Browsing by Subject "oceanography"

Now showing 1 - 15 of 15
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    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.
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    An analysis of anomalously wet summers in the South Western Cape of South Africa
    (2023) De Kock, Wade Matthew; Reason, Christopher; Blamey, Ross
    Unlike the rest of South Africa, the southwestern Cape (SWC) experiences most its rainfall in the austral winter (May-September). Due to interannual, intraseasonal and interdecadal variability, drought is a familiar occurrence. The SWC recently suffered from an extended dry period, known as the ‘Day Zero Drought' during 2015-2018, where greater Cape Town nearly ran out of piped water supply. Despite most rainfall in the SWC occurring from MaySeptember, considerable rainfall events have been known to occur during the summer (October-March). Such events could play a substantial role in mitigating winter droughts and multiyear droughts the region suffers from. Large Rainfall Events (LREs) during the summer of 2018/19 caused average dam levels in all major dams of the SWC to increase by more than 1%. The dam level increase is significant during the driest period of the year where dam levels decrease by several % per month. This study investigates all LREs during the summer (October-March) from 1979-2019 and their effects on major dam levels. Most summer LREs are found to be linked to atmospheric rivers (ARs) or cut-off lows (COLs), which together account for up to 88% of the top 75 LREs. Apart from one study characterising the considerable effect of ARs on winter rainfall, to date little research on ARs has been done for the region. Furthermore, COLs have been suggested to occur mostly during transition seasons. This thesis reveals that although ARs last shorter than COLs and lead to a smaller area receiving rainfall in the SWC, they both yield intense rainfall amounts with ARs concentrated around Greater Cape Town. After LREs have occurred, average dam volumes were shown to increase by up to 5% making LREs essential in drought recovery. Anomalously wet summers, which typically contain more LREs than average, are also mostly associated with cooler temperatures and less extreme hot days (90% decile). Rainfall totals are inversely correlated (r=-0.44) with extreme hot days. In addition, extreme hot days also show a significant increasing trend of 2.8 days/decade from 1979-2019. Along with increased cloud cover, weaker winds over dam catchment areas can be associated with 4 out the 5 wettest summer seasons. Of the 5 wettest summer seasons, only one (2013/14) occurred in the last two decades. Anomalously cool and wet summers, reduce the water consumption impact on dam volumes as well as help reduce the impacts of drier than normal winter seasons. Wet and cool summer seasons also reduce fire risk in the region which is important considering that the region is agriculturally productive and has experienced several devastating fires in recent decades, both in agricultural areas as well as in greater Cape Town. Although the extended summer contributes only about 30% of the year's annual rainfall, summer LREs occur during the most water demanding part of the year. Notably, increased summer LREs usually correspond with anomalously wet summers. This thesis finds that anomalously wet summers can be characterized by increased rainfall days which are linked to increased cyclonic anomalies over the region and westerly moisture fluxes shifted anomalously equatorward in the South Atlantic. These changes in circulation patterns are found to be linked to a negative Southern Annular Mode pattern and in the late summer, also linked to ENSO and the zonal wave number 3 pattern. Overall, trends suggest decreases in rainfall days in the Greater Cape Town region and in the nearby mountain areas where most major dams are located for the mid to late summer (December-March). These decreases in rainfall days can be related to poleward expansions of the South Atlantic High Pressure (SAHP) which then lead to decreases in storms impacting the SWC. With storm tracks occurring further poleward due to moisture corridor shifts and SAHP poleward expansions during recent years, there is a decrease in summer LREs in the SWC. Some of these poleward shifts are related to the tendency of the Southern Annular Mode to be in positive phase in recent decades. Since summer LREs are important in mitigating droughts in the region, future work needs to consider rainfall in all seasons rather than just the historical focus on winter rainfall which has been relatively well studied. This thesis shows the potential importance of anomalously wet summers as essential contributors to moisture in the region during the driest period of the year.
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    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.
