Occurrence and characteristics of mesoscale eddies in the tropical northeastern Atlantic Ocean

Abstract

Abstract. Coherent mesoscale features (referred to here as eddies) in the tropical northeastern Atlantic Ocean (between 12-22 • N and 15-26 • W) are examined and characterized. The eddies' surface signatures are investigated using 19 years of satellite-derived sea level anomaly (SLA) data. Two automated detection methods are applied, the geometrical method based on closed streamlines around eddy cores, and the Okubo-Weiß method based on the relation between vorticity and strain. Both methods give similar results. Mean eddy surface signatures of SLA, sea surface temperature (SST) and sea surface salinity (SSS) anomalies are obtained from composites of all snapshots around identified eddy cores. Anticyclones/cyclones are identified by an elevation/depression of SLA and enhanced/reduced SST and SSS in their cores. However, about 20 % of all anticyclonically rotating eddies show reduced SST and reduced SSS instead. These kind of eddies are classified as anticyclonic mode-water eddies (ACMEs). About 146 ± 4 eddies per year with a minimum lifetime of 7 days are identified (52 % cyclones, 39 % anticyclones, 9 % ACMEs) with rather similar mean radii of about 56 ± 12 km. Based on concurrent in situ temperature and salinity profiles (from Argo float, shipboard, and mooring data) taken inside of eddies, distinct mean vertical structures of the three eddy types are determined. Most eddies are generated preferentially in boreal summer and along the West African coast at three distinct coastal headland regions and carry South Atlantic Central Water supplied by the northward flow within the Mauretanian coastal current system. Westward eddy propagation (on average about 3.00 ± 2.15 km d −1 ) is confined to distinct zonal corridors with a small meridional deflection dependent on the eddy type (anticyclones -equatorward, cyclones -poleward, ACMEs -no deflection). Heat and salt fluxes out of the coastal region and across the Cape Verde Frontal Zone, which separates the shadow zone from the ventilated subtropical gyre, are calculated.

Impact of mesoscale eddies on water mass and oxygen distribution in the eastern tropical South Pacific

“…The sum of all these boxes is taken to compute the seasonal cycle of eddy generation. The Argo float profiles and the mooring time series are separated into data conducted within cyclones, anticyclones and the "surrounding area" which is not associated with eddylike structures also following the approach of Schütte et al (2016a). In addition the relative position of 25 the mooring or Argo float profile in relation to the eddy center and eddy boundary could be computed.…”

“…Eddies are tracked forward and backward in time following the approach described by Schütte et al (2016a). To estimate the percentage of eddy coverage in the ETSP eddies are identified and tracked between 1993 and 2015.…”

Bathypelagic particle flux signatures from a suboxic eddy in the oligotrophic tropical North Atlantic: production, sedimentation and preservation

“…Benitez-Nelson and McGillicuddy, 2008). The eddies form in spring and summer at the African coast, in the area between Cape Blanc and Cape Vert, Senegal, and propagate westward at about 5 km per day (Schütte et al, 2016a). Some of the eddies, in particular the ACMEs, exhibit low dissolved oxygen (DO) concentrations at very shallow depth (< 40 m; Karstensen et al, 2015).…”

“…1), about 100 km north of the Cabo Verde island of São Vicente, is located in such an eddy corridor. Considering rotation as well as the vertical structure of eddies, three types may be distinguished (Schütte et al, 2016a): cyclonic, anticyclonic, and anticyclonic modewater eddies (ACMEs). In particular, ACMEs have been reported in the past as supporting high productivity and chlorophyll standing stock, primarily related to a very shallow mixed layer base in the eddy and the efficiency in vertical transport of nutrients into the euphotic zone (McGillicuddy et al, 2007;Karstensen et al, 2016).…”

“…In the quiescent shadow zone region of the eastern tropical North Atlantic (Luyten et al, 1983), mesoscale eddies originate mostly from energetic flow in the coastal/open ocean transition zone of the West African coast. After formation, the eddies propagate westward into the open North Atlantic, typically at certain latitudes which may be considered to be eddy corridors (Schütte et al, 2016a). The CVOO mooring site ( Fig.…”