OpenUCT :: Browsing by Author "Vichi, Marcello"

Browsing by Author "Vichi, Marcello"

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Open Access
A novel approach to investigating chlorophyll-a fluorescence quantum yield variability in the Southern Ocean
(2019) Bone, Emma Lewis; Vichi, Marcello; Thomalla, Sandy J; Bernard, Stewart; Smith, Marié E; Ryan-Keogh, Thomas. J
The apparent fluorescence quantum yield of chlorophyll-a (ΦF ), i.e. the ratio of photons emitted as chlorophyll-a fluorescence to those absorbed by phytoplankton, serves as a first order measure of photosynthetic efficiency and a photophysiological indicator of the resident phytoplankton community. Drivers of ΦF variability, including taxonomy, nutrient availability, and light history, differ in magnitude of influence across various biogeographic provinces and seasons. A Multi-Exciter Fluorometer (MFL, JFE Advantech Co., Ltd.) was selected for use in in situ ΦF derivation and underwent an extensive radiometric calibration for this purpose. Wavelength-specific ΦF was determined for 66 in situ field stations, sampled in the Atlantic Southern Ocean during the austral winter of 2012 and summer of 2013/ 2014. Phytoplankton pigments, macronutrient concentrations, and light levels were simultaneously measured to investigate their influence on ΦF . While no relationship was observed between macronutrient levels and ΦF , an inverse relationship between light and ΦF was apparent. This was likely due to the influence of speciesspecific fluorescence quenching mechanisms employed by local populations. ΦF derived from ocean colour products (Φsat) from the Moderate Resolution Imaging Spectroradiometer (MODIS) were compared to in situ ΦF to assess the performance of three existing Φsat algorithms. Results indicate that accounting for chlorophyll-a fluorescence reabsorption, the inherent optical properties of the surrounding water column, and the sensor angle of observation, is crucial to reducing Φsat uncertainty. A hybrid combination of two of the algorithms performed best, and was used to derive Φsat for stations co-located to in situ iron measurements in the Atlantic Southern Ocean. A significant negative relationship was observed, indicative of the effects of iron availability on quantum yield and its potential as a proxy for iron limitation. However, separating the individual contributions of light, taxonomy, and iron limitation to Φsat variability remains a challenge. A time series analysis of Φsat was also undertaken, which revealed a prominent Φsat seasonal cycle. Ultimately, increased in situ sampling would expedite the development of improved Φsat algorithms; the routine retrieval of Φsat would offer insight into phytoplankton dynamics in undersampled regions such as the climate relevant Southern Ocean.
Open Access
A preliminary investigation on the relationships between upwelling and commercial hake fishery in the Southern Benguela
(2021) Memela, Nkuleleko; Vichi, Marcello; Hall, Russel
The hake bottom trawl division is the largest component of the fishing industry in South Africa and it is one of the strong pillars of the food industry and the national economy. It is the main source of livelihood for many people in the West Coast and as such, finding ways of advancing it towards the direction of fourth industrial revolution is at the top of societal interests and a top priority for the major companies that are key players in the industry. Sea Harvest Group Limited is one of these key players and as such, it has undertaken to be a part of the study to improve predictability of fishing by collecting data which will contribute towards the scientific study of the patterns which determine the viability of some fishing locations over the others at different times under various conditions. The studied region is the West Coast grounds located in the southern Benguela at grid (32°S: 34°S, 16°E :19°E). The study is based on the hypothesis that the main driver of the availability of hake is the upwelling, separated into its coastal Ekman transport and curl-driven components. These two components of upwelling are driven by winds and they are known to stimulate primary production and support a larger marine food web. The correlations between these upwelling types and the mean monthly catch per unit effort (CPUE) of this region is assessed. The relationship between chlorophyll abundance and hake CPUE hypothesises lagged association of hake abundance to the underlying biological food chain driven by the upwelling events. A multiple regression model is then produced as a basic step towards quantification. The results suggest that Hake CPUE is lag correlated with upwelling and that some degree of predictability can be derived from the observation of combined upwelling patterns.
Open Access
Assessment of early 20th Century climate model simulations of Antarctic Sea ice using historical commercial humpback whale catch data
(2021) Mazomba, Thando; Vichi, Marcello
The story of Southern Hemisphere humpback whales through time presents multiple narratives. This study integrated two of those narratives to better understand our climate over time - the ecological behaviour of this species as well as their exploitation in the last century. The changing of our climate is largely better understood from the introduction of satellites in the late 20th century when data could be collected at higher spatial and temporal levels. Prior to this, data were scarce, especially for remote areas such as the Southern Ocean. The trust on climate models to produce valuable projections rely on how skilled they are in reproducing the historical climate; therefore their results require assessments against as many observations as possible to further increase their reliability. The Southern Ocean being an integral component to climate regulation, it is important to try understand its oceanographic features. The seasonal sea ice cover represents one major feature of this system. This study proposes to use other sources of data for the early 20th century that will help closing the gap prior to satellite observations. Humpback whales migrate poleward during the austral summer to feed on Antarctic krill at a proximity to the ice edge. Humpback whale catch locations in the early 20th century corroborate with this foraging behaviour. Using humpback whale catch location data as a benchmark, the study aims to assess the skill of climate models in simulating sea ice edge location for the early 20th century. Sea ice edge is directly related to sea-ice extent, which is an important variable in the research of sea ice dynamics over time. This is especially true in the face of rapid climate change where accuracy of sea ice changes is very important. The study therefore also aims to assess climate model climatological seasonal cycle of sea ice extent results from climate models against literature and contemporary observations. Comparisons between each model's results are also carried out. The humpback whale catch effort, as per IWC data, mostly covered the Atlantic and the Indian sector of the Southern Ocean. For this reason, the study focused on these two sectors for the analyses. Decade 1930-1939 showed the highest catch numbers consistently throughout the months of the study. The simulated past century and recent climatological seasonal cycle sea ice extent show a wide variety of responses between the models, with the majority of them underestimating the seasonal cycle based on previous literature and contemporary observations. This indicates the need to improve the sea ice physical processes in models to better capture the specific Southern Ocean processes. The ensemble median of ice edge location from the models apparently follow the latitudinal pattern of the whale catch locations, which are assumed in this study to mark the topography of the sea ice edge. However, they simulate a sea ice edge equatorward of the edge derived from an ensemble analysis of humpback whale catch locations. The variance explained by the coefficient of determination between the models and the whale catch distribution is low, with the highest value of one month being low as well. This indicates that only a portion of the simulated edge follows the reconstructed sea ice features.
