Monitoring the MOC at 26.5°N
There is a northward transport of heat throughout the Atlantic, reaching a maximum of 1.3PW (25% of the global heat flux)
around 24.5°N. The heat transport is a balance of the northward flux of a warm Gulf Stream, and a southward flux of cooler
thermocline and cold North Atlantic Deep Water that is known as the meridional overturning circulation (MOC). The heat transported by the MOC is given off
to the atmosphere and much of it is carried eastward by westerly winds. This
is an important contribution to northwestern Europe's mild climate.
Numerical models suggest that the MOC is likely to weaken by about
30% in the coming century as a consequence of greenhouse gas emissions.
Furthermore, paleoclimate records suggest that during the last Ice Age the MOC
has undergone abrupt rearrangements that were responsible for a cooling of
European climate of between 5-10°C.
A principal objective of the RAPID programme is the development and
maintenance of a pre-operational prototype system that will
continuously observe the strength and structure of the MOC. An
initiative has been formed to fulfill this objective and consists of
three interlinked projects:
A more detailed
description of the initiative is found in the initial project proposal (Marotzke et al., 2002).
- Nineteen moorings were deployed in March 2004 across the Atlantic at 26.5°N to measure the southward branch of the MOC (Hirschi et al., 2003, Baehr et al., 2004,
Cunningham et al.,
2007, Kanzow et al., 2007) .
- Additional moorings were deployed on the western boundary along 26.5°N in the framework of the U.S. Meridional Overturning Circulation and Heatflux Array (MOCHA) project (Prof. Bill Johns, University of Miami)
to resolve transport in the Deep Western Boundary Current (Johns et al., 2005, 2008, Bryden et al., 2005). These moorings allow surface-to-bottom density
profiles along the western boundary, Mid-Atlantic Ridge, and eastern boundary to be observed. As a result, the transatlantic
pressure gradient can be continuously measured.
- Dr Molly Baringer (NOAA/AOML) leads the monitoring of the northward branch of the MOC using submarine telephone cables in the
Florida Straits (Baringer et al., 2001).
Since 2004 the MOC monitoring array has been recovered and redeployed
annually. The observations from the first year have shown that even
on subannual timescales a large variability is found for the MOC (Cunningham et al.,
2007, Kanzow et al., 2007). The transport timeseries have now been
extended and cover the period from April 2004 to October 2007 (Figure
1). After its first phase from 2004 to 2008 the MOC monitoring project
has now been funded until 2014 in the framework of RAPID-WATCH
. This will provide a unique, decade long timeseries of the MOC.
Figure 1: Transport timeseries obtained from the first 10 years
of observations at 26.5°N. The different curves show the MOC (red
line) and its constituents, i.e. the transport through the Florida
Straits (blue line), the Ekman transport (black line), and the density
driven transport obtained from the mooring data (pink line). The
transport units are Sverdrups (Sv, 1Sv =
106m3s-1). The mean and standard
deviations for the different transports are 18.5 ±4.9Sv (MOC),
31.7 ±2.8Sv (Florida Straits), 3.5 ±3.4Sv (Ekman), and
-16.6 ±3.2Sv (transport from mooring densities). Brief
descriptions of how the transports are calculated are given below.
Monitoring the MOC at 26.5°N
An estimate of the meridional flow relating to the MOC along 26.5°N can be obtained by decomposing it into three components:
- transport through the Florida Straits
- flow induced by the interaction between wind and the ocean surface (Ekman transport)
- transport related to the difference in sea water density between the American and African continents
Transport through the Florida Strait
The northward flowing Florida Current (T) contains salty seawater that can conduct electricity. As charged particles in the seawater
pass through the Earth's magnetic field (B), an electrical field is generated. This field induces a voltage (U) in submerged telephone lines
crossing the Florida Strait (see Figure 2). After proper calibration, the induced voltage can be used as a continuous indicator
of the strength of the ocean current through the strait. More details can be found at the project's website.
Figure 2: Monitoring currents in the Florida Straits using submerged
telephone cables between the US and Bahamas.
The zonal wind stress acting at the ocean surface generates a meridional(north-south) Ekman transport (see Figure 3). This mass transport is
typically confined to the upper 50m of the ocean. At 26°N, the Ekman contribution to the MOC is relatively small (2-4 Sv on average).
However, it is responsible for the largest short-term (subannual) variability occurring in the MOC. In the Rapid MOC monitoring
system initiative, the winds will be estimated using satellite and ship measurements.
Figure 3: Wind stress and Ekman transport.
Density Related Transport
If, for a particular ocean location, both the force of the wind acting on the ocean surface and the vertical change in sewater
density is known, then vertical change in the meridional flow induced by the wind can be approximated. A goal of the RAPID MOC
mooring array is to measure the vertical change in seawater density at a series of different longitudes between the Bahamas and
the African continent. The differences in these measured density profiles can allow one to estimate the current velocities with
respect to depth along 26.5°N (see Figure 4).
Figure 4: Calculated velocities between pairs of adjacent vertical density profiles
The combination of the velocity fields from the three components (Florida Strait, Ekman, and Density-driven transport) forms a rough
approximation of the meridional velocity across 26.5°N. In general the corresponding computed net meridional mass transport is not
zero and in order to obtain an MOC estimate a spatially (but not temporally) constant correction is added to the velocity field. Figure 5 illustrates the three
components of the MOC: Florida Strait transport (red arrow), Ekman transport (green arrow), and density-induced transport
(light blue arrows). The velocity correction is shown as the yellow arrows.
Figure 5: Estimate of the MOC.