Fig 1.
Dipolar circulation traced by surface drifters during weak Tehuano conditions.
Red (blue) trajectories show an anticyclonic (cyclonic) eddy. Vectors show the average wind field and the associated variance ellipses for 20 days (from 25 June to 15 July 2000). The gray scale (solid grey lines) shows the topography (bathymetry) of the region (m) from GEBCO dataset (GEBCO_08 Grid, version 20100927: http://www.gebco.net). Drifter data was obtained from the Lagrangian experiment performed by Trasvina and Barton [9] and the wind from the Cross-Calibrated Multi-Platform (CCMP).
Fig 2.
(A) Trajectories of 30 drifters released in the Gulf of Tehuantepec and (B) their lifetimes.
Fig 3.
Four-day average of the wind (black vectors), geostrophic current (gray vectors), and Ekman pumping velocity (Wtot) fields (red-blue scale) for 24 days covering periods of the development and erosion of the first dipole traced mainly by drifter trajectories. The variability ellipses were computed from the standard deviation of the wind velocity components. (A) Black dots indicate the initial drifter positions, with most of the drifters starting on 26 Jun. (B) Dipole development under Tehuano wind conditions. (C) The cyclonic side of the dipole begins to erode. (D) The erosion of the cyclonic side of the dipole is complete. (E-F) The anticyclonic eddy strengthens. (G) Wind speed along a section of the GT (~94–95°W, 15°N). Tehuano wind events (shading) and vertical dashed lines indicate periods corresponding to the panels (A-F). Drifter speed is shown in m s-1.
Fig 4.
Four-day average of the wind (black vectors), geostrophic current (gray vectors), and Ekman pumping velocity (Wtot) fields (red-blue scale) for 24 days covering the triggering and propagation of the second dipole. The variability ellipses were computed from the standard deviation of the wind velocity components. (A-B) Under Tehuano wind conditions, the anticyclonic eddy forms (~96° W, ~15° N) while the cyclonic eddy is maintained in the eastern portion of the GT. (C) The dipole develops, and one drifter traces it. (D) Toward the end of the Tehuano wind period, the dipole is fully developed. (E-F) The vortices of the dipole propagate. (G) Wind speed along a section of the GT (~94–95°W, 15°N). Tehuano wind events (shading) and vertical dashed lines indicate periods corresponding to the panels (A-F). The drifter speed is indicated in m s-1.
Fig 5.
Four-day average of the wind (black vectors), geostrophic current (gray vectors), and Ekman pumping velocity (Wtot) fields (red-blue scale) for 24 days covering the triggering and evolution of the third dipole. The variability ellipses were computed from the standard deviation of the wind velocity components. (A) Under stronger Tehuano wind conditions (~12 m s-1), the dipole is fully developed. (B-C) Tehuano winds are persistent, and the vortices are strengthened and propagated. (D) With minimum wind speeds, the anticyclonic eddy is located to the southwest of the GT while the cyclonic eddy covers most of the GT. (E) The wind re-intensifies, and closed cyclonic circulation breaks down. (F) After which, only the anticyclonic eddy persists. (G) Wind speed along of a section of the GT (~94–95°W, 15°N). Tehuano wind events (shading) and vertical dashed lines indicate periods corresponding to the panels (A-F). The drifter speed is shown in m s-1.
Fig 6.
Dipole development and evolution in the GT: drifters and geostrophic currents.
(A-C). Dipole eddies are indicated by the letter A (C) for anticyclonic (cyclonic) eddies followed by the corresponding dipole number. The first dipole was identified by visual inspection, based on the trajectories of surface drifters (red/blue vectors for A/C) and geostrophic currents, as is showed in A. The anticyclonic eddy was fully developed and detected by the Nencioli Algorithm on Jun 05. The second and third dipoles were detected by the Nencioli Algorithm on the indicated date. The properties of the A and C eddies are as follows: (D-F) kinetic energy, (G-I) relative vorticity, (J-L) size, and (M-O) eddy translation speed. (P-R) Wind speed along a section of the GT (~94-95° W, 15° N), the shaded areas indicate Tehuano wind periods. The maximum value of each parameter normalized the eddy properties.
Fig 7.
Vertical velocities inside the eddies of the second dipole are shown under Tehuano wind conditions.
(A-E) The spatial structure for the linear and (F-J) non-linear components of Ekman pumping, and (K-O) the total vertical velocity inside eddies are shown in m d-1. The geostrophic (wind) field is shown with gray (black) vectors. The eddies have been normalized by their radii.
Fig 8.
The azimuthal average for an anticyclonic (cyclonic) eddy is shown for every component of Ekman pumping for three Tehuano wind stages: (A-B) under the influence of Tehuano winds, (C-D) the wind relaxation period, and (E-F) post-Tehuano conditions (see Fig 4). Bars indicate the standard deviation for Wtot.
Fig 9.
Global Ekman pumping inside eddies: (A) eddy A1, (B) eddy A2, (C) eddy A3, (D) eddy C2, and (E) eddy C3 as calculated from Eq 5. (F) Time series of the wind speed in the region of maximum intensity (~94–95°W, 15°N) where shaded areas indicate periods of Tehuano winds. The global Ekman pumping (Wtot) inside eddies was estimated during the Tehuano wind event and indicated by parentheses.