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    Deriving a policy document towards an early warning system for estuaries in South Africa: case study Great Brak estuary, Eden District, Southern Cape
    (2020) Stander, Johan; Ansorge, Isabel; Hermes, Juliet
    South Africa's estuaries and their surrounding communities are becoming increasingly vulnerable to storm surges and accompanied estuary flooding. These events are largely due to increasing severity of storm surges combined with growing housing and commercial developments. A particularly severe weather event in 2007/2008 highlighted the pressing need to understand the processes involved and the urgency to develop proactive response and management actions to mitigate the effects of future storm events on these coastal areas. Scientific research on estuarine flooding is limited not only for South Africa but within the international community as well and only recently has received committed attention from policy makers. It is clear that our current knowledge of South African estuary flooding events remains rudimentary; while necessary action to mitigate such events are poorly understood and planned. The aim of this PhD thesis is to devise and implement an Estuary Early Warning – Emergency Preparedness and Response Guide for stakeholders and government policymakers. This guide will target South Africa's coastal region by analysing past information on storm surges and estuary flooding, particularly in the low-lying southern coast region of the Western Cape, South Africa. The key objective of this thesis is to assess the best processesfor the issuing of estuary alerts and to better standardise them so that the response remains in line with multi-hazard early warning standard procedures and practices within South Africa. A further aim is to provide a comprehensive national guideline on how best to effectively disseminate and communicate such information and to establish an Estuary Early Warning (EEW) – Emergency Preparedness and Response Guide (EPRG), which forms part of the South African Multi-Hazard Early Warning System (MHEWS). It is critical that this EEW meets general principles accepted internationally for an effective Early Warning System. This thesis addresses the following key elements namely: (1) Risk identification, (2) Key drivers and contributions to estuary flooding, (3) Monitoring and alert early warning system, (4) Alert dissemination and (5) Response actions. Such pioneering work is an essential tool to translate science into policy, a crossover field, which remains poorly implemented.
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    Interannual variability and long-term trends of surface hydrography around the Prince Edward Island Archipelago, Southern Ocean
    (2022) Toolsee, Tesha; Lamont, Tarron; Rouault, Mathieu
    The Prince Edward Islands (PEIs) Archipelago are situated in a prime location for the study of ecosystem response to intrinsic climate variability in the Southern Ocean and the impact of climate change. They are positioned in the Polar Frontal Zone, which is constrained by the subAntarctic Front and the Antarctic Polar Front, all of which are part of the strong, uninterrupted Antarctic Circumpolar Current (ACC). Due to its remoteness and challenging accessibility, there is a severe lack of data in the Southern Ocean and at the PEIs. The existing data are only available as single points observations or snapshots from past research cruises. This study thus makes use of 23 years (1993 – 2016) of satellite and reanalysis data to determine the annual/interannual and long-term variability of Sea Surface Temperature (SST), wind forcing and surface circulation at the PEIs and determine whether natural modes of climate variability like the El Niño Southern Oscillation (ENSO), Southern Annular Mode (SAM) or SemiAnnual Oscillation (SAO) were affecting these parameters. SST, wind speed, wind stress curl and the Ekman current did not express any long-term trend. A significant increasing but very small trend was only perceived in the geostrophic current and total surface current which was concluded to not be associated with the intensification of the ACC caused by a more positive SAM phase. The anomalies in SST showed striking interannual variability at a periodicity of 0.8, 2.8 and 7.5 years showing a similar pattern to that of ENSO with a periodicity of 1.5, 2.9 and 6 years. There has however been no relationship established between SST and any of the climate modes, but the Antarctic Circumpolar Wave (ACW), which is one of ENSO's teleconnection, could be responsible for the interannual changes seen in the SST anomalies. The anomalies in wind speed did not show any apparent periodicity and no relationship with ENSO. More so, while the impact of SAM and SAO has been seen on the westerly wind belt which governs the latitude of the PEIs, no correlation was established between the wind speed at the islands and SAM or SAO. The anomalies in wind stress curl presented no visible interannual variability but some sign of short-term variability. There was also no link 2 established between wind stress curl at the PEIs and any of the climate modes. Finally, a periodicity of 1.3 and 4 years was seen in the geostrophic current anomalies which also coincided with the pattern of ENSO but only showed minor correlation with ENSO. The ACW was deduced to perhaps also be responsible for the surface currents anomalies since the ACW is primarily propagated within the ACC. The trends perceived in the parameters considered for this study and the impact of climate modes on them appeared to be different to patterns which has been historically observed across the Southern Ocean. This further confirms the fact that the neighbouring oceanography and surface wind speed variability surrounding the PEIs differ from other regions of the Southern Ocean, most probably due to the frequent mesoscale instability such as eddies and frontal movement influencing the region. The impact of climate change on the PEIs ecosystem thus cannot be expected to be the same as the rest of the Southern Ocean.