Open Access
Assessment of the synoptic variability of the Antarctic marginal ice zone with in Situ observations
(2019) de Jong, Ehlke; Vichi, Marcello
Knowledge of sea ice variability, which contributes to the detection of climate change trends, stems primarily from remote sensing information. However, sea ice in the Southern Ocean is characterised by large variability that remains unresolved and limits our confidence on the remotely sensed products. Although one of the biggest seasonal changes on Earth is the annual advance and retreat of the Antarctic sea ice cover, relatively little attention has been given to the processes by which the marginal ice zone (MIZ) edge forms and responds to synoptic events. This study aimed to assess the seasonal sea ice extent (SIE) of the MIZ by comparing sea ice observations estimated from aboard ship to high resolution passive microwave (PM) satellite imagery when transecting the MIZ. To achieve this, sea ice concentration (SIC) was derived from two AMSR (Advanced Microwave Scanning Radiometer ) products; the ARTIST (Arctic Radiation and Turbulence Interaction STudy) Sea Ice (ASI-AMSR ) and the bootstrap (BST-AMSR ). Theice concentration estimated from these PM satellite products was assessed against SIC observations collected from the S.A. Agulhas II (using the Antarctic Sea Ice Processes and Climate (ASPeCt) protocol). This assessment took place over summer and winter for the years 2016 and 2017. After evaluating how well these PM-SIC estimates compared against the ASPeCt SIC observations, we found that there was good correlation over summer MIZ conditions, while over winter MIZ conditions the correlation was relatively poor. This highlighted winter limitations inherent in PM SIC estimates. Therefore, from these comparison results, an analysis of the seasonal SIE was accomplished while being aware of the winter limitations linked to the PM products. We inferred that the MIZ acts as an indicator for what the evolution of winter SIE might look like over the following months. In addition to winter limitations associated with PM-SIC retrievals, the ASPeCt SIC estimates, based on human interpretation of the sea ice conditions, was limited because of subjective bias. This resulted in the development of an algorithm to automatically acquire SIC from image stills and videos. This method can be used to obtain quantitative seaice data from vessels of opportunity without the need to have trained personnel on-board. In summary, this study assesses seasonal MIZ SIE within the Atlantic sector after highlighting the limitations associated with various SIC-retrieval methods.
Open Access
Atmospheric drivers of ice drift in the Antarctic marginal ice zone
(2021) Womack, Ashleigh Catherine Stevenson; Vichi, Marcello
Sea-ice drift in the Antarctic marginal ice zone (MIZ) was investigated using an array of five drifting ice buoys, deployed during the winter sea-ice expansion, in July 2017. An initial 15- day analysis of pancake ice drift is presented, using the cluster of buoys, which shows: (1) exceptionally fast ice drift speeds and increased meandering of the buoys during cyclone activity; (2) high correlation of drift velocities with the surface wind velocities, even at 100% remotely sensed ice concentration, indicating free drift conditions where ice drift is primarily governed by wind; and (3) the presence of a clear energy peak (»13.5 hour period), which is suggested to be excited by the passage of cyclones through the transfer of momentum from wind. Additionally, one of the buoys (buoy U1) drifted for approximately four months from the South Atlantic sector to the Indian Ocean sector of the Southern Ocean. The analysis of this buoy revealed that it remained within the MIZ even during the winter ice expansion, as the mixed pancake-frazil field was maintained. This allowed for a continued assumption of free drift conditions for buoy U1's full drift, where it continued to respond linearly to the momentum transfer from surface winds. The analysis of buoy U1 also indicated a strong inertial signature at a period of 13.47 hours however, the wavelet analysis indicated majority of the power remained within the lower frequencies. This strong influence at the lower (multiday) frequencies has therefore been identified as the primary effect of atmospheric forcing. When these lower frequencies were filtered out using the Butterworth high-pass filter it allowed the inertial oscillations to become more significant within the wavelet power spectrum, where it can be seen that these inertial oscillations were often triggered by the passage of cyclones. The initiation of inertial oscillations of sea ice has therefore been identified as the secondary effect of atmospheric forcing, which dominates ice drift at sub-daily timescales and results in the deviation of ice drift from a straight-line path. This comprehensive analysis suggests that the general concentration-based definition of the MIZ is not enough to describe the sea-ice cover, and that the MIZ, where ice is in free drift and under the influence of cyclone induced inertial motion, and presumably waves, can extend up to »200 km.
