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

摘要

We study the contribution of eastern-boundary density variations tosub-seasonal and seasonal anomalies of the strength and vertical structureof the Atlantic Meridional Overturning Circulation (AMOC) at 26.5° N, bymeans of the RAPID/MOCHA mooring array between April 2004 and October 2007.The major density anomalies are found in the upper 500 m, and they are oftencoherent down to 1400 m. The densities have 13-day fluctuations that areapparent down to 3500 m. The two strategies for measuring eastern-boundarydensity – a tall offshore mooring (EB1) and an array of moorings on thecontinental slope (EBH) – show little correspondence in terms of amplitude,vertical structure, and frequency distribution of the resulting basin-wideintegrated transport fluctuations, implying that there are significanttransport contributions between EB1 and EBH. Contrary to the originalplanning, measurements from EB1 cannot serve as backup or replacement forEBH: density needs to be measured directly at the continental slope tocompute the full-basin density gradient. Fluctuations in density at EBHgenerate transport variability of 2 Sv rms in the AMOC, while the overallAMOC variability is 4.8 Sv rms. There is a pronounced deep-reaching seasonalcycle in density at the eastern boundary, which is apparent between 100 mand 1400 m, with maximum positive anomalies in spring and maximum negativeanomalies in autumn. These changes drive anomalous southward upper mid-oceanflow in spring, implying maximum reduction of the AMOC, and vice-versa inautumn. The amplitude of the seasonal cycle of the AMOC arising from theeastern-boundary densities is 5.2 Sv peak-to-peak, dominating the 6.7 Svpeak-to-peak seasonal cycle of the total AMOC. Our analysis suggests thatthe seasonal cycle in density may be forced by the strong near-coastalseasonal cycle in wind stress curl.