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    Living with great white sharks in Cape Town
    (2013) Kock, Alison
    Coordinated by Alison Kock, Research Manager, Shark Spotters. For surfers, conservationists, lifeguards and anyone interested in the topic of living with great white sharks in Cape Town.
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    Numerical modelling of Tropical Cyclone Dineo and its rainfall impacts over north-eastern South Africa
    (2019) Meyiwa, Sbongile; Reason, Chris
    Widespread flooding over parts of Mozambique, Zimbabwe, Malawi, Botswana as well as north-eastern South Africa was experienced in February 2017. The flooding was associated with Tropical Cyclone Dineo that was generated in the Mozambique Channel on 12 February 2017 and made landfall over the south-central coast of Mozambique on 15 February. This study investigates the atmospheric circulation and potential mechanisms responsible for the heavy rainfall that occurred during the passage of ex-Tropical Cyclone Dineo inland from the Mozambican coast with focus on the rainfall patterns over north-eastern South Africa. Output from the Weather Research and Forecasting (WRF) model, the Climate Forecast System Reanalyses version 2 (CFSv2) atmospheric reanalysis, satellite derived rainfall and wind data, and station rainfall data are used for this purpose. Tropical Rainfall Measuring Mission (TRMM) rainfall estimates, WRF model rainfall and rainfall station data indicated that many parts of north-eastern South Africa experienced large amounts of rainfall during the final stages of Dineo (16-17 February 2017) while Mozambique experienced heavy rainfall soon after the cyclone made landfall. An inland trough ahead of Dineo led to substantial rainfall at this time over Malawi and Botswana. Furthermore, analysis of the station data revealed that in north-eastern South Africa some stations recorded about 80 % of their total monthly rainfall from this event. The WRF model run indicated low level monsoonal north-easterly moisture fluxes feeding into Dineo in the Mozambique Channel. Subsequent convergence over south-eastern Africa between this flow and the south-easterly cyclonic flux associated with Dineo led to substantial rainfall over Mozambique, Zimbabwe, Botswana and north-eastern South Africa. Although the 2016/17 tropical cyclone season recorded below average numbers of storms, it is suggested that the conditions prior to the storm iv formation were favourable for the track of Tropical Cyclone Dineo and landfall on the southcentral Mozambican coast.
