/CO2 program

Orkney Passage:•Moorings deployed in collaboration with LDEO since 2005•Swath survey in 2010•More moorings going out in 2011

Filchner Sill:•Mooring deployed in collaboration with Bjerknes Center, UoB, since 2003•Measures northward flow of ISW off the continental shelf

The end

Runs using all density points• All grid cells are used to calculate maximum overturning.• Boundary points are also used (ignoring internal bathymetry)

BAS in brief• BAS is a research institute, under the Natural Environment

Research Council, UK, based in Cambridge• Our main area of focus is Antarctica and the Arctic• The Polar Oceans program is currently expanding, and also looking

further afield – including the and impacts of Antarctica on the World Ocean

• Operates two ships (RRS James Clark Ross and RRS Ernest Shackleton), five airplanes, and five bases (four during winter)

• Long-term monitoring plans include monitoring the outflow of Antarctic Bottom Water through Orkney Passage, in collaboration with Lamont-Doherty Earth Observatory, Columbia University