Absolute Brazil Current transport variability at 34.5°S from a long-term moored array

Abstract

<p>The southward flowing Brazil Current is the western limb of the subtropical gyre of the South Atlantic Ocean, carrying relatively warm and salty waters along the continental margin of South America. While the mean strength of the Brazil Current is fairly well-known from observations, there are still many open questions about its structure and variability, the dynamical mechanisms that drive it, as well as its influence on the regional and large-scale circulation patterns. Here the Brazil Current transport variability is analyzed using continuous-in-time daily data collected along a line of four pressure-equipped inverted echo sounders (PIESs) in the western South Atlantic at 34.5°S between 51.5°W and 44.5°W from 2009 to 2015. In addition, the mooring line was augmented in December 2012 with two current-and-pressure-equipped inverted echo sounders (CPIESs) and in December 2013 with a bottom pressure recorder and a bottom moored Acoustic Doppler Current Profiler to enhance the spatial coverage of observations of the Brazil Current on the continental shelf and upper slope. High-resolution hydrographic transects also provide estimates of the flow on the shelf region. Daily time series of absolute meridional transport between neighboring pairs of PIES/CPIES are estimated by vertically integrating the calculated geostrophic velocities (baroclinic referenced to the bottom plus barotropic) from the sea surface to the interface between South Atlantic Central Water and Antarctic Intermediate Water (on average at 600 dbar) and are combined with information on the shelf and slope. The Brazil Current flow at this location is confined west of 49.5°W. The time-mean absolute Brazil Current transport at 34.5°S is -12.6 ± 1.1 Sv (negative indicates southward flow), and the temporal standard deviation is 8.8 Sv. Peak-to-peak transport variations of 30 Sv occur over periods as short as 3 weeks. Fluctuations with periods shorter than 120 days account for 80% of the total transport variance. Due to the observed high frequency variability of the Brazil Current, to obtain a monthly value accurate to within one standard error of the mean level (of 1.1 Sv) at least 25 daily observations within that month are needed. In order to obtain an annual mean value accurate to 1.1 Sv (or better) at least 60 randomly sub-sampled observations are required during that year. These results point to the need of continuous measurements to resolve monthly to interannual time scales. The baroclinic component of the transport accounts for the largest fraction of the absolute transport variance (75%), but the barotropic component is not negligible. The baroclinic and barotropic transports are uncorrelated, highlighting the need of measuring both transport components independently. No significant seasonal cycle in absolute transport is found in the ~8 year record, and the annual means of the absolute transport vary only slightly. Our results suggest that the transport variability is highly influenced by eddies impinging on the array’s location, consistent with earlier observations in the Deep Western Boundary Current at this same latitude.</p>