Southeastern Tropical Atlantic Changing From Subtropical to Tropical Conditions

A warming and freshening trend of the mixed layer in the upper southeastern tropical Atlantic Ocean (SETA) is observed by the Argo float array during the time period of 2006–2020. The associated ocean surface density reduction impacts upper-ocean stratification that intensified by more than 30% in the SETA region since 2006. The initial typical subtropical stratification with a surface salinity maximum is shifting to more tropical conditions characterized by warmer and fresher surface waters and a subsurface salinity maximum. During the same period isopycnal surfaces in the upper 200 m are shoaling continuously. Observed wind stress changes reveal that open ocean wind curl-driven upwelling increased, however, partly counteracted by reduced coastal upwelling due to weakened alongshore southerly winds. Weakening southerly winds might be a reason why tropical surface waters spread more southward reaching further into the SETA region. The mixed layer warming and freshening and associated stratification changes might impact the marine ecosystem and pelagic fisheries in the Angolan and northern Namibian upwelling region.

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Southeast tropical Atlantic changing from subtropical to tropical conditions

10.5194/egusphere-egu21-833 ◽

2021 ◽

Author(s):

Marisa Roch ◽

Peter Brandt ◽

Sunke Schmidtko

Keyword(s):

Wind Stress ◽

Surface Waters ◽

Primary Productivity ◽

Coastal Upwelling ◽

Ocean Dynamics ◽

Upper Ocean ◽

Tropical Atlantic ◽

Ocean Stratification ◽

Tropical Conditions ◽

Stress Changes

<div> <p><span>A warming and freshening trend of the mixed layer in the upper southeast tropical Atlantic Ocean (SETA) is observed by the Argo observation array during the time period of 2006 to 2019. Thus, the ocean surface density is reducing. This has an impact on the upper-ocean stratification which intensified by more than 30 % in the SETA region since 2006. The initial typical subtropical stratification with a salinity maximum at the surface is shifted to more tropical conditions characterized by warmer and fresher surface waters and a subsurface salinity maximum. </span></p> </div><div> <p><span>A more detailed analysis of isopycnals shows a continuous upward displacement of isopycnal surfaces suggesting that wind stress curl-driven upwelling has to play an essential role. Therefore, ASCAT wind stress changes are examined, revealing that increased open ocean wind curl-driven upwelling but also partly counteracting reduced coastal upwelling due to weakened alongshore southerly winds are present. Changing alongshore winds might be a reason why tropical surface waters spread further southward reaching more into the SETA region. Besides, atmospheric fluxes could further impact upper ocean characteristics. </span></p> </div><div> <p><span>Changes in the upper-ocean stratification matter as they affect not only physical ocean dynamics such as ocean ventilation processes but also biogeochemical and ecological activities such as nutrient fluxes and fisheries. Nevertheless, the consequences of increased stratification for upwelling regions are not yet fully understood. The SETA upwelling system is a key region for enhanced nutrient supply to the euphotic zone and hence, a core nutrient source for high coastal primary productivity. </span></p> </div><p>We aim to assess the recent change of upper-ocean stratification towards tropical conditions at the sea surface in the SETA region and explore its driving mechanisms as well as possible consequences for the primary productivity and fisheries off Angola and Namibia, in order to improve our understanding of what is happening as a result of intensified upper-ocean stratification in upwelling regions.</p>

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Using Eulerian and Lagrangian Approaches to Investigate Wind-Driven Changes in the Southern Ocean Abyssal Circulation

Journal of Physical Oceanography ◽

10.1175/jpo-d-13-0108.1 ◽

2013 ◽

Vol 44 (2) ◽

pp. 662-675 ◽

Cited By ~ 8

Author(s):

Paul Spence ◽

Erik van Sebille ◽

Oleg A. Saenko ◽

Matthew H. England

Keyword(s):

Southern Ocean ◽

Wind Stress ◽

Climate Model ◽

Meridional Overturning Circulation ◽

Global Ocean ◽

Lagrangian Particle ◽

Stress Changes ◽

Abyssal Circulation ◽

Flow Pathways ◽

Ocean Wind

Abstract This study uses a global ocean eddy-permitting climate model to explore the export of abyssal water from the Southern Ocean and its sensitivity to projected twenty-first-century poleward-intensifying Southern Ocean wind stress. The abyssal flow pathways and transport are investigated using a combination of Lagrangian and Eulerian techniques. In an Eulerian format, the equator- and poleward flows within similar abyssal density classes are increased by the wind stress changes, making it difficult to explicitly diagnose changes in the abyssal export in a meridional overturning circulation framework. Lagrangian particle analyses are used to identify the major export pathways of Southern Ocean abyssal waters and reveal an increase in the number of particles exported to the subtropics from source regions around Antarctica in response to the wind forcing. Both the Lagrangian particle and Eulerian analyses identify transients as playing a key role in the abyssal export of water from the Southern Ocean. Wind-driven modifications to the potential energy component of the vorticity balance in the abyss are also found to impact the Southern Ocean barotropic circulation.

