scholarly journals Relation between Upwelling Intensity and the Variability of Physical and Chemical Parameters in the Southern Benguela Upwelling System

2015 ◽  
Vol 2015 ◽  
pp. 1-8 ◽  
Author(s):  
Hassan Ebrahiem Ismail ◽  
Johannes Jacobus Agenbag ◽  
Stephanie de Villiers ◽  
Bhekumusa Jabulani Ximba
Keyword(s):  
Temperature Gradient ◽  
Vertical Mixing ◽  
Surface Heating ◽  
Nutrient Concentrations ◽  
Upper Ocean ◽  
Ocean Temperature ◽  
Upwelling System ◽  
Benguela Upwelling ◽  
Upwelling Intensity ◽  
Upwelling Index

The extent to which wind-driven seasonal upwelling cycles manifest in surface ocean temperature and nutrient variability along a monitoring line in the Southern Benguela upwelling system was investigated. Monitoring conducted monthly over a six-year period shows that surface temperature and nutrient concentrations exhibit very poor seasonality and weak correlation with the upwelling index. This is, despite clear evidence for spatial inshore-offshore gradients in temperature, nutrients, and chlorophyll, consistent with an upwelling regime. The upper ocean temperature gradient shows a much better correspondence to the upwelling index but at the same time demonstrates that surface heating, and not vertical mixing related to upwelling, controls the upper ocean temperature gradient. The results suggest that remote sensing techniques would be inadequate tools to monitor upwelling events in the Southern Benguela. Also, the incidence of phytoplankton blooms is more likely triggered by stratified conditions associated with surface heating than relaxation of upwelling winds.

scholarly journals Spatio-temporal patterns of C : N : P ratios in the northern Benguela upwelling system

2014 ◽  
Vol 11 (3) ◽  
pp. 885-897 ◽  
Author(s):  
A. Flohr ◽  
A. K. van der Plas ◽  
K.-C. Emeis ◽  
V. Mohrholz ◽  
T. Rixen
Keyword(s):  
N2 Fixation ◽  
South Atlantic ◽  
Subtropical Gyre ◽  
Total Alkalinity ◽  
Nutrient Concentrations ◽  
The South ◽  
Upwelling System ◽  
Benguela Upwelling ◽  
Bottom Waters ◽  
The Impact

Abstract. On a global scale the ratio of fixed nitrogen (N) and phosphate (P) is characterized by a deficit of N with regard to the classical Redfield ratio of N : P = 16 : 1 reflecting the impact of N loss occurring in the oceanic oxygen minimum zones. The northern Benguela upwelling system (NBUS) is known for losses of N and the accumulation of P in sub- and anoxic bottom waters and sediments of the Namibian shelf resulting in low N : P ratios in the water column. To study the impact of the N : P anomalies on the regional carbon cycle and their consequences for the export of nutrients from the NBUS into the oligotrophic subtropical gyre of the South Atlantic, we measured dissolved inorganic carbon (CT), total alkalinity (AT), oxygen (O2) and nutrient concentrations in February 2011. The results indicate increased P concentrations over the Namibian shelf due to P efflux from sediments resulting in a C : N : P : -O2 ratio of 106 : 16 : 1.6 : 138. N reduction further increase C : N and reduce N : P ratios in those regions where O2 concentrations in bottom waters are < 20 μmol kg−1. However, off the shelf along the continental margin, the mean C : N : P : -O2 ratio is again close to the Redfield stoichiometry. Additional nutrient data measured during two cruises in 2008 and 2009 imply that the amount of excess P, which is created in the bottom waters on the shelf, and its export into the subtropical gyre after upwelling varies through time. The results further reveal an inter-annual variability of excess N within the South Atlantic Central Water (SACW) that flows from the north into the NBUS, with highest N values observed in 2008. It is postulated that the N excess in SACW occurred due to the impact of remineralized organic matter produced by N2 fixation and that the magnitude of excess P formation and its export is governed by inputs of excess N along with SACW flowing into the NBUS. Factors controlling N2 fixation north of the BUS need to be addressed in future studies to better understand the role of the NBUS as a P source and N sink in the coupled C : N : P cycles.

scholarly journals Oxygen and Nutrient Trapping in the Southern Benguela Upwelling System

2021 ◽  
Vol 8 ◽  
Author(s):  
Tim Rixen ◽  
Niko Lahajnar ◽  
Tarron Lamont ◽  
Rolf Koppelmann ◽  
Bettina Martin ◽  
...  
Keyword(s):  
Global Warming ◽  
Global Change ◽  
Trapping Efficiency ◽  
Nutrient Concentrations ◽  
Ecosystem Structure ◽  
Austral Spring ◽  
Critical Feature ◽  
Upwelling System ◽  
Benguela Upwelling ◽  
The Impact

