Seasonal and interannual variability of surface circulation in the Cape Verde region from 8 years of merged T/P and ERS-2 altimeter data

Lázaro, Clara; Fernandes, Joana; Santos, A. Miguel P.; Oliveira, Paulo B.

doi:10.1016/j.rse.2005.06.005

Cited by 37 publications

(63 citation statements)

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“…Barotropic and baroclinic instabilities of the near-coastal currents (Pantoja et al, 2012) triggered by, e.g., the passage of poleward propagating coastal trapped waves (Zamudio et al, 2001(Zamudio et al, , 2007, wind perturbations (Pares-Sierra et al, 1993), or interactions of the large-scale circulation with the bottom topography (Kurian et al, 2011) are the main processes identified for the eddy generation in eastern boundary upwelling regions. In the TANWA, the period of maximum eddy generation (June/July) is characterized by a strong near-surface boundary current, the MC (Lázaro et al, 2005) suggesting dynamic instabilities of the boundary current as an important generation mechanism. However, there is a difference in peak generation of cyclones and anticyclones.…”

Section: Seasonal Variability Of Eddy Generationmentioning

confidence: 99%

“…This current is referred to as Mauretania Current (MC) and reaches latitudes up to 20 • N (Mittelstaedt, 1991). The strength of the MC is strongly related to the seasonally varying NECC with a time lag of 1 month (Lázaro et al, 2005). During boreal winter and spring when the NECC is pushed to the Equator and becomes unstable, the MC becomes weak and unsteady and only reaches latitudes south of Cap-Vert (Mittelstaedt, 1991;Lázaro et al, 2005).…”

Section: Introductionmentioning

confidence: 99%

“…The strength of the MC is strongly related to the seasonally varying NECC with a time lag of 1 month (Lázaro et al, 2005). During boreal winter and spring when the NECC is pushed to the Equator and becomes unstable, the MC becomes weak and unsteady and only reaches latitudes south of Cap-Vert (Mittelstaedt, 1991;Lázaro et al, 2005). During this period the wind induced coastal upwelling is at its maximum.…”

Section: Introductionmentioning

confidence: 99%

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Occurrence and characteristics of mesoscale eddies in the tropical northeastern Atlantic Ocean

2016

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Abstract. Coherent mesoscale features (referred to here as eddies) in the tropical northeastern Atlantic Ocean (between 12-22 • N and 15-26 • W) are examined and characterized. The eddies' surface signatures are investigated using 19 years of satellite-derived sea level anomaly (SLA) data. Two automated detection methods are applied, the geometrical method based on closed streamlines around eddy cores, and the Okubo-Weiß method based on the relation between vorticity and strain. Both methods give similar results. Mean eddy surface signatures of SLA, sea surface temperature (SST) and sea surface salinity (SSS) anomalies are obtained from composites of all snapshots around identified eddy cores. Anticyclones/cyclones are identified by an elevation/depression of SLA and enhanced/reduced SST and SSS in their cores. However, about 20 % of all anticyclonically rotating eddies show reduced SST and reduced SSS instead. These kind of eddies are classified as anticyclonic mode-water eddies (ACMEs). About 146 ± 4 eddies per year with a minimum lifetime of 7 days are identified (52 % cyclones, 39 % anticyclones, 9 % ACMEs) with rather similar mean radii of about 56 ± 12 km. Based on concurrent in situ temperature and salinity profiles (from Argo float, shipboard, and mooring data) taken inside of eddies, distinct mean vertical structures of the three eddy types are determined. Most eddies are generated preferentially in boreal summer and along the West African coast at three distinct coastal headland regions and carry South Atlantic Central Water supplied by the northward flow within the Mauretanian coastal current system. Westward eddy propagation (on average about 3.00 ± 2.15 km d −1 ) is confined to distinct zonal corridors with a small meridional deflection dependent on the eddy type (anticyclones -equatorward, cyclones -poleward, ACMEs -no deflection). Heat and salt fluxes out of the coastal region and across the Cape Verde Frontal Zone, which separates the shadow zone from the ventilated subtropical gyre, are calculated.

