Monitoring the integrated deep meridional flow in the tropical North Atlantic

Meridional transport of heat is accomplished by fundamentally different mechanisms in the atmosphere and the ocean. While in the atmosphere eddies exhibit a dominant role, the largest fraction of northern hemisphere poleward heat transport in the ocean is related to the Atlantic meridional overturning circulation (MOC). The evolution of the MOC and its impact on climate have been subject to intensive theoretical and numerical studies, however continuous measurements of MOC variability have not been carried out. In this study results from an observational pilot project to monitor fluctuations of the deep southward branch of the MOC across a latitude circle in the tropical North Atlantic are presented. Within the framework of the Meridional Overturning Variability Experiment (MOVE) a four year long time series of deep meridional volume transport fluctuations has been recorded.The backbone of the experiment design is an end point measurement method, which makes use of the fact that the deep ocean flow field is to first order in geostrophic balance: Fluctuations of deep zonally integrated meridional transports in the western trough1 of the Atlantic are efficiently monitored by continuous moored measurements of the evolution of the zonal density and bottom pressure difference between the eastern and western end point of the section. One main aspect of this study comprises data calibration and processing as well as a thorough technical performance assessment of the different measurement components of the monitoring array. It is found that two components (density and current meter measurements) provide robust estimates of transport fluctuations. As a consequence of sensor characteristics and data processing the third element (bottom pressure) is found to suppress low frequency variability. Simulations suggest that changes in the deployment scheme might help to overcome these problems to a large extent. Bottom pressure fluctuations derived from space-borne gravity field measurements at 16N deviate substantially from the in-situ observations and thus do not provide robust estimates of the evolution of deep transports. For the interpretation of the observed mean and time variable velocities and volume transports and the verification of the monitoring design comparisons to independent observational data and numerical model output have been carried out and spectral analysis as well as basic theoretical aspects of fluid dynamics have been applied. Since only the western trough of the Atlantic is covered by the array, westward propagating Rossby waves from the eastern trough represent a major source of noise, which may mask the MOC signal. An extension of the zonal integration scale from the western boundary from 400 to 1000 km leads to a substantial suppression of the wave signal, thus confirming the monitoring strategy. The best estimate of mean southward transport of North Atlantic Deep Water is 14.9 ± 3.0 Sv, its inter-annual variability amounts to 2.4 Sv. A verification of the experiment design using model simulations attest the transport signal observed by MOVE to be moderately representative for MOC and meridional heat transport fluctuations at 16N at inter-annual time scales. An eastward extension of the array into the eastern trough might lead to a drastic increase in the signal-to-noise ratio. However it is found that only at longer than decadal time scales coherent MOC fluctuation over the entire meridional extent of the Atlantic can be found. To separate locally and remotely forced MOC fluctuations on shorter time scales, it is suggested to operate two end point monitoring systems simultaneously at different latitude circles of the North Atlantic. Additional monitoring elements specifically designed to quantify the impact of different mechanisms responsible for MOC fluctuations should be added