scholarly journals The relation of wind-driven coastal and offshore upwelling in the Benguela Upwelling System

2021 ◽  
Author(s):  
Mohammad Hadi Bordbar ◽  
Volker Mohrholz ◽  
Martin Schmidt
Keyword(s):  
Chlorophyll A ◽  
Wind Stress ◽  
Coastal Upwelling ◽  
Marine Ecosystem ◽  
Surface Wind ◽  
Phase Relationship ◽  
Wind Stress Curl ◽  
Upwelling System ◽  
Benguela Upwelling ◽  
Ekman Theory

AbstractSpatial and temporal variations of nutrient-rich upwelled water across the major eastern boundary upwelling systems are primarily controlled by the surface wind with different, and sometimes contrasting, impacts on coastal upwelling systems driven by alongshore wind and offshore upwelling systems driven by the local wind-stress-curl. Here, concurrently measured wind-fields, satellite-derived Chlorophyll-a concentration along with a state-of-the-art ocean model simulation spanning 2008-2018 are used to investigate the connection between coastal and offshore physical drivers of the Benguela Upwelling System (BUS). Our results indicate that the spatial structure of long-term mean upwelling derived from Ekman theory and the numerical model are fairly consistent across the entire BUS and closely followed by the Chlorophyll-a pattern. The variability of the upwelling from the Ekman theory is proportionally diminished with offshore distance, whereas different and sometimes opposite structures are revealed in the model-derived upwelling. Our result suggests the presence of sub-mesoscale activity (i.e., filaments and eddies) across the entire BUS with a large modulating effect on the wind-stress-curl-driven upwelling off Lüderitz and Walvis Bay. In Kunene and Cape Frio upwelling cells, located in the northern sector of the BUS, the coastal upwelling and open-ocean upwelling frequently alternate each other, whereas they are modulated by the annual cycle and mostly in phase off Walvis Bay. Such a phase relationship appears to be strongly seasonally dependent off Lüderitz and across the southern BUS. Thus, our findings suggest this relationship is far more complex than currently thought and seems to be sensitive to climate changes with short- and far-reaching consequences for this vulnerable marine ecosystem.

On the connection between coastal Ekman upwelling and wind-stress-curl-driven upwelling off the southwest African coasts

2021 ◽  
Author(s):  
Mohammad Hadi Bordbar ◽  
Volker Mohrholz ◽  
Martin Schmidt
Keyword(s):  
Chlorophyll A ◽  
Wind Stress ◽  
Coastal Upwelling ◽  
Model Simulation ◽  
Marine Ecosystem ◽  
Phase Relationship ◽  
Open Ocean ◽  
Wind Stress Curl ◽  
Upwelled Water ◽  
Ekman Theory

<p>Spatial and temporal variations of nutrient-rich upwelled water across the major eastern boundary upwelling systems are primarily controlled by the surface atmospheric flow with different, and sometimes contrasting, impacts on coastal and open-ocean upwelling systems. Here, concurrently measured wind-fields, satellite-derived Chlorophyll-a concentration along with a state-of-the-art ocean model simulation spanning 2008-2018 are used to investigate the connection between coastal and offshore physical drivers of the Benguela Upwelling System (BUS). Our results indicate that the spatial structure of long-term mean upwelling derived from Ekman theory and the numerical model are fairly consistent across the entire BUS and closely followed by the Chlorophyll-a pattern. The variability of the upwelling from the Ekman theory is proportionally diminished with offshore distance, whereas different and sometimes opposite structures are revealed in the model-derived upwelling. Our result suggests the presence of sub-mesoscale activity (i.e. filaments and eddies) across the entire BUS with a large modulating effect on the wind-stress-curl-driven upwelling off Lüderitz and Walvis Bay. In Kunene and Cape Frio upwelling cells, located in the northern sector of the BUS, the coastal upwelling and open-ocean upwelling frequently alternate each other, whereas they are modulated by the annual cycle and mostly in phase off Walvis Bay. Such a phase relationship appears to be strongly seasonal dependent off Lüderitz and across the southern BUS. Thus, our findings suggest this relationship is far more complex than currently thought and seems to be sensitive to climate changes with short- and far-reaching consequences for this vulnerable marine-ecosystem.</p>

scholarly journals The Benguela Upwelling System: Quantifying the Sensitivity to Resolution and Coastal Wind Representation in a Global Climate Model*

2015 ◽  
Vol 28 (23) ◽  
pp. 9409-9432 ◽  
Author(s):  
R. Justin Small ◽  
Enrique Curchitser ◽  
Katherine Hedstrom ◽  
Brian Kauffman ◽  
William G. Large
Keyword(s):  
Wind Stress ◽  
Climate Model ◽  
Coastal Upwelling ◽  
Ocean Model ◽  
Wind Stress Curl ◽  
Upwelling System ◽  
Wind Structure ◽  
Eastern Boundary ◽  
Benguela Upwelling ◽  
Atmosphere Model