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Local Coupled Equatorial Variability versus Remote ENSO Forcing in an Intermediate Coupled Model of the Tropical Atlantic

Journal of Climate ◽

10.1175/jcli3922.1 ◽

2006 ◽

Vol 19 (20) ◽

pp. 5227-5252 ◽

Cited By ~ 14

Author(s):

Serena Illig ◽

Boris Dewitte

Keyword(s):

Mixed Layer ◽

El Niño ◽

Time Scale ◽

El Nino ◽

Coupled Model ◽

Tropical Atlantic ◽

Equatorial Atlantic ◽

Layer Model ◽

Intermediate Coupled Model ◽

Mixed Layer Model

Abstract The relative roles played by the remote El Niño–Southern Oscillation (ENSO) forcing and the local air–sea interactions in the tropical Atlantic are investigated using an intermediate coupled model (ICM) of the tropical Atlantic. The oceanic component of the ICM consists of a six-baroclinic mode ocean model and a simple mixed layer model that has been validated from observations. The atmospheric component is a global atmospheric general circulation model developed at the University of California, Los Angeles (UCLA). In a forced context, the ICM realistically simulates both the sea surface temperature anomaly (SSTA) variability in the equatorial band, and the relaxation of the Atlantic northeast trade winds and the intensification of the equatorial westerlies in boreal spring that usually follows an El Niño event. The results of coupled experiments with or without Pacific ENSO forcing and with or without explicit air–sea interactions in the equatorial Atlantic indicate that the background energy in the equatorial Atlantic is provided by ENSO. However, the time scale of the variability and the magnitude of some peculiar events cannot be explained solely by ENSO remote forcing. It is demonstrated that the peak of SSTA variability in the 1–3-yr band as observed in the equatorial Atlantic is due to the local air–sea interactions and is not a linear response to ENSO. Seasonal phase locking in boreal summer is also the result of the local coupling. The analysis of the intrinsic sustainable modes indicates that the Atlantic El Niño is qualitatively a noise-driven stable system. Such a system can produce coherent interdecadal variability that is not forced by the Pacific or extraequatorial variability. It is shown that when a simple slab mixed layer model is embedded into the system to simulate the northern tropical Atlantic (NTA) SST variability, the warming over NTA following El Niño events have characteristics (location and peak phase) that depend on air–sea interaction in the equatorial Atlantic. In the model, the interaction between the equatorial mode and NTA can produce a dipolelike structure of the SSTA variability that evolves at a decadal time scale. The results herein illustrate the complexity of the tropical Atlantic ocean–atmosphere system, whose predictability jointly depends on ENSO and the connections between the Atlantic modes of variability.

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Impact of mesoscale eddies on salinity and CO2 ocean parameters in the western tropical Atlantic in February 2020

10.5194/egusphere-egu21-1253 ◽

2021 ◽

Author(s):

Léa Olivier ◽

Jacqueline Boutin ◽

Nathalie Lefèvre ◽

Gilles Reverdin ◽

Peter Landschützer ◽

...

Keyword(s):

Dissolved Inorganic Carbon ◽

Salinity Gradient ◽

Sea Surface Salinity ◽

Amazon River ◽

Tropical Atlantic ◽

Surface Salinity ◽

The North ◽

Brazil Current ◽

North Brazil

<p>Large oceanic eddies are formed by the retroflection of the North Brazil Current (NBC) near 8&#176;N in the western tropical Atlantic. The EUREC<sup>4</sup>A-OA/Atomic cruise took place in January - February 2020, and extensively documented two NBC rings. The NBC flows northward across the Equator and pass the mouth of the Amazon River, entraining fresh and nutrient-rich water along its nearshore edge. From December to March, the Amazon river discharge is low but a freshwater filament stirred by a NBC ring was nevertheless observed. The strong salinity gradient can be used to delineate the NBC ring during its initial phase and its westward propagation. Using satellite sea surface salinity and ocean color associated to in-situ measurements of salinity, temperature, dissolved inorganic carbon, alkalinity and fugacity of CO<sub>2</sub> we characterize the salinity and biogeochemical signature of NBC rings.</p>

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Salinity-induced mixed and barrier layers in the southwestern tropical Atlantic Ocean off the northeast of Brazil

Ocean Science ◽

10.5194/os-7-63-2011 ◽

2011 ◽

Vol 7 (1) ◽

pp. 63-73 ◽

Cited By ~ 6

Author(s):

M. Araujo ◽

C. Limongi ◽

J. Servain ◽

M. Silva ◽

F. S. Leite ◽

...