Open Access
Characterising the response of the mixed and the transitional layers to the passage of storms in the Sub-Antarctic Zone
(2019) Mpalweni, Ayanda; Vichi, Marcello; Nicholson, Sarah
Mid-latitude storms are common in the Southern Ocean (SO) and have been shown to drive substantial vertical mixing, leaving behind wakes of perturbed upper ocean. The vertical extent and duration of the impact of these storms on the upper ocean remains unknown in this region, partly due to lack of observations in this remote part of the world. The mixed-layer depth (MLD) is used widely as proxy for vertical extent of upper-ocean mixing, with the assumption that it integrates the variability of atmospheric forcing. Recent studies have shown that this shear-driven mixing associated with storms can actually extend below the base of the MLD into the transitional layer (TL). Knowledge about the TL would help improve the mixing models of the upper ocean because it acts as a window/mediator between the deep ocean and the surface mixed layer (ML). However, the responses of the MLD and the transitional layer depth (TLD) have been shown to vary substantially between different storm events at similar locations. In this study, these two diagnostics, the MLD and TLD, have been used to investigate the response of the upper ocean mixing to storms in the Sub-Antarctic Zone (SAZ) and to further interrogate the relevance of the MLD as a proxy for mixing extent at these short temporal scales. This is explored during the summer period when the storm-driven mixing is thought to maintain primary production via enhanced nutrient supply. I used data collected from high-resolution autonomous gliders in pseudo-mooring mode, which remotely sampled the SAZ from spring to summer documenting the passage of storm events. Four storms of different magnitude were analysed in summer, and two different modes of the upper ocean response were identified. In the first mode, the MLD deepened during a storm, with little or no changes in the vertical structure of temperature and salinity in the layer below. The second mode was characterized by changes in the TL properties, which deepened at times; the MLD however did not respond to this storm forcing. In the pair of storms that was more in line with the classical response (i.e first mode), the vertical stratification in the upper ocean structure was eroded during the storm and after the storm. In the other case (i.e the 5second mode), however, the vertical stratification was enhanced during the passage of the storm and after the storm. These contrasting responses from both these storms can be linked to a number of atmospheric and oceanic factors; the atmospheric factor was the wind forcing extent (magnitude and duration). The oceanic factor that might have played a role is the pre-existing vertical stratification (depth and strength) within the water column. These two factors conspired to bring about upper ocean changes associated with the passing of storms. It has been shown here that most of the changes are indeed occurring in the transitional layer below the MLD. The MLD, which is used widely by the oceanographic community as a proxy for the integrated effect of surface mixing on many temporal scales, does not always capture the full response of upper-ocean mixing driven by the transient synoptic events.
Open Access
A Computational Fluid Dynamics Model for the Small-Scale Dynamics of Wave, Ice Floe and Interstitial Grease Ice Interaction
(2021-04-29) Marquart, Rutger; Bogaers, Alfred; Skatulla, Sebastian; Alberello, Alberto; Toffoli, Alessandro; Schwarz, Carina; Vichi, Marcello
The marginal ice zone is a highly dynamical region where sea ice and ocean waves interact. Large-scale sea ice models only compute domain-averaged responses. As the majority of the marginal ice zone consists of mobile ice floes surrounded by grease ice, finer-scale modelling is needed to resolve variations of its mechanical properties, wave-induced pressure gradients and drag forces acting on the ice floes. A novel computational fluid dynamics approach is presented that considers the heterogeneous sea ice material composition and accounts for the wave-ice interaction dynamics. Results show, after comparing three realistic sea ice layouts with similar concentration and floe diameter, that the discrepancy between the domain-averaged temporal stress and strain rate evolutions increases for decreasing wave period. Furthermore, strain rate and viscosity are mostly affected by the variability of ice floe shape and diameter.
Open Access
Estimates of Phytoplankton carbon from high resolution optical sensors in the Southern Ocean
(2015) Ogunkoya, Ayodele Gilbert; Vichi, Marcello; Thomalla, Sandy
Phytoplankton is an important component of the oceanic carbon cycle, and deriving a good estimate of its carbon biomass (Cphyto) at ocean scale is difficult due to the lack of automatic sampling procedures. This is particularly difficult in the Southern Ocean, where winter conditions limit the sampling. This study explored the opportunity of using a high resolution data from the glider tracks in the Sub-Antarctic Zone of the Southern Ocean. The data consisted of particulate backscattering and chlorophyll and four different methods of estimating phytoplankton carbon were used, three of them based on backscattering (named 30%POC, B05 and M13) and one on chlorophyll (S09). The methods are different in their empirical formulations and source of original data. Three methods showed similar results despite the fact that one of them makes use of chlorophyll to derive Cphyto. Method M13 doubles that of the 3 other methods (~80mg C m-³ vs 40-50 mg C m-³). It was observed that discrepancy between M13 and the other 3 methods decreases with depth and when biomass was low (~0.25 mg Chl-a m-³) e.g., at depth 80 m. Investigating the drivers of variability in chl-a:C phyto ratios with depth and MLD shows little response and highlighted the need for more research in this region. Although M13 has a very low chl-a:Cphyto ratios, the range of variability was similar to that of the 30%POC and B05 methods and likely driven by variability in light and Fe limitation and changes in community structure. Despite a similar magnitude, the S09 method show a tight constrain in chl-a:Cphyto ratios that were methodologically driven and thus less sensitive to physiological adjustments in cellular chl-a:Cphyto ratios. The analysis also confirms that each oceanic region has factors that drive their variability and care needs to be taken when applying a method that was derived from one oceanic region to another.
