Description: Large-scale fully coupled Earth system models (ESMs) are usually applied in climate projections like the IPCC (Intergovernmental Panel on Climate Change) reports. In these models internal variability is often within the correct order of magnitude compared with the observed climate, but due to internal variability and arbitrary initial conditions they are not able to reproduce the observed timing of climate events or shifts as for instance observed in the El Niño–Southern Oscillation (ENSO), the Pacific Decadal Oscillation (PDO), or the Atlantic Meridional Overturning Circulation (AMOC). Additional information about the real climate history is necessary to constrain ESMs; not only to emulate the past climate, but also to introduce a potential forecast skill into these models through a proper initialisation. We attempt to do this by extending the fully coupled climate model Max Planck Institute Earth System Model (MPI-ESM) using a partial coupling technique (Modini-MPI-ESM). This method is implemented by adding reanalysis wind-field anomalies to the MPI-ESM's inherent climatological wind field when computing the surface wind stress that is used to drive the ocean and sea ice model. Using anomalies instead of the full wind field reduces potential model drifts, because of different mean climate states of the unconstrained MPI-ESM and the partially coupled Modini-MPI-ESM, that could arise if total observed wind stress was used. We apply two different reanalysis wind products (National Centers for Environmental Prediction, Climate Forecast System Reanalysis (NCEPcsfr) and ERA-Interim reanalysis (ERAI)) and analyse the skill of Modini-MPI-ESM with respect to several observed oceanic, atmospheric, and sea ice indices. We demonstrate that Modini-MPI-ESM has a significant skill over the time period 1980–2013 in reproducing historical climate fluctuations, indicating the potential of the method for initialising seasonal to decadal forecasts. Additionally, our comparison of the results achieved with the two reanalysis wind products NCEPcsfr and ERAI indicates that in general applying NCEPcsfr results in a better reconstruction of climate variability since 1980.

Description: The tropical impact on the East Asian winter monsoon (EAWM) is examined in an ensemble of atmospheric general circulation model runs that use relaxation towards the ERA-40 reanalysis in the tropics for winters between 1960/61 and 2001/02 and performed with a recent version of the European Centre for Medium-Range Weather Forecasts model. 25% of the interannual variance of the EAWM can be reproduced in the ensemble mean by the model experiments with relaxation, even though the influence from ENSO appears to be weak. The implication is that there is the possibility of enhanced predictability for the EAWM resulting from improved forecast skill in the tropics as a whole. Prescribing observed sea surface temperature and sea ice without using relaxation cannot reproduce the interannual variability of the EAWM in our experiments, questioning the usefulness of uncoupled atmosphere models in this region, consistent with previous studies.

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.