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    On-shelf nutrient trapping enhances the fertility of the southern Benguela upwelling system
    (2019) Flynn, Raquel; Fawcett, Sarah; Granger, Julie
    The southern Benguela upwelling system (SBUS), located off the southwest coast of Africa, supports high rates of primary productivity that sustain important commercial fisheries. The exceptional fertility of this system is reportedly fuelled not only by upwelled nutrients, but also by nutrients regenerated on the broad and shallow continental shelf. We present the first nitrate nitrogen (N) and oxygen (O) isotope data (δ15N and δ18O, respectively) from the SBUS, generated for samples collected along four hydrographic lines in February (summer) and May (early winter) of 2017. During summer upwelling, a decrease in nitrate δ 18O on the shelf reveals that on average, 30% of the subsurface nutrients derive from in situ remineralization of sinking phytoplankton biomass. In the more quiescence winter, an average of 35% of the on-shelf nitrate is regenerated, with the signal propagating further westward along the mid-shelf region such that the total regenerated nitrate burden is greater during this season. In both seasons, a shoreward increase in subsurface nitrate δ 15N and decrease in N* (i.e., total dissolved nitrogen - 16 x phosphate + 2.9) suggests N loss to benthic denitrification coincident with the on-shelf remineralization, which implies that an even higher quantity of nitrate is regenerated than we calculate. Our data show that remineralized nutrients get trapped on the SBUS shelf in summer and early winter, enhancing the nutrient pool that can be upwelled to support surface productivity and decreasing bottom water oxygen concentrations. The proposed mechanism for this “nutrient trapping” involves upwelled nutrients being removed from surface waters and converted into organic biomass that is sequestered and remineralized on the shelf while the now nutrient-deplete surface waters are advected offshore by Ekman transport. This process is aided by a number of equatorward-flowing fronts that impede the lateral exchange of waters in the upper 200 m of the water column, increasing their residence time on the shelf. The extent to which remineralized nutrients are trapped on the SBUS shelf has implications for bottom water hypoxia. Trapped nutrients will be supplied to the surface during upwelling, supporting high rates of primary productivity and a large sinking biomass flux. The subsequent on-shelf remineralization of this organic matter has the potential to further decrease already-low bottom water oxygen concentrations.
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    Sentinels to climate change. The need for monitoring at South Africa’s Subantarctic laboratory
    (2014) Ansorge, Isabelle J; Durgadoo, Jonathan V; Treasure, Anne M
    The International Society for Burns Injuries (ISBI) has published guidelines for the management of multiple or mass burns casualties, and recommends that 'each country has or should have a disaster planning system that addresses its own particular needs.' The need for a national burns disaster plan integrated with national and provincial disaster planning was discussed at the South African Burns Society Congress in 2009, but there was no real involvement in the disaster planning prior to the 2010 World Cup; the country would have been poorly prepared had there been a burns disaster during the event. This article identifies some of the lessons learnt and strategies derived from major burns disasters and burns disaster planning from other regions. Members of the South African Burns Society are undertaking an audit of burns care in South Africa to investigate the feasibility of a national burns disaster plan. This audit (which is still under way) also aims to identify weaknesses of burns care in South Africa and implement improvements where necessary.
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    The biophysical processes controlling the South-East Madagascar Bloom
    (2018) Dilmahamod Ahmad Fehmi; Reason Christopher
    Phytoplankton blooms are ecological hotspots in the ocean, and are fundamental to the biogeochemical cycling of elements, the storage of carbon and the ability to regulate the atmospheric carbon dioxide; and the life in the ocean. The South-East Madagascar Bloom, one of the largest blooms in the global ocean, coexists with the poleward flowing South-East Madagascar Current (SEMC), the eastward flowing South Indian Ocean Countercurrent (SICC) as well as westward-propagating surface and subsurface-intensified eddies. This austral summer bloom extends largely towards the open ocean, from the Madagascan coasts up to ~65°E and it exhibits an intriguing interannual variability. A variety of observational datasets as well as a high resolution coupled physical-biogeochemical model, based on CROCOPISCES, are used to explore the biophysical processes associated with the bloom and these westward-propagating eddies. Based on historical observational data, the bloom is shown to occur in a region of shallow mixed layer, with the surface layer exhibiting lower salinity, a possible signature of the coastal poleward flowing SEMC waters. The testing of various hypotheses revealed a dampening of the coastal current-driven upwelling south-east of Madagascar during bloom months. A dipole mesoscale feature is also prevalent close to the Madagascan coast during the bloom, from which a new hypothesis emerges. This new hypothesis states that the region south/south-east of Madagascar, influenced by local mesoscale turbulence, acts as a gate for the SEMC to flow either towards the African continent, or into the bloom region through an early retroflection, hence fertilizing the bloom. The model produces a sporadic enhancement of chlorophyll-a in the subsurface levels, associated with a low-salinity surface signature. The mean local circulation associated with the simulated bloom also reveals a dipole structure, as in observed datasets. Nitrate from subsurface levels (upwelling) as well as from the Madagascan coast (advection) is shown to influence the simulated bloom. A Lagrangian experiment shows dispersion of higher percentages of particles in the bloom region during bloom years and south of Madagascar during non-bloom years. Mesoscale eddies, originating close to Australia and which propagate westward towards southern Africa, can potentially impact the South-East Madagascar Bloom. In this study, a vast majority of these features have been shown to be subsurface-intensified eddies. A co-located eddy tracking dataset with Argo profiling floats are used to devise a subsurface-eddy identification method, which is based on the steric dynamic height anomaly of a specific eddy. Adding to the `eddy-zoo', these eddies are termed `SIDDIES' (South Indian ocean eDDIES), occurring as surface (surfSIDDIES) and subsurface (subSIDDIES) features. They travel along the latitudinal band range of 15°S to 35°S which we name the ‘SIDDIES corridor’. Advecting warm and fresh water during their propagation, cyclonic (anticyclonic) subSIDDIES contribute about 58% (32%) of the total eddy-heat flux in the South Indian Ocean. Anticyclonic subSIDDIES have also been found to be the sole, high-saline water eddy-conveyor towards the western South Indian Ocean. These eddies could also possibly transport nutrients throughout their journey, impacting the biogeochemistry of the ocean near Madagascar.
