Browsing by Author "Rouault, Mathieu"
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- ItemOpen AccessA 1000-year carbon isotope rainfall proxy record from South African baobab trees (Adansonia digitata L.)(Public Library of Science, 2015) Woodborne, Stephan; Hall, Grant; Robertson, Iain; Patrut, Adrian; Rouault, Mathieu; Loader, Neil J; Hofmeyr, MicheleA proxy rainfall record for northeastern South Africa based on carbon isotope analysis of four baobab ( Adansonia digitata L.) trees shows centennial and decadal scale variability over the last 1,000 years. The record is in good agreement with a 200-year tree ring record from Zimbabwe, and it indicates the existence of a rainfall dipole between the summer and winter rainfall areas of South Africa. The wettest period was c. AD 1075 in the Medieval Warm Period, and the driest periods were c. AD 1635, c. AD 1695 and c. AD1805 during the Little Ice Age. Decadal-scale variability suggests that the rainfall forcing mechanisms are a complex interaction between proximal and distal factors. Periods of higher rainfall are significantly associated with lower sea-surface temperatures in the Agulhas Current core region and a negative Dipole Moment Index in the Indian Ocean. The correlation between rainfall and the El Niño/Southern Oscillation Index is non-static. Wetter conditions are associated with predominantly El Niño conditions over most of the record, but since about AD 1970 this relationship inverted and wet conditions are currently associated with la Nina conditions. The effect of both proximal and distal oceanic influences are insufficient to explain the rainfall regime shift between the Medieval Warm Period and the Little Ice Age, and the evidence suggests that this was the result of a northward shift of the subtropical westerlies rather than a southward shift of the Intertropical Convergence Zone.
- ItemOpen AccessA study of Benguela Niños and Niñas from 1958 to 2015(2018) Imbol, Koungue Rodrigue Anicet; Rouault, Mathieu; Illig, SerenaPrediction and Research Moored Array in the Tropical Atlantic (PIRATA) records in combination with outputs from an Ocean Linear Model (OLM) and altimetric data are used to investigate the link between the equatorial Atlantic Ocean dynamics and the variability in the coastal region of Angola-Namibia at interannual timescales over 1998 to 2012. The PIRATA records help to define an index of equatorial Kelvin wave activities in the Equatorial Atlantic. There is a good agreement between PIRATA monthly dynamic height anomalies, altimetric monthly sea surface height anomalies (SSHA), and sea level anomalies calculated with an OLM at interannual time scales. This allows the interpretation of PIRATA records in terms of equatorial Kelvin wave propagations. Extreme warm or cold events in the Angola – Namibia area lag strong anomalous eastward equatorial propagations by 1–2 months. Remote equatorial forcing via equatorial Kelvin waves which propagate poleward along the west African coast as coastal trapped waves is at the origin of their developments. Results show a seasonal phasing, with significantly higher correlations between the equatorial index and coastal sea surface temperature anomalies (SSTA) off Angola-Namibia in October - April season. Then, a systematic study of all the Benguela Niño and Benguela Niña events before 1982 is done using an Ocean general circulation model in combination with the OLM outputs from 1958 to 2015. 26 anomalous strong coastal events (16 warm and 10 cold) are identified. The analysis of their evolution confirms the remote equatorial origin of most of these coastal anomalous strong events. Modelled meridional transport anomalies across the Angola Benguela Front (ABF) contribute to the development of these anomalous coastal warm events. Across the ABF, the results obtain with the net temperature transport are similar to the ones with net mass transport. Most anomalous events peak in October - April season. Lagged composites of surface temperature and wind stress anomalies in the equatorial and southeastern Atlantic reveal that both local and remote forcings develop simultaneously 1-2 months before the peak of Benguela Niño or Niña. At the monthly scale, local atmospheric forcing is more correlated with anomalous coastal events occurring in Southern Angola which is a non-wind-upwelling driven region. The results from this thesis open the possibility to predict Benguela Niño and Benguela Niña events using an index depicting the equatorial interannual variability associated with Interannual Equatorial Kelvin Wave propagation, especially from October to April when the coastal stratification is favourable to the imprint of coastal trapped waves in the surface layer.
