Upwelling limitation by onshore geostrophic flow

Marchesiello, Patrick; Estrade, Philippe

doi:10.1357/002224010793079004

Cited by 99 publications

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“…Our results show that compensation of coastal upwelling by cross-shore geostrophic currents is an important process, which needs to be taken into account [Colas et al, 2008;Marchesiello and Estrade, 2010]. In all of our simulations, the onshore geostrophic velocity plays an important role all year round and compensates up to one third of the Ekman transport ( Figure 11).…”

Section: Discussionmentioning

confidence: 76%

Peru‐Chile upwelling dynamics under climate change

et al. 2015

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The consequences of global warming on the Peru-Chile Current System (PCCS) ocean circulation are examined with a high-resolution, eddy-resolving regional oceanic model. We performed a dynamical downscaling of climate scenarios from the IPSL-CM4 Coupled General Circulation Model (CGCM), corresponding to various levels of CO 2 concentrations in the atmosphere. High-resolution atmospheric forcing for the regional ocean model are obtained from the IPSL atmospheric model run on a stretched grid with increased horizontal resolution in the PCCS region. When comparing future scenarios to preindustrial (PI) conditions, the circulation along the Peru and Chile coasts is strongly modified by changes in surface winds and increased stratification caused by the regional warming. While the coastal poleward undercurrent is intensified, the surface equatorial coastal jet shoals and the nearshore mesoscale activity are reinforced. Reduction in alongshore wind stress and nearshore wind stress curl drive a year-round reduction in upwelling intensity off Peru. Modifications in geostrophic circulation mitigate this upwelling decrease in late austral summer. The depth of the upwelling source waters becomes shallower in warmer conditions, which may have a major impact on the system's biological productivity.

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Section: Discussionmentioning

confidence: 76%

Peru‐Chile upwelling dynamics under climate change

et al. 2015

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confidence: 99%

“…Currently, there is no generalized conceptual model for the upwelling structure that considers the region near the coast, the coastal boundary and the open ocean (Mellor, 1986;Marchesiello and Estrade, 2010). Traditionally a simple relationship based on wind stress along the coast has been used as an index of the coastal upwelling intensity (Bakun, 1973); this approximation does not consider other more complex physical processes, such as the wind curl (Pickett and Paduan, 2003;Capet et al, 2004;Jacox and Edwards, 2012) and the geostrophic flow toward the coast, which is in balance with the alongshore pressure gradient and could potentially limit upwelling Marchesiello and Estrade, 2010). In the case of the wind curl, several modeling studies from different upwelling systems suggest that wind stress decreases within a narrow coastal band of 10-80 km called wind "drop-off" (Capet et al, 2004;Bane et al, 2005;Perlin et al, 2007;Renault et al, 2012Renault et al, , 2015 that is highly sensitive to the resolution of the model.…”

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confidence: 99%

Seasonal variability of the Ekman transport and pumping in the upwelling system off central-northern Chile (∼  30° S) based on a high-resolution atmospheric regional model (WRF)

et al. 2016

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Abstract. Two physical mechanisms can contribute to coastal upwelling in eastern boundary current systems: offshore Ekman transport due to the predominant alongshore wind stress and Ekman pumping due to the cyclonic wind stress curl, mainly caused by the abrupt decrease in wind stress (drop-off) in a cross-shore band of 100 km. This wind drop-off is thought to be an ubiquitous feature in coastal upwelling systems and to regulate the relative contribution of both mechanisms. It has been poorly studied along the central-northern Chile region because of the lack in wind measurements along the shoreline and of the relatively low resolution of the available atmospheric reanalysis. Here, the seasonal variability in Ekman transport, Ekman pumping and their relative contribution to total upwelling along the central-northern Chile region ( ∼ 30 • S) is evaluated from a high-resolution atmospheric model simulation. As a first step, the simulation is validated from satellite observations, which indicates a realistic representation of the spatial and temporal variability of the wind along the coast by the model. The model outputs are then used to document the fine-scale structures in the wind stress and wind curl in relation to the topographic features along the coast (headlands and embayments). Both wind stress and wind curl had a clear seasonal variability with annual and semiannual components. Alongshore wind stress maximum peak occurred in spring, second increase was in fall and minimum in winter. When a threshold of −3 × 10 −5 s −1 for the across-shore gradient of alongshore wind was considered to define the region from which the winds decrease toward the coast, the wind dropoff length scale varied between 8 and 45 km. The relative contribution of the coastal divergence and Ekman pumping to the vertical transport along the coast, considering the estimated wind drop-off length, indicated meridional alternation between both mechanisms, modulated by orography and the intricate coastline. Roughly, coastal divergence predominated in areas with low orography and headlands. Ekman pumping was higher in regions with high orography and the presence of embayments along the coast. In the study region, the vertical transport induced by coastal divergence and Ekman pumping represented 60 and 40 % of the total upwelling transport, respectively. The potential role of Ekman pumping on the spatial structure of sea surface temperature is also discussed.

