Annual to interannual barotropic variability in the Atlantic Western Boundary

A method for estimating deep, depth independent current variability is described. The procedure uses XBT derived dynamic heights to remove the near surface signal from altimetric sea surface height (SSH). The difference between SSH and dynamic height is operationally defined as barotropic height (BRT). Currents are obtained from BRT slopes using the geostrophic balance. The method requires the variability below the deepest XBT measurement to be small. Results are restricted to temporal variability, as geoid uncertainties in the SSH data render absolute current estimations impossible. The technique is originally developed for use in the Atlantic Western Boundary Current (WBC). Comprehensive verification of the methodology requires simultaneous SSH, XBT and current meter measurements. There are no available Atlantic data sets that meet these requisites. The alternative is to use synthetic data from the CLIPPER Atlantic model (1/6° resolution). Correlation (r) between estimated and modelled near bottom velocities in WBC areas of interest vary from 0.7 to 0.8. Further tests are conducted with observed data from the Shikoku Basin boundary current in southern Japan, where the method is capable of reproducing the directly measured near bottom current variability (r = 0.6). The procedure is also tested using north Pacific (5°--55°N) data from the OCCAM model. Correlations between model derived and BRT estimated velocities are around r = 0.7 for the Shikoku Basin northern boundary and for the Pacific WBC. Values reach r = 0.9 in large areas of the basin's interior, especially over smooth topography. The above method is used to generate time series of the barotropic variability in two areas of the Atlantic Western Boundary. One site is located at 38°N, inshore of the Gulf Stream. The other is at 8°S, off the Brazilian coast. Both series are a approximately 6 years long. Empirical Orthogonal Function analysis results conducted on SSH and sea surface temperature data are used to confirm the feasibility of applying the method in the chosen South Atlantic area. Currents are compared to scatterometer derived local along-shore wind stress and basin wide wind stress curl. In both areas, current variability is significantly correlated to basin averaged wind stress curl and also to local along-shore wind stress. The relationship between currents and wind curl is coherent with the WBC response to interior Sverdrup flow. We propose that local wind stress exerts control over the flow by divergence of the Ekman flow at the coast. In the north, the variability is dominated by interannual oscillations of the wind curl. The effects of the local stress are secondary and have annual frequency. Both wind stress curl and along-shore wind are significantly correlated to the currents on the southern site, but the local effect appears to be the dominant forcing. The main observed results are confirmed by data from a numerical model with 1/6° horizontal resolution.