Description: The eastern equatorial Atlantic hosts a productive marine ecosystem that depends on the upward supply of nutrients. The main process that transports nutrients into the surface mixed layer is turbulent mixing induced in the shear zone between the surface mixed layer and the core of the Equatorial Undercurrent (EUC). Here we present experimental data from two trans-Atlantic cruises along the equator as well as moored observations allowing to characterize the seasonal cycle of velocity shear and turbulence. These data in combination with hydrographic data allow the analysis of the seasonal cycle of equatorial mixing and upward nitrate flux. The core of the EUC migrates vertically following an annual cycle. It reaches its shallowest position in boreal spring and its deepest position in boreal fall. The seasonal cycle of the maximum nitrate gradient instead shows a primary upward movement during boreal summer and a secondary upward movement in boreal winter bringing the nitrate gradient into the region of enhanced shear and turbulent mixing. During boreal spring, the nitrate gradient is located below the EUC core that is characterized by a minimum of turbulence. It prevents an upward supply of nitrate into the surface mixed layer and results in low productivity. The dynamic behavior of the shear zone is driven by a resonant equatorial basin mode associated with the east- and westward propagation of equatorial Kelvin and Rossby waves making the seasonal cycle of upward nutrient supply and productivity in the equatorial Atlantic distinct from that in the Pacific Ocean.

Description: The upper-ocean circulation of the tropical Atlantic is a complex superposition of thermohaline and wind-driven flows. The zonally and vertically integrated upper-ocean meridional flow is associated with the upper branch of the Atlantic Meridional Overturning Circulation (AMOC) — a major component of the global climate system. In the tropics, the northward AMOC flow is superimposed by the shallower overturning associated with the wind-driven Subtropical cells (STC). At the western boundary, the TRACOS (TRopical Atlantic Circulation & Overturning at 11°S) array consists of tall moorings monitoring the strong western boundary current (WBC) system - more specifically, the North Brazil Undercurrent (NBUC) and the Deep Western Boundary Current (DWBC) as part of the AMOC and STC. Mean transports seem rather stable throughout the observations (between 2000-2004 and 2013 up to date), while long-term changes in water mass properties are detected and related to changes in remote areas. At the eastern side the Angola current is observed since 2013 and shows weak mean circulation, whereas the variability seems dominated by remotely forced waves from the equatorial region. In addition to the boundary observations, the program at 11°S was extended to observe the basin wide circulation with bottom pressures sensors on both sides of the basin. The available data is used to estimate AMOC variability at 11°S on seasonal to interannual time scales. Together with other AMOC arrays, this array has great potential for understanding mechanisms relevant for tropical Atlantic variability, as well as meridional coherence and long-term changes of AMOC variability.

Description: The tropical Angolan upwelling system (tAUS) is a highly productive ecosystem with a distinct seasonal variability with productivity peaking in austral winter. The tAUS is connected to equatorial dynamics via the propagation of equatorial and coastal trapped waves (CTWs). We use hydrographic, ocean turbulence and satellite data to investigate the role of CTWs in controlling the seasonal cycle of productivity in the tAUS. During austral winter associated with the passage of an upwelling CTW, the nitracline is displaced upward by about 50 m. Through this vertical advection nitrate-rich waters passes onto the shelf. Due to the elevated mixing rates on the shelf, this movement of the nitracline results an increased vertical nitrate flux into the ocean mixed layer. Our analysis further shows that interannual variability in the strength of the austral winter net primary production correlates with the amplitude of the seal level anomaly signal of the corresponding upwelling CTW. The signal of sea level depression leads the maximum productivity signal by about 40 days. It is suggested that this time lag arises, among other factors, from the vertical structure of the CTWs arriving in the tAUS. While the sea level anomaly is dominated by the faster low-baroclinic mode CTWs, the displacement of the nitracline is mainly influenced by the slower high-baroclinic mode CTWs that arrive later in the tAUS. Our results highlight the crucial role CTWs play for the productivity in the tAUS. The strong connection between equatorial dynamics and productivity further introduces a possibility for predicting interannual variability.

Description: Important effects of mesoscale eddies in eastern boundary upwelling systems (EBUS) are offshore transport of coastal water and vertical exchange between the mixed layer and below. In our contribution we reconstruct the 3-D structure of several cyclonic eddies and anticyclonic modewater eddies in the Canary EBUS from in situ velocity observations, based on different survey patterns of different durations. The reconstructions include derived dynamic properties such as vorticity and divergence, so that trapping radii and shallow secondary circulation can be estimated. Common features of the eddies are used to extrapolate on the offshore transport and vertical exchange caused by EBUS eddies. Some collateral insights from these investigations include that (i) a few inertial periods of observation time are needed to estimate vertical velocity, and (ii) all observed eddies have a radius very similar to the Rossby radius, as opposed to the larger variability in size that satellite products suggest.