Keyword(s):

Barrier Layer ◽

Austral Summer ◽

Late Winter ◽

Brazilian Coast ◽

Tropical Atlantic ◽

Austral Winter ◽

Surface Salinity ◽

Barrier Layer Thickness ◽

Near Surface ◽

Mixed Layers

Abstract. High-resolution hydrographic observations of temperature and salinity are used to analyze the formation and distribution of isothermal depth (ZT), mixed depth (ZM) and barrier layer thickness (BLT) in a section of the southwestern Atlantic (0°30´ N–14°00´ S; 31°24´–41°48´ W), adjacent to the northeastern Brazilian coast. Analyzed data consists of 279 CTD casts acquired during two cruises under the Brazilian REVIZEE Program. One occurred in late austral winter (August–October 1995) and another in austral summer (January–April 1997). Oceanic observations are compared to numerical modeling results obtained from the French Mercator-Coriolis Program. Results indicate that the intrusion of subtropical Salinity Maximum Waters (SMW) is the major process contributing to the seasonal barrier layer formation. These waters are brought by the South Equatorial Current (SEC), from the subtropical region, into the western tropical Atlantic boundary. During late austral winter southeastern trade winds are more intense and ITCZ precipitations induce lower surface salinity values near the equator. During this period a 5–90 m thick BLT (median = 15 m) is observed and BLT > 30 m is restricted to latitudes higher than 8° S, where the intrusion of salty waters between 8°–12.3° S creates shallow mixed layers over deep (ZT ≥ 90 m) isothermal layers. During austral summer, shallow isothermal and mixed layers prevail, when northeasterly winds are predominant and evaporation overcomes precipitation, causing saltier waters at the surface/subsurface layers. During that period observed BLT varies from 5 to 70 m and presents thicker median value of 35 m, when comparing to the winter. Furthermore, BLT ≥ 30 m is observed not only in the southernmost part of the study area, as verified during late winter, but in the latitude range 2°–14° S, where near-surface salty waters are transported westward by the SEC flow. These results indicate that the inclusion of salinity dynamics and its variability are necessary for studying mixed and barrier layer behaviors in the tropical Atlantic, where ocean-atmosphere coupling is known to be stronger.

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

Ocean Science ◽

10.5194/os-11-1-2015 ◽

2015 ◽

Vol 11 (1) ◽

pp. 1-11 ◽

Cited By ~ 16

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.

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Plankton blooms in vortices: the role of biological and hydrodynamic timescales

Nonlinear Processes in Geophysics ◽

10.5194/npg-14-443-2007 ◽

2007 ◽

Vol 14 (4) ◽

pp. 443-454 ◽

Cited By ~ 43

Author(s):

M. Sandulescu ◽

C. López ◽

E. Hernández-García ◽

U. Feudel

Keyword(s):

Coastal Upwelling ◽

Marine Ecosystem ◽

Coupled System ◽

Component Model ◽

Residence Times ◽

Key Factors ◽

Mesoscale Structures

Abstract. We study the interplay of hydrodynamic mesoscale structures and the growth of plankton in the wake of an island, and its interaction with a coastal upwelling. Our focus is on a mechanism for the emergence of localized plankton blooms in vortices. Using a coupled system of a kinematic flow mimicking the mesoscale structures behind the island and a simple three component model for the marine ecosystem, we show that the long residence times of nutrients and plankton in the vicinity of the island and the confinement of plankton within vortices are key factors for the appearance of localized plankton blooms.

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Environmental control on the variability of DMS and DMSP in the Mauritanian upwelling region

Biogeosciences ◽

10.5194/bg-9-1041-2012 ◽

2012 ◽

Vol 9 (3) ◽

pp. 1041-1051 ◽

Cited By ~ 22

Author(s):

C. Zindler ◽

I. Peeken ◽

C. A. Marandino ◽

H. W. Bange

Keyword(s):