Open Access
Exploring South Africa’s southern frontier: A 20-year vision for polar research through the South African National Antarctic Programme
(CrossMark, 2017-06) Ansorge, Isabelle J; Skelton, Paul; Bekker, Annie; de Bruyn, P J Nico; Butterworth, Doug S; Cilliers, Pierre; Cooper, John; Cowan, Don A; Dorrington, Rosemary; Fawcett, Sarah; Fietz, Susanne; Findlay, Ken P; Froneman, P William; Grantham, Geoff H; Greve, Michelle; Hedding, David; Hofmeyr, G J Greg; Kosch, Michael; le Roux, Peter; Lucas, Mike; MacHutcho, Keith; Meiklejohn, Ian; Nel, Werner; Pistorius, Pierre; Ryan, Peter; Stander, Johan; Swart, Sebastiaan; Treasure, Anne; Vichi, Marcello; Jansen van Vuuren, Bettine
Antarctica, the sub-Antarctic islands and surrounding Southern Ocean are regarded as one of the planet’s last remaining wildernesses, ‘insulated from threat by [their] remoteness and protection under the Antarctic Treaty System’1 . Antarctica encompasses some of the coldest, windiest and driest habitats on earth. Within the Southern Ocean, sub-Antarctic islands are found between the Sub-Antarctic Front to the north and the Polar Front to the south. Lying in a transition zone between warmer subtropical and cooler Antarctic waters, these islands are important sentinels from which to study climate change.2 A growing body of evidence3,4 now suggests that climatically driven changes in the latitudinal boundaries of these two fronts define the islands’ short- and long-term atmospheric and oceanic circulation patterns. Consequently, sub-Antarctic islands and their associated terrestrial and marine ecosystems offer ideal natural laboratories for studying ecosystem response to change.5 For example, a recent study6 indicates that the shift in the geographical position of the oceanic fronts has disrupted inshore marine ecosystems, with a possible impact on top predators. Importantly, biotic responses are variable as indicated by different population trends of these top predators.7,8 When studied collectively, these variations in species’ demographic patterns point to complex spatial and temporal changes within the broader sub-Antarctic ecosystem, and invite further examination of the interplay between extrinsic and intrinsic drivers.
Open Access
Frazil Ice in the Antarctic Marginal Ice Zone
(2021-06-10) Paul, Felix; Mielke, Tommy; Schwarz, Carina; Schröder, Jörg; Rampai, Tokoloho; Skatulla, Sebastian; Audh, Riesna R; Hepworth, Ehlke; Vichi, Marcello; Lupascu, Doru C
Frazil ice, consisting of loose disc-shaped ice crystals, is the first ice that forms in the annual cycle in the marginal ice zone (MIZ) of the Antarctic. A sufficient number of frazil ice crystals form the surface “grease ice” layer, playing a fundamental role in the freezing processes in the MIZ. As soon as the ocean waves are sufficiently damped by a frazil ice cover, a closed ice cover can form. In this article, we investigate the rheological properties of frazil ice, which has a crucial influence on the growth of sea ice in the MIZ. An in situ test setup for measuring temperature and rheological properties was developed. Frazil ice shows shear thinning flow behavior. The presented measurements enable real-data-founded modelling of the annual ice cycle in the MIZ.
Open Access
Hydrographic conditions of a continental shelf region in the Northwest African Upwelling System from in-situ data
(2015) Juby, Paul Robert; Vichi, Marcello; Deschamps, Anne; Meunier, Thomas; Floc'h, France
This study investigates the hydrographic and dynamic properties of the continental shelf region between Cape Juby (28.5 °N) and Agadir (30.4 °N) within the Moroccan Sub-region of the North West African Upwelling System. Data came from two cruises conducted in June (beginning of summer) and November (end of autumn) 2013. Coastal upwelling was obvious in both cruises in the in-situ temperature and salinity data as well as in remotely sensed sea surface temperature maps. ADCP data showed the presence of a strong jet like current associated with enhanced upwelling off Cape Juby. This strong quasi-permanent upwelling center was observed during both cruises. It results from the orientation of this portion of coast which is aligned with the dominant wind direction, as well as a wind intensification near the cape. The presence of a secondary upwelling front was also observed near the shelf break. It was accompanied by an intense baroclinic jet. The EK60 data showed evidence of internal waves as well as small and mesoscale turbulence that were probably strongly interacting with the mean upwelling circulation and made a straightforward interpretation of the data quite challenging. However, this study revealed the main physical processes of this poorly studied region, as well as their seasonal variability.
