Influence of Salinity and Temperature Gradients on the Variability of the North Brazil Undercurrent

Ingo Richter,Hao Liu,Xunwei Nie,Zexun Wei,Chuanshun Li

doi:10.3389/fmars.2021.744833

Abstract

The North Brazil Undercurrent (NBUC) is a narrow (&lt;1°) northward western boundary current in the tropical South Atlantic Ocean. It carries a large volume of water (&gt;16 Sv) and plays an important role in the Atlantic Meridional Overturning Circulation and the South Atlantic Subtropical Cell. Strong salinity and temperature fronts occur over the NBUC region. The role of temperature and salinity gradients on the genesis of NBUC variability has never been explored. This study uses three high-resolution (≤0.1°) and one low-resolution (=0.25°) model outputs to explore the linear trend of NBUC transport and its variability on annual and interannual time scales. We find that the linear trend and interannual variability of the geostrophic NBUC transport show large discrepancies among the datasets. Thus, the linear trend and variability of the geostrophic NBUC are associated with model configuration. We also find that the relative contributions of salinity and temperature gradients to the geostrophic shear of the NBUC are not model dependent. Salinity-based and temperature-based geostrophic NBUC transports tend to be opposite-signed on all time scales. Despite the limited salinity and temperature profiles, the model results are consistent with the in-situ observations on the annual cycle and interannual time scales. This study shows the relationship of salinity-based and temperature-based geostrophic NBUC variations in the annual and interannual variability and trend among different models and highlights the equal important roles of temperature and salinity in driving the variability of NBUC transport.

Highlights

The western tropical South Atlantic is a key conduit of the Atlantic Meridional Overturning Circulation (AMOC, Garzoli and Matano, 2011), involving a deep southward flow of cold and salty North Atlantic Deep Water (NADW) and compensating northward flows (Figures 1A,B) above the NADW
We showed that the correlation coefficients of density-based North Brazil Undercurrent (NBUC) geostrophic transport on the interannual time scales are small between models
We find that the density-based geostrophic NBUC transport integrated from the surface to 700 m is not very different from that averaged from the surface to 1,200 m (Figure 10), with temporal correlation coefficients higher than 0.9 in all models

Summary

Introduction

The western tropical South Atlantic is a key conduit of the Atlantic Meridional Overturning Circulation (AMOC, Garzoli and Matano, 2011), involving a deep southward flow of cold and salty North Atlantic Deep Water (NADW) and compensating northward flows (Figures 1A,B) above the NADW. Variability of the North Brazil Undercurrent coast, the surface flow has a weak northward or even southward expression (Figure 1B) due to the toward shore Ekman drift (Figure 12 from da Silveira et al, 1994). The surface and above the NADW, the flow is northward, and the maximum velocity core is at approximately 200 m with the magnitude estimated between 0.5 m/s (da Silveira et al, 1994) and 1 m/s (Dossa et al, 2021). The subsurface current between 5 and 11◦S is called the North Brazil Undercurrent (NBUC, Figures 1A,B). At approximately 4.8◦S, the NBUC merges with the central South Equatorial Current (a surface strengthened flow, Dossa et al, 2021), forming the North Brazil Current (NBC)

Methods

Findings

Discussion

Conclusion