- ItemOpen AccessAssessing the impacts of assimilating satellite SST in addition to along-track SLA into a HYCOM of the Agulhas System(2017) Rapeti, Tharone; Backeberg, Bjorn Christoph; Rouault, MathieuThe greater Agulhas Current System, is considered to be the largest western boundary current in the Southern Hemisphere, with only the Gulf Stream, and possibly the Kuroshio, considered to be larger globally (Bryden et al., 2005). The Current System is a crucial factor for determining the mean state and variability of the regional marine environment, resources and ecosystems in the region, regional weather, as well as the global climate on a broad range of temporal and spatial scales. Due to an absence of a coherent in situ and satellite-based observing system in the area, modelling and data assimilation techniques are utilised. These both further the quantitative understanding of the ocean dynamics as well as providing better forecasts of this complicated western boundary current system. In this study, we compare two assimilation experiments using the Ensemble Optimal Interpolation (EnOI) data assimilation scheme in a regional implementation of the Hybrid Coordinate Ocean Model (HYCOM). In the first experiment, we assimilate along-track satellite sea level anomaly (SLA) data only, and in the second experiment we assimilate both along-track sea level anomaly (SLA) as well as satellite sea surface temperature (SST) data. The objectives of the study are to investigate the impacts of assimilating SST along with SLA into the regional HYCOM model, with the hopes of improving the model performance. The long term aim of this experiment is to develop a regional ocean prediction system. The additional assimilation of SST along with SLA into the HYCOM model, has improved upon the representation of the SST field across the region by reducing the error. However, with regards to velocity, surface eddy kinetic energies (EKE), as well as subsurface velocities, the updated SST model shows less improvement. A velocity bias can be seen as the reason for underperformance in these aspects. The model still struggles to recreate subsurface water masses, underestimating salinity in the upper 500 m; assimilating T/S profiles in the future could improve on this. The assimilation of SST has improved upon the SST-SSH correlation in the model, as well as the spatial distribution and accuracy. The assimilation of SST along with SLA has had many positive impacts, with unfortunately, a few negatives. The shortcomings of the numerical model will have to be improved upon and additional assimilation variables should be tested in further studies, to provide a solid forecasting system.
- ItemOpen AccessThe atmospheric boundary layer above the Agulhas current(1996) Lee-Thorp, Andrew Michael; Lutjeharms, Johann R E; Rouault, MathieuThis thesis describes the atmospheric boundary layer above the Agulhas Current using shipboard meteorological measurements and rawinsonde ascents. The juxtaposition of the warm Agulhas Current and cool shelf waters is shown to have far-reaching effects on the overlying atmosphere. Air-sea fluxes of momentum, sensible and latent heat and resultant boundary layer characteristics demonstrate high horizontal inhomogeneity. The results suggest that this inhomogeneity is permanent. The spatial heat flux gradient is reflected in the overlying atmosphere by a transition in stability of the boundary layer and potential cumulus formation from the cool shelf to the warm current. For airflow perpendicular to the Agulhas Current an internal boundary layer was observed to develop at the inshore sea surface temperature front. Onshore-moving air accumulated a significant quantity of moisture during its trajectory over the current. When airflow is parallel to the current an atmospheric moisture front exists along the axis of the inshore sea surface temperature front. The mean thermodynamic structure of the atmosphere was investigated. An inversion capped the boundary layer whilst a second, higher-level subsidence inversion was found which acts to limit the vertical development of cumulus clouds and therefore the redistribution of heat and moisture above the boundary layer. The results presented in this thesis are useful in two ways. The Agulhas Current has frequently been linked to South African climate. This is the first dedicated study which quantifies and characterizes the atmospheric boundary layer in this region. Secondly, maritime airmasses are dramatically modified above the Agulhas Current. The resultant large horizontal inhomogeneity, its vertical extent and permanence suggest that its inclusion is vital to any successful climate model. Atmospheric general circulation models have been criticized for not taking into account regions of strong horizontal inhomogeneity. The results of this thesis support this argument and highlight the need for similar studies. Bibliography: pages 116-123.
- ItemOpen AccessAtmospheric circulation and sea surface temperature anomalies during the dry summers of 2001/02, 2002/03 and 2003/04 in Southern Africa(2005) Mehari, Michael Futwi; Rouault, MathieuDrought is a phenomenon associated with the lack or shortage of water. It means less than normal or no water is available. It primarily originates from lack of precipitation. Lack of precipitation leads to depletion of storage of I. soil moisture that results in dry land crop failure and dying-off grazing and other vegetations, II. of ground water, which results in drying up of springs, streams and boreholes and Ill. of water in man-made reservoirs, which results in stress to households, industry, power stations and irrigation schemes (Davis, 1983). There are three types of drought {Thomas, 1965): meteorological, agricultural and hydrological droughts. Meteorological drought occurs when the rainfall is abnormally low. Agricultural drought exists when the soil is depleted to the extent that crop harvests are reduced significantly (Davis, 1983). Agricultural drought has a common time scale of a season (3 to 6 months) (Harsch E, 1992). Agricultural drought can also be caused by excessive rain or flood leading to a damage of crops. Hydrological drought is associated with scarcity of precipitation on a longer time scale (1- 2 years or more) and its effect is on ground water supply (Meigh et al, 1992). Meteorological drought can be seen as a subset of agricultural drought. If there is agricultural drought then there is also meteorological drought. On the other hand, agricultural and hydrological drought can be out of phase, each having different signatures (Rouault and Richard, 2003).
