Subregional characterization of mesoscale eddies across the Brazil‐Malvinas Confluence

Abstract

Horizontal and vertical motions associated with coherent mesoscale structures, including eddies and meanders, are responsible for significant global transports of many properties, including heat and mass. Mesoscale vertical fluxes also influence upper ocean biological productivity by mediating the supply of nutrients into the euphotic layer, with potential impacts on the global carbon cycle. The Brazil‐Malvinas Confluence (BMC) is a western boundary current region in the South Atlantic with intense mesoscale activity. This region has an active role in the genesis and transformation of water masses and thus is a critical component of the Atlantic meridional overturning circulation. The collision between the Malvinas and Brazil Currents over the Patagonian shelf/slope creates an energetic front that translates offshore to form a vigorous eddy field. Recent improvements in gridded altimetric sea level anomaly fields allow us to track BMC mesoscale eddies with high spatial and temporal resolutions using an automated eddy tracker. We characterize the eddies across fourteen 5° × 5° subregions. Eddy‐centric composites of tracers and geostrophic currents diagnosed from a global reanalysis of surface and in situ data reveal substantial subregional heterogeneity. The in situ data are also used to compute the evolving quasi‐geostrophic vertical velocity (QG‐ω) associated with each instantaneous eddy instance. The QG‐ω eddy composites have the expected dipole patterns of alternating upwelling/downwelling, however, the magnitude and sign of azimuthally averaged vertical velocity varies among subregions. Maximum eddy values are found near fronts and sharp topographic gradients. In comparison with regional eddy composites, subregional composites provide refined information about mesoscale eddy heterogeneity.

Malvinas Current Volume Transport at 41°S: A 24 Yearlong Time Series Consistent With Mooring Data From 3 Decades and Satellite Altimetry

“…In contrast, during transport minima, the SAF was displaced southward, back on the mooring line in extreme cases, as large and intense (>30 cm) positive sea level anomalies shed by the overshoot managed to propagate westward onto the slope. The sea level anomaly propagation velocities (6 km/d along a northwest path closely following the 4,000 m isobaths in the southern Argentine Basin leading to transport maxima and 2–3 km/d westward in the highest EKE region of the Confluence about 41°S–43°S leading to transport minima) are similar to estimates from Fu () and Mason et al (). Regardless of the sign of the intense MC transport anomalies, the separation of the BCF from the 1,000 m isobaths remained at the same location, in agreement with Saraceno et al ().…”

“…Hovmöller diagrams of SLA along a section parallel to the track for two time periods that precede transport minima (Figures c and d) suggest a slow northwestward propagation of large positive SLA toward the slope. The propagation speed of the positive anomaly is slow (about 2–3 and 1 km/d), in agreement with mesoscale westward propagation speed in the vicinity of the BMC estimates from Fu () and Mason et al (). Overall, it is difficult to trace anomalies back further in space and time in this extremely high eddy kinetic energy environment, and the development of the positive anomaly may differ from one transport minimum to the other.…”

“…The propagation speed is about 6 km/d. This is the same order of magnitude as mesoscale eddy northward propagation speed along the 4,000 m isobath estimated by Fu () and Mason et al (). The cyclonic anomalies are traced back to the south of the Argentine Basin.…”

Comparing a Multivariate Global Ocean State Estimate With High-Resolution in Situ Data: An Anticyclonic Intrathermocline Eddy Near the Canary Islands

“…Figures 2, 3 highlight the difference in eddy sizes with each data set. In order to better compare horizontal distributions of density using the in situ 3D fields, in Figure 4 we perform a normalization of the horizontal axes using the respective estimated eddy radii: r is = 46 km for the PUMP eddy, and r ar = 2r is = 92 km from ARMOR3D (Chelton et al, 2011;Gaube et al, 2013Gaube et al, , 2014Gaube et al, , 2015Gaube and McGillicuddy, 2017;Mason et al, 2017). The in situ and ARMOR3D eddies have similar elliptical shapes of the isopycnals with the major axis oriented NW-SE in both data sets (Figure 4).…”

“…Pascual et al (2015) and Barceló-Llull et al (2016) derive the QG mesoscale vertical velocity from the ARMOR3D fields in order to analyze its distribution in different regions of the global ocean as well as its contribution to nutrient advection and phytoplankton growth. Mason et al (2017) construct eddy-centric composites of tracers, geostrophic currents and QG vertical velocity diagnosed from ARMOR3D in the energetic Brazil-Malvinas Confluence region. Through subregional composites they are able to reveal and analyze mesoscale eddy heterogeneity.…”