Description: The oxygen minimum zone (OMZ) of the tropical North East Atlantic (TNEA) is located in the region between the oxygen-rich equatorial band and the Cape Verde Frontal Zone at about 20°N at a depth of 300 – 600 m. The focus of our study is on the lateral ventilation of the OMZ across its southern boundary. This boundary is characterized by a maximum of the mean meridional oxygen gradient at about 5°N. In this region energetic mesoscale activity with periods from one to two months is present. The interaction of the mesoscale activity with the mean meridional oxygen gradient gives rise to a rich oxygen variance from finescale to mesoscale (O(1km) – O(100km)). An intense measurement program along 23°W cutting through the TNEA OMZ has been executed during recent years. Oxygen variability on time scales of O(1h) to O(1month) was observed with moored optode sensors from PIRATA moorings at 4°N and 11.5°N as well as from moorings located at the equator, 2°N, 5°N and 8°N. High variability of the current field was measured via moored acoustic Doppler current profilers. Additionally, repeat ship sections along the 23°W meridian were performed with standard CTD, oxygen and shipboard current measurements. The observed oxygen variability as a function of depth and latitude shows characteristic patterns consisting of an intermediate maximum at the upper boundary of the OMZ, a decay of oxygen variability with depth in the upper OMZ and a second weaker maximum in the lower OMZ. Comparing latitudes shows, that the oxygen variability at the OMZ core depth is strongest at about 4°N to 5°N and weaker towards the interior OMZ at 8°N and 11.5°N as well as towards the equator. A high-resolution advection-diffusion model was developed to simulate oxygen fluxes as a result of the mesoscale eddy field acting on the mean meridional oxygen gradient. While this model is able to describe the production of oxygen variability on length and time scales as small as O(1km) and O(1h), respectively, it reproduces several characteristics of the observed oxygen variability pattern.

Description: The equatorial deep jets (EDJ) are a striking feature of the equatorial ocean circulation. In the Atlantic Ocean, the EDJ are associated with a vertical scale of between 300 and 700 m, a time scale of roughly 4.5 years and upward energy propagation to the surface and thus are contributing to the interannual climate variability in the equatorial Atlantic region. However, it has been found that the meridional width of the EDJ is roughly 1.5 times larger than expected based on their vertical scale. Here a representation of a equatorial basin mode excited in a shallow water model for a single high order baroclinic vertical normal mode is used as a simple model for the EDJ. The model is linearised about both a state of rest and a barotropic mean flow resembling the Atlantic Equatorial Intermediate Current System with eastward flow at roughly 2◦ N and 2◦ S and westward flow in between and poleward of it. We argue that mixing of momentum along isopycnals can explain the enhanced width and a lateral eddy viscosity of 300 m^2 s−1 is found to be sufficient to account for the width implied by observations. The underlying eastward mean flow effectively shields the equator from off-equatorial Rossby waves, blocking the westward propagation of these waves that are generated by the reflection of equatorial Kelvin waves at the eastern boundary.

Description: The equatorial deep jets (EDJ) are a striking feature of the equatorial ocean circulation. In the Atlantic Ocean, the EDJ are associated with a vertical scale of between 300 and 700 m, a time scale of roughly 4.5 years and upward energy propagation to the surface. It has been found that the meridional width of the EDJ is roughly 1.5 times larger than expected based on their vertical scale. Here a representation of a equatorial basin mode excited in a linear shallow water model for a high order baroclinic vertical normal mode is used as a simple model for the EDJ. We argue that mixing of momentum along isopycnals can explain the enhanced width and a lateral eddy viscosity of 300 m^2s^-1 is found to be sufficient to account for the width implied by observations. Additionally, the effect of barotropic mean flow on the spatial and temporal structure of the wave field is studied. A mean flow resembling the Atlantic Equatorial Intermediate Current System with eastward jets at 2°N/S and westward flow in between results in a wave shielding of the equatorial band from adjacent regions.

