Comment on "Eddy/Wind Interactions Stimulate Extraordinary Mid-Ocean Plankton Blooms"

doi:10.1126/science.1152111

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Science 25 April 2008:
Vol. 320. no. 5875, p. 448
DOI: 10.1126/science.1152111

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Technical Comments

Comment on "Eddy/Wind Interactions Stimulate Extraordinary Mid-Ocean Plankton Blooms"

Amala Mahadevan¹^*, Leif N. Thomas² and Amit Tandon³

¹ Department of Earth Sciences, Boston University, Boston, MA 02215, USA.
² Woods Hole Oceanographic Institution, Woods Hole, MA 02543, USA.
³ Physics Department and Department of Estuarine and Ocean Sciences, University of Massachusetts, Dartmouth, North Dartmouth, MA 02747, USA.

Fig. 1. The nonlinear Ekman effect generates upwelling and downwelling in a Northern hemisphere anticyclonic eddy, as schematically depicted. The Ekman transport in the surface layer is at 90 degrees to the right of the wind and inversely proportional to the net rotation of the fluid. The rotation of the eddy is anticyclonic and opposite to Earth's rotation. It reduces the net spin, (f + ${zeta}$ )/2, felt by the fluid toward the inside of the eddy. At the periphery, the shear between the eddy and ambient fluid generates a spin in the fluid that is in the same sense as Earth's rotation, thus enhancing the net spin of the fluid. Hence, the Ekman transport is enhanced on the inside of the eddy and weakened toward the outside. The divergence/convergence of the Ekman transport drives up/down motion as shown. The vertical motion associated with an anticyclonic eddy is greater than that with a cyclonic eddy of similar strength because decreasing the magnitude of the net rotation solicits a greater response than increasing it by the same amount. [View Larger Version of this Image (67K GIF file)]

Fig. 2. A snapshot from a numerical simulation of a front with a uniform westerly wind stress of 0.1 N/m² (wind speed $~$ 10 m/s) showing, in the top row, surface views of (A) density, (B) phytoplankton resulting from new production or the fresh supply of nutrients from beneath, and (C) phytoplankton that has been in the euphotic layer longer than 3 days or is formed from nutrients recycled within the euphotic layer. In (C), the phytoplankton and nutrients upwelled along the front are being entrapped in an anticyclonic eddy. In the lower row, a vertical section A-B through the eddying structure marked in (A) shows (D) the ratio of the relative to planetary vorticity ${zeta}$ /f, with dashed contours denoting density; (E) vertical velocity; and (F) phytoplankton. Contours of ${zeta}$ /f are overlaid in (E) and (F) to demonstrate that the largest vertical velocities are where the vorticity changes sign and thus result from submesoscale effects. The nonlinear Ekman effect results in upwelling and downwelling at the eddy's periphery, as depicted in Fig. 1. Although this simulation does not represent a specific coherent eddy, it demonstrates how submesoscale processes intensify vertical velocities and phytoplankton accumulates at the center of an anticyclonic eddy structure. [View Larger Version of this Image (74K GIF file)]