Southeast tropical Atlantic changing from subtropical to tropical conditions

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>

scholarly journals Southeastern Tropical Atlantic Changing From Subtropical to Tropical Conditions

2021 ◽  
Vol 8 ◽  
Author(s):  
Marisa Roch ◽  
Peter Brandt ◽  
Sunke Schmidtko ◽  
Filomena Vaz Velho ◽  
Marek Ostrowski
Keyword(s):  
Mixed Layer ◽  
Surface Waters ◽  
Coastal Upwelling ◽  
Marine Ecosystem ◽  
Tropical Atlantic ◽  
Surface Salinity ◽  
Density Reduction ◽  
Tropical Conditions ◽  
Stress Changes ◽  
Ocean Wind

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.

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

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’.

Control of Decadal and Bidecadal Climate Variability in the Tropical Pacific by the Off-Equatorial South Pacific Ocean

2013 ◽  
Vol 26 (17) ◽  
pp. 6524-6534 ◽  
Author(s):  
Hiroaki Tatebe ◽  
Yukiko Imada ◽  
Masato Mori ◽  
Masahide Kimoto ◽  
Hiroyasu Hasumi
Keyword(s):  
Time Scales ◽  
Wind Stress ◽  
Climate Model ◽  
South Pacific ◽  
Ocean Model ◽  
Ocean Dynamics ◽  
Wind Stress Curl ◽  
Mode Water ◽  
Tropical Variability ◽  
Stress Changes

Abstract Delayed negative feedback processes determining intrinsic decadal and bidecadal time scales for the tropical variability in the Pacific are investigated based on climate model experiments. By comparing a control run driven by preindustrial forcing and partial blocking runs driven by the same forcing but with ocean temperature and salinity restored to climatology in selected regions, subsurface oceanic signals of South Pacific origin are shown to precede SST variability in the Niño-3.4 region. Using a linear reduced-gravity ocean model driven only by wind stress changes and an offline tracer model, oceanic wave adjustment triggered by changes of wind stress curl in the South Pacific extratropics is suggested to be essential for the decadal component of the equatorial SST, while slower isopycnal advection of subsurface temperature anomalies from the formation region of South Pacific Eastern Subtropical Mode Water controls the bidecadal component. The intrinsic time scales of the tropical variability are regulated by simple linear ocean dynamics.

scholarly journals Observational evidence of stratification control of upwelling and pelagic fishery in the eastern Arabian Sea

2021 ◽  
Vol 11 (1) ◽  
Author(s):  
Jayu Narvekar ◽  
Riyanka Roy Chowdhury ◽  
Diksha Gaonkar ◽  
P. K. Dinesh Kumar ◽  
S. Prasanna Kumar
Keyword(s):  
Arabian Sea ◽  
Coastal Upwelling ◽  
Physical Phenomenon ◽  
Upper Ocean ◽  
Ocean Stratification ◽  
Water Influx ◽  
Eastern Arabian Sea ◽  
First Time ◽  
Pelagic Fishery

AbstractUpwelling is a physical phenomenon that occurs globally along the eastern boundary of the ocean and supports pelagic fishery which is an important source of protein for the coastal population. Though upwelling and associated small pelagic fishery along the eastern Arabian Sea (EAS) is known to exist at least for the past six decades, our understanding of the factors controlling them are still elusive. Based on observation and data analysis we hypothesize that upwelling in the EAS during 2017 was modulated by freshwater-induced stratification. To validate this hypothesis, we examined 17 years of data from 2001 and show that inter-annual variability of freshwater influx indeed controls the upwelling in the EAS through stratification, a mechanism hitherto unexplored. The upper ocean stratification in turn is regulated by the fresh water influx through a combination of precipitation and river runoff. We further show that the oil sardine which is one of the dominant fish of the small pelagic fishery of the EAS varied inversely with stratification. Our study for the first time underscored the role of freshwater influx in regulating the coastal upwelling and upper ocean stratification controlling the regional pelagic fishery of the EAS.

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

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’.