- ItemOpen AccessCoastal climate change and variability in the Benguela current system(2022) Tomety, Folly Serge; Rouault, Mathieu; Illig, SerenaThe thesis aims to seek to document the long-term change and decadal variability in the Benguela Upwelling System and study the possible mechanisms behind these changes. The Benguela Upwelling System is one of the four most productive fisheries areas in the world, and it is therefore important to understand the mechanisms leading to changes at different time and space scales before developing scenarios or forecasts for the future of the region. The first part of the thesis (chapter 3) uses four satellite-derived Sea Surface Temperature (SST) datasets combined with various climate reanalysis data to investigate the long-term SST trends in the Benguela Upwelling System over the period 1982-2017. The use of different datasets shows different trends depending on the dataset, which is a concern. However, after a thorough examination, there is some consensus. Results show that the Angola-Benguela Upwelling System has significantly changed during the last three decades. The changes vary in space and depend on season. Cooling trends are observed in the southern part of the Benguela Upwelling System in the austral summer and autumn. The cooling trend is consistent with a positive trend in upwelling-favourable equatorward winds due to the intensification and poleward expansion of the South Atlantic Subtropical high-pressure atmospheric system. A warming trend is observed in Southern Angola and Northern Benguela in late spring and summer. Results also show that the warming or cooling trends in the Benguela Upwelling System are not as linear as the trend in global air temperature. Indeed, when studying trends for the 1982-2017 period, trends tend to slow down and can reverse sign in some regions and recent time, suggesting decadal variability. Most discrepancy between SST datasets occurs from 1982 to 1985, the start of the satellite era. The second part of the thesis (chapter 4) focuses on understanding the mechanisms leading to the warming trends along the Angolan and Northern Benguela coast. To do so, the Ocean General Circulation Model NEMO (OGCM NEMO) is used. The model produces an unrealistic cooling trend in the Northern Benguela due to a positive trend in upwelling-favourable wind model forcing. The modelled warming trend in Southern Angola is properly simulated which allows me to use the model to study the mechanisms leading to the warming trend in Angola. Analysis of the model net heat budget components and their contribution to the overall SST trend suggests that the warming trend observed along the Angolan and Namibian coasts through the austral summer is primarily associated with the intensification of the poleward flow along the coast, bringing more warm water from the tropics to the region and also due to weakening of the vertical flow of cold water to the surface. Locally, the net surface heat flux has decreased and tends to create a negative SST trend but does not offset the warming trend created by the intensification of the flow. The poleward intensification of the Angola Current is attributed to the intensification of the cyclonic circulation around the Angola Dome. Lastly, in chapter 5, the decadal variability in the Benguela upwelling system, identified in Chapter 3, is investigated using a long-term ocean model simulation of 110 years (1900 - 2010) of the global ocean-ice components of the Norwegian Earth System Model (NorESM). The results reveal the presence of three dominant scales of variability: the interannual (2-8 years), quasi-decadal (9-14 years) and interdecadal (19-26 years) variability in the Southern Benguela upwelling system. The Southern Benguela SST correlations with the global SST reveal that at quasi-decadal scale the Southern Benguela SST is linked to the south Atlantic SST and the north-east Pacific SST fluctuations, while at the interdecadal scale the Southern Benguela SST modulation is linked to the equatorial and northern Pacific SST, Indian SST and Atlantic SST fluctuations except the equatorial Atlantic SST.
- ItemOpen AccessA comparative study of Phoebetria albatrosses' interactions with mesoscale oceanographic features south of the African continent(2015) Rasehlomi, Tshikana Phillip; Rouault, MathieuTwo sympatric Phoebetria albatrosses, P. fusca and palpebrata breeding at Marion Island in the South Indian Ocean were studied. Adult individuals were tracked between 2008 and 2011. The study examined the foraging distribution of the two species in relation to environmental parameters such as sea surface temperature. Interaction with mesoscale features expressed by sea surface temperatures, was tested with statistical models. Tracked P. fusca demonstrated positive association with sea surface temperatures in the Southern Indian Ocean, in particular during incubating and chick-provisioning periods, by travelling shorter distances to the interfaces of mesocale features created as a result of the Agulhas Return Current located << 500 km to the north of breeding islands. During nest construction, tracked P. fusca travelled greater distances in search of food possibly because they had no chick to return to at the colonies. Contrastingly, tracked P. palpebrata did not demonstrate any positive association with sea surface temperatures. During incubating and chick-provisioning periods, tracked P. palpebrata travelled shorter distances to foraging grounds located to the south of breeding islands possibly to maximise returns to chicks while minimising efforts. During nest construction, tracked P. palpebrata travelled to distant foraging grounds to the south of the Antarctic Polar Front in areas of low mesoscale variability suggesting that greater distances can be achieved when they have no chick to feed at the colonies. Individuals of both tracked species foraged within close proximity during energy-demanding periods indicating their ability to employ an efficient foraging strategy that ensures minimal effort whilst maximising returns. Generalised Linear Models with the response variable being species and explanatory variables comprised of sea surface temperatures, annual breeding stages, distance traversed and activity, were conducted to examine the effect of environmental factors on seabird foraging distribution. These models were subjected to robust fitness tests and those that represented ecological reality of the two tracked albatrosses were chosen. The study demonstrates that the most important foraging areas for these two tracked seabird species overlap with the Indian Ocean Tuna Commission Convention area making them vulnerable from incidental capture from high longline fisheries. These results have implications to the conservation of these albatrosses suggesting a need for a multi-faceted approach on fisheries management particularly on mitigation of seabird bycatch in the Indian Ocean Tuna Convention area.
