Anatomy and evolution of a cyclonic mesoscale eddy observed in the northeastern Pacific tropical-subtropical transition zone

Abstract

In November 2005 off Cabo Corrientes, a cyclonic eddy was sampled from (i) remotely sensed data of sea level anomaly, near-surface wind, chlorophyll-a concentration and sea surface temperature and (ii) direct observations from a lowered Acoustic Doppler profiler, and a conductivity, temperature, and depth sensor. The sea level anomaly data set and an automated eddy detection method showed that this vortex formed near the Mexican coast and traveled westward/northwestward, ∼1000 km into the open ocean for ∼8 months. Surface winds and currents indicated that the cyclone was generated during a coastal upwelling event that occurred simultaneously with an equatorward flow located close to the coast. During its growing phase, the eddy extended from the surface to 750 m depth and mixed the water column in the first ∼100 m, showing an eastward shift of the eddy axis toward the sea surface of 1.5° of longitude, that generated a southward meridional heat transport in the upper layers. Integrated in the vertical, this heat transport accounted for a total of −105 × 1012 W, within the region of the eddy. The estimation of the available heat and salt anomaly contents revealed that the eddy transported relatively cold, salty, and anoxic waters associated with the Subtropical Subsurface water mass. While traveling offshore, the cyclonic eddy also exhibited relatively higher chlorophyll a concentrations at the sea surface than its surroundings. Comparison of some surface properties estimated from satellite and in situ data showed that satellite data tend to underestimate its amplitude and orbital geostrophic velocity.