Abstract Of all the major coastal upwelling systems in the world’s oceans, the Benguela, located off southwest Africa, is the one that climate models find hardest to simulate well. This paper investigates the sensitivity of upwelling processes, and of sea surface temperature (SST), in this region to resolution of the climate model and to the offshore wind structure. The Community Climate System Model (version 4) is used here, together with the Regional Ocean Modeling System. The main result is that a realistic wind stress curl at the eastern boundary, and a high-resolution ocean model, are required to well simulate the Benguela upwelling system. When the wind stress curl is too broad (as with a 1° atmosphere model or coarser), a Sverdrup balance prevails at the eastern boundary, implying southward ocean transport extending as far as 30°S and warm advection. Higher atmosphere resolution, up to 0.5°, does bring the atmospheric jet closer to the coast, but there can be too strong a wind stress curl. The most realistic representation of the upwelling system is found by adjusting the 0.5° atmosphere model wind structure near the coast toward observations, while using an eddy-resolving ocean model. A similar adjustment applied to a 1° ocean model did not show such improvement. Finally, the remote equatorial Atlantic response to restoring SST in a broad region offshore of Benguela is substantial; however, there is not a large response to correcting SST in the narrow coastal upwelling zone alone.

scholarly journals SST–Wind Interaction in Coastal Upwelling: Oceanic Simulation with Empirical Coupling

2009 ◽  
Vol 39 (11) ◽  
pp. 2957-2970 ◽  
Author(s):  
Xin Jin ◽  
Changming Dong ◽  
Jaison Kurian ◽  
James C. McWilliams ◽  
Dudley B. Chelton ◽  
...  
Keyword(s):  
Wind Stress ◽  
Coastal Upwelling ◽  
Surface Wind ◽  
Wind Stress Curl ◽  
Intermediate Space ◽  
Surface Wind Stress ◽  
Wind Stress Field ◽  
Induced Changes ◽  
Anticyclonic Eddies ◽  
Current Systems

Abstract Observations, primarily from satellites, have shown a statistical relationship between the surface wind stress and underlying sea surface temperature (SST) on intermediate space and time scales, in many regions inclusive of eastern boundary upwelling current systems. In this paper, this empirical SST–wind stress relationship is utilized to provide a simple representation of mesoscale air–sea coupling for an oceanic model forced by surface winds, namely, the Regional Oceanic Modeling System (ROMS). This model formulation is applied to an idealized upwelling problem with prevailing equatorward winds to determine the coupling consequences on flow, SST, stratification, and wind evolutions. The initially uniform wind field adjusts through coupling to a cross-shore profile with weaker nearshore winds, similar to realistic ones. The modified wind stress weakens the nearshore upwelling circulation and increases SST in the coastal zone. The SST-induced wind stress curl strengthens offshore upwelling through Ekman suction. The total curl-driven upwelling exceeds the coastal upwelling. The SST-induced changes in the nearshore wind stress field also strengthen and broaden the poleward undercurrent. The coupling also shows significant impact on the developing mesoscale eddies by damaging cyclonic eddies more than anticyclonic eddies, which leads to dominance by the latter. Dynamically, this is a consequence of cyclones with stronger SST gradients that induce stronger wind perturbations in this particular upwelling problem and that are therefore generally more susceptible to disruption than anticyclones at finite Rossby number. The net effect is a weakening of eddy kinetic energy.

scholarly journals Phytoplankton blooms on the western shelf of Tasmania: evidence of a highly productive ecosystem

Ocean Science ◽  
2015 ◽  
Vol 11 (1) ◽  
pp. 1-11 ◽  
Author(s):  
J. Kämpf
Keyword(s):  
Satellite Data ◽  
Coastal Upwelling ◽  
Phytoplankton Bloom ◽  
Hot Spot ◽  
Marine Ecosystem ◽  
Austral Summer ◽  
Phytoplankton Blooms ◽  
Study Region ◽  
Upwelling System ◽  
Fluorescence Line

Abstract. Satellite-derived chlorophyll a data using the standard NASA-OC3 (ocean colour) algorithm are strongly biased by coloured dissolved organic matter and suspended sediment of river discharges, which is a particular problem for the western Tasmanian shelf. This work reconstructs phytoplankton blooms in the study region using a quadratic regression between OC3 data and chlorophyll fluorescence based on the fluorescence line height (FLH) data. This regression is derived from satellite data of the nearby Bonney upwelling region, which is devoid of river influences. To this end, analyses of 10 years of MODIS-aqua satellite data reveal the existence of a highly productive ecosystem on the western Tasmanian shelf. The region normally experiences two phytoplankton blooms per annum. The first bloom occurs during late austral summer months as a consequence of upwelling-favourable coastal winds. Hence, the western Tasmanian shelf forms a previously unknown upwelling centre of the regional upwelling system, known as Great South Australian Coastal Upwelling System. The second phytoplankton bloom is a classical spring bloom also developing in the adjacent Tasman Sea. The author postulates that this region forms another important biological hot spot for the regional marine ecosystem.