- ItemOpen AccessThe effect of the Agulhas Current on synthetic aperture radar derived wind fields(2016) Schilperoort, Daniel E; Krug, Marjolaine; Rouault, Mathieu; Hansen, MortenIn this study, 5 years (987 swaths) of high resolution wind speeds, derived from Advanced Synthetic Aperture Radar data collected over the Agulhas Current region, are studied to investigate the effect of warm, high intensity currents on the ocean's surface roughness and resulting derived wind fields. The wind data are derived using the CMOD5.n GMF with CFS reanalysis wind data as direction input. The CFS direction data are validated using ASCAT derived wind observations Globcurrent ocean current velocity data is used to investigate the difference between the satellite derived wind speeds compared to surface velocities of the current and the true wind speed. The, so called, current-relative effect is investigated for different wind direction regimes, namely: upcurrent, downcurrent, crosscurrent west and crosscurrent east. Our analyses are conducted for 6 locations of interest, evenly spaced along the Northern Agulhas Current. MODIS, SEVIRI and OSTIA SST data are used as proxy for locating the core of the Agulhas and it's temperature fronts, as well as to investigate wind speed modifications as a result of ocean-atmosphere energy transfer. It is found that higher resolution SAR derived winds have a greater ability to represent higher intensity and smaller scale wind features in comparison to winds derived from Scatterometers. A combination of the current relative effect and SST-atmospheric heating for upcurrent wind directions results in a sharp increase in mean wind speeds over the inshore boundary of the current of between 5m/s and 7m/s (50−60%). Individual events can reach as high as 15m/s (100%) over 10′s of kilometres. For downcurrent winds, the expected current relative effect is overridden by increased wind speeds of up to 5m/s (40%) across the entire current due to the influence of SSTs. The mean effect of SSTs on wind speeds has a stronger effect than the current relative effect on wind speed changes over the current. The wind speed differences are best represented under moderate wind speeds, between 5−15m/s. This investigation will contribute to future satellite wind speed derivations to identifying new wind speed and surface roughness altering effects. It will also serve to increase understanding of high resolution wind features and sharp changes over ocean features.
- ItemOpen AccessImpact of the Agulhas Current on storm development(University of Cape Town, 2020) O'Connor, Jason; Rouault, MathieuA high-resolution atmospheric model (WRF) is used to investigate the impact of the Agulhas Current on synoptic storm development. A sensitivity experiment is conducted to analyse the influence of the Agulhas Current's sea surface temperature (SST) on rain producing, synoptic scale weather features. Two model configurations: Control (CTL) and Smooth (SMTH) are analysed to understand the effect of the Agulhas Current's SST and high latent heat fluxes on storms that develop or track over the Current. The two configurations are identical except that the SMTH simulation has the SST signature of the Agulhas reduced by smoothing out the strong SST gradients associated with the Current. This results in the Agulhas Current core having SSTs reduced by roughly 1.5°C in the SMTH configuration. Consequently, lower (100 - 150 W.m¯²) latent heat fluxes are also simulated at the Current core's location in the SMTH run. Using daily South Africa Weather Service synoptic charts from 2001 - 2005, when the model output is available, two hundred (200) synoptic scale storms are found to track over the Current. Using the TRMM 3B42 3-hourly 0.25 x 0.25° precipitation rain rate product, 70 (of the 200) are found to have produced rainfall. Five model variables are used as proxies for the storm intensity of these 70 storms. Ten storms are found to show storm intensification when passing over the Current. In the CTL simulation, of these ten storms, ten show lower 850mb geopotential heights (m), nine show higher surface wind speeds (m.sˉ¹), seven show higher rain rates (mm.hrˉ¹), eight show higher Eddy Kinetic Energy (EKE) (m².sˉ²) and nine show greater upward moisture flux at the surface (g.mˉ².sˉ¹) compared to the SMTH run once each storm has propagated over the Current. Model output analysis shows sustained or dissipating storm intensity of the other 60 storms while passing over the Current. Nonetheless, these results provide a strong case for the influence of the Agulhas Current on the intensification of synoptic scale, rain producing events.
- ItemOpen AccessImpact of Wind Driven Variability on Sea Surface Temperature and Ocean Colour in False Bay(2019) Seymour, Sian; Krug, Marjolaine; Smith, Marie; Mouche, Alexis; Rouault, MathieuFalse Bay is the largest true bay in South Africa and is an important area for conservation, the local fishing industry and marine based recreational activities. A large amount of studies, both recent and historical, have been carried out on the biology of the bay, but studies on the physics of the bay are very few in comparison. In this study high resolution satellite imagery is used to investigate wind variability and its impact on sea surface temperature (SST) and chlorophyll concentration (Chl-a) variability within False Bay and the Cape Peninsula region. High resolution (1 km) coastal winds derived from the Sentinel-1 satellite Synthetic Aperture Radar (SAR) show that winds are strongly influenced by topography under the predominantly south-easterly wind regime. The Hottentots Holland mountain range and Cape Peninsula mountain range create wind shadows as well as areas of increased wind speed within False Bay and west of the Cape Peninsula. Our observations also show that global atmospheric models, such as ECMWF, are not able to capture the spatial variability in the wind fields driven by the orography. Analyses of the SST and ocean colour imagery show that wind shadows are generally associated with warmer surface waters and higher Chl-a. In contrast, regions of enhanced wind speeds show colder surface waters and decreased chlorophyll concentration. Our results suggest that spatial variation in the horizontal wind fields have direct and significant impact on the water properties within False Bay. This study highlights the need for high resolution wind observations and simulations to force regional oceanic models of False Bay and the Cape Peninsula region.
