Abstract
Abstract. The Brazil Current transports from observations and the Hybrid Coordinate Model (HYCOM) model are analyzed to improve our understanding of the current's structure and variability. A time series of the observed transport is derived from a three-dimensional field of the velocity in the South Atlantic covering the years 1993 to 2015 (hereinafter called Argo & SSH). The mean transports of the Brazil Current increases from 3.8 ± 2.2 Sv (1 Sv is 106 m3 s−1) at 25∘ S to 13.9 ± 2.6 Sv at 32∘ S, which corresponds to a mean slope of 1.4 ± 0.4 Sv per degree. Transport estimates derived from HYCOM fields are somewhat higher (5.2 ± 2.7 and 18.7 ± 7.1 Sv at 25 and 32∘ S, respectively) than those from Argo & SSH, but these differences are small when compared with the standard deviations. Overall, the observed latitude dependence of the transport of the Brazil Current is in agreement with the wind-driven circulation in the super gyre of the subtropical South Atlantic. A mean annual cycle with highest (lowest) transports in austral summer (winter) is found to exist at selected latitudes (24, 35, and 38∘ S). The significance of this signal shrinks with increasing latitude (both in Argo & SSH and HYCOM), mainly due to mesoscale and interannual variability. Both Argo & SSH, as well as HYCOM, reveal interannual variability at 24 and 35∘ S that results in relatively large power at periods of 2 years or more in wavelet spectra. It is found that the interannual variability at 24∘ S is correlated with the South Atlantic Subtropical Dipole Mode (SASD), the Southern Annular Mode (SAM), and the Niño 3.4 index. Similarly, correlations between SAM and the Brazil Current transport are also found at 35∘ S. Further investigation of the variability reveals that the first and second mode of a coupled empirical orthogonal function of the meridional transport and the sea level pressure explain 36 and 15 % of the covariance, respectively. Overall, the results indicate that SAM, SASD, and El Niño–Southern Oscillation have an influence on the transport of the Brazil Current.
Highlights
The circulation in the South Atlantic has been studied extensively because it is an important part of the Atlantic Meridional Overturning Circulation, which consists of a northward transport of relatively warm and fresh upper-ocean water of southern origin across the equator into the northern North Atlantic and a southward transport of relatively cold and salty deep water from the North Atlantic into the South Atlantic
A summary of the circulation in the South Atlantic as well as the pathways of the flow and its role in the Atlantic Meridional Overturning Circulation has been presented by Schmid (2014) and many others
The wavelet transform amplitude for the Malvinas Current near 40◦ S presented by Spadone and Provost (2009), which overlaps with the time series presented has no similarity in terms of annual or semiannual signals with the wavelet transform amplitude derived for the Brazil Current transport at 38◦ S
Summary
The circulation in the South Atlantic has been studied extensively because it is an important part of the Atlantic Meridional Overturning Circulation, which consists of a northward transport of relatively warm and fresh upper-ocean water of southern origin across the equator into the northern North Atlantic and a southward transport of relatively cold and salty deep water from the North Atlantic into the South Atlantic. An important challenge for Atlantic Meridional Overturning Circulation transport calculations is the estimation of the transport in the western boundary current (the Brazil Current in the subtropical South Atlantic) All estimates of this transport face the challenge of deriving the contributions on and often near the shelf break. This study will build on the earlier results with the focus on improving the knowledge about the mean transport of the Brazil Current and its variability In preparation for this analysis a monthly observations-based time series of three-dimensional fields of the horizontal velocity was derived.
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