Surface Waters ◽

Coastal Upwelling ◽

Environmental Control ◽

Hot Spot ◽

Local Source ◽

Surface Seawater ◽

Near Surface ◽

Northwest Africa ◽

N:P Ratios ◽

A Minor

Abstract. Dimethylsulphide (DMS) and dissolved and particulate dimethylsulfoniopropionate (DMSPd, DMSPp) were measured in near-surface waters along the Mauritanian coast, Northwest Africa, during the upwelling season in February 2008. DMS, DMSPd and DMSPp surface concentrations of up to 10 nmol L−1, 15 nmol L−1 and 990 nmol L−1, respectively, were measured. However, the DMS concentrations measured are in the low range compared to other upwelling regions. The maximum DMSPp concentration is the highest reported from upwelling regions so far, which might indicate that the Mauritanian upwelling is a hot spot for DMSP. Within the phytoplankton groups, dinoflagellates were identified as important contributors to DMS concentrations, while other algae seemed to have only a minor or no influence on DMS and DMSP concentrations. A pronounced switch from high DMSP to high DMS concentrations was observed when the nitrogen to phosphorus ratio (N:P) was below 7. The high DMS/DMSP ratios at N:P ratios <7 indicate that nitrogen limitation presumably triggered a switch from DMSP to DMS independent of the species composition. Our results underline the importance of coastal upwelling regions as a local source for surface seawater sulphur.

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External forcing mechanisms controlling the North Atlantic coastal upwelling regime during the mid-Holocene

Geology ◽

10.1130/g48112.1 ◽

2020 ◽

Author(s):

Armand Hernández ◽

Mário Cachão ◽

Pedro Sousa ◽

Ricardo M. Trigo ◽

Jürg Luterbacher ◽

...

Keyword(s):

North Atlantic ◽

Coastal Upwelling ◽

Marine Ecosystem ◽

External Forcing ◽

The North ◽

New Findings ◽

The North Atlantic ◽

Eastern North Atlantic ◽

Forcing Mechanisms ◽

The Impact

Nearshore upwelling along the eastern North Atlantic margin regulates regional marine ecosystem productivity and thus impacts blue economies. While most global circulation models show an increase in the intensity and duration of seasonal upwelling at high latitudes under future human-induced warmer conditions, projections for the North Atlantic are still ambiguous. Due to the low temporal resolution of coastal upwelling records, little is known about the impact of natural forcing mechanisms on upwelling variability. Here, we present a microfossil-based proxy record and modeling simulations for the warmest period of the Holocene (ca. 9–5 ka) to estimate the contribution of the natural variability in North Atlantic upwelling via atmospheric and oceanic dynamics. We found that more frequent high-pressure conditions in the eastern North Atlantic associated with solar activity and orbital parameters triggered upwelling variations at multidecadal and millennial time scales, respectively. Our new findings offer insights into the role of external forcing mechanisms in upwelling changes before the Anthropocene, which must be considered when producing future projections of midlatitude upwelling activity.

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Investigation of the abundance, distribution and composition of microplastics at coastal upwelling sites in the Atlantic Ocean

10.7287/peerj.preprints.1629v1 ◽

2016 ◽

Author(s):

La Daana K Kanhai ◽

Rick Officer ◽

Ian O'Connor ◽

Richard C Thompson

Keyword(s):

Atlantic Ocean ◽

Surface Waters ◽

Primary Productivity ◽

Null Hypothesis ◽

Latitudinal Gradient ◽

Coastal Upwelling ◽

Abundance Distribution ◽

Significant Difference ◽

Benguela Upwelling ◽

Canary Upwelling

Microplastics are an issue of international concern due to the fact that these substances may potentially threaten biota by (i) causing physical harm, (ii) transporting persistent, bioaccumulating and toxic (PBT) substances and, (iii) leaching plastic additives. Within the world’s oceans, areas which experience coastal upwelling are biota rich due to their high levels of primary productivity. The assessment of microplastic presence in areas which experience coastal upwelling is vital as it will indicate whether microplastics are an issue of concern in areas which support key biological resources. The null hypothesis of the present study is that microplastic abundance will be lower in areas where there is upwelling. As such, the present study aims to investigate whether microplastic abundance in upwelled areas in the Atlantic Ocean is significantly different from non-upwelled areas. Based on an opportunistic voyage aboard the RV Polarstern, microplastics will be sampled in sub-surface waters along a diverse latitudinal gradient in the Atlantic Ocean i.e. from Bremerhaven (Germany) to Cape Town (South Africa). Based on the proposed route, it will be possible to determine microplastic levels at two areas of coastal upwelling in the Atlantic Ocean (i) Canary Upwelling Ecosystem (CUE) and (ii) Benguela Upwelling Ecosystem (BUE). The results will then be analysed to determine whether there was a statistically significant difference between ‘upwelled areas’ and ‘non-upwelled areas’.

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