Claudia Schmid

Mean meridional currents in the central and eastern equatorial Atlantic

Ship-based acoustic Doppler current profiler (ADCP) velocity measurements collected by several major field programs in the tropical Atlantic are averaged and combined with estimates of the mean near-surface velocity derived from drifters and Argo float surface drifts (ADCP+D) to describe the mean cross-equatorial and vertical structure of the meridional currents along 23°W and 10°W. Data from moored ADCPs and fixed-depth current meters, a satellite-derived velocity product, and a global ocean reanalysis were additionally used to evaluate the mean ADCP+D meridional velocity. The dominant circulation features in the long-term mean ADCP+D meridional velocity in the upper 100xa0m are the tropical cells (TCs) located approximately between 5°S and 5°N, with near-surface poleward flow and subsurface equatorward flow that is stronger and shallower in the northern cell compared to the southern cell. The thickness of the surface limb of the TCs decreases and the northern cell is found to shift further south of the equator from the central to eastern tropical Atlantic. Analysis of two-season means estimated from the ship-based ADCP, near-surface drift, and moored velocity data, as well as the simulated fields, indicates that the maximum poleward velocity in the surface limb of the TCs intensifies during December–May along 23°W largely due to seasonal compensation between the geostrophic and ageostrophic (or wind-driven) components of the meridional velocity, whereas the maximum equatorward flow in the subsurface limb of the northern cell intensifies during June–November along both 23°W and 10°W due to the seasonality of the geostrophic meridional velocity.

Effects of eddies on an ocean observing system with profiling floats: Idealized simulations of the Argo array

[1] This study aims at evaluating effects of the mesoscale variability on the expected accuracy of reconstruction of temperature, salinity, and velocities from the Argo measurements and trajectories. For this purpose, an idealized observing system with profiling floats is simulated in a high‐resolution ocean model of the North Atlantic set up to produce annual mean hydrography and circulation. The simulations with and without mesoscale variability are compared, and the effects of the time mean and mesoscale eddy–induced advection are effectively separated and investigated. The results demonstrate several effects of mesoscale eddies on the expected accuracy of the Argo‐based reconstructions of temperature, salinity, and horizontal velocities. In most of the domain, the eddies help to achieve uniform spatial coverage. The effects of eddy advection on reconstruction errors are, however, complex but moderate in most of the domain. High‐frequency variability in temperature and salinity leads to enhancement of reconstruction errors, especially if the sampling is carried out for only a few years. The reconstruction of horizontal velocities from trajectories of the profiling floats is capable of detecting multiple zonal jets which have been observed already. The reconstruction of the meridional velocities is significantly less reliable, primarily due to a small signal‐to‐noise ratio in the in the interior of domain.

An observations and model‐based analysis of meridional transports in the South Atlantic

A three-dimensional velocity field constructed from Argo observations and sea surface heights (called Argo and SSH, hereinafter) is used to estimate meridional overturning volume transport and meridional heat transport (MHT) across 20°S, 25°S, 30°S, and 35°S for the years 2000–2014 in the South Atlantic. Volume transport in the upper branch of Meridional Overturning Circulation (MOC) and MHT from the observations are consistent with the previous observations, but are higher than the estimates derived from three data assimilative ocean models, at some of the latitudes. Both the observations and models show strong correlations between the strength of MOC and MHT at all the latitudes. The corresponding change in MHT for 1 Sv change of MOC strength, in the observations, increases from 0.046 PW in 25°S, 30°S, and 35°S to 0.056 PW across 20°S. A comparison of model-based transports at 35°S at the boundaries and in the interior with those from Argo and SSH shows significant differences between them with respect to the contributions in the three segments of the section. In addition, the contributions also vary greatly between the different models. An analysis of the seasonality of MOC in the models and in the observations reveals that MOC anomalies in the models mostly show strong annual cycles at all the latitudes, whereas those derived from Argo and SSH exhibit annual cycles at three latitudes (35°S, 30°S, and to a lesser extent at 25°S) and a semiannual cycle at 20°S.

An Enhanced PIRATA Dataset for Tropical Atlantic Ocean–Atmosphere Research

AbstractThe Prediction and Research Moored Array in the Tropical Atlantic (PIRATA) provides measurements of the upper ocean and near-surface atmosphere at 18 locations. Time series from many moorings are nearly 20 years in length. However, instrumental biases, data dropouts, and the coarse vertical resolutions of the oceanic measurements complicate their use for research. Here an enhanced PIRATA dataset (ePIRATA) is presented for the 17 PIRATA moorings with record lengths of at least seven years. Data in ePIRATA are corrected for instrumental biases, temporal gaps are filled using supplementary datasets, and the subsurface temperature and salinity time series are mapped to a uniform 5-m vertical grid. All original PIRATA data that pass quality control and that do not require bias correction are retained without modification, and detailed error estimates are provided. The terms in the mixed-layer heat and temperature budgets are calculated and included, with error bars. As an example of ePIRATA’s applicati...