The contribution of density variations at the eastern boundary of the North Atlantic to the meridional overturning circulation at 26.5N

CHIDICHIMO, M. P.; KANZOW, T.; JOHNS, W. E.; CUNNINGHAM, S.; MAROTZKE, J.

Montreal

Congreso; 2009 MOCA-09 IAMAS/IAPSO/IACS/ Joint Assembly "Our Warming Planet"; 2009

IAMAS/IAPSO/IACS

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 the contribution of eastern boundary density to sub-seasonal and seasonal variations of the strength and vertical structure of the Meridional Overturning Circulation (MOC) at 26.5°N by means of RAPID mooring data between April 2004 and October 2007. 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 processes near the coast like Ekman pumping or upwelling. A previous study using only limited data suggested that variability at the eastern boundary is an order of magnitude less than that at the west (Longworth et al., 2006). Here we show that the amplitude and frequency distribution of eastern boundary density variability is an important contribution to the inter-annual evolution of the MOC. The standard deviation of inter-annual variability of the upper mid-ocean transport due to variations in the eastern boundary vertical density distribution is ±2.1Sv, while the overall MOC variability is ± 4.9 Sv (Kanzow et al., 2009). The amplitude of the seasonal cycle of overturning variability arising from the eastern boundary is 5.2 Sv (peak-to-peak). The seasonal cycle in density at the eastern boundary is coherent between 100 m and 1000 m with maximum density anomalies at fixed depth in spring and minima in autumn; corresponding to a raising and lowering of the thermocline. These changes drive an enhanced southward upper mid-ocean flow in spring resulting in a minimum in the MOC and vice-versa for the autumn. The pronounced seasonal variations in density right at the eastern boundary dominate the upper mid-ocean transport seasonality of the MOC. Western boundary transports do not display such a clear seasonal cycle.  A simple model suggests that the seasonal cycle in eastern boundary densities may be linked to seasonal variations of the eastern boundary wind stress curl.