- ItemOpen AccessImpacts of ENSO on coastal South African sea surface temperatures(2020) Nhesvure, Belinda; Rouault, MathieuThe impact of El Niño Southern Oscillation (ENSO) on the Southern African inland climate is well documented and provides skill in the seasonal forecast of rainfall but little is known of the impact of ENSO on the ocean surrounding South Africa. The aim of this study is to assess the impact of ENSO on sea surface temperatures around the coast of South Africa and to calculate SST trends around the coast. I start by updating the study of Rouault et al (2010) on the very topic with an additional 10 years of data and two additional newer datasets which allow sampling closer to the coast where wind-driven upwelling is more active. The new highresolution ERA 5 reanalyzed climate dataset is also used to look at the atmospheric forcing of sea surface temperatures by ENSO. As in Rouault et al. (2010), I study five similar threedegree-long coastal regions around South Africa, namely: West Coast, South Coast, Port Elizabeth/Port Alfred, Transkei, Kwazulu-Natal and a larger offshore Agulhas Current area domain. Three SST datasets are evaluated in this study: the 1 ̊x1 ̊Optimal Interpolation sea surface temperature (OI SST) used by Rouault et al (2010), the 0.25 ̊x 0.25 ̊ Optimal Interpolation SST and the 4 km x 4 km Advanced Very High-Resolution Radiometer (AVHRR) Pathfinder SST version 5.3. The 0.25 ̊x 0.25 ̊OI SST resolvesSST anomalies better in these coastal regions as compared to 1 ̊x1 ̊ OISST. The difference in results among the three products concerning trends and correlation with ENSO is a cause for concern. The 4 km x 4 km AVHRR Pathfinder allows for SST to be extracted even closer to the coast but missing values are numerous and hamper the use of this dataset for ENSO composites and trend analyses. Results show a significant positive correlation with El Niño in summer at the monthly scale, reaching a maximum correlation of 0.45 at 3 months lag. Correlation is the highest in late summer. There is a negative correlation in the Agulhas Current area, opposite to those with ENSO and West Coast. The impact of ENSO on the coast of South Africa, West Coast and South Coast is due to change in surface wind speed with weaker upwelling favorable during El Niño leading to warmer than normal coastal water SST and stronger than normal Southeasterly winds during la Niña leading to cooler than normal coastal water. The wind perturbation is part of largescale basin-wide perturbations in the tropical Atlantic, in the South Atlantic high-pressure atmospheric system and in the westerly wind pattern of the midlatitude to the south. Non-ENSO related impact can be as important as ENSO related SST perturbation and is also linked to large scale perturbations in the South Atlantic. There is no relation between the strength of ENSO and the strength of the perturbation, and some ENSO events do not lead to the expected canonical warming or cooling. The large-scale SST perturbations seem to be caused by anomalous surface turbulent flux of latent and sensible heat and abnormal wind speed and direction. This study opens the possibility of seasonal forecasting of SST in the South Benguela upwelling system because of the positive lag correlation between ENSO and SST with ENSO leading SST.
- ItemOpen AccessThe influence of the Agulhas Current on two South African extreme weather events(2000) White, Sarah April; Rouault, MathieuSurface station, satellite and NCEP re-analysis data are used to examine the evolution of two severe storms that occurred over the eastern coastal regions during South Africa's summer season 1998/99. The storms in November and December were both accompanied by heavy rainfall in two widely separated locations. The storm in December proved to be more severe as it resulted in flooding while tornadoes were reported in the Umtata and Hogsback regions of the Eastern Cape. Both storms appeared to result from interaction between a continental heat low, advection of warm moist air around an anticyclone in the South-west Indian Ocean and an approaching midlevel westerly trough. NCEP derived moisture flux diagrams and back trajectories of air parcels constructed from ECMWF data suggest that the Agulhas Current region was a major source of low level moisture for both storms. TRMM satellite imagery captured heavy rainfall above the high sea surface temperatures of the Agulhas Current. TRMM measurements of rainfall and latent heat in the atmosphere show that the high sea surface temperatures of the Agulhas Current modified the mesoscale environment above the current. To what extent the mesoscale environment above the Agulhas Current modified the synoptic situations over land could be answered using regional modeling and more frequent radiosonde data.
- ItemOpen AccessInterannual variability and long-term trends of surface hydrography around the Prince Edward Island Archipelago, Southern Ocean(2022) Toolsee, Tesha; Lamont, Tarron; Rouault, MathieuThe 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.