Open Access
Ice - ocean - atmosphere interactions in the Southern Ocean and implications for phytoplankton phenology
(2021) Hague, Mark; Vichi, Marcello
The annual advance and retreat of sea ice in the Southern Ocean is recognised as one of the largest seasonal events on Earth. Such considerable physical changes have profound effects on the vertical structure of the water column, and hence controls the availability of both light and nutrients to phytoplankton. This means that in the region seasonally covered by sea ice (the SSIZ), the timing of the growth and decline (phenology) of phytoplankton is determined to a large degree by the dynamic interactions between ice, ocean and atmosphere. However, this region is simultaneously one of the most poorly observed in the global ocean, and one of the most complex. This has led to significant gaps in our understanding of how sea ice modulates the exchanges of heat and momentum between atmosphere and ocean, as well as the implications this has for phytoplankton phenology in the SSIZ. This study seeks to address these gaps by combining both model and observationallybased methods. The lack of observational data are directly tackled through an analysis of BGC-Argo float data sampling under ice. Such data reveal high growth rates in the presence of near full ice cover and deep mixed layers, conditions previously thought to prevent growth. These results suggest a revision of our current understanding of the drivers of under ice phytoplankton phenology, which should take into account the unique character of Antarctic sea ice and its effect on the under ice light environment. In addition, results obtained from several numerical process studies indicates that phytoplankton may have a higher affinity for low light conditions than previously thought. From a modelling perspective, an analysis and intercomparison of 11 Earth System Models (ESMs) and their representation of vertical mixing and phenology is presented. This revealed that misrepresentations in phenology where driven by model biases in sea ice cover and vertical mixing. That is, only models with either too much or too little ice cover were able to simulate phenology close to observations. Furthermore, a strong correlation between the location of the ice edge and the extent of vertical mixing suggested that ESMs overly dampen ocean-atmosphere fluxes as mediated by sea ice. This led to the development of a regional ocean-sea ice model of the Atlantic sector of the Southern Ocean, from which experiments enhancing both heat and momentum fluxes could be conducted. It was found that the model responded more uniformly to enhanced heat flux, generally deepening the mixed layer closer to observations in winter. On the other hand, the effects of enhanced momentum flux (implemented by increased air-ice drag) where more complex and spatially heterogeneous, with contrasting responses depending on the initial vertical density structure of the water column. Overall, the argument is made that the unique features of Antarctic sea ice should be included in models if we are to improve the representation of the SSIZ mixed layer, and hence phenology
Open Access
Improved estimates and understanding of interannual trends of CO₂ fluxes in the Southern Ocean
(2017) Gregor, Luke; Monteiro, Pedro M S; Vichi, Marcello; Kok, Schalk
The Southern Ocean plays an important role in mitigating the effects of anthropogenically driven climate change. The region accounts for 43% of oceanic uptake of anthropogenic carbon dioxide (CO₂). This is foreseen to change with increasing greenhouse gas emissions due to ocean chemistry and climate feedbacks that regulate the carbon cycle in the Southern Ocean. Studies have already shown that Southern Ocean CO₂ is subject to interannual variability. Measuring and understanding this change has been difficult due to sparse observational data that is biased toward summer. This leaves a crucial gap in our understanding of the Southern Ocean CO₂ seasonal cycle, which needs to be resolved to adequately monitor change and gain insight into the drivers of interannual variability. Machine learning has been successful in estimating CO₂ in may parts of the ocean by extrapolating existing data with satellite measurements of proxy variables of CO₂. However, in the Southern Ocean machine learning has proven less successful. Large differences between machine learning estimates stem from the paucity of data and complexity of the mechanisms that drive CO₂. In this study the aim is to reduce the uncertainty of estimates, advance our understanding of the interannual drivers, and optimise sampling of CO₂ in the Southern Ocean. Improving the estimates of CO₂ was achieved by investigating: the impact of increasing the gridding resolution of input data and proxy variables, and Support vector regression (SVR) and Random Forest Regression (RFR) as alternate machine learning methods. It was found that the improvement gained by increasing gridding resolution was minimal and only RFR was able to improve on existing error estimates. Yet, there was good agreement of the seasonal cycle and interannual trends between RFR, SVR and estimates from the literature. The ensemble mean of these methods was used to investigate the variability and interannual trends of CO₂ in the Southern Ocean. The interannual trends of the ensemble confirmed trends reported in the literature. A weakening of the sink in the early 2000's, followed by a strengthening a strengthening of the sink into the early 2010's. Wind was the overall driver of dominant decadal interannual trends, being more important during winter due to the increased efficacy of entrainment processes. Summer interannual variability of CO₂ was driven primarily by chlorophyll, which responded to basin scale changes in drivers by the complex interaction with underlying physics and possibly sub-mesoscale processes. Lastly CO₂ sampling platforms, namely ships, profiling floats and moorings, were tested in an idealised simulated model environment using a machine learning approach. Ships, simulated from existing cruise tracks, failed to adequately resolve CO₂ below the uncertainty threshold that is required to resolve the seasonal cycle of Southern Ocean CO₂. Eight high frequency sampling moorings narrowly outperformed 200 profiling floats, which were both able to adequately resolve the seasonal cycle. Though, a combination of ships and profiling floats achieved the smallest error.