scholarly journals SEASONAL PATTERN OF WIND INDUCED UPWELLING OVER JAVA-BALI SEA WATERS AND SURROUNDING AREA

2010 ◽  
Vol 5 (1) ◽  
Author(s):  
Siswanto ◽  
Suratno
Keyword(s):  
Wind Stress ◽  
Coastal Upwelling ◽  
Sea Water ◽  
Mixed Layer Depth ◽  
Surface Wind ◽  
Ekman Transport ◽  
Monsoon Circulation ◽  
Wide Field ◽  
Surrounding Area ◽  
Noaa Avhrr

The influence of monsoonal wind to coastal upwelling mechanism which is generated by Ekman transport was studied here by analyzing wind stress curl (WSC) distribution over Java-Bali Sea waters and its surrounding area. Surface wind data were used as input data to calculate curl of wind stress in barotropic model. Confirmation with Corioli effect in the Southern Hemisphere, it could be known that negative curl value has relation with vertical motion of sea water as resulted by Ekman transport. Result of analysis showed that negative curl near coast over Java Sea which is stretching to Lombok Sea occurred in December to April when westerly wind of the North West Monsoon actives. It can be guidance and related with season of coastal upwelling in the region. Reversal condition, the occurrance of coastal upwelling in the south coast of JAva island related with the negative value of WSC that occurs since easterlies wind take place in May to August as a part of South East Monsoon episode. Generally, upwelling occurrance in the field of study is a response to the Monsoon circulation. This study with related data such as sea surface temperature, chlorophyll concetration and mixed layer depth that derived from satellite imaging data National Oceanic and Atmospheric Administration Advanced Very High Resolution Radiometer (NOAA-AVHRR), Aqua/Modis and sea viewing Wide Field-of-view Sensor(Sea WiFS) shows as magnificent confirmation pattern. So applying WSC to recoqnize upwelling zone is alternatively way as climatic approach to maps potential fertilizing of sea water in maritime-continent Indonesia. Key words: coastal upwelling, Ekman transport, Java-Bali Sea, Monsoon circulation, upwelling.

Spatiotemporal variation of vertical particle fluxes and modelled chlorophyll a standing stocks in the Benguela Upwelling System

2018 ◽  
Vol 180 ◽  
pp. 59-75 ◽  
Author(s):  
Maria-Elena Vorrath ◽  
Niko Lahajnar ◽  
Gerhard Fischer ◽  
Viktor Miti Libuku ◽  
Martin Schmidt ◽  
...  
Keyword(s):  
Chlorophyll A ◽  
Spatiotemporal Variation ◽  
Upwelling System ◽  
Benguela Upwelling ◽  
Standing Stocks ◽  
Particle Fluxes

scholarly journals From monsoon to marine productivity in the Arabian Sea: insights from glacial and interglacial climates

2017 ◽  
Vol 13 (7) ◽  
pp. 759-778 ◽  
Author(s):  
Priscilla Le Mézo ◽  
Luc Beaufort ◽  
Laurent Bopp ◽  
Pascale Braconnot ◽  
Masa Kageyama
Keyword(s):  
Stress Intensity ◽  
Summer Monsoon ◽  
Wind Stress ◽  
Arabian Sea ◽  
Coastal Upwelling ◽  
Boreal Summer ◽  
Tropospheric Temperature ◽  
Ekman Pumping ◽  
Wind Stress Curl ◽  
Jet Axis

Abstract. The current-climate Indian monsoon is known to boost biological productivity in the Arabian Sea. This paradigm has been extensively used to reconstruct past monsoon variability from palaeo-proxies indicative of changes in surface productivity. Here, we test this paradigm by simulating changes in marine primary productivity for eight contrasted climates from the last glacial–interglacial cycle. We show that there is no straightforward correlation between boreal summer productivity of the Arabian Sea and summer monsoon strength across the different simulated climates. Locally, productivity is fuelled by nutrient supply driven by Ekman dynamics. Upward transport of nutrients is modulated by a combination of alongshore wind stress intensity, which drives coastal upwelling, and by a positive wind stress curl to the west of the jet axis resulting in upward Ekman pumping. To the east of the jet axis there is however a strong downward Ekman pumping due to a negative wind stress curl. Consequently, changes in coastal alongshore stress and/or curl depend on both the jet intensity and position. The jet position is constrained by the Indian summer monsoon pattern, which in turn is influenced by the astronomical parameters and the ice sheet cover. The astronomical parameters are indeed shown to impact wind stress intensity in the Arabian Sea through large-scale changes in the meridional gradient of upper-tropospheric temperature. However, both the astronomical parameters and the ice sheets affect the pattern of wind stress curl through the position of the sea level depression barycentre over the monsoon region (20–150° W, 30° S–60° N). The combined changes in monsoon intensity and pattern lead to some higher glacial productivity during the summer season, in agreement with some palaeo-productivity reconstructions.