- ItemOpen AccessInvestigating the seasonal and interannual variability of the poleward undercurrent in the northern Benguela system(2012) Yates, Sarah Elizabeth; Rouault, Mathieu; Veitch, Jennifer AnneThe Benguela Current System is unique as both the equatorward and poleward boundaries are warm water current systems. Between 15° S – 37° S the surface currents are generally equatorwards, with 7 distinct upwelling cells, narrow equatorward shelf-edge jets and a poleward undercurrent along the continental slope. Model data was used to determine the seasonal and interannual variability of the poleward undercurrent (PUC) in the northern Benguela system. The PUC is the southward extension of the Angolan Current that carries low oxygen water (LOW) originating from the Angola Dome. The LOW flows from the Angolan region southwards in the Benguela system. The focus of the study is on the PUC associated with the Sverdrup relation. The model ORCA-025 was used to reproduce zonal transects from 17° S to 30° S to determine the changing characteristics of the PUC with latitude as well as seasonal and interannual variability of this current. The PUC is faster moving in the north (~17° S) and decreases in velocity moving south (~30° S). The PUC is shallower in the north increasing in depth in the south. The model data shows the velocity of the PUC has a seasonal cycle that is faster in the austral summer and autumn and weakens in the winter. The transport of the PUC is amplified during austral winter and spring, which is consistent with the increased negative wind stress curl during those seasons. The wind stress curl in the region exhibits a strong connection with the transport of the PUC via the Sverdrup relation. The PUC exhibits interannual variability when comparing to the Benguela Niño events, but does not show a correlation with El Niño Southern Oscillation.
- ItemOpen AccessLong-term climate variability at the Prince Edward Islands in the Southern Ocean(2021) Shangheta, Anna Liisa Penelao Tulimevava; Lamont, Tarron; Ansorge, Isabel; Rouault, MathieuA warming Southern Ocean (SO), due to climate change and global warming, has many implications on the sub-Antarctic Islands in the SO. Due to the distance away from continental land these islands experience an oceanic climate, making them the perfect sentinels to climate change in this sector of the Southern Ocean. Studies have proposed that climate changes reported at the Prince Edward Islands (PEIs) correspond in time to a southward shift of the Antarctic Circumpolar Current (ACC) particularly the Subantarctic Front (SAF). While other studies have shown distinctive trends in ocean and atmospheric parameters such as sea surface temperature (SST), air temperature, sunshine, rainfall, air sea level pressure and wind speed and direction from the 1950s to the early 2000s, the aim of this study is to update those studies to a more recent time with updated time series. Among the changes recorded is an increase in SST and air temperature, which is a strong indication of the changing local and global climate. Using linear regression, this study showed that the rates of increase from 1949 to 2018 of the SST (0.022°C/year), minimum (0.0072°C/year) and maximum air temperatures (0.016°C/year) are smaller than estimated in previous studies. The increasing trend in SST and air temperature reported by previous papers has actually stopped since the 2000s, which reduces the formerly reported trend (0.028°C/year). Although the in-situ measured SST data had gaps, a good correlation with in-situ SST and large scale satellite derived Reynolds SST help to corroborate the covariation between SST, in-situ SST and air temperature giving weight to the hypothesis of a reversal of the positive temperature trends reported by others. The change in decadal variability a decrease in air pressure of 4 hPa since the late 1990s to late 2000s, which coincided with a decrease in minimum and maximum air temperatures of 1°C over the same period; decrease in westerly wind and an increase in the northerly component of the wind, which would explain the decrease of inshore sea surface temperature a while thereafter. This study further corroborates previous findings of a continued decrease in rainfall, while the sunshine has largely remained the same. The seasonal cycle of the air pressure is significantly associated with that of rainfall, showing that the bimodal high air pressure signature resulting from the Semi-annual Oscillation (SAO) is associated with a decrease in rainfall. The Southern Annual Mode (SAM) was significantly yet weakly correlated with the SST (0.24), rainfall (-0.25) and air pressure (0.16), indicating that it does have an impact at the PEIs but not as strong as previously speculated. The El Niño Southern Oscillation (ENSO) has very weak and insignificant relationships with the parameters examined except for a weak relationship with in-situ SST, sunshine and air pressure. These new insights, especially at the decadal timescale, could further our insight on how subAntarctic islands have responded to climatic changes.
- ItemOpen AccessMechanisms for the wet and dry years over Eritrea during the summer rainfall season(2003) Kidanemariam, Mussie T; Rouault, Mathieu; Brundrit, GeoffThe economy of Eritrea depends on rain-fed agriculture, which is highly sensitive to seasonal and inter-annual variations of rainfall. Adequate amount of rainfall is required every year for adequate agricultural production and drinking water. Therefore, the distribution of seasonal rainfall in space and time is critical to the country's economy. Drought has great influence on the socio-economic aspect and livelihood of the rural population. Drought occurs in Eritrea due to anomalous regional atmospheric circulation. This study focuses on the summer rainy season of June, July and August (JJA). It examines atmospheric and oceanographic mechanism patterns associated to summer rainfall anomalies. The investigation mainly concentrates on wet and dry spells that occurred over Eritrea during the rainy seasons (June to August) of 1950's-1990's. Rainfall data was available from the Climatic Research Unit (CRU) Hulme dataset for a period of 1950 -1996 and this together with rainfall data from the Africa Data Dissemination Service was used to determine the characteristics of wet and dry spells. Finally, NCEP-NCAR reanalysis climate data was used to analyse the atmospheric circulation associated with wet and dry years. It has been found that the pattern of the SST along the eastern Pacific and the Indian oceans seem to influence the occurrence of the anomaly rainfall years. Correlation analysis shows that Eritrean summer rainfall is more strongly correlated with the eastern Pacific and Indian Oceans SST. Thus, the drought records in Eritrea are mainly associated with EI Nino events.