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Section: Seasonal Variability Of Eddy Generationmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

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Occurrence and characteristics of mesoscale eddies in the tropical northeastern Atlantic Ocean

2016

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“…It is located near 5 • N in winter and reaches 10 • N in summer. During this season, it continues north as the Mauritania Current (MC), which flows northward until about 20 • N [15]. Offshore, the Guinea Dome (GD) is another important physical characteristic of the area, defined by a dome of the isotherms, and FIGURE 1 | Map of the schematic surface circulation in the Atlantic North-eastern Tropical Upwelling System characterized by permanent currents (dark gray), seasonal currents in winter-spring (green), another seasonal currents in summer-autumn (blue) and upwelling area (gray shading).…”

Section: Introductionmentioning

confidence: 99%

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A model study of the seasonality of sea surface temperature and circulation in the Atlantic North-eastern Tropical Upwelling System

et al. 2015

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The climatological seasonal cycle of the sea surface temperature (SST) in the north-eastern tropical Atlantic (7)(8)(9)(10)(11)(12)(13)(14)(15)(16)(17)(18)(19)(20)(21)(22)(23)(24)(25) • N, 26-12 • W) is studied using a mixed layer heat budget in a regional ocean general circulation model. The region, which experiences one of the larger SST cycle in the tropics, forms the main part of the Guinea Gyre. It is characterized by a seasonally varying open ocean and coastal upwelling system, driven by the movements of the intertropical convergence zone (ITCZ). The model annual mean heat budget has two regimes schematically. South of roughly 12 • N, advection of equatorial waters, mostly warm, and warming by vertical mixing, is balanced by net air-sea flux. In the rest of the domain, a cooling by vertical mixing, reinforced by advection at the coast, is balanced by the air-sea fluxes. Regarding the seasonal cycle, within a narrow continental band, in zonal mean, the SST early decrease (from September, depending on latitude, until December) is driven by upwelling dynamics off Senegal and Mauritania (15-20 • N), and instead by air-sea fluxes north and south of these latitudes. Paradoxically, the later peaks of upwelling intensity (from March to July, with increasing latitude) essentially damp the warming phase, driven by air-sea fluxes. The open ocean cycle to the west, is entirely driven by the seasonal net air-sea fluxes. The oceanic processes significantly oppose it, but for winter north of ∼18 • N. Vertical mixing in summer-autumn tends to cool (warm) the surface north (south) of the ITCZ, and advective cooling or warming by the geostrophic Guinea Gyre currents and the Ekman drift. This analysis supports previous findings on the importance of air-sea fluxes offshore. It mainly offers quantitative elements on the modulation of the SST seasonal cycle by the ocean circulation, and particularly by the upwelling dynamics.

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On the temporal memory of coastal upwelling off NW Africa

et al. 2014

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We use a combination of satellite, in situ, and numerical data to provide a comprehensive view of the seasonal coastal upwelling cycle off NW Africa in terms of both wind forcing and sea surface temperature (SST) response. Wind forcing is expressed in terms of both instantaneous (local) and time-integrated (nonlocal) indices, and the ocean response is expressed as the SST difference between coastal and offshore waters. The classical local index, the cross-shore Ekman transport, reproduces reasonably well the time-latitude distribution of SST differences but with significant time lags at latitudes higher than Cape Blanc. Two nonlocal indices are examined. One of them, a cumulative index calculated as the backward averaged Ekman transport that provides the highest correlation with SST differences, reproduces well the timing of the SST differences at all latitudes (except near Cape Blanc). The corresponding time lags are close to zero south of Cape Blanc and range between 2 and 4 months at latitudes between Cape Blanc and the southern Gulf of Cadiz. The results are interpreted based on calculations of spatial and temporal auto and cross correlations for wind forcing and SST differences. At temporal scales of 2-3 weeks, the alongshore advection of alongshore momentum compensates for interfacial friction, allowing the upwelling jet and associated frontal system to remain active. We conclude that the coastal jet plays a key role in maintaining the structure of coastal upwelling, even at times of relaxed winds, by introducing a seasonal memory to the system in accordance with the atmospheric-forcing annual cycle.

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Seasonal and interannual variability of surface circulation in the Cape Verde region from 8 years of merged T/P and ERS-2 altimeter data

Cited by 37 publications

References 37 publications

Occurrence and characteristics of mesoscale eddies in the tropical northeastern Atlantic Ocean

Occurrence and characteristics of mesoscale eddies in the tropical northeastern Atlantic Ocean

A model study of the seasonality of sea surface temperature and circulation in the Atlantic North-eastern Tropical Upwelling System

On the temporal memory of coastal upwelling off NW Africa

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