An offshore eddy in the California current system Part I: Interior dynamics

Abstract

--From January 9 to 17, 1981, detailed observations of the horizontal and vertical structure beneath one of the quasi-permanent semi-stationary mesoscale offshore eddy signatures in the California Current System (CCS) discussed by Bernstein, Breaker and Whritner (1977), Burkov andPavlova (1980), andSimpson (1982) were made. The vertical sections of temperature and density show the presence of a three-layer system. A subsurface warm-core eddy, whose diameter is about 150 km at the 7°C isotherm, is the dominant feature. A warm surface layer, which extends to a depth of 75 m, lies over the eddy. Between the warm surface layer and the subsurface warm-core eddy, there is a cold-core region which extends to a depth of about 200 m. There is a high degree of symmetry about the vertical axis of rotation. Vertical sections of salinity and dissolved oxygen are entirely different from sections of temperature and density. Diagrams of water mass characteristics confirm that the core of the eddy, found between 250-600 m, consists of inshore water from the California Undercurrent (CU). Below about 700 m, local waters from the Deep Poleward Flow (DPF) have been incorporated into the eddy. The observed distributions of properties (T, S, o-0, 02) are inconsistent with a single, local generation process for the eddy system. Radial distributions of angular velocity, normalized gradient velocity and relative vorticity support the use of a Gaussian radial height field as an initial condition in eddy models. Possible reasons why CCS eddies may differ dynamically from Gulf Stream rings are given in the text. At the time the observations were made, the system as a whole was in near geostrophic balance. Local geostrophic balance, however, cannot explain the observed distribution of properties and structure. The observed symmetry in the structure of the eddy system, chemical evidence (Simpson, 1984), biological distributions (Haury, 1984) and satellite images of the CC (Koblinsky, Simpson and Dickey, 1984) suggest that lateral entrainment of warm (oceanic) and cold (coastal) water into the upper two layers of the three-layer system by the subsurface eddy is a likely generation mechanism for the cold-core region. The coastal origin of the frontal structure along the northeastern quadrant and the oceanic origin of the frontal structure along the southwestern quadrant of the eddy system further support lateral entrainment as a generation mechanism for the cold core. This entrainment makes the CCS eddy system different from cold-core rings in the Gulf Stream and rather similar to some warm-core eddies found in the East Australian Current. The presence of CU water in the core of this eddy raises the question of how CU water was transported from the continental slope. Eddy generation mechanisms, other than baroclinic instability of the CC, may be required to explain the distribution, persistence, and core composition of offshore mesoscale eddies in the CCS. There is evidence that barotropic, in addition to baroclinic, processes may be i...

Main eddy vertical structures observed in the four major Eastern Boundary Upwelling Systems

“…For the PCUS and the CALUS in the Pacific basin, the observed eddy-types were close to those previously documented. In both of these upwelling systems, the presence of strong subsurface-intensified anticyclones, carrying warm and salty water from their nearcoastal poleward undercurrents, are well sampled by the Argo floats and match the previous observations [Simpson et al, 1984;Johnson and McTaggart, 2010;Chaigneau et al, 2011;Kurian et al, 2011;Colas et al, 2012;Collins et al, 2013]. However, we also highlighted the presence of weak surface-intensified anticyclones in the PCUS that represent 45% of the sampled eddies.…”

“…These sites have been previously recognized as hot spots for eddy generation [Chaigneau et al, 2009;Kurian et al, 2011]. The observed negative S 0 (Figure 8c) is probably due to the local eddy-induced downwelling of the surface layers composed of fresh Pacific Subarctic Water [Simpson et al, 1984;Emery and Meincke, 1986;Huyer et al, 1991;Checkley and Barth, 2009;Dong et al, 2012] that can be clearly identified in Figure 9a (color shading). The temporal evolution of the corresponding composite anomalies is consistent with their mean profiles and their intensity does not present strong variations between the growth, mature and decay phases (Figures 10a-10c).…”

“…by the California Current [Simpson et al, 1984;Emery and Meincke, 1986;Huyer et al, 1991;Checkley and Barth, 2009;Dong et al, 2012]. Both the shoaling of the isopycnals associated with these surface-intensified eddies and the advection of saltier near-coastal water across fresher offshore waters can lead to this observed positive S 0 .…”

Spatial and temporal variation of natural bacterioplankton assemblages studied by total genomic DNA cross‐hybridization

“…2) indicates a surface mixed layer down to -20-30 m, which was typical over the course of the cruise. The studied area is close to the western boundary of the California Current System, which can be divided into surface and subsurface waters typically at -200-m depth by characteristic constituent water masses (Simpson et al 1984 and references therein).…”

Recurrent patterns in surface thermal fronts associated with cold filaments along the West Coast of North America

“…5-8) contain surface signatures of cold filaments and mesoscale eddies. Complex mixing of inshore and oceanic waters occurs in the transition zone as evidenced by water property distributions (e.g., Simpson et al, 1984;Simpson and Lynn, 1990), as well as chemical distributions (Simpson, 1984) and biological populations (Haury, 1984). Visual interpretation and quantification of thermal fronts in such imagery, however, is difficult because of the overall complexity of the California Current system.…”