On the contribution of eastern boundary density variability to the MOC at 26.5N

CHIDICHIMO, M. P.; KANZOW, T.; CUNNINGHAM, S. A.; MAROTZKE, J.

Cambridge

Congreso; RAPID annual meeting 2008; 2008

Natural Environment Research Council

The RAPID-MOC array makes use of moored time series measurements of vertical density profiles at the western and eastern boundaries at 26.5°N of the Atlantic to estimate the transatlantic, absolute zonally integrated meridional geostrophic transport. Here we study thecontribution of eastern boundary density to the Meridional Overturning Circulation (MOC), based on mooring data between April 2004 and October 2006. Among the mechanisms that may change densities at the eastern boundary (and thus the strength of the MOC) are Kelvin waves propagating poleward and wind-driven upwelling. It is generally expected that the density variability near the eastern boundary of the North Atlantic is smaller than near the western boundary. However, neither the amplitude nor the frequency distribution of eastern boundary densities contribution to MOC variability have been studied systematically. To highlight the eastern boundary variability, the MOC is calculated assuming that density at the western boundary is time-invariant and only the eastern boundary density varies over time. At the eastern boundary there are two methods of sampling density profiles: by two 5000 m long (full water column) moorings located at the base of the Africancontinental slope, and with an array of small moorings distributed between the African shelf and the base of the continental slope. The eastern boundary contribution to the basin-scale meridional transports inferred from the inshore (small moorings) and offshore (tall moorings) data sets are investigated for potential redundancy. There are considerable differences between the two data sets in terms of amplitude, vertical structure and frequency distribution of the resulting mid ocean geostrophic transport fluctuations.The vertical correlation scale of the density anomalies that account for the major changes in thetransports is much larger at the offshore site (throughout the entire water column) with maximum density changes at ~1000 m. Density anomalies in the inshore data set display larger density anomalies than offshore but these are mostly confined to depths above 1000 m. Transports inferred from the inshore data show pronounced variance in the high frequency limit, with dominant periods of 3 and 13 days. Near boundary processes appear to play an important role in setting the time scale. Transports inferred from the offshore data exhibit maximum variability in the low frequency limit with dominant periods of 5 months. The two transports signals are uncorrelated. Mechanisms which are unrelated to the MOC (such as eddies) may mask MOC related density signals in the offshore data set on the time scales under consideration. The full water column mooring is too far offshore to detect potential boundary waves and / or upwelling signal. Therefore we conclude that theinshore data set should be used to compute the eastern boundary density contribution to the MOC. Contribution of eastern boundary density variability to MOC variability is ± 2.1 Sv. The overall MOC variability is ± 5.6 Sv (Cunningham et al., 2007).