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

Abstract

McGillicuddy et al. (Reports, 18 May 2007, p. 1021 proposed that eddy/wind interactions enhance the vertical nutrient flux in mode-water eddies, thus feeding large mid-ocean plankton blooms. We argue that the supply of nutrients to ocean eddies is most likely affected by submesoscale processes that act along the periphery of eddies and can induce vertical velocities several times larger than those due to eddy/wind interactions.H ow do eddies, such as those described in McGillicuddy et al. (1), sustain their extraordinary concentrations of phytoplankton and biological productivity in an ocean whose surface is bereft of nutrients? As an explanation, McGillicuddy et al. invoke the mechanism of eddy/wind interaction (2), whereby the difference in the relative air-water velocity (and, consequently, wind stress) felt on diametrically opposite sides of an anticyclonic eddy, induces an upward Ekman pumping velocity. McGillicuddy et al. assert that the upward velocity, on the order of about 1 m/day at the eddy center, supports the nutrient flux to sustain the observed productivity.Here, we point out that submesoscale effects (3-5), which include intensification of the ageostrophic secondary circulation (ASC) (6) and nonlinear Ekman transport (7-10), can result in vertical velocities on the order of 10 to 100 m/day. These velocities are 10 to 100 times as large as the linear Ekman pumping velocity due to the eddy/wind interaction mechanism. Submesoscale effects come into play for flows whose relative vorticity z, defined as the curl or rotary component of the horizontal velocity field, is not much smaller in magnitude than the planetary vorticity f, arising from Earth's rotation. At ocean eddies and fronts, the quantity z/f, known as the Rossby number (Ro), typically takes on values of 0.1 to 1.0. For such flows, the loss of geostrophy, the balance between pressure gradient and Coriolis effects, is restored by an overturning circulation across lateral density variations in the presence of straining. The strength of the overturning at a front, as described by the semigeostrophic Sawyer-Eliassen equation (11), continues to grow as the front intensifies until limited by mixing. Such submesoscale intensification is typically manifest on horizontal length scales on the order of 1 to 10 km. A further effect of the relatively large relative vorticity z is that the windforced horizontal Ekman mass transport, M E = −t/[r( f +z)], depends on the net (i.e., planetary plus relative) vorticity of the flow, ( f + z) (12). Consequently, lateral variations in the relative vorticity can result in a modulation of the Ekman transport, the divergence of which drives vertical motions even if the wind stress t is spatially uniform (Fig. 1).To quantify the relative contributions of the nonlinear Ekman effect and eddy/wind interaction on the induction of vertical motions, we derived the ratio of scalings for their respective vertical velocities (see Supporting Online Material) as Ro (u a /u o ), where u a is the wind speed, u o is the maximum az...

Imprint of Southern Ocean eddies on chlorophyll

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Open ocean dead zones in the tropical North Atlantic Ocean

“…In particular, intense phytoplankton blooms have been reported for cyclonic and anticyclonicmodewater eddies (Mahadevan et al, 2008;McGillicuddy et al, 2007;Chelton et al, 2011a;Gaube et al, 2014). Phytoplankton blooms are likely important for creating a low-DO zone, because subsequent sinking of detritus is accompanied by oxygen consumption.…”

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“…Chlorophyll a (Chl a) concentration deduced from satellite data (O'Reilly et al 1998, Nieves et al 2007) and from fluorometers (Coble et al 2007) in oceanographic campaigns is normally used as a proxy for ocean primary production. Eddies, sub-mesoscale structures (McGillicuddy et al 2007, Mahadevan et al 2008, McGillicuddy et al 2008) and vertical veloci-ties in upwelling areas (Estrada et al 1985, GEOHAB 2005, Patti et al 2008 can have a determinant role in Chl blooms. The role played by vertical velocities in modulating the biological activity through phytoplankton growth, mainly by bringing nutrients to the photic zone, has been long recognized (McGillicuddy et al 2008).…”