Variability of Intraseasonal Kelvin Waves in the Equatorial Pacific Ocean

2008 ◽  
Vol 38 (5) ◽  
pp. 921-944 ◽  
Author(s):  
Toshiaki Shinoda ◽  
Paul E. Roundy ◽  
George N. Kiladis
Keyword(s):  
Wind Stress ◽  
General Circulation ◽  
Phase Speed ◽  
Circulation Model ◽  
Kelvin Waves ◽  
Upper Ocean ◽  
Equatorial Pacific Ocean ◽  
Model Experiments ◽  
Stress Changes ◽  
Background State

Abstract Previous observational work has demonstrated that the phase speed of oceanic equatorial Kelvin waves forced by the Madden–Julian oscillation (MJO) appears to vary substantially. Processes that are responsible for systematic changes in the phase speed of these waves are examined using an ocean general circulation model. The model was integrated for 26 yr with daily wind stress derived from the NCEP–NCAR reanalysis. The model is able to reproduce observed systematic changes of Kelvin wave phase speed reasonably well, providing a tool for the analysis of their dynamics. The relative importance of the upper ocean background state and atmospheric forcing for phase speed changes is determined based on a series of model experiments with various surface forcings. Systematic changes in phase speed are evident in all model experiments that have different slowly varying basic states, showing that variations of the upper ocean background state are not the primary cause of the changes. The model experiments that include and exclude intraseasonal components of wind stress in the eastern Pacific demonstrate that wind stress changes to the east of the date line can significantly alter the speed of Kelvin waves initially generated over the western Pacific, which often results in a phase propagation faster than the free wave speed. These faster waves contribute to the systematic changes of phase speed evident in observations. Similar results are also obtained using a linear stratified model, eliminating nonlinearity as a possible cause of the phase speed changes.

scholarly journals Subsurface Tropical Instability Waves in the Atlantic Ocean in Model and Observations 

2021 ◽  
Author(s):  
Mia Sophie Specht ◽  
Johann Jungclaus ◽  
Jürgen Bader
Keyword(s):  
Atlantic Ocean ◽  
Vertical Mixing ◽  
Ocean Surface ◽  
Ocean Dynamics ◽  
Upper Ocean ◽  
Tropical Atlantic ◽  
Mean Flow ◽  
Instability Waves ◽  
Tropical Atlantic Ocean ◽  
Tropical Instability Waves

<div> <p><span>Tropical instability waves (TIWs) near the ocean surface are present in all tropical oceans and are known to be important for air-sea interactions and regional climate variability. Recent studies based on observations in the Pacific Ocean found that apart from TIWs at the surface, there also exist subsurface TIWs (subTIWs) which can alter vertical mixing. To date, most studies have focused on TIW related dynamics near the ocean surface. However, to properly assess vertical mixing in the upper ocean, improved understanding of the vertical structure of TIWs and the influence of subTIWs is needed. In this study, </span>we show subTIW<span> presence</span> in the Atlantic Ocean for the first time using mooring observations.<span>Further, we characterize subTIWs in the tropical Atlantic Ocean with a special focus on subTIW spatial and temporal variability and their effect on mixing. For this, data covering almost two decades are used that were generated from a comprehensive, global, high-resolution ocean model forced by the reanalysis ERA5. We find subTIWs between 40 m depth and the thermocline in both model and observations and unlike TIWs, subTIWs are frequently active both north and south of the Equator. The results of our study suggest that subTIWs induce a multi-layer shear structure which has the potential to destabilize the mean flow and thereby cause mixing. These effects are strongest north of the Equator where TIWs and subTIWs act simultaneously, implying possible TIW/subTIW interactions. </span>We conclude that subTIWs are a feature of the tropical Atlantic Ocean with regionally varying implications for vertical mixing and heat fluxes. <span>In addition, subTIWs differ from TIWs in their temporal and regional occurrences Therefore, subTIWs should be considered in </span>f<span>u</span>ture assessments of upper ocean dynamics, particularly in subTIW dominated regions.</p> </div>

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