Decadal variability and trends of the Benguela upwelling system as simulated in a high-resolution ocean simulation

Tim, Nele; Zorita, Eduardo; Hünicke, Birgit

doi:10.5194/os-11-483-2015

Cited by 65 publications

(42 citation statements)

References 64 publications

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“…The upwelling itself, defined here as the simulated vertical velocity at 100 m depth, shows an annual cycle and strength, comparable to other model studies (Small et al, ; Tim et al, , ). Direct comparison to observations is not possible as there are no long‐term direct observations of upwelling intensity.…”

Section: Resultssupporting

confidence: 85%

“…The Benguela upwelling region is bounded by the Angola‐Benguela‐Front at around 15–17°S in the north and the Cape region at 34°S in the south. The Lüderitz upwelling cell at 27°S separates the upwelling system into a northern and a southern part (Tim et al, ) with distinct characteristics and seasonality. Observational studies suggest that the central waters of the South Benguela region are dominated by ESACW, whereas central waters in North Benguela are mainly fed by SACW (Mohrholz et al, ).…”

Section: Introductionmentioning

confidence: 99%

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The Impact of Agulhas Leakage on the Central Water Masses in the Benguela Upwelling System From A High‐Resolution Ocean Simulation

et al. 2018

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We analyze the contribution of the Agulhas Current on the central water masses of the Benguela upwelling system (BUS) over the last decades in a high‐resolution ocean simulation driven by atmospheric reanalysis. The BUS is an Eastern Boundary Upwelling System where upwelling of cold nutrient‐rich water favors biomass growth. The two distinct subregions, North and South Benguela, differ in nutrient and oxygen properties of the upwelling water mass. Our analysis indicates that the contribution of Agulhas water to the upwelling is very strong in both subregions. Although the water masses feeding the upwelling have a common origin, their pathways are distinct in both regions. Whereas for the central waters of South Benguela the path is rather direct from where it is formed, the central waters of North Benguela takes a longer route through the equatorial current system. Not only the travel time from the Agulhas Current to the BUS is longer but also the central water mass is twice as old for the northern part when compared to the southern. Our analysis traces the pathways, history, and origin of the central water masses feeding upwelling in the BUS and emphasizes the direct impact of the Agulhas Current on the upwelling region. The variability of that link between the Indian Ocean and the South Atlantic is likely to change the nutrient and oxygen content, as well as temperature and salinity of the water masses in the upwelling region.

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Section: Resultssupporting

confidence: 85%

Section: Introductionmentioning

confidence: 99%

The Impact of Agulhas Leakage on the Central Water Masses in the Benguela Upwelling System From A High‐Resolution Ocean Simulation

et al. 2018

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“…In the future simulations, the main upwelling season in Benguela is also June-August. Strongest upwelling in Benguela takes place between July and October (Tim et al, 2015). The main upwelling season could be both of the standard seasons (JJA or SON), since the mean difference is small.…”

Section: Upwelling Indexmentioning

confidence: 99%

The importance of external climate forcing for the variability and trends of coastal upwelling in past and future climate

et al. 2016

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Abstract. The eastern boundary upwelling systems, located in the subtropics at the eastern boundary of the Atlantic and Pacific oceans and mainly driven by the trade winds, are the major coastal upwelling regions. Previous studies have suggested that the intensity of upwelling in these areas in the past centuries may have been influenced by the external radiative forcing, for instance by changes in solar irradiance, and it will also be influenced in the future by the increasing atmospheric greenhouse gases. Here, we analyse the impact of the external climate forcing on these upwelling systems in ensembles of simulations of two Earth system models. The ensembles contain three simulations for each period covering the past millennium (900-1849) and the 20th century . One of these Earth system models additionally includes the near future . Using a set of simulations, differing only in their initial conditions, enables us to test whether the observed variability and trends are driven by the external radiative forcing. Our analysis shows that the variability of the simulated upwelling is largely not affected by the external forcing and that, generally, there are no significant trends in the periods covering the past and future. Only in future simulations with the strongest increase of greenhouse gas concentrations the upwelling trends are significant and appear in all members of the ensemble.

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“…According to the authors, this apparently unexpected relationship might be related to a large-scale heat flux from atmosphere-ocean interactions. Indeed SST is not only affected by ocean upwelling, but also depends on complex ocean-atmosphere interactions (McCabe et al 2015;Tim et al 2015).…”

Section: Wind Stress and Upward Ocean Mass Transportmentioning

confidence: 99%

The future of coastal upwelling in the Humboldt current from model projections

Oyarzún

Brierley

2018

Clim Dyn

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The Humboldt coastal upwelling system in the eastern South Pacific ocean is one of the most productive marine ecosystems in the world. A weakening of the upwelling activity could lead to severe ecological impacts. As coastal upwelling in eastern boundary systems is mainly driven by wind stress, most studies so far have analysed wind patterns change through the 20th and 21st Centuries in order to understand and project the phenomenon under specific forcing scenarios. Mixed results have been reported, and analyses from General Circulation Models have suggested even contradictory trends of wind stress for the Humboldt system. In this study, we analyse the ocean upwelling directly in 13 models contributing to phase 5 of the Coupled Model Intercomparison Project (CMIP5) in both the historical simulations and an extreme climate change scenario (RCP8.5). The upwelling is represented by the upward ocean mass flux, a newly-included variable that represents the vertical water transport. Additionally, wind stress, ocean stratification, Ekman layer depth and thermocline depth were also analysed to explore their interactions with coastal upwelling throughout the period studied. The seasonal cycle of coastal upwelling differs between the Northern and Southern Humboldt areas. At lower latitudes, the upwelling season spans most of the autumn, winter and spring. However, in the Southern Humboldt area the upwelling season takes place in spring and the summertime with downwelling activity in winter. This persists throughout the Historical and RCP8.5 simulations. For both the Northern and Southern Humboldt areas an increasing wind stress is projected. However, different trends of upwelling intensity are observed away from the sea surface. Whereas wind stress will continue controlling the decadal variability of coastal upwelling on the whole ocean column analysed (surface to 300 m depth), an increasing disconnect with upwelling intensity is projected below 100 m depth throughout the 21st Century. This relates to an intensification of ocean stratification under global warming as shown by the sea water temperature profiles. Additionally, a divergence between the Ekman layer and thermocline depths is also evidenced. Given the interaction of upwelled nutrients and microscopic organisms essential for fish growth, a potential decline of coastal upwelling at depth could lead to unknown ecological and socio-economical effects.

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Decadal variability and trends of the Benguela upwelling system as simulated in a high-resolution ocean simulation

Cited by 65 publications

References 64 publications

The Impact of Agulhas Leakage on the Central Water Masses in the Benguela Upwelling System From A High‐Resolution Ocean Simulation

The Impact of Agulhas Leakage on the Central Water Masses in the Benguela Upwelling System From A High‐Resolution Ocean Simulation

The importance of external climate forcing for the variability and trends of coastal upwelling in past and future climate

The future of coastal upwelling in the Humboldt current from model projections

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