Description: The tropical North East Atlantic (TNEA) is characterized by an oxygen minimum zone (OMZ) that is located at intermediate depth (300m – 700m) and latitudinally spreads between the oxygen-rich equatorial Atlantic and the Cape Verde Frontal Zone at about 20°N. Recent studies have shown that local oxygen fluctuations and the associated ventilation of the TNEA OMZ are mainly caused by diapycnal mixing and mesoscale eddies. Zonal currents additionally ventilate the TNEA by advecting oxygen-rich water from the well-ventilated western boundary eastwards. The spatial and temporal variability of these zonal currents is thought to contribute to the oxygen variability in this regime. An intense measurement program along 23°W cutting through the TNEA OMZ has been executed during recent years. Moored observations and repeat ship sections were performed with CTD/O2 (conductivity, temperature, depth, oxygen) and current measurements. Here, we analyze the spatial and temporal variability of the zonal currents in the TNEA at intermediate depths and discuss their respective role for the spatial and temporal oxygen variability as well as the ventilation of the OMZ. Particularly, the observed annual cycle of the North Equatorial Undercurrent (NEUC) at 5°N, which is several cm/s at intermediate depth, causes phase-shifted (zonal velocity leading oxygen) annual oxygen fluctuations in a range of about 10 µmol/kg. In general, time-varying zonal currents advect oxygen eastwards that is meridionally redistributed by mesoscale eddies. The overall effect of those currents for the ventilation of the OMZ is discussed.

Description: The oxygen minimum zone (OMZ) of the tropical North East Atlantic (TNEA) is located in the region between the oxygen rich equatorial band and the Cape Verde Frontal Zone at about 20°N at a depth of 300 – 600 m. Its horizontal extent is predominantly defined by two major current systems: 1) the northern boundary of the OMZ is given by the southward extent of the North Equatorial Current, that transports oxygen rich waters of the northern subtropical gyre, and 2) the southern boundary is given by the extent of the system of mean and variable zonal currents near the equator, in which the eastward flows supply oxygen from the well-ventilated western boundary regime resulting in an equatorial oxygen maximum. The focus of our study is on the lateral ventilation of the OMZ through its southern boundary. This boundary is given by a maximum of the mean meridional oxygen gradient at about 5°N. In this region energetic mesoscale activity with periods from one to two months is present. The interaction of the mesoscale activity with the mean meridional oxygen gradient gives rise to a rich oxygen variance from finescale to mesoscale (O(1km) – O(100km)). An intense measurement program along 23°W cutting through the TNEA OMZ has been executed during recent years. Repeat ship sections along the 23°W meridian were performed with standard CTD (conductivity, temperature, depth) and shipboard current measurements. Additionally, high temporal variability of the oxygen and current field was observed with moored optodes and acoustic Doppler current profilers, respectively, along 23°W at the equator, 2°N, 5°N and 8°N. The observed oxygen variability as a function of depth and latitude shows characteristic patterns consisting of an intermediate maximum at the upper boundary of the OMZ, a decay of oxygen variability with depth in the upper OMZ and a second weaker maximum in the lower OMZ. Comparing latitudes shows, that the oxygen variability in the OMZ is strongest at 5°N and weaker at 8°N, 2°N and the equator as well. To corroborate the observational results, a high-resolution advection-diffusion model was developed to simulate oxygen fluxes being the result of the mesoscale eddy field acting on the mean meridional oxygen gradient. While this model is able to describe the production of oxygen variability on length and time scales of O(1km) and O(1h), respectively, it reproduces several characteristics of the observed oxygen variability pattern.

Description: For certain, but realizable, states of the thermohaline and wind driven circulation of the North Atlantic Ocean, we demonstrate the possibility of making statements regarding the likely range of values to be taken by the annual average of the NAO-index on time scales out to a decade. Given that the North Atlantic is currently in such a predictable state, a simple surrogate model yields a prediction that the NAO index is more likely to be positive than negative for the next couple of years, followed by several years in which the NAO index is more likely to be negative.

Description: The traditional point of view is that in the ocean, the meridional transport of heat is achieved by the wind-driven and meridional overturning circulations. Here we point out the fundamental role played by ocean mixing processes. We argue that mixing (i.e., water mass conversion) associated with eddies, especially in the surface mixed layer, can play an important role in closing the ocean heat budget. Our results argue that the lateral mixing applied at the surface of ocean/climate models should be playing an important role in the heat balance of these models, indicating the need for physically-based parameterizations to represent this mixing.