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    The Hydroclimate Variability of Central Africa: seasonal cycle, mechanisms, teleconnections and impacts on neighbouring regions
    (2018) Longandjo, Georges-Noel Tiersmondo; Rouault Mathieu; Reason Christopher
    Central Africa is, climatologically speaking, a poorly studied region (Clivar, 2000; Dezfuli and Nicholson, 2012; Nicholson and Dezfuli, 2012; Todd and Washington, 2004). It is considered as a knowledge gap in the understanding of the tropical climate system (Todd and Washington, 2004). Drivers of Central Africa rainfall are not well documented and deserve more attention. The aims of thesis are to enhance our fundamental understanding of Central Africa rainfall and the mechanisms involved in its seasonal and interannual variability as well as to assess how an atmospheric general circulation model forced by observed sea surface temperature (SST), the ECHAM5.3 model, does represent the main features of Central Africa hydroclimate variability. The seasonal cycle of Central Africa rainfall is primarily driven by change in the atmospheric low-pressure system of Central Africa landmass, water vapor and latent heat release rather than change of local temperature. From October to April, over Central Africa and its neighbouring regions, we highlight the existence in the mid-lower troposphere, between 1000 and 500 hPa of a dominant cyclonic and quasipermanent circulation pattern that drives the atmospheric large-scale circulation and its associated water vapor transports, namely the Central Africa Low. The Central Africa Low, with its variation strongly modulated by El Niño Southern Oscillations (ENSO), is characterized by strong convective activity due to an unstable atmosphere over central Africa, leading to high rainfall with less variance. Nevertheless, when the Central Africa Low prevails, Central Africa is a sink of water vapor, with the Indian Ocean as the main supplier. The weakening of the Central Africa Low, in May to September, is associated with the reversal of the water vapor transport at the northern boundary channel, leading Central Africa to become a source of moisture. During this season, both surrounding oceans are suppliers of moisture, with some additional contribution from the Congo basin rainforest. Central Africa rainfall variability is controlled by large-scale circulation variation, rather than variation in tropospheric water vapor. Year-round, the large-scale circulation is characterized by dominant easterly jets at middle (African easterly jets, AEJs) and upper (tropical easterly jets, TEJ) levels, owed by the Central Africa Low. At low-levels, there is a shallow zonal overturning circulation thermally direct, namely the Congo Basin Cell, driven by near-surface land-ocean thermal contrast between the warm central Africa landmass and the relatively cold Atlantic Ocean. The Congo Basin Cell, characterizes by eastward flow, persists year-round, with a maximum strength (-196.92±32.89 Sv) and width (30o degree) in August/September and minimum strength (-24.80± 17.83 Sv) and width (~6o degree) in May. The Congo Basin Cell does not play any crucial role in modulating Central Africa rainfall but it does regulate the rainfall distribution, through the seasonal position of the ITCZ. At midlevel, the atmospheric convective instability over Central Africa is controlled by the southward import of high moist static energy from the warmer Sahel associated with the AEJ over Central Africa. The saturation of the rising moist air at midlevel determines the location of high rainfall over central Africa year-round. Nevertheless, the absence of significant trend (- 0.013 mm per decade) of the Central Africa rainfall is associated with the weakening of the Central Africa Low in recent decades (1979 to 2015), consistent with Lau and Wu (2006). Further investigations on physical mechanisms affecting the Central Africa hydroclimate reveals that the Central Africa Low and land-ocean thermal contrasts are the main drivers of Central Africa rainfall variability at seasonal and interannual time scale, through the control of AEJs and the Congo