Eastern boundary baroclinic variability and the MOC at 26.5N,

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

Plymouth

Congreso; RAPID annual meeting 2007; 2007

Natural Environment Research Council

The aim of this work is to study the contribution of eastern boundary density to the MOC, based on the RAPID 26.5°N mooring data between April 2004 and October 2006. 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 density contributions to MOC variability have been studied systematically. Here we show that they are important. The RAPID array makes use of moored time series measurements of density at the western and eastern boundaries of the North Atlantic to estimate the basin scale, absolute zonally integrated mid-ocean geostrophic transport. To highlight the eastern boundary variability, calculations are made assuming that density at the western boundary is constant and only the eastern boundary density varies over time. At the eastern boundary there are two methods of sampling density profiles: using either of two 5000 m long (full water column) moorings located at the base of the African continental slope, or with an array of small moorings distributed between the African shelf and the base of the continental slope. Using the tall moorings, the largest variability is found in the thermocline layer (surface to 800 db, standard deviation of 1.1Sv) and in lower North Atlantic Deep Water (below 3000 db, standard deviation of 1.34 Sv). The dominating vertical velocity mode exhibits large amplitudes in these layers with a zero crossing at ~ 1770 db. Moreover, flows in these two layers compensate each other.The eastern boundary contribution to the basin-scale velocity inferred from the inshore (small moorings) and offshore (tall moorings) data sets are investigated for potential redundancy. Preliminary results suggest that 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 spectrum based onthe inshore data shows much more variance in the high frequency limit. Thus, near boundary processes appear to play an important role. The analysis of the dominating vertical velocity modes associated with the inshore data set shows that variability is more concentrated in the upper ocean, and is very small in the deep ocean. Contribution of eastern boundary density variability to MOC variability is 1.9 Sv if the offshore data are used, and 1.5 Sv if the inshore data are used. The different sources of the eastern boundary transport variability as well as the associated processes will be addressed.