A number of projects have been funded by UK NERC to monitor the MOC, some of which are collaborations with US NSF funded projects.
Monitoring the Atlantic Meridional Overturning Circulation at 26.5°N
Much of the heat transported northward in the Atlantic is given off to the atmosphere over the Gulf Stream extension from where it is transported north-eastward toward Europe by the atmosphere. Fluctuations in heat transport (and, by implication, transports of other quantities such as freshwater and carbon) are expected to be dominated by fluctuations in the transporting velocity field, and only to a lesser extent by variability in heat (or property) content. As one consequence, the basic monitoring of the MOC should occur near the heat transport maximum. 26.5°N has the triple advantage of being close to the heat transport maximum in the Atlantic, of being the latitude of four modern hydrographic occupations, and of offering a long time series of boundary current observations not existing anywhere else. At 26.5°N the western boundary current (flow through Florida Strait) can be measured relatively straightforwardly by cable (existing long-term programme by the US) and regular calibration cruises. This makes the monitoring of the entire MOC equivalent to the task of monitoring the depth profile at which the flow through the Florida Straits returns southward. Currently, its contribution to the MOC returns southward at depths between 1000m and 4000m.
Figure 24. MOC monitoring at 26.5oN.
The monitoring consists of continuous full-depth density profiles at and near the eastern and western boundaries. In total, 8 full-depth moorings, six of which equipped with a McLane Moored Profiler (MMP) taking roughly one CTD profile every other day. The use of profilers has the big advantage over individual, fixed-location CTD sensors that only a single instrument
needs to be calibrated. Several moorings are deployed near each boundary, for obtaining boundary current measurements through thermal wind, improving the signal-to-noise ratio, and as failsafe measures. All moorings are equipped not only with CTDs but also with bottom pressure sensors, and some with current meters. This gives added information for estimating the depth-independent part of the MOC that is not in thermal wind balance but is rather dominated by high-frequency barotropic dynamics. The presence of the Mid-Atlantic Ridge (MAR) complicates the endpoint monitoring of the MOC, because a pressure drop may exist across the ridge. Below the ridge crest, the sub-basins to the east and west therefore are monitored separately. The array consist of one MMP mooring on each side of the MAR, but the back-up fixed-depth CTD moorings only reach to the ridge crest. The MMP moorings will tell us how the shallow Gulf Stream return flow is divided between eastern and western basins. In addition to the full-depth sampling, the sloping shelfbreak topography is instrumented, from the deep water to shallow depths, with CTDs, bottom pressure recorders (BPR), and current meters (CM), to obtain continuous observations at fixed depths. This provides an alternative vertical sampling strategy, and also help solve the bottom triangle problem
Figure 25a and b. A monitoring array along the western margin of the North Atlantic
The aim of this observing system is to monitor the communication of MOC signals along the western margin of the Atlantic on time-scales of months to years. There are a number of motivations for focusing on this boundary communication:
• The boundary waves represent the rapid, integrated response of the mid- and low-
latitude ocean to deepwater formation events at high latitudes, and should allow changes in MOC at mid-latitudes to be attributed to their high-latitude sources.
• With the boundary wave signal taking weeks to propagate between sites along the US
continental slope, it is possible to identify the coherent part of the signal within a four- year dataset. Contrast this with a tracer signal that takes years to propagate and is strongly modified by mixing and recirculations.
• The well-defined speed of propagation facilitates the identification of the relevant signal
amongst the "noise" and aliasing of more localized ocean processes, for example generated through local wind forcing.
• the observing system is designed to capture the propagation of information from the
Grand Banks to the Gulf Stream where the continental slope is steep and comparatively uncomplicated, and thus the wave signals propagates in a relatively well-ordered manner.
The prototype array of instruments is designed to measure both integrated and local properties of the ocean circulation in this region, with the aim of identifying propagation of signals along the
western boundary, and of attributing these propagating signals to variability in the thermohaline circulation. This array doesn’t represent a complete monitoring system in itself for the MOC. However such an array is essential component of any complete monitoring system, which would ideally include a MOC monitoring line at 26N.
III. The in-situ observing system in the tropical Atlantic
In Section I a basin-scale overview was provided of Atlantic sector observations. This section is focused more specifically on these observation with respect to their relevance and status in the tropical Atlantic. During the past few decades, real-time observations from the tropical Atlantic in-situ observing system were derived primarily from volunteer observing ship (VOS) program, coastal and island tide-gauges, and a small number of drifting buoys. Considered as a second priority during the Tropical Ocean and Global Atmosphere (TOGA) program (1985-1994), which was mainly focused on the Pacific Ocean and El Niño – Southern Oscillation (ENSO) climatic features, the in-situ observing system in the tropical Atlantic registered progress only from the mid-1990s. This progress was first dedicated to increasing the number of classical expendable instrumentation (surface drifters, XBT, HD-XBT, …) launched by the VOS system and other oceanographic vessels. Regarded as the centre piece of the tropical Atlantic observing system, the Pilot Research moored Array in the Tropical Atlantic (PIRATA) (Servain et al., 1998) is a
network of in-situobservations enable to monitor changes in oceanic weather conditions in the
tropical Atlantic. It completes the similar system already set up in the Pacific during the TOGA’s years and known as TAO/TRITON (Tropical Atmosphere-ocean Array/TRIangle Trans-Ocean buoy Network). In the year 2000, a new and important component was added, the Argo program. As discussed in Section I, Argo is an international program whose goal is to deploy in the global ocean an array of 3,000 free-drifting profiling floats that measure the temperature and salinity of the upper 2000 m in a period of 5 years. Approximately 700 floats were operating in 2003 in the Atlantic, of which about one third in the tropical basin.