- ItemOpen AccessOn the role of the Agulhas Current on weather and climate of South Africa(2018) Nkwinkwa, Njouodo Arielle Stela; Rouault, Mathieu; Keenlyside, NoelThe Agulhas Current is the strongest western boundary current of the Southern Hemisphere. The aim of this thesis is to understand the impact of ocean-atmosphere interaction in the Agulhas Current on the atmosphere and to investigate its importance for Southern African rainfall. This warm Current creates a high temperature gradient with the surrounding ocean, leading to a large turbulent flux of moisture from ocean to atmosphere (also called the turbulent latent heat flux). The dynamics of ocean-atmosphere interaction above the Agulhas Current and its impact on the weather and climate of Southern Africa are not well known. This is due to a) climate reanalyses that do not include the Agulhas Current and b) the lack of regional capacity in ocean-atmosphere modeling. I use ocean observations, various climate reanalyses, and several satellite remote sensing data sets to find out if the new reanalyses (cited below) do represent the intense exchange of moisture that occurs above the Agulhas Current and the Agulhas Retroflection region. The largest turbulent latent heat flux is found in the Retroflection region in winter, while the lowest is off Port Elizabeth in summer. The Climate Forecast System Reanalysis (CFSR) and the ModernEra Retrospective analysis for Research and Applications version-2 (MERRA-2) do represent the turbulent latent heat flux well when compared to high-resolution satellite data. ERA-Interim Reanalysis underestimates the turbulent latent heat flux due to reduced wind speeds. The observation-based National Oceanography Centre Southampton (NOCS) is different from the satellites and the reanalysis products because its annual cycle is reversed, and NOCS underestimates the turbulent latent heat flux compared to the former products. The study of the satellite product air-sea turbulent fluxes (SEAFLUX) shows that east of the Agulhas Current, the specific humidity difference is the main driver of the annual cycle variations of turbulent latent heat flux, while the main driver in the Retroflection is the wind speed and both the specific humidity difference and the wind speed in between (around Port Elizabeth). I use high-resolution annual mean observations from satellites, atmospheric reanalysis, and the Weather Research and Forecasting (WRF) model to show that the warm core of the Agulhas Current drives a band of precipitation along the east coast of South Africa when the Current is adjacent to the coast. To do that, I conduct a classic modeling experiment with one configuration representing the sea surface temperature (SST) of the Agulhas Current relatively well. This WRF experiment reproduces the turbulent latent heat flux well. The second simulation is with SST of the Agulhas Current reduced by up to 2°C compared to the first experiment. From a diagnostic of the pressure adjustment mechanism, results show that the warm SST of the Agulhas Current enhances the formation of coastal precipitation along and above the Current. Finally, I look at the seasonality of oceanatmosphere interaction in the Agulhas Current and its impact on Southern African precipitation. In winter, the impact of the Agulhas Current is confined to the atmosphere above it and mechanisms are similar to those described for annual mean. In summer and autumn, SST differences between the two simulations where the Agulhas Current system is more than 25°C, leads to differences in geopotential height above the ocean, extending along the eastern coast and over the land area. The higher temperature of the control simulation leads to cyclonic circulation anomalies and larger moisture flux anomalies from the Agulhas Current to the continent from the south. The analysis of the high-level moisture flux indicates that the Agulhas Current influences the rainfall and humidity flux of Southern Africa. More moisture flux is then brought inland at a higher level. In the northeast of the region, there is an export of moisture anomalies from land to the ocean, and an import of moisture anomalies from above the Agulhas Current to the landmass. This is due to the wind anomalies between the two simulations. However, the overall result leads to more precipitation over the interior of the continent. The study shows that it is important to integrate the fine structure of the ocean temperature of the Agulhas Current in modeling studies and climate reanalyses. Results of this thesis have implications for the prediction of South African weather and climate, and for understanding past and present climate.
