The Benguela Upwelling System: Quantifying the Sensitivity to Resolution and Coastal Wind Representation in a Global Climate Model (original) (raw)
Related papers
2016
We quantify the wind contribution to the development of interannual sea surface temperature (SST) anomalies along the shelf of southern Africa. We compare numerical simulations that differ only in the amount of variability kept in the ERS1/2-derived surface wind forcing. Surprisingly, most of the cold and warm episodes over the Agulhas Bank are strictly related to local fluctuations of the forcing, whereas the shelf of the west coast extending 400 km north of Cape Columbine is equally sensitive to open-sea wind fluctuations. We diagnose the respective role of mesoscale eddy activity and of low frequency and intra-monthly wind fluctuations in generating interannual SST variability. The fair degree of correlation obtained at a few locations between the model and concomitant observations confirms the interest of a regional numerical tool to study anomalous events in the Benguela system.
Linking wind and interannual upwelling variability in a regional model of the southern Benguela
Geophysical Research Letters, 2002
1] We quantify the wind contribution to the development of interannual sea surface temperature (SST) anomalies along the shelf of southern Africa. We compare numerical simulations that differ only in the amount of variability kept in the ERS1/2-derived surface wind forcing. Surprisingly, most of the cold and warm episodes over the Agulhas Bank are strictly related to local fluctuations of the forcing, whereas the shelf of the west coast extending 400 km north of Cape Columbine is equally sensitive to open-sea wind fluctuations. We diagnose the respective role of mesoscale eddy activity and of low frequency and intra-monthly wind fluctuations in generating interannual SST variability. The fair degree of correlation obtained at a few locations between the model and concomitant observations confirms the interest of a regional numerical tool to study anomalous events in the Benguela system.
Journal of Geophysical Research, 2005
The structure and patterns of variability of the southern Benguela coastal upwelling system are investigated with a high-resolution regional model forced by QuikSCAT winds over 1999QuikSCAT winds over -2003 The relevance of this global wind product is tested, at first, for the specific nearshore southeast Atlantic; then, the wind products are spatially or temporally degraded from the original 0.5° daily fluxes and are used to diagnose the main scales of the surface dynamical forcing variability. Time resolution appears as a crucial factor in the wind stress to retrieve the patterns of interannual anomalies in sea surface temperatures in a good agreement with independent NASA Pathfinder observations. Various upwelling indices are also calculated in the model to study specific warm or cold coastal events whose circulation schemes are detailed from a Lagrangian interpretation of the model time-varying threedimensional velocity field. Our study emphasizes the connections established between the Benguela upwelling and the large-scale circulation in relation with, for instance, the Agulhas Current retroflection and associated ring shedding.
Continental Shelf Research, 1988
The spatial distribution ofwind-driven upwelling along the west coast of Africa (29-34°S) in the early summer is described using intermittent and sequential aerial survey observations, supported by surface-based time series. Three semi-permanent upwelling centers, at 29-31, 33 and 34°S were examined for response to local topographic and meteorological forcing, both before and after the active phase of upwelling. Differing scales, intensities and response lags in wind-driven coastal upweUing were documented through the analysis of selected aerial survey data sets. The upwelling area centered at 30°S was found to be of greater spatial extent than the upwelling plumes associated with the capes at 33 and 34°S. Variations in the depth of the marine atmospheric boundary layer affected the structure of equatorwards winds and upwelling activity off the capes. When the air temperature inversion was relatively low, wind stress was enhanced seawards of the capes, while equatorwards, a wind shadow reduced offshore Ekman transport and upwelling. The case studies highlight persistent mesoscale features in the wind and sea surface temperature fields which are formed by the relationship between coastal upwelling, wind forcing and local topography.
Linking wind and interannual upwelling variability in a regional model of the southern
2002
We quantify the wind contribution to the development of interannual sea surface temperature (SST) anomalies along the shelf of southern Africa. We compare numerical simulations that differ only in the amount of variability kept in the ERS1/2-derived surface wind forcing. Surprisingly, most of the cold and warm episodes over the Agulhas Bank are strictly related to local fluctuations of the forcing, whereas the shelf of the west coast extending 400 km north of Cape Columbine is equally sensitive to open-sea wind fluctuations. We diagnose the respective role of mesoscale eddy activity and of low frequency and intra-monthly wind fluctuations in generating interannual SST variability. The fair degree of correlation obtained at a few locations between the model and concomitant observations confirms the interest of a regional numerical tool to study anomalous events in the Benguela system.
