Description: The processing was done as described in Bunge et al. (2008) and in Bourlès Bernard et al (2020): French PIRATA cruises: MOORING ADCP data. SEANOE. https://doi.org/10.17882/51557

Description: Since 2001, current velocities have been measured continuously as part of a multilateral collaboration, the Prediction and Research Moored Array in the Tropical Atlantic (PIRATA), that regularly services a moored observatory located at 0°N, 23°W. Here, we present an update of 20 years of full-depth current velocity observations at 0°N, 23°W. With the presented current velocity data product, we aim to provide an important and accessible reference data set against which models and reanalysis output could be validated. The velocity time series will also be helpful for studies focusing on long-term climate variability to search for connections with changes in the equatorial circulation over the last 20 years. Earlier versions of this data product have already been used in a variety of studies and provided a significant contribution to an overall improved understanding of equatorial ocean dynamics. The moored observatory at 0°N, 23°W is an ongoing example of a successful multinational collaboration extending over more than two decades.

Description: Current velocities of the upper water column along the cruise track of R/V Maria S. Merian cruise MSM117 were collected by a vessel-mounted 38 kHz RDI Ocean Surveyor ADCP. The ADCP transducer was located at 6.0 m below the water line. The instrument was operated in two different configurations: 1) broadband mode with 32 m bins and a blanking distance of 16 m, with a total of 50 bins, 2) narrowband mode with 32 m bins and a blanking distance of 16 m, with a total of 50 bins. Beam velocities as recorded by the data acquistion software VmDAS were transformed to ship coordinates and after merging with the navigation data from the ship's Motion Reference Unit and Global Positioning systems into earth coordinates. Single-ping data were screened for bottom signals and, where appropriate, a bottom mask was manually processed. The ship's velocity was calculated from position fixes obtained by the Global Positioning System (GPS). Accuracy of the ADCP velocities mainly depends on the quality of the position fixes and the ship's heading data. Further errors stem from a misalignment of the transducer with the ship's centerline. Data post-processing included water track calibration of the misalignment angle (configuration 1: 0.3196° +/- 0.8714°, configuration 2: 0.3603° +/- 0.6433°) and scale factor (configuration1: 1.0007 +/- 0.0151, configuration 2: 1.0024 +/- 0.0107) of the Ocean Surveyor signal. The velocity data were averaged in time using an average interval of 60 s. Velocity quality flagging is based on following threshold criteria: abs(UC) or abs(VC) 〉 2.0 m/s, rms(UC_z) or rms(VC_z) 〉 0.3.

Description: The upper-ocean circulation of the tropical Atlantic is a complex superposition of thermohaline and wind-driven flow components. The resulting zonally- and vertically-integrated upper-ocean meridional flow is referred to as the upper branch of the Atlantic Meridional Overturning Circulation (AMOC) - a major component and potential tipping element of the global climate system. We investigate the tropical part of the northward AMOC branch, i.e. the return flow covering the upper 1,200 m, based on Argo data and repeated shipboard velocity measurements. The western boundary mean circulation at 11°S is realistically reproduced from high-resolution Argo data showing a remarkably good representation of the vertical structure of meridional velocity and the volume transport of water mass layers when compared to results from direct velocity measurements along a repeated ship section. Thus, we extend the analysis to the inner tropical Atlantic. Within the AMOC return flow, a diapycnal upwelling of central water into the thermocline layer of ~2 Sv is derived between 11°S and 10°N which is about half the magnitude of previous estimates, likely due to improved horizontal resolution. The mean strength of the AMOC return flow is ~16 Sv across 11°S and 10°N. At 11°S, northward transport is concentrated at the western boundary where the AMOC return flow enters the tropics at all vertical layers above 1,200 m. At 10°N, northward transport is observed both at the western boundary and in the interior predominantly in the surface and intermediate layer indicating recirculation and transformation of thermocline and central water within the tropics.