Description: The direct response of the tropical mixed layer to near-inertial waves (NIWs) has only rarely been observed. Here, we present upper-ocean turbulence data that provide evidence for a strongly elevated vertical diffusive heat flux across the base of the mixed layer in the presence of a NIW, thereby cooling the mixed layer at a rate of 244 W m−2 over the 20 h of continuous measurements. We investigate the seasonal cycle of strong NIW events and find that despite their local intermittent nature, they occur preferentially during boreal summer, presumably associated with the passage of atmospheric African Easterly Waves. We illustrate the impact of these rare but intense NIW induced mixing events on the mixed layer heat balance, highlight their contribution to the seasonal evolution of sea surface temperature, and discuss their potential impact on biological productivity in the tropical North Atlantic.

Description: The main goal of cruise MSM61 was to install an autonomous multidisciplinary observatory at the summit of Senghor Seamount off the Cape Verdean archipelago. A suite of different mobile and moored instrument platforms equipped with physical, biological and biogeochemical instruments was deployed during the cruise in order to investigate spatio-temporal variability of physical and biogeochemical conditions and how these affect the local ecosystem at this openocean seamount. The research program further included hydrographic work, biological and biogeochemical sampling as well as video transects in the meso- and bathypelagic zones both at Senghor Seamount and at the Cape Verde Ocean Observatory (CVOO).

Description: Zooplankton organisms are a central part of pelagic ecosystems. They feed on all kinds of particulate matter and their egested fecal pellets contribute substantially to the passive sinking flux to depth. Some zooplankton species also conduct diel vertical migrations (DVMs) between the surface layer (where they feed at nighttime) and midwater depth (where they hide at daytime from predation). These DVMs cause the active export of organic and inorganic matter from the surface layer as zooplankton organisms excrete, defecate, respire, die, and are preyed upon at depth. In the Eastern Tropical North Atlantic (ETNA), the daytime distribution depth of many migrators (300–600 m) coincides with an expanding and intensifying oxygen minimum zone (OMZ). We here assess the day and night-time biomass distribution of mesozooplankton with an equivalent spherical diameter of 0.39–20 mm in three regions of the ETNA, calculate the DVM-mediated fluxes and compare these to particulate matter fluxes and other biogeochemical processes. Integrated mesozooplankton biomass in the ETNA region is about twice as high at a central OMZ location (cOMZ; 11° N, 21° W) compared to the Cape Verde Ocean Observatory (CVOO; 17.6° N, 24.3° W) and an oligotrophic location at 5° N, 23° W (5N). An Intermediate Particle Maximum (IPM) is particularly strong at cOMZ compared to the other regions. This IPM seems to be related to DVM activity. Zooplankton DVM was found to be responsible for about 31–41% of nitrogen loss from the upper 200m of the water column. Gut flux and mortality make up about 31% of particulate matter supply to the 300–600 m depth layer at cOMZ, whereas it makes up about 32% and 41% at CVOO and 5N, respectively. Resident and migrant zooplankton are responsible for about 7–27% of the total oxygen demand at 300–600 m depth. Changes in zooplankton abundance and migration behavior due to decreasing oxygen levels at midwater depth could therefore alter the elemental cycling of oxygen and carbon in the ETNA OMZ and impact the removal of nitrogen from the surface layer.