Zonal structure of the mean flow and eddies in the Azores Current system

Abstract

[1] The surface circulation and eddy field from the Azores Current system are studied here by analyzing surface drifters records and altimetry maps collected over more than 16 years. Clear differences in mean flow and eddy characteristics allow for a classification of the Azores Current in three zonal sectors: west of 30°W between the Mid-Atlantic Ridge and the Hyères-Atlantis seamount system, between 30°W and the longitude of Madeira, and east of Madeira. A detailed and quantitative characterization of each sector is given. The transition between the western and central parts is controlled by the Hyères-Atlantis ridge. In the transition to the eastern sector there is a change in the dynamics and the flow is forced by the cyclonic recirculation driven by the Mediterranean outflow. There is no clear expression of a continuous surface northern counterflow, but three distinct westward flows are seen: one along the southeastern border of the Azores plateau, another one southwest of Madeira, and a third one along 31°N west of the Hyères seamount. No clear tilt in the axis of the Azores Current is found in the calculated mean surface flow from surface drifters. Observations from both drifters and altimetry data show that anticyclonic features dominate in the northern region, whereas cyclonic ones dominate in the southern region. Overall, cyclones are more numerous, constituting 60% of large eddies. The eddy features detected in the west tend to be larger than those in the east, both in size and sea level anomaly. There is indication of different mean propagation speeds east and west of 25°W-30°W, with the mean speeds consistently increasing by 25%-45% in the west. Estimates for the rate of formation of strong eddies range from 1.4 to 2.4 year −1 for cyclones, and 1.2 to 1.7 year −1 for anticyclones.

The Azores Confluence Zone

“…Table 1 summarizes the physical characteristics of the farfield eddies that reached the archipelago. Although the eddy characteristics compare well with those calculated by Barbosa et al (2011), the Chelton and Schlax dataset has higher spatial (1/4 • ) and temporal resolution (daily) when compared with the 1 × 1 • results that are calculated every 4-days by Barbosa et al (2011). This resulted in the visual detection of 152 "loopers" (eddies), using Lagrangian drifters for a period of 18 years in a much broader region (Barbosa et al, 2011), compared with 459 eddies that reach the Azores detected using the newly processed altimetry data (Chelton and Schlax).…”

“…Although the eddy characteristics compare well with those calculated by Barbosa et al (2011), the Chelton and Schlax dataset has higher spatial (1/4 • ) and temporal resolution (daily) when compared with the 1 × 1 • results that are calculated every 4-days by Barbosa et al (2011). This resulted in the visual detection of 152 "loopers" (eddies), using Lagrangian drifters for a period of 18 years in a much broader region (Barbosa et al, 2011), compared with 459 eddies that reach the Azores detected using the newly processed altimetry data (Chelton and Schlax). Nevertheless, the Barbosa et al (2011) Lagrangian analysis (1990 can be considered as a representative sub-sample of the altimetry data (1992-2012) since it overlaps in time (16 years) and in space.…”

“…Results suggested that the west group of islands was mostly affected by north and eastward flow i.e., Gulf Stream; whereas the east group was strongly affected by a westward propagating flow. The latter is hypothesized to be the westward propagating eddies pinching-off from the Azores Current (previously reported by Pingree et al, 1999;Sangrá et al, 2009;Barbosa et al, 2011).…”

“…This resulted in the visual detection of 152 "loopers" (eddies), using Lagrangian drifters for a period of 18 years in a much broader region (Barbosa et al, 2011), compared with 459 eddies that reach the Azores detected using the newly processed altimetry data (Chelton and Schlax). Nevertheless, the Barbosa et al (2011) Lagrangian analysis (1990 can be considered as a representative sub-sample of the altimetry data (1992-2012) since it overlaps in time (16 years) and in space.…”

“…The oceanographic conditions in the Mid-Atlantic Ridge (MAR) region adjacent to the Azores Archipelago has been the subject of many scientific studies (e.g., Gould, 1985;Mailly et al, 1997;Alves and Verdière, 1999;Pingree et al, 1999;Alves et al, 2002;Juliano and Alves, 2007;Søliland et al, 2008;Barbosa et al, 2011;Lázaro et al, 2013). Fewer studies however, center on the oceanographic processes affecting the Azores Islands per se.…”