The Benguela Upwelling System in the southeast Atlantic Ocean is of crucial socio-economic importance due to its high productivity. However, predicting its response to global change and understanding past changes are still great challenges. Here, we compile data obtained from a research cruise and an oceanographic mooring to demonstrate that a topographically steered nutrient trapping zone develops in a narrow belt along the coast during the main upwelling season in austral spring and summer in the southern Benguela Upwelling System. High nutrient concentrations within this zone increase the impact of upwelling on the productivity of the southern Benguela Upwelling System, but the efficient nutrient trapping operates at the expense of decreasing oxygen concentrations. This enhances the probability of anoxic events emerging toward the end of the upwelling season. However, at the end of the upwelling season, the front that separates the coastally trapped waters from open shelf waters weakens or even collapses due to upwelling cessation and the reversing current regime. This, in addition to a stronger vertical mixing caused by winter cooling, fosters the ventilation of the nutrient trapping zone, which reestablishes during the following upwelling season. The postulated intensification of upwelling and changes in the ecosystem structure in response to global warming seem to reduce the nutrient trapping efficiency by increasing offshore advection of surface waters and plankton blooms. The intensified upwelling and resulting lower biological oxygen consumption appears to mask the expected impacts of global warming on the oxygen minimum zone (OMZ) in the southern Benguela Upwelling System. In contrast to other OMZs, including those in northern Benguela Upwelling Systems, the OMZ in the southern Benguela Upwelling System reveals so far no detectable long-term decrease in oxygen. Thus, the nutrient trapping efficiency seems to be a critical feature mitigating global change impacts on the southern Benguela Upwelling System. Since it is topographically steered, regional impacts on the nutrient trapping efficiency appear also to explain varying responses of upwelling systems to global change as the comparison between southern and northern Benguela Upwelling System shows. This emphasizes the need for further and more comparable studies in order to better understand the response of Eastern Boundary Upwelling Systems and their ecosystem services to global change.

scholarly journals Performance of long-chain mid-chan diol based temperature and productivity proxies at test: a five-years sediment trap record from the Mauritanian upwelling

2021 ◽  
Author(s):  
Gerard J. M. Versteegh ◽  
Karin A. F. Zonneveld ◽  
Jens Hefter ◽  
Oscar E. Romero ◽  
Gerhard Fischer ◽  
...  
Keyword(s):  
Wind Speed ◽  
Sediment Trap ◽  
Saturation Index ◽  
Sea Surface ◽  
Nutrient Concentrations ◽  
Upper Ocean ◽  
Trap Site ◽  
Trap Location ◽  
Upwelling Intensity ◽  
Long Chain

Abstract. Long-chain mid-chain diol (shortly diol) based proxies obtain increasing interest to reconstruct past upper ocean temperature and productivity. Here we evaluate performance of the sea surface temperature proxies; long chain diol index (LDI), the diol saturation index (DSI) and the diol chain-length index (DCI), productivity/upwelling intensity proxies: the two diol indices DIR (Rampen et al., 2008) and DIW (Willmott et al., 2010) and the combined diol index (CDI), as well as the nutrient diol index (NDI) as proxy for phosphate and nitrate levels. This evaluation is based on comparison of the diols in sediment trap samples from the upwelling region off NW Africa collected at 1.28 km water depth with daily satellite derived sea surface temperatures (SSTSAT), subsurface temperatures, productivity, the plankton composition from the trap location, monthly PO43− and NO3− concentrations, wind speed and wind direction from the nearby Nouadhibou airport. The diol based SST reconstructions are also compared the long chain alkenones based UK’37 proxy reconstructions (SSTUK). At the trap site, most diol proxies lag wind speed (phase φ = 30 days) and can be related to upwelling. Correlation with the abundance of upwelling species and wind speed is best for the DCI, DSI and NDI whereas the DI and CDI perform comparatively poorly. The nutrient proxy NDI shows no significant correlation to monthly PO43− and NO3− concentrations in the upper waters and a negative correlation with wind-induced upwelling (r2 = 0.28, φ = 32 days) as well as the abundance of upwelling species (r2 = 0.38; Table 4). It is suggested that this proxy reflects upwelling intensity rather than upper ocean nutrient concentrations. At the trap site, SSTSAT lags wind speed forced upwelling by about 4 months (φ = 129 d). The LDI based SST (SSTLDI) correlate poorly (r2 = 0.17) to SSTSAT which we attribute to variability in 1,13 diol abundance unrelated to SST such as productivity. The SSTUK correlates best with SSTSAT (r2 = 0.60). Also amplitude and absolute values agree very well and the flux corrected SSTUK time series average equals the SSTSAT annual average.

Late-Holocene dynamics of sea-surface temperature and terrestrial hydrology in southwestern Africa

The Holocene ◽  
2017 ◽  
Vol 28 (5) ◽  
pp. 695-705 ◽  
Author(s):  
Robyn Granger ◽  
Michael E Meadows ◽  
Annette Hahn ◽  
Matthias Zabel ◽  
Jan-Berend W Stuut ◽  
...  
Keyword(s):  
Surface Temperature ◽  
Sea Surface Temperature ◽  
Late Holocene ◽  
Sea Surface ◽  
Temperate Climate ◽  
Upwelling System ◽  
Important Region ◽  
Benguela Upwelling ◽  
Geochemical Analyses ◽  
Upwelling Intensity