A regional hydrodynamic model of upwelling in the Southern Benguela
2001
Hydrodynamic modelling is an active field in oceanography. The increase in available computing power as well as the incorporation of advanced features in numerical codes allow hydrodynamic models efficiently and robustly to simulate the oceanic circulation at high spatial resolution in the coastal domain. 1 The ability of the more advanced numerical codes to resolve mesoscale circulation features such as eddies and fronts is of particular interest for investigating the dynamics of coastal circulation and its impact on coastal ecosystems. There have been few attempts to model ocean dynamics over the coastal shelf and slope off South Africa. Recently, the Princeton Ocean Model (POM) was used to simulate the circulation around southern Africa from about 46°S to 12°S and 4°E to 30°E with a spatial resolution of 20 km. 3 This model reproduces most of the characteristic, large-scale features of the circulation over the Benguela region, but the resolution is too coarse and the numerical choices are such that it is difficult to resolve accurately the mesoscale patterns that develop over the shelf off the west coast as well as off the Cape Peninsula and western Agulhas Bank.
Journal of Physical Oceanography
In addition to their well-known seasonal cycle, eastern boundary upwelling systems (EBUS) undergo modulation on shorter synoptic to intraseasonal time scales. Energetic intensifications and relaxations of upwelling-favorable winds with 5–10-day typical time scales can impact the EBUS dynamics and biogeochemical functioning. In this work the dynamical effects of wind-forced synoptic fluctuations on the South Senegalese Upwelling Sector (SSUS) are characterized. The region geomorphology is unique with its wide continental shelf and a major coastline discontinuity at its northern edge. The ocean response to synoptic events is explored using a modeling framework that involves applying idealized synoptic wind intensification or relaxation to a five-member climatological SSUS ensemble run. Model evaluation against sparse midshelf in situ observations indicates qualitative agreement in terms of synoptic variability of temperature, stratification, and ocean currents, despite a moderate but ...
Journal of Geophysical Research: Oceans, 2014
1] Satellite scatterometers provide continuously valuable surface wind speed and direction estimates over the global ocean on a regular grid both in space and time. The Level 3 data derived from the Advanced Scatterometer (ASCAT), available at 1/4 spatial resolution (hereafter AS25), and Quick Scatterometer (QuikSCAT), available on 1/2 and 1/4 horizontal grids (QS50 and QS25, respectively), are studied at regional scales in both the Benguela and Canary upwelling systems. They are compared to the European Center for Medium-Range Weather Forecast surface wind analysis, with insight into their intrinsic and actual spatial resolutions. In the coastal band, the finest spatial patterns are found in the QS25 winds and are O(75 km). This demonstrates the sensitivity of the high-resolution satellite-derived winds to coastal processes related to sea surface temperature (SST) perturbations and land-sea transition. Next, short-lived upwelling episodes (SUEs) calculated from SST anomalies are defined consistently with the QS25 actual resolution. These cold events refer to local, short-lived perturbations that add to seasonal upwelling variability. We characterize concomitant atmospheric synoptic conditions for SUEs identified at chosen latitudes and highlight two subregions in both upwelling systems, with contrasted patterns for the alongshore wind stress component and curl. The complexity of the latter patterns is closely linked to local, short-term SST variability. Closer to the shore, numerical sensitivity experiments show that the imbalance between Ekman transport and Ekman pumping has an impact on ocean dynamics: wind reduction in the coastal QS25 forcing, partially induced by orography, tends to reduce coastal SST cooling.
We investigate the respective roles of equatorial remote (Equatorial Kelvin Waves) and local atmospheric (wind, heat fluxes) forcing on coastal variability in the SouthEast Atlantic Ocean extending up to the Benguela Upwelling System (BUS) over the 2000–2008 period. We carried out a set of six numerical experiments based on a regional ocean model, that differ only by the prescribed forcing (climatological or total) at surface and lateral boundaries. Results show that at subseasonal timescales (<100 days), the coastal oceanic variability (currents, thermocline, and sea level) is mainly driven by local forcing, while at interan-nual timescales it is dominated by remote equatorial forcing. At interannual timescales (13–20 months), remotely forced Coastal-Trapped Waves (CTW) propagate poleward along the African southwest coast up to the northern part of the BUS at 248S, with phase speeds ranging from 0.8 to 1.1 m.s 21. We show that two triggering mechanisms limit the southward propagation of CTW: interannual variability of the equatorward Benguela Current prescribed at the model's southern boundary (308S) and variability of local atmospheric forcing that modulates the magnitude of observed coastal interannual events. When local wind stress forcing is in (out) of phase, the magnitude of the interannual event increases (decreases). Finally, dynamical processes associated with CTW propagations are further investigated using heat budget for two intense interannual events in 2001 and 2003. Results show that significant temperature anomalies (628C), that are mostly found in the subsurface, are primarily driven by alongshore and vertical advection processes.