Open Access
Intra-seasonal variability of Southern Ocean primary production: the role of storms and mesoscale turbulence
(2016) Nicholson, Sarah-Anne; Lévy, Marina; Monteiro, Pedro; Swart, Sebastiaan; Vichi, Marcello
The Southern Ocean is one of the stormiest places on earth; here strong mid-latitude storms frequently traverse large distances of this ocean. Underlying these passing storms, the Southern Ocean is characterized by having some of the highest eddy kinetic energy ever measured (eddies occupying the meso to sub-mesoscale). The presence of the passage of intense storms and meso to sub-mesoscale eddy variability has the potential to strongly impact the intraseasonal variability of the upper ocean environment where phytoplankton live. Yet, exactly how phytoplankton growth rates and its variability are impacted by the dominance of such features is not clear. Herein, lies the problem addressed by the core of this thesis, which seeks to advance the understanding of intra-seasonal variability of Southern Ocean primary production. The drivers of this intra-seasonal variability have been explored from two points of view: the local-scale and the remote-scale perspectives, with a suite of physicalbiogeochemical (NEMO-PISCES) numerical models of varying complexity. At the local-scale, these model experiments have suggested that intra-seasonal stormlinked physical supplies of dissolved iron (DFe) during the summer played a considerably more active and influential role in explaining the sustained summer productivity in the surface waters of the Southern Ocean than what was thought previously. This was through two important insights: 1. Storm-eddy interactions may strongly enhance the magnitude and extent of upperocean vertical mixing in both the surface mixed layer as traditionally understood as well as in the subsurface ocean. These two mixing regimes have different dynamics but act in concert to amplify the DFe fluxes to the surface ocean. 2. Storm initiated inertial motions may, through interaction with eddies, greatly reinforce w and thus, enhance the vertical advection of DFe to the surface ocean, an effect that may last several days after the storm. At the local-scale, such storm-eddy dynamics may greatly increase the intra-seasonal variability of primary production, a step towards helping to explain why this variability is so strong in large regions of the Southern Ocean. At the remote-scale, the cumulative impact of these short-term storm-eddy interactions have unexpected implications in respect of the larger-scale mean flow and its influence on the effectiveness of intra-seasonal forcing of DFe fluxes. This counter intuitive feedback is a reduced strength of the intra-seasonal variability in primary production despite what was shown at the local-scale. Moreover, the addition of storms intensified the main clockwise cell of the meridional overturning circulation particularly the downward branch thus, reducing DFe inventory from the upper-ocean. Such an impact could potentially be enhanced with increasing storm intensities as suggested by climate projections. Understanding these remote-scale and local-scale responses of primary productivity to storms and their interaction with the underlying ocean mesoscale turbulence may be key to better understanding the sensitivities of the carbon cycle to short-term variability and long-term trends in atmospheric forcing.
Open Access
Investigating the relationship between volume transport and sea surface height in the Agulhas Current System
(2018) Vermeulen, Estee; Hermes, Juliet C; Backeberg, Björn; Elipot, Shane; Vichi, Marcello
The relationship between the volume transport of the Agulhas Current at 34°S (the position of the Agulhas Current Time-series array) and the gradient of sea surface height across the current was investigated using a regional Hybrid Coordinate Ocean Model. Previous studies have suggested a high correlation between SSH slope and Agulhas Current transport and, based on 3 years of in situ measurements, a transport proxy between along-track satellite data and in situ data was developed. The purpose of this modelling study was to re-create the Agulhas Current transport proxy in a virtual modelling environment, to test the validity of the underlying assumption on which the satellite-altimeter proxy was based. The Agulhas transport proxy assumed nine, constant linear relationships between SSH slope and integrated transport per unit distance over the 22-year transport time-series, based on the 3-year sampling period and a constant vertical stratification. The 34-year regional-hindcast from HYCOM provided the means to test the sensitivity of the transport proxy to vertical changes in the current and the length scale of observations used to build a constant, linear relationship between transport and SSH slope. During the investigation it was found that HYCOM contained exaggerated levels of offshore variability. This resulted in stronger correlations for the inshore linear regression models with a decreasing trend moving offshore. Based on the overall performance of the 34-year transport proxies it was concluded that the proxy was more capable of estimating the net transport of the Agulhas Current across the array instead of only the southwest transport component. Therefore, transport estimates inshore were more accurate than the transport estimates offshore, when the current is in a meandering state, and the poorer performance of the southwest transport proxy, specifically developed to capture the transport during offshore meander events, was less capable of estimating an accurate transport estimate. Results showed that calculating the proxy over longer time periods did not significantly improve the skill of the Agulhas transport proxy, suggesting the 3-years was a sufficient time-period used to develop the transport proxy in HYCOM. This study motivates the need to improve long-term monitoring methods, where the usage of numerical ocean models could help understand the sensitivities and limitations involved in the development of transport proxies in future.