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“…The cross-shelf circulation over the nearshore region plays a key role on the distribution of plankton, nutrients, heat, salt and sediments, and has been the subject of many recent studies Lentz et al, 2008;Hendrickson and MacMahan, 2009;Marchesiello and Estrade, 2010;Ganju et al, 2011;Lentz and Fewings, 2012;Liu and Gan, 2014). Wind-driven upwelling and downwelling systems are particularly important because of the cross-shelf exchange that is forced in these wind conditions, which promotes transport of material across the shelf, especially over stratified shelves (Austin and Lentz, 2002).…”

Section: Introductionmentioning

confidence: 99%

Diurnal variability of inner-shelf circulation in the lee of a cape under upwelling conditions

Lamas

Peliz

Dias

et al. 2017

Continental Shelf Research

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to study the diurnal variability of the cross-shelf circulation. This period was chosen because it had a steady upwellingfavourable wind condition modulated by a diurnal cycle much similar to sea breeze.The daily variability of the observed cross-shelf circulation consisted of three distinct periods: a morning period with a 3-layer vertical structure with onshore velocities at mid-depth, a mid-day period where the flow is reversed and has a 2-layer structure with onshore velocities at the surface and offshore flow below, and, lastly, in the evening, a 2-layer period with intensified offshore velocities at the surface and onshore flow at the bottom. The observed cross-shelf circulation showed a peculiar vertical shape and diurnal variability different from several other systems described in literature. We hypothesize that the flow reversal of the cross-shelf circulation results as a response to the rapid change of the wind magnitude and direction at mid-day with the presence of the cape north of the mooring site influencing this response.A numerical modelling experiment exclusively forced by winds simulated successfully most of the circulation at the ADCP site, especially the mid-day reversal and the evening's upwelling-type structure. This supports the hypothesis that the cross-shelf circulation at diurnal timescales is mostly wind-driven. By analysing the 3D circulation in the vicinity of Cape Sines we came to the conclusion that the diurnal variability of the wind and the flow interaction with topography are responsible for the circulation variability at the ADCP site, though only a small region in the south of the cape showed a similar diurnal variability.The fact that the wind diurnally undergoes relaxation and intensification strongly affects the circulation, promoting superficial onshore flows in the leeside of Cape Sines. Despite the small-scale nature of the observed cross-shelf circulation, onshore flows as the ones described in this study can be particularly helpful to understand the transport and settlement of larvae in this region and in other regions with similar topography and wind characteristics.

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Upwelling limitation by onshore geostrophic flow

Cited by 99 publications

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Peru‐Chile upwelling dynamics under climate change

Peru‐Chile upwelling dynamics under climate change

Seasonal variability of the Ekman transport and pumping in the upwelling system off central-northern Chile (∼  30° S) based on a high-resolution atmospheric regional model (WRF)

Diurnal variability of inner-shelf circulation in the lee of a cape under upwelling conditions

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Upwelling limitation by onshore geostrophic flow

Cited by 99 publications

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Peru‐Chile upwelling dynamics under climate change

Peru‐Chile upwelling dynamics under climate change

Seasonal variability of the Ekman transport and pumping in the upwelling system off central-northern Chile (∼ 30° S) based on a high-resolution atmospheric regional model (WRF)

Diurnal variability of inner-shelf circulation in the lee of a cape under upwelling conditions

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Seasonal variability of the Ekman transport and pumping in the upwelling system off central-northern Chile (∼  30° S) based on a high-resolution atmospheric regional model (WRF)