Seasonal to interannual variations of wind forcing in the Peruvian upwelling system

2020 ◽  
Author(s):  
Sadegh Yari ◽  
Volker Mohrholz
Keyword(s):  
Wind Stress ◽  
Coastal Upwelling ◽  
Temporal Variations ◽  
Wind Forcing ◽  
Enso Cycle ◽  
Spatial And Temporal Variability ◽  
Ekman Pumping ◽  
North West ◽  
Upwelling System ◽  
Peruvian Coast

<p>The Humboldt (Peruvian) Upwelling System (HUS) is the most productive among the main Eastern Boundary Upwelling Systems (EBUS), namely California, North West Africa, Benguela and itself. In spite of comparable upwelling intensity its fisheries production exceeds that of the other upwelling systems considerably (Chavez and Messie 2009). Wind is the major driving force of the coastal and curl driven upwelling, that controlls the nutrient supply from the deep water pool to the euphotic surface layer. Strength, spatial and temporal variability of the wind forcing are subjected to seasonal and interannual changes. The core of this study is describe the wind driven upwelling cells in the Peruvian coastal area in detail using long-term data which is not well understood. A better understanding of the state and dynamics of HUS seems essential for fututre regional climate predictions. ASCAT wind stress data for the period of 11 years (2008-2018) is analyzed to assess the spatio-temporal variations of the wind stress field, coastal upwelling and Ekman pumping along the Peruvian coast. The meridional component of wind stress off the peruvian coast, which is the main driver of offshore transport, has been marginally inensified over the entire priod. However, a high level of interannual variability is evident. The El-Niño years show anomalously high wind stress and associated Ekman transoprt. Our results indicate that the southern sector is more influenced by ENSO cycle than the northern sector. Additionally, a strong seasonality in the wind stress is observed. During the austral summer (December-February) the wind stress show the minimum value while the high values are observed in July-September.</p>

Climatological annual cycle of Benguela upwelling system using different upwelling indices

2020 ◽  
Author(s):  
Mohammad Hadi Bordbar ◽  
Volker Mohrholz ◽  
Martin Schmidt
Keyword(s):  
Chlorophyll A ◽  
Vertical Velocity ◽  
Annual Cycle ◽  
Ekman Transport ◽  
Spatial And Temporal Variation ◽  
Clear Correlation ◽  
Ekman Pumping ◽  
Chlorophyll A Concentration ◽  
Upwelling System ◽  
Benguela Upwelling

<p>Several long-established upwelling indices derived from the observed wind fields, Chlorophyll-a concentration, sea surface temperature (SST) are used to investigate the climatology annual cycle of Benguela Upwelling System (BUS). Chlorophyll-a concentration is taken as an indicator of ocean primary production. In addition, we analyze a multi-decadal simulation of a state-of-the-art eddy resolving ocean model which was forced by observed atmospheric heat and momentum fluxes. We take the vertically averaged of simulated vertical velocity in water column as a direct measure of upwelling strength.</p><p>The Ekman offshore transport tends to have two distinctive upwelling cells near the coast of Lüderitz (26.3°S) and Cape Frio (17°S) with large seasonal cycles. The former peaks between September and December. The latter features a biannual cycle with two peaks over April-June and September-December, which is concurrent with meridional migration of Angola-Benguela SST front. The offshore (30-200 km) vertical velocity, primarily induced by Ekman transport divergence, depicts a similar annual cycle, but with smaller magnitude. It becomes broader from south to north with four distinctive upwelling cells located near the coast of Cape Columbine (33°S), Orange River (28°S), Walvis Bay (23°S) and northern part of Cape Frio (16°S). The spatial and temporal variation of Ekman pumping and Chlorophyll-a, as measures of upwelling, show a clear correlation. However, such a correlation is not evident when Ekman coastal transport is taken. SST-based index depicts a very similar spatial pattern. However, the seasonal cycle does not match with other observational and simulated indices. Our finding suggests that the local SST anomalies are strongly influenced by horizontal heat advection and surface heat flux anomaly which can dominate over the anomalies associated with the upwelling; meaning that SST-index alone may not give a realistic estimate of upwelling strength over the region.</p>

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