Open Access
Investigating the sensitivity of one-dimensional turbulence schemes in the sub-Antarctic Southern Ocean
(2018) Boodhraj, Kirodh; Smit, Jacoba E; Vichi, Marcello
The sub-Antarctic Zone (SAZ) is a zone of vigorous vertical mixing in the Southern Ocean where it is difficult to obtain data for model validation on the turbulence conditions. In this study, a onedimensional configuration from the Nucleus for the European Modelling of the Ocean (NEMO) model was implemented in order to determine the sensitivity and turbulence response of an idealized SAZ water column. Various turbulence scheme parameterizations that are available for ocean models were tested. Furthermore, the number of vertical levels were varied in order to ascertain the sensitivity of the grid. The forcing data were obtained from various reanalyses (ERA-Interim, NASA, NCEP and JRA55) and were likewise tested. Different turbulence diagnostics and univariate indicators were chosen to ascertain the turbulence response and to analyse the energetics of the water column. It was found that using different reanalyses produced different tracer (salinity and temperature) results. Even though the results varied considerably, very high correlations were found for the potential energy anomaly between reanalyses and insignificant correlations were found for the other indicators. This suggested that it was a valuable descriptor which captured the buoyancy fluxes and wind stress information and can be efficiently used to assess the vertical turbulent state with data such as ARGO profiles. It was further found that for a single reanalysis, the turbulence schemes had produced similar results (with small variability and not to the extent as changing the reanalysis) for the turbulence diagnostics and univariate indicators. An important finding of an entrapped warm water parcel beneath cooler waters was found in simulation outputs as well as ARGO validation data. For realistic conditions observed from the ARGO floats, as the season progressed, there were no more instances of a warm water parcel. There was no reason however, to why there should not have been eddies passing by the region. In simulations, the warm water parcel persisted throughout the season for simulated data, likely causing the early stratification that affects ocean models in the SAZ. The stratification was found to have an approximate one month early onset observed from comparing the ARGO data profiles to simulated profiles. The Brunt Väisälä frequency, potential energy anomaly as well as the buoyancy flux were analysed and these diagnostics indicated that an approximate one month early stratification was found during November. It was likely that this false stratification signal may have influenced the summer stratification leading to a poor representation of the Mixed Layer Depth (MLD) and various other indicators. It was found that during the austral winter months, the model simulated comparable MLD's to the ARGO float data as well as theWinter Cruise data (obtained from the SA Agulhas II), capturing the winter dynamics well.
Open Access
Investigation of Wind Variability in the South Atlantic Sector of the Southern Ocean and the Influence on the Upper Ocean in a Numerical Ocean Model
(2019) Moalusi, Tumelo Comfort; Vichi, Marcello; Chang, Nicolette; Nicholson, Sarah
Several papers have linked climate variability in the Southern Ocean (SO) with the Southern Annular Mode (SAM), which has seen an increase in the positive phase since the mid-1960s, due to the Antarctic ozone depletion and emissions of greenhouse gases. The SAM is recognized as the main mode of atmospheric variability in the SO. The SAM index allows an understanding of the latitudinal movement (south-north) of the westerly wind belt circling Antarctica and has significant impacts on Antarctic surface temperatures, ocean circulation, and many other aspects of Southern Hemisphere climate and thus the global ocean. During negative phases of the SAM Index, westerlies intensify and move north, bringing about more (or stronger) storms, and low pressure systems over southern Australia. The changes associated with SAM forcing may have impacts on carbon uptake and storage in the SO directly through upwelling and outgassing, and indirectly, by influencing nutrient cycles and phytoplankton activity. Understanding the variability of the wind field in the SO and how it affects ocean circulation, climatic and oceanic variables is important. Thus, this thesis presents the relationship of the SAM index and the upper ocean, specifically analysing sea surface salinity (SSS), sea surface temperature (SST) and the mixed layer depth (MLD), in the Southern Atlantic sector of the SO as presented in numerical ocean models. Two resolutions of NEMO ocean model are compared: a) eddy-permitting (SATLANTIC05), b) eddy-resolving (SATLANTIC12) models, with horizontal resolutions of ½ and 1/12 °, respectively. In situ data from 2013 World Ocean Atlas is used as a benchmark for the analysis. Our model‐based analysis confirms previous studies done on the influence of the SAM on the SO, that a strong relationship exists. The SAM index is positively correlated with wind speed in the Antarctic Zone (AZ) and negatively correlated in the Subantarctic Zone (SAZ). The impacts of this is clear in the upper ocean. These correlations between SAM index and the selected variables at these selected locations confirms that the SAM index corresponds with cool surface temperatures at higher latitudes and a weak cooling at midlatitudes during positive phase, which differs regionally.
Open Access
The meridional and seasonal variability of the carbonate system in the Southeast Atlantic sector of the Southern Ocean: SCALE 2019 Experiment
(2023) Binase, Zanele; Vichi, Marcello; Monteiro Pedro
Unlike well-characterized regions that have emerging pH and carbonate data, distant areas like polar oceans remain seasonally undersampled. Consequently, there may be a significant gap in our understanding of biological implications of future ocean acidification (OA). This study seeks to examine the seawater state conditions of the carbonate system that are currently experienced by marine organisms that depend on pH and Aragonite saturation (ΩAr) for calcification. The main focus was on how the physical and carbonate characteristics are sensitive to seasonal variability in the S.E Atlantic sector of the Southern Ocean. We analyzed the temperature, salinity, Dissolved Inorganic Carbon (CT), alkalinity data (AT), pH and Omega (ΩAr) at the surface and in the upper 1000m. We then compared the seasonal meridional gradients in the surface layer and in the upper 1000m water column. The CT and AT were measured using a VINDTA instrument while pH and ΩAr were derived using the CO2SYS program. The initial hypothesis was that the southernmost part of the Southern Ocean would acidify more in winter than in spring. This was based on the idea that colder waters hold more CO2. However, we found remarkable results showing that the surface CT was consistently higher in spring than in winter, with mean seasonal differences ranging from 5.86-21.90µmol/kg although SST was consistently higher in spring than winter, with mean seasonal differences ranging from 0.67-2.12˚C within the ocean front boundaries. It was then hypothesized that the temperature, biological production and CO2 flux seasonal cycles may have been out of phase. Consistent with the CT gradient, the surface pH and ΩAr were ±0.05 units lower in the warmer waters of spring and comparatively higher in winter. The seasonal lag was seen even within the interior layers of the column. There were three main findings in this study: Firstly, since the biogeochemically controlled seasonal CO2 seasonal transition variability lagged the heat flux influence on SST, the expected winter and spring seasonal conditions for carbonate were not reflected in the timing of the winter and spring cruises. Secondly, we observed an uncoupling of pH and ΩAr surface meridional gradients. While pH had no significant meridional gradient trend apart from frontal variability, ΩAr followed the meridional trend that was driven by CT. This was due to the impact of the meridional temperature gradient on K2 that compensated for the impact of the CT gradient on the pH. Lastly, we found that the impact of the seasonal cycle of carbonate stretched down to 1000 m and it was attributed to physical processes. Our findings led us to infer that from a carbonate perspective, the winter cruise was in fact the tail end of autumn while the spring cruise was the tail end of winter.