Basin Cell strength and width. The analysis of ECHAM5.3 experiments provide a support to these mechanisms. Finally, to unravel what are the physical mechanisms shaping the rainfall anomalies patterns associated with the interannual variability of Central Africa rainfall, we found out that the Central Africa does reflect the regional-scale response of the atmosphere to the variation of the interbasin SST anomalies gradient (ΔSST) between tropical Atlantic and Indian Oceans. Likely, the zonal contrast of central Africa rainfall is owed by the Central Africa Low, which separates central Africa in two distinct regions of opposite polarity by regulating the strength of the low-level westerly and mid-upper easterly jets and their associated water vapor transports. This east-west dipole-like pattern of Central Africa rainfall is similar to the second leading mode obtained by empirical orthogonal functions (EOF) analysis of rainfall anomalies during the long rainy season. Thus, during the positive phase of ΔSST, the Central Africa Low area change induces an anomalous clockwise zonal overturning cell over Central Africa, with ascending branch over Atlantic, indicative of deep convection leading to rainfall surplus, and sinking branch over Indian Ocean, indicative of subsistence, which suppress convection and lead to rainfall deficit, consistent with the mechanism proposed by Dezfuli et al. (2015). However, the impact of ΔSST on Central Africa rainfall variability is asymmetrical during positive and negative phases of ΔSST.
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    The southern termination of the East Madagascar current
    (2007) Webb, Adrian Myles; Lutjeharms, Johann
    The Agulhas Current system is a vital part of the global thermohaline circulation. This global thermohaline overturning of the oceans has in turn an inextricable link to world climate. Although this current is so important, relatively little is known on its sources. One of these proposed sources of the Agulhas Current is the southern limb of the East Madagascar Current (EMC). Previously the EMC was primarily studied through the use of drifters, remote sensing and ships' drift methods. In an attempt to increase the understanding of this possible source region a first dedicated cruise was organised for the southern termination of the EMC. The cruise took place in March 2001. The hydrographic and other data from the cruise have the potential to describe the EMC termination in a seminal way. The EMC is shown to consist of Tropical Surface Water, Subtropical Surface Water and South Indian Central Water. In the region of the EMC Antarctic Intermediate Water, North Indian Deep Water and North Atlantic Deep Water/Circumpolar Deep Water were found. There is clear evidence of the presence of Red Sea Intermediate Water with salinities greater than 34.5 psu at temperatures between 4 and 6.5°C from 800-1250 m, inshore of the EMC. The width of the current during the cruise period was 100 km, the maximum speed of the current was 1.1 m/s and the depth of the current was 1000 m. The maximum transport of the EMC was calculated to be 39.3 Sv. An EMC undercurrent was found with a depth range of 1000-2000 m. The maximum speed of the undercurrent was seen to be 0.3 mis and the transport of the undercurrent was 0.8 Sv. No retroflection of the EMC emerges from this data set. Satellite geostrophics over a period of 13 years give evidence of eastward flow being associated with eddies in the region of the EMC termination. References have suggested that such eddies may give the appearance of a retroflection. From the observations there was ample evidence for the presence of a relatively weak upwelling cell inshore of the EMC. The temperature at the surface of the upwelling cell was 26°C (2 degrees less than the surrounding water) and the chlorophyll concentration was 0.63 mg Chl-a/m3 . Nitrate concentration was 6.4 µmol/kg at 100 m, phosphate 0.53 µmol/kg, silicate 5.69 µmol/kg and oxygen was 107.5 µmol/kg. At 45 km from the coast there was an average movement of water offshore down to a depth of 40 m. The maximum speed of the offshore flow was 0.5 m/s.
