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Five years of data from a line of dynamic height moorings (DHM), bottom-pressure recorders (BPR), and pressure-equipped inverted echo sounders (PIES) near the Atlantic Ocean western boundary at 26.51N are used to evaluate the structure and variability of the Deep Western Boundary Current (DWBC) during 2004–2009. Comparisons made between transports estimated from the DHM þ BPR and those made by the PIES demonstrate that the two systems are collecting equivalent volume transport information (correlation coefficient r¼ 0.96, root-mean-square difference ¼6 Sv; 1 Sv ¼10 6 m 3 s � 1 ). Integrated to � 450 km off from the continental shelf and between 800 and 4800 dbar, the DWBC has a mean transport of approximately 32 Sv and a standard deviation during these five years of 16 Sv. Both the barotropic (full-depth vertical mean) and baroclinic flows have significant variability (changes exceeding 10 Sv) on time scales ranging from a few days to months, with the barotropic variations being larger and more energetic at all time scales. The annual cycle of the deep transport is highly dependent on the horizontal integration distance; integrating � 100 km offshore yields an annual cycle of roughly similar magnitude but shifted in phase relative to that found from current meter arrays in the 1980–1990s, while the annual cycle becomes quite weak when integrating � 450 km offshore. Variations in the DWBC transport far exceed those of the total basin-wide Meridional Overturning Circulation (standard deviations of 16 Sv vs. 5 Sv). Transport integrated in the deep layer out to the west side of the Mid-Atlantic Ridge still demonstrates a surprisingly high variance, indicating that some compensation of the western basin deep variability must occur in the eastern basin. Published by Elsevier Ltd.

The rapid climate change programme (RAPID) has established a prototype system to continuously observe the strength and structure of the Atlantic meridional overturning circulation (MOC) at 26.5°N. Here we provide a detailed description of the RAPID-MOC monitoring array and how it has evolved during the first four deployment years, as well as an overview of the main findings so far. The RAPID-MOC monitoring array measures: (1) Gulf Stream transport through Florida Strait by cable and repeat direct velocity measurements; (2) Ekman transports by satellite scatterometer measurements; (3) Deep Western Boundary Currents by direct velocity measurements; (4) the basin wide interior baroclinic circulation from moorings measuring vertical profiles of density at the boundaries and on either side of the Mid-Atlantic Ridge; and (5) barotropic fluctuations using bottom pressure recorders. The array became operational in late March 2004 and is expected to continue until at least 2014. The first 4 years of observations (April 2004–April 2008) have provided an unprecedented insight into the MOC structure and variability. We show that the zonally integrated meridional flow tends to conserve mass, with the fluctuations of the different transport components largely compensating at periods longer than 10 days. We take this as experimental confirmation of the monitoring strategy, which was initially tested in numerical models. The MOC at 26.5°N is characterised by a large variability—even on timescales as short as weeks to months. The mean maximum MOC transport for the first 4 years of observations is 18.7 Sv with a standard deviation of 4.8 Sv. The mechanisms causing the MOC variability are not yet fully understood. Part of the observed MOC variability consists of a seasonal cycle, which can be linked to the seasonal variability of the wind stress curl close to the African coast. Close to the western boundary, fluctuations in the Gulf Stream and in the North Atlantic Deep Water (NADW) coincide with bottom pressure variations at the western margin, thus suggesting a barotropic compensation. Ongoing and future research will put these local transport variations into a wider spatial and climatic context.

Direct oceanographic observations at the strait of Bab el Mandeb, which connects the Red Sea with the Indian Ocean, are used to estimate the heat and freshwater budgets of the Red Sea. The observations consist of 18-month records from transport resolving current-meter arrays and temperature–salinity chain moorings that allow accurate heat and salt fluxes through the strait to be determined. The new measurements account for the seasonally reversing surface-layer flow in the strait and the summertime intrusion of low-salinity thermocline water from the Gulf of Aden. The annual mean freshwater loss to the atmosphere over the Red Sea is estimated to be 2.06±0.22 m yr −1 , while the annual mean heat loss is 11±5 W m −2 . These results are compared with previous estimates of the Red Sea heat and freshwater budgets derived from both direct and indirect methods, which show large differences between each other and much larger uncertainties than the present estimates.