Anatomy of a subtropical intrathermocline eddy

Abstract

An interdisciplinary survey of a subtropical intrathermocline eddy was conducted within the Canary Eddy Corridor in September 2014. The anatomy of the eddy is investigated using near submesoscale fine resolution two-dimensional data and coarser resolution threedimensional data. The eddy was four months old, with a vertical extension of 500 m and 46 km radius. It may be viewed as a propagating negative anomaly of potential vorticity (PV), 95% below ambient PV. We observed two cores of low PV, one in the upper layers centered at 85 m, and another broader anomaly located between 175 m and the maximum sampled depth in the three-dimensional dataset (325 m). The upper core was where the maximum absolute values of normalized relative vorticity (or Rossby number), |Ro| = 0.6, and azimuthal velocity, U = 0.5 m s-1 , were reached and was defined as the eddy dynamical core. The typical biconvex isopleth shape for intrathermocline eddies induces a decrease of static stability, which causes the low PV of the upper core. The deeper low PV core was related to the occurrence of a pycnostad layer of subtropical mode water that was embedded within the eddy. The eddy core, of 30 km radius, was in solid body rotation with period of 4 days. It was encircled by a thin outer ring that was rotating more slowly. The kinetic energy (KE) content exceeded that of available potential energy (APE), KE/APE = 1.58; this was associated with a low aspect ratio and a relatively intense rate of spin as indicated by the relatively high value of Ro. Inferred available heat and salt content anomalies were AHA = 2.9 × 10 18 J and ASA = 14.3 × 10 10 kg, respectively. The eddy AHA and ASA contents per unit volume largely exceed those corresponding to Pacific Ocean intrathermocline eddies. This suggests that intrathermocline eddies may play a significant role in the zonal conduit of heat and salt along the Canary Eddy Corridor.

Phytoplankton Size Structure in Association with Mesoscale Eddies off Central-Southern Chile: The Satellite Application of a Phytoplankton Size-Class Model

“…There is also a particular type of anticyclonic eddy, called intrathermocline (ITE) or subsurface mode water eddy, which represents 30-55% of the anticyclonic eddy population in Eastern Boundary Current Systems (EBCS) [13]. These eddies (ITEs) are characterized by a typical radius of~20-60 km and have a vertical extent of~500 m, together with dome-shaped isopleths in the upper layers and a bowl shape in the lower layers, a minimum in oxygen, and high rates of phytoplankton productivity [14][15][16][17].…”

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

“…In the frame of the project PUMP (Study of the Vertical Oceanic Pump in mesoscale eddies), high-resolution in situ data were obtained from an anticyclonic intrathermocline eddy in the subtropical northeast Atlantic (Barceló-Llull et al, 2017b). The eddy was shed by the island of Tenerife in May 2014, and the survey took place 4 months later in a region 550 km to the southwest (Figure 1).…”

“…With these transects, they aimed to locate the eddy center and to make a first estimate of its radius and depth from density vertical sections with a precision of ±4 km, the averaged distance (20.95-19.30 • W, 25.33-26.87 • N) highlights the eddy region considering an eddy radius of 92 km derived from ARMOR3D ADT and density fields. Magenta box (20.54-19.71 • W, 25.71-26.49 • N) delimits the real eddy size considering an eddy radius of 46 km derived from the in situ high-resolution meridional section (gray triangles in the zoom) of density anomaly (Barceló-Llull et al, 2017b). The trajectory of the eddy center from its formation is obtained through an automated eddy tracker and is depicted by gray dots (the time interval between dots is 1 day); the blue dot shows the eddy center position on the date of the comparison (10-09-2014).…”

“…The eddy was shed by the island of Tenerife in May 2014, and the survey took place 4 months later in a region 550 km to the southwest (Figure 1). Guided by the eddy signal in sea level anomaly (SLA) maps provided by AVISO, Barceló-Llull et al (2017b) first crossed the eddy with two perpendicular transects sampled with continuous tows of a Conductivity Temperature Depth (CTD) probe on a SeaSoar (undulating vehicle). With these transects, they aimed to locate the eddy center and to make a first estimate of its radius and depth from density vertical sections with a precision of ±4 km, the averaged distance (20.95-19.30 • W, 25.33-26.87 • N) highlights the eddy region considering an eddy radius of 92 km derived from ARMOR3D ADT and density fields.…”

“…The objective of the present study is to characterize this underestimation using 3D high-resolution in situ CTD and horizontal current observations obtained from the intensively sampling of an anticyclonic intrathermocline eddy (ITE) inside the Canary Eddy Corridor in the northeast Atlantic (Sangrà et al, 2009;Barceló-Llull et al, 2017b). Barceló-Llull et al (2017a) used these 3D in situ fields to infer the QG vertical velocity distribution within the anticyclonic intrathermocline eddy, as well as the vertical velocity obtained from the integration of a generalized omega equation valid for high Rossby numbers which includes the forcing contribution of the ageostrophic horizontal velocities.…”