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    The upper ocean structure of the Antarctic circumpolar current in the vicinity of the South-West Indian ridge
    (2005) Gulekana, Mthuthuzeli Kenneth; Lutjeharms, J. R. E.; Ansorge, I. J.
    A survey was undertaken in the southwest Indian sector of the Southern Ocean, at a fracture zone situated within the South-West Indian Ridge (SWIR). The fracture zone, formally called the Andrew Bain fracture zone (ABFZ), is centred at 50°S;30°E. The main focus of this investigation, which was strategically located around the ABFZ, was to study the dynamics and the structural modification of the Antarctic Circumpolar Current (ACC) when it encounters the complex SWIR. High sea surface variability has been observed at the southwest Indian sector of the Southern Ocean, specifically at and around 50°S;30°E. As the strong eastward flowing ACC encountered the elevated and complex bottom topography of the SWIR it was obstructed and deflected from its pathway and forced to flow through the ABFZ (50°S;30°E). Consequently the ACC was significantly narrowed at the fracture zone and immediately widened after it had flowed through the fracture zone. Its structure, therefore, was modified upstream of the ABFZ prior to entering the fracture zone, and was modified further downstream of the fracture zone. The fronts observed were the northern and southern branches of the Subantarctic Front, namely the northern Subantarctic Front (SAF), the southern Subantarctic Front (SSAF) and the Antarctic Polar Front (APF). Firstly, the SAF was occasionally encountered at the north of the survey grid, secondly, the SSAF was found to be highly variable with extreme meandering patterns across the survey grid; and lastly the APF which was observed along the 51 °S latitudinal band with minor northward and southward meanders. The SSAF and the APF partially converged at 50°30'S; 30°E, that is, at the position of the ABFZ. Coincidentally, at the same location, maximum surface geostrophic velocities of >25 cm/s were observed. The geostrophic velocities tended to be high at the vicinity of frontal bands, particularly at the convergence of the SSAF and the APF. The ACC was therefore, found to undergo structural modification, both zonally and meridionally, upstream and downstream of the ABFZ as a result of the constriction. Nutrients and dissolved oxygen (DO) had a defined distribution pattern: a decrease southwards and downwards within the survey grid. Maximum and minimum nutrients and DO were recorded south and north of the APF, respectively. Four water masses were encountered firstly the Subantarctic Surface Water (SASW), which was clearly defined north of the SSAF only upstream of the ABFZ, secondly the Antarctic Intermediate Water (AAIW) which was characterised by a salinity minimum (34.2), thirdly the Antarctic Surface Water (AASW) which was characterised by a temperature minimum of 2°C at 200m depth mainly located south of the APF in the Antarctic Zone (AAZ). Lastly, the Circumpolar Deep Water (CDW) which was the only present deep water due to the data limitations. The interaction and mixing between these water masses was evident particularly at the ABFZ. The circulation pattern of the ACC was observed to be controlled by the bottom topography. The conservation of vorticity generated latitudinal mesoscale meanders and cyclonic (cold) and anticyclonic (warm) eddies. Elevated surface velocities of >60 cm/s were observed at the ABFZ and at regions closer to the frontal bands.