Open Access
Modelling waves and near-shore circulation around the Cape Peninsula: towards enhanced predictions for South African coastal activities
(2022) de Vos, Marc; Vichi, Marcello; Fawcett, Sarah
South Africa's coastal oceans support a multitude of human and ecological interests. My work at the confluence of academia, industry and government highlighted a gap in knowledge and capacity in the marine environmental information space. Broad research questions were developed and addressed through a combination of statistical review, data mining and numerical modelling: how do marine weather and oceanography impinge on South African coastal activities, what are the main circulation characteristics in a critical coastal region, and which environmental data are needed for predictions? Hazard modelling of environmental conditions and coastal user safety in South Africa revealed a wide sensitivity to weather-ocean hazards among different users and places. The Cape Peninsula sub-region exhibited the highest incident frequency related to increasing hazard severity. I implemented a coupled numerical ocean model (Delft3D FLOW and SWAN), with 700 m horizontal resolution to simulate waves and currents in this area. The model is driven by realistic forcing, striking a previously lacking balance between geographical coverage, spatial granularity, and process complexity. It has the potential to be deployed in forecast mode in future. Modelling confirmed the importance of wind in establishing broad circulation patterns and inducing nearshore upwelling. However, bay-scale cyclonic and anticyclonic flow in False Bay has low-medium predictability. These patterns developed as expected at times but failed to do so at others. The model's inclusion of two-way wave-current interactions enabled the identification of a novel cyclonic gyre in Table Bay during large wave events. Investigation revealed the gyre to be driven by wave induced radiation stress gradients. Model experiments with and without wave-coupling demonstrated that the gyre causes material differences in virtual drift trajectories and affects the monthly mean general circulation. This highlights its potential impact on coastal users and its implications for understanding Table Bay's circulation, where the importance of waves beyond the nearshore may be crucial for predictability. Enhanced velocity measurements, sufficient to resolve the alternating rotational circulation in False Bay and wave-driven gyre in Table Bay, are recommended. It is further suggested that appropriate wave forcing be included in any further circulation modelling in the study area.
Open Access
Not all models are created equal: assessing parameterisations of iron dynamics in ocean biogeochemical models
(2020) Rogerson, Jonathan J; Vichi, Marcello
Iron is one of the most commonly studied trace metals as it exerts a significant influence on ocean productivity, carbon sequestration as well as modulating atmospheric CO2 concentrations. As iron is such a vital nutrient for biogeochemical processes it is often included as a variable in ocean biogeochemical models. In representing the iron cycle, biogeochemical models must parameterise the major processes of uptake by phytoplankton, remineralisation and scavenging. However, there is no generally accepted set of equations to represent iron dynamics and thus a variety of different parameterisations are employed across the modelling community. The thesis work focussed on the inorganic iron parameterisations with an emphasis on the scavenging formalisms which are employed in current biogeochemical models. Using an open-source numerical model (Biogeochemical Flux Model, BFM) as a background model, a more advanced inorganic iron parameterisations that simulates free iron scavenging and ligands linked to dissolved organic carbon (DOC) (from the open-source model PISCES) was included and compared to assess the implications on iron cycling and plankton community structure. The parameterisations were compared by running box models (0D) in four different regions: Southern Ocean, Equatorial Pacific, North Atlantic gyre and North-east Pacific, representing different types of iron dynamics. The free scavenging model (FePISCES) resulted in dissolved iron concentrations being two to three times greater than with the standard formulation (FeBFM), which used a simpler formalism for scavenging. Consequently, the elevated iron concentrations in FePISCES resulted in altered community compositions for phytoplankton which impacted the seasonal cycle of macronutrients and chlorophyll concentrations. Furthermore, the prognostic appreciation of ligand dynamics in FePISCES lead to a decoupling of dissolved iron from its organic species with the DOC content for a region being indirectly implicated in driving the iron system by affecting the scavenging regime. Therefore, using a different set of iron parameterisations will alter the biogeochemical behaviour of a model. The results suggest that the testing of parameterisations should be initially done within 0D models in order to assess any non-linear behaviours and ultimately embedded in 3D models to study how they interact with physics.