Southwest Africa is an important region for paleo-climatic studies, being influenced by both tropical and temperate climate systems and thus reflecting the interplay of variable controls. The aim of this study was to unravel the interaction of sea-surface temperature (SST) changes in the southernmost Benguela upwelling system with precipitation changes in South Africa’s winter rainfall zone (WRZ) during the late Holocene. Therefore, a marine sediment core from the southernmost Benguela upwelling system was investigated to reconstruct climate changes in this region for the past ~2000 years. Grain size and geochemical analyses were conducted to reconstruct changes in fluvial sediment discharge and weathering intensity, while SST changes were estimated using alkenone paleo-thermometry. Results show that the southernmost Benguela behaves distinctly in comparison with the rest of the Benguela system reflecting amplified SST changes. Decreasing SSTs accompanied increasing river discharge during times of increased precipitation in the WRZ, reflecting northerly shifted westerly winds during austral winter. We infer a control of past SST changes by processes not analogous to modern processes driving seasonal SST changes by changes through upwelling intensity. The findings suggest that late-Holocene SST changes in the southernmost Benguela upwelling system and the precipitation in the WRZ were both driven by latitudinal shifts of the austral westerly wind belt and associated changes in advection of cold sub-Antarctic waters and/or changes in Agulhas leakage of warm Indian Ocean waters.

scholarly journals Is the World Ocean Warming? Upper-Ocean Temperature Trends: 1950–2000

2007 ◽  
Vol 37 (2) ◽  
pp. 174-187 ◽  
Author(s):  
D. E. Harrison ◽  
Mark Carson
Keyword(s):  
Vertical Structure ◽  
World Ocean ◽  
Time Variability ◽  
Upper Ocean ◽  
Ocean Temperature ◽  
Similar Magnitude ◽  
Subsurface Temperature ◽  
Temperature Trends ◽  
Uncertainty Estimates ◽  
The World

Abstract Subsurface temperature trends in the better-sampled parts of the World Ocean are reported. Where there are sufficient observations for this analysis, there is large spatial variability of 51-yr trends in the upper ocean, with some regions showing cooling in excess of 3°C, and others warming of similar magnitude. Some 95% of the ocean area analyzed has both cooled and warmed over 20-yr subsets of this period. There is much space and time variability of 20-yr running trend estimates, indicating that trends over a decade or two may not be representative of longer-term trends. Results are based on sorting individual observations in World Ocean Database 2001 into 1° × 1° and 2° × 2° bins. Only bins with at least five observations per decade for four of the five decades since 1950 are used. Much of the World Ocean cannot be examined from this perspective. The 51-yr trends significant at the 90% level are given particular attention. Results are presented for depths of 100, 300, and 500 m. The patterns of the 90% significant trends are spatially coherent on scales resolved by the bin size. The vertical structure of the trends is coherent in some regions, but changes sign between the analysis depths in a number of others. It is suggested that additional attention should be given to uncertainty estimates for basin average and World Ocean average thermal trends.

Seasonal and interannual phytoplankton dynamics and forcing mechanisms in the Northern Benguela upwelling system

2016 ◽  
Vol 157 ◽  
pp. 124-134 ◽  
Author(s):  
Deon C. Louw ◽  
Anja K. van der Plas ◽  
Volker Mohrholz ◽  
Norbert Wasmund ◽  
Tim Junker ◽  
...  
Keyword(s):  
Phytoplankton Dynamics ◽  
Upwelling System ◽  
Benguela Upwelling ◽  
Forcing Mechanisms

Basin-Wide Connections of Upper-Ocean Temperature Variability in the Equatorial Indian Ocean

2021 ◽  
pp. 1-50
Author(s):  
Ge Song ◽  
Bohua Huang ◽  
Rongcai Ren ◽  
Zeng-Zhen Hu
Keyword(s):  
Indian Ocean ◽  
Equatorial Indian Ocean ◽  
Reanalysis Data ◽  
Upper Ocean ◽  
Ocean Temperature ◽  
Ocean Reanalysis ◽  
Near Surface ◽  
Surface Warming ◽  
Transition Mechanism ◽  
Wide Range

AbstractIn this paper, the interannual variability of upper-ocean temperature in the equatorial Indian Ocean (IO) and its basin-wide connections are investigated using 58-year (1958-2015) comprehensive monthly mean ocean reanalysis data. Three leading modes of an empirical orthogonal function (EOF) analysis dominate the variability of upper-ocean temperature in the equatorial IO in a wide range of timescales. A coherent interannual band within the first two EOF modes identifies an oscillation between the zonally tilting thermocline across the equatorial IO in its peak phases and basin-wide displacement of the equatorial thermocline in its transitional phases. Consistent with the recharge oscillation paradigm, this oscillation is inherent of the equatorial IO with a quasi-periodicity around 15 months, in which the wind-induced off-equatorial Rossby waves near 5°S-10°S provide the phase-transition mechanism. This intrinsic IO oscillation provides the biennial component in the observed IOD variations. The third leading mode shows a nonlinear long-term trend of the upper-ocean temperature, including the near-surface warming along the equatorial Indian Ocean, accompanied by cooling trend in the lower thermocline originating further south. Such vertical contrary trends may lead to an enhanced stratification in the equatorial IO.

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