Advanced insights into sources of vertical velocity in the ocean

Giordani, Hervé; Prieur, Louis; Caniaux, Guy

doi:10.1007/s10236-005-0050-1

Cited by 75 publications

(86 citation statements)

References 23 publications

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“…Other processes-such as wind-induced frictional forces, diabatic processes, time-dependent near-and super-inertial motions, or higher-order corrections to the quasigeostrophic omega equation-could also generate vertical motion (Viú dez et al 1996;Giordani et al 2006;Nagai et al 2006;Thomas et al 2010). For example, a spatially uniform wind blowing over a current with lateral variations in vertical vorticity will generate Ekman pumping/suction owing to the modification of the Ekman transport by the vorticity (Stern 1965;Niiler 1969).…”

Section: February 2010 N O T E S a N D C O R R E S P O N D E N C Ementioning

confidence: 99%

Subduction on the Northern and Southern Flanks of the Gulf Stream

Thomas

Joyce

2010

Journal of Physical Oceanography

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Sections of temperature, salinity, dissolved oxygen, and velocity were made crossing the Gulf Stream in late January 2006 to investigate the role of frontal processes in the formation of Eighteen Degree Water (EDW), the Subtropical Mode Water of the North Atlantic. The sections were nominally perpendicular to the stream and measured in a Lagrangian frame by following a floating spar buoy drifting in the Gulf Stream's warm core. During the survey, EDW was isolated from the mixed layer by the stratified seasonal pycnocline, suggesting that EDW was not yet actively being formed at this time in the season and at the longitudes over which the survey was conducted (648-708W). However, in two of the sections, the seasonal pycnocline in the core of the Gulf Stream was broken by an intrusion of cold, fresh, weakly stratified water, nearly saturated in oxygen, that appears to have been subducted from the surface mixed layer north of the stream. The intrusion was identified in three of the sections in profiles with a nearly identical temperature-salinity relation. From the western-toeasternmost sections, where the intrusion was observed, the depth of the intrusion's salinity minimum descended by ;90 m in the 71 h it took to complete this part of the survey. This apparent subduction occurred primarily on the upstream side of a meander trough, where the cross-stream velocity was confluent and frontogenetic. Using a variant of the omega equation, the vertical velocity driven by the confluent flow was inferred and yielded downwelling in the vicinity of the intrusion spanning 10-40 m day 21, a range of values consistent with the intrusion's observed descent, suggesting that frontal subduction was responsible for the formation of the intrusion. In the easternmost section located downstream of the meander trough, the flow was diffluent, driving an inferred vertical circulation that was of the opposite sense to that in the section upstream of the trough. In transiting the two sides of the trough, the intrusion was observed to move toward the center of the stream between the downwelling branches of the opposing vertical circulations, resulting in a downward Lagrangian mean vertical velocity and net subduction. Hydrographic evidence of the subduction of weakly stratified surface waters was seen in the southern flank of the Gulf Stream as well. The solution of the omega equation suggests that this subduction was associated with a relatively shallow vertical circulation confined to the upper 200 m of the water column in the proximity of the front marking the southern edge of the warm core.

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Section: February 2010 N O T E S a N D C O R R E S P O N D E N C Ementioning

confidence: 99%

Subduction on the Northern and Southern Flanks of the Gulf Stream

Thomas

Joyce

2010

Journal of Physical Oceanography

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“…The thermocline is diagnosed as the first level from the top of the ocean that experiences a density difference larger than 0.02 kg m −3 from the reference ocean level (5 m). This method has been applied successfully in the past by Paci et al (2005) and Giordani et al (2005Giordani et al ( , 2006 the POMME experiment datasets (Weill et al, 2003;Caniaux et al, 2005aCaniaux et al, , 2005bMémery et al, 2005).…”

Section: The One-dimensional Ocean Modelmentioning

confidence: 99%

“…In our study, MESO-NH is coupled to a one-dimensional ocean model (Gaspar et al, 1990) that has been shown to represent relatively well the vertical mixed layer structure and the SST evolution (Blanke and Delecluse, 1993;Alexander et al, 2000). Several ocean turbulence models are based on the Gaspar et al (1990) model equations (D'Alessio et al, 1998;Giordani et al, 2006). Using this air-sea coupled modelling system, the aim of the present article is to examine the ocean response to the high-resolution atmospheric forcing imposed during Mediterranean heavy precipitation events.…”

Section: Introductionmentioning

confidence: 99%

Two‐way one‐dimensional high‐resolution air–sea coupled modelling applied to Mediterranean heavy rain events

Brossier

Ducrocq

Giordani

2008

Quart J Royal Meteoro Soc

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South-eastern France is prone to heavy rain events during the autumn. For these extreme precipitation events, the Mediterranean Sea fuels the atmospheric boundary layer in heat and moisture and sometimes contributes to flooding, owing to the large swell and waves produced in these situations. The aim of this study is to examine how the severe atmospheric conditions over the sea associated with these events alter the ocean mixed layer, and what feedback the ocean contributes to the precipitation events.To address these questions, a high-resolution air-sea coupled system is developed between the atmospheric MESO-NH model and a one-dimensional ocean model. It is applied for short range (24 h) and high-resolution (2-3 km) simulations of three representative torrential rainfall events: 12-13 November 1999 (Aude case), 8-9 September 2002 (Gard case) and 3 December 2003 (Hérault case). In those meteorological situations characterized by moderate to intense low-level winds, the Mediterranean Sea globally loses energy, to the benefit of the convective precipitating systems. The result is an overall decrease of the thermal content all along the simulation of the events. Significant cooling and deepening of the ocean mixed layer are found in the areas of intense low-level winds. A notable result of the study concerns the impact of the torrential rainfall on the ocean mixed layer. The most important disturbances of the ocean mixed layer are indeed found underneath the heavy precipitation. The salt content is decreased all along the ocean mixed layer depth, but more significantly in the first ten metres near the air-sea interface, with the formation of a salt barrier.By performing both two-way and one-way coupled simulations, it is found that the interactive coupling tends to moderate both the atmospheric and ocean responses compared with the one-way mode. The differences between the two-way and one-way coupled simulations are, however, found to be relatively small considering the atmospheric short-range forecast.

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“…For Ro = 0.1, a typical value for mesoscale eddies, the nonlinear Ekman effect would be about 10 times as important for the windinduced vertical circulation as stress asymmetry from the air-sea velocity difference. Further, the nonlinear Ekman effect would be greatly enhanced in regions where the relative vorticity is locally intensified and is coupled with lateral density variations (3,6). High-resolution modeling studies show that the largest relative vorticity in eddying flows occurs in filament-like features along fronts and at the edges of eddies, rather than at eddy centers (4, 13), and routinely equals or exceeds the planetary vorticity in magnitude, bringing submesoscale effects into play.…”

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confidence: 99%

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

Mahadevan

Thomas

Tandon

2008

Science

150

149

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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...

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Advanced insights into sources of vertical velocity in the ocean

Cited by 75 publications

References 23 publications

Subduction on the Northern and Southern Flanks of the Gulf Stream

Subduction on the Northern and Southern Flanks of the Gulf Stream

Two‐way one‐dimensional high‐resolution air–sea coupled modelling applied to Mediterranean heavy rain events

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

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