Lagrangian study of the Panama Bight and surrounding regions

Abstract

Near-surface circulation of the Panama Bight and surrounding regions [0-9°N; 73°W-90°W] was studied using satellite-tracked drifter trajectories from 1979-2004. This region encompasses three major currents showing typical velocities of ∼30 cm s-1: (1) the eastward North Equatorial Counter Current (NECC), (2) the near-circulat Panama Bight Cyclonic Gyre. (PBCG), and (3) the westward South Equatorial Current (SEC). We do not observe significant modification of the mean surface circulation during El Niño Southern Oscillation events, even if the SEC is slightly reinforced during relatively warm El Niño periods. At seasonal scales, the circulation is strongly controlled by the activity of the Panama wind-jet: in boreal winter, the currents are stronger and an anticyclonic cell is present west of the PBCG. This dipole leads to a strong ∼206 km wide southward current which then disappears during the rest of the year. In summer, the three major currents have reduced intensity by 30%-40%. Large-scale current vorticity shows that the upwelling associated with the PBCG is also 3-4 times stronger in winter than during summer months. The kinetic energy is largely dominated by eddy activity and its intensity is double in winter than during summer. Ageostrophic motions and eddy activity appear to have a substantial impact on the energy spatial distribution. In the NECC and SEC regions, Lagrangian scales are anisotropic and zonally enhanced in the direction of the mean currents. The typical integral time and length scales of these regions are 2.5 days and 50-60 km in the zonal direction and 1.5 days and 25-30 km in the meridional direction. Lateral eddy diffusivity coefficients are on the order of 11-14 107 cm2 s-1 zonally and 5-6 107 cm2 s-1 meridionally. In contrast, in the PBCG region, the Lagrangian characteristics are isotropic with typical timescales of 1.7 days, space scales of 30 km and eddy diffusivity coefficieiits of 6 107 cm2 s-1 in both directions. Copyright 2006 by the American Geophysical Union.