Description: PREFCLIM is a mixed-layer climatology for the Eastern Tropical Atlantic. The climatology contains a high-resolution (0.25 degrees) monthly-mean mixed-layer hydrography (mixed-layer depth, temperature, salinity), and coarse-resolution (2.5 degrees) estimates of the mixed-layer heat and salt balance, as well as of near-surface velocities and of air-sea fluxes. All existing hydrographic products of the region were hampered by the sparse availability of near-shore data owned by the West-African coastal countries, which could, however, be included in the new climatology.

Description: This dataset is a collection of moored velocity data used in the study “Sources and pathways of intraseasonal meridional kinetic energy in the equatorial Atlantic Ocean” (Körner et al., 2022) to analyze the representation of meridional intraseasonal velocity variability in a general circulation model. The observational velocity data were collected at five different locations along the equator. At 0°N, 35°W velocity data were collected by single-point current meters in four different depths from October 1992 until May 1994. Between August 2004 and June 2006 an acoustic Doppler current profiler (ADCP) and single-point current meters collected data at 0°N, 35°W. Moorings at 0°N, 23°W provide velocity measurements from December 2001 to June 2021 (apart from a period from December 2002 to February 2004 when no mooring was in place). At 0°N, 10°W velocity data were collected between May 2003 to March 2019 using ADCPs and single-point current meters. However, the mooring was not consecutively installed leading to data gaps of up to two years. Between 2007 and 2011 ADCPs recorded velocity data at 0°N, 0°E.

Description: The tropical Angolan upwelling system is a highly productive ecosystem with a distinct seasonal cycle in surface temperature and primary production. The lowest sea surface temperature, strongest cross-shore temperature gradient, and maximum productivity occur in austral winter when seasonally prevailing upwelling favorable winds are weakest. A multi cruise dataset of microstructure profiles collected between 2013 and 2022 in the tropical Angolan upwelling system was used to analyze the importance of mixing for cooling of the mixed layer. The data were collected during six cruises on board of the R/V Meteor. The results show that cooling due to turbulent heat fluxes at the base of the mixed layer is an important cooling term. This turbulent cooling, that is strongest in shallow shelf regions, is capable of explaining the observed negative cross-shore temperature gradient.

Description: Multiyear moored velocity observations of the Angola Current near 11°S reveal a weak southward mean flow superimposed by substantial intraseasonal to seasonal variability, including annual and semiannual cycles with distinct baroclinic structures. In the equatorial Atlantic these oscillations are associated with basin-mode resonances of the fourth and second baroclinic modes, respectively. Here, the role of basin-mode resonance and local forcing for the Angola Current seasonality is investigated. A suite of linear shallow-water models for the tropical Atlantic is employed, each model representing a single baroclinic mode forced at a specific period. The annually and semiannually oscillating forcing is given by 1) an idealized zonally uniform zonal forcing restricted to the equatorial band corresponding to a remote equatorial forcing or 2) realistic, spatially varying Fourier components of wind stress data that include local forcing off Angola, particularly alongshore winds. Model-computed modal amplitudes are scaled to match moored velocity observations from the equatorial Atlantic. The observed annual cycle of alongshore velocity at 11°S is well reproduced by the remote equatorial forcing. Including local forcing slightly improves the agreement between observed and simulated semiannual oscillations at 11°S compared to the purely equatorial forcing. However, the model-computed semiannual cycle lacks amplitude at middepth. This could be the result of either underestimating the strength of the second equatorial basin mode of the fourth baroclinic mode or other processes not accounted for in the shallow-water models. Overall, the findings underline the importance of large-scale linear equatorial wave dynamics for the seasonal variability of the boundary circulation off Angola.

Description: High primary productivity in the equatorial Atlantic and Pacific oceans is one of the key features of tropical ocean biogeochemistry and fuels a substantial flux of particulate matter towards the abyssal ocean. How biological processes and equatorial current dynamics shape the particle size distribution and flux, however, is poorly understood. Here we use high-resolution size-resolved particle imaging and Acoustic Doppler Current Profiler data to assess these influences in equatorial oceans. We find an increase in particle abundance and flux at depths of 300 to 600 m at the Atlantic and Pacific equator, a depth range to which zooplankton and nekton migrate vertically in a daily cycle. We attribute this particle maximum to faecal pellet production by these organisms. At depths of 1,000 to 4,000 m, we find that the particulate organic carbon flux is up to three times greater in the equatorial belt (1° S–1° N) than in off-equatorial regions. At 3,000 m, the flux is dominated by small particles less than 0.53 mm in diameter. The dominance of small particles seems to be caused by enhanced active and passive particle export in this region, as well as by the focusing of particles by deep eastward jets found at 2° N and 2° S. We thus suggest that zooplankton movements and ocean currents modulate the transfer of particulate carbon from the surface to the deep ocean.