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    Toward an improved understanding of the Southern Ocean's biological pump: phytoplankton group-specific contributions to nitrogen and carbon cycling across the Subantarctic Indian Ocean
    (2021) Forrer, Heather; Fawcett, Sarah
    Iron (and silicate) (co-)limitation of phytoplankton is considered a primary cause of the Southern Ocean's inefficient biological pump. However, the role of phytoplankton community structure and response to nutrient cycling remains poorly understood. In a mass balance sense, phytoplankton consumption of new nitrogen (N; e.g., allochthonous nitrate) is proportional to net carbon (C) export, while growth fueled by recycled N (e.g., ammonium) yields no net C flux. The N isotope ratio (δ15N) of surface biomass has long been used as an integrative tracer of new versus regenerated uptake. This approach is rendered more accurate by coupling either fluorescence-activated cell sorting (FACS; of nano- and picophytoplankton; 0.4-20 μm) or microscopy (for microphytoplankton; >20 um) with groupspecific δ15N measurements. Samples were collected for the analysis of nutrients and nitrate-, FACS-, and microscopy-δ15N on a mid-summer transect of the Subantarctic Indian basin during the 2016/17 Antarctic Circumnavigation Expedition (ACE) cruise. The data show that all phytoplankton populations preferentially utilize nitrate (≥55%) across the Indian Sector of the Subantarctic, potentially driving higher C export potential than previously estimated. Indeed, near the Subantarctic islands, 72% of microand >80% of nano- and picophytoplankton growth is supported by nitrate. This is likely due to the partial alleviation of phytoplankton iron and silicate stress, largely as a result of bathymetric upwelling, which constitutes a manifestation of the island mass effect. C export potential is lower in the open ocean region away from the islands where iron stress has been shown to be higher; here, nitrate supports >55% of micro- and picophytoplankton and 7 to 79% of nanophytoplankton growth. In terms of relative abundance (RA), the open Subantarctic is dominated by picoeukaryotes (64%), although there exists a large disconnect between relative abundance and potential contribution to C export. The three largest surface-ocean phytoplankton populations included in this study – microphytoplankton, cryptophytes, and nanoeukaryotes – each contribute ~30% to the total C export potential across the Subantarctic Indian sector while picophytoplankton contribute ~5%. Thus, as has been concluded previously, the larger phytoplankton size classes are disproportionately important drivers of the Subantarctic biological pump. Other interesting ecological findings include diatom-dominated microphytoplankton populations apparently fueled by a significant fraction of regenerated N, even in areas of iron supply, and Synechococcus relying near-exclusively on new N, in contrast to subtropical observations. Additionally, the abundance of Synechococcus appears to be controlled by the availability of iron across the Subantarctic, with silicate and temperature playing a supporting role.
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    Understanding the interannual variability of pCO2 in the sea-ice impacted Southern Ocean
    (2024) Jojo, Bongiwe; Vichi, Marcello; Nicholson, S; du Plessis, M
    Sea-ice is permeable and plays an active role in the marine carbon cycle via biological and physio-chemical processes. The carbon cycle in seasonally sea-ice-covered waters needs to be better understood due to a lack of observational data and the system's complexity. To characterize the interannual variability of oceanic pCO2 in the sea-ice-impacted Southern Ocean and identify their potential primary drivers, this thesis combines in situ observations with remotely sensed data and reanalysis models during the austral summer months. The region of focus is divided into three sections: the Southern Ocean, three ocean basins, and the Goodhope line transect. Averaged over the Southern Ocean, the range of year-to-year variability of pCO2 between 2000 to 2018 was between 290 atm (2004) and 355 atm (2003). It is also noted that the interannual variability in pCO2 does not correspond to that of the Southern Annular Mode (SAM) index; however, there are some indications that the SAM may be an essential driver on longer time scales. Noticeably, the year 2016 stands out as one of the warmest and has the smallest Antarctic sea-ice extent (SIE) recorded since 1979 in the Southern Hemisphere. This SIE reduction has been attributed to positive sea surface temperature anomalies, the zonal wave pattern 3, and a SAM negative phase. pCO2 decreased in response to this ice loss event highlighting its sensitivity to rapid changes in sea ice. Overall, salinity obtains the highest correlation to the annually averaged pCO2 in the Southern Ocean and various basins. Along the Goodhope Line, variability of pCO2 indicated a higher magnitude and interannual variability of pCO2 during early summer than late summer. Non-thermal drivers primarily explain the variability of pCO2. These results suggest that the leading causes of the interannual variability of pCO2 in the sea ice-impacted Southern Ocean are those associated with non-thermal drivers of pCO2.