- ItemOpen AccessThe process of conserving biodiversity: From planning to evaluating conservation actions on private land in the Cape Lowlands, South Africa(2009) von Hase, Amrei; Rouault, MathieuConservation can be conceptualised as a process of linked phases that contribute to bringing about effective biodiversity protection: (i) a conservation assessment that identifies spatially explicit conservation priorities to provide strategic guidance on where best to invest conservation resources; (ii) a planning phase that takes the spatial priorities forward into implementation processes by setting out a strategy and schedule for undertaking conservation action; (iii) an implementation phase during which conservation interventions are executed; and (iv) an evaluation phase to investigate whether conservation has been successful. In practice, conservation is rarely conducted in this way. The interrelated phases are often undertaken separately, links are neglected, and conservation science to date has focused primarily on the conservation assessment. This has led to the development of highly sophisticated principles and techniques for locating priority conservation areas, but planning and evaluation have received limited research attention: few published studies demonstrate collaborative planning processes that assist with putting conservation assessments into practice, or show on-theground conservation success linked to effective conservation planning and implementation processes. My PhD research aimed to address these knowledge gaps by conducting a conservation assessment and collaborative planning phase that would lead to effective conservation action as determined by an evaluation. The study area was in the critically endangered Cape Lowlands, a conservation priority area in the Cape Floristic Region, South Africa. The highly transformed agricultural production landscape is mostly privately owned; formal biodiversity protection is low; and remnants of natural vegetation (< 9% is left) harbour an exceptionally diverse flora. Strategic conservation interventions coordinated across the Cape Floristic Region (CFR) provided the overall implementation context in the Cape Lowlands. My research was conducted in this real-world practical situation and addresses the whole conservation process, from assessment to evaluation of conservation actions. I first developed a conservation assessment guided by three key questions: âWhat are feasible, efficient, defensible and efficacious solutions for (i) deriving a surrogate layer that represents biodiversity in a region which is characterised by exceptional plant species richness and endemism ; and (ii) considering the connectivity of natural areas in an ecosystem that is highly transformed, fragmented and largely unprotected?â; and âHow can a selection method be developed for identifying and prioritising key biodiversity areas in a landscape identified as 100% irreplaceable?â To answer these questions I identified feasible, efficient, defensible methods focusing on three key aspects: (i) producing a biodiversity surrogate map of original vegetation cover using two alternative approaches: simple expert mapping and statistical modelling integrating plant species and environmental data; (ii) designing selection units based on vegetation connectivity in a simple technique to include spatial attributes of conservation areas before identifying key biodiversity areas; (iii) developing a prioritisation method based on a simple scoring system and verifying results with MARXAN-selected priority areas. In all vi three cases I found that the simple conservation assessment methods produced suitable outputs for further integration in the assessment and in decision-making during planning. (i) The expert map was as effective as the vegetation model and required fewer resources to be produced since the model relied on resource-intensive species data collection. (ii) In comparison with commonly used cadastre-based units, connectivity-based selection units captured connected vegetation more effectively and area-efficiently in units that served as the basis for priority area selection. (iii) Scoring provided a feasible, defensible mechanism for prioritising key biodiversity areas in the Cape Lowlands where all remaining vegetation has been identified as 100% irreplaceable. The planning phase complemented the assessment. Key guiding questions here were âHow can collaborative planning be used to translate the conservation assessmentâs technical outputs into timebased conservation goals and into useful products for implementation?â and âWhat constitutes effective planning in the conservation process? Through a collaborative scheduling process, I developed timebased conservation goals for action in the Cape Lowlands. This was undertaken in two work sessions with scientists, planners and conservation practitioners from the implementing agency, CapeNature. Scheduling was guided by (i) scoring-derived biodiversity-driven spatial priorities that made intuitive sense to implementers; and (ii) conservation opportunities and constraints (including resources) identified by the practitioners. Scheduling was conducted with reference to the on-going development of a private land conservation strategy for the CFR to be piloted in the Cape Lowlands. The scheduling process was an effective platform for taking spatial priorities from the assessment towards implementation: the discourse-based collaborative planning was constructive and led to consensusbased final products, including a 20-year and 5-year conservation plan setting out spatially explicit goals for conservation interventions in the Cape Lowlands. The main limitation of the process was that resource planning was not integrated explicitly enough to identify realistic goals. This highlighted the importance of integrating detailed resource considerations in future planning. Finally, to address the question âTo what extent has the Cape Lowlands conservation plan been implemented after five years of off-reserve conservation interventions in the region?â I developed a protocol for evaluating the effectiveness of conservation action in the Cape Lowlands. I assessed (i) the extent to which the goals conservation plans produced in the planning phase had been implemented; and (ii) the achievements of incentive-based conservation stewardship interventions on private land in the Cape Lowlands and CFR. Achievements were measured as hectares of vegetation protected through voluntary and legally-binding contractual conservation agreements between landowners and conservation organisations. The evaluation revealed that (i) CapeNatureâs stewardship interventions in the Cape Lowlands focused on priority areas identified in the 5- and 20-year conservation plans, thus demonstrating effective execution of the plans; (ii) private land conservation interventions have been remarkably successful and cost-effective: 68604ha priority vegetation were protected in the CFR under conservation agreements by end 2007, rivalling private land biodiversity conservation in the U.S.A. and Latin America, and more than 8000ha in the critically endangered Cape Lowlands at a cost of R 6.8 vii Million (< 1 million US$). The evaluation identified the long-term financial sustainability of current implementation programmes as the most significant threat to future success in private land conservation interventions in the Cape Lowlands and CFR. There is significant scope to design future monitoring and evaluation systems to measure ecological gains due to specific conservation actions, not done in the Cape Lowlands study, and to tailor approaches to suit specific programme stages. This PhD provides a rare overview of an entire conservation cycle with linked phases that has led to biodiversity protection. The study highlights that an effective long-term process demands significant investment in (i) a diverse (growing) set of skills and expertise to solve complex conservation situations; (ii) time, especially for visible implementation success; and (iii) well-allocated resources (money, time, skills, research attention) across all phases in the conservation process. This is necessary as each phase is needed to achieve the ultimate conservation goal: I show in the Cape Lowlands that a simple conservation assessment with limited funds (R1.8 million over 3 years) can be highly effective in guiding action towards priority areas. Important here is to develop rapid, defensible methods for cost-effective assessments and linking these with in-depth planning processes. Planning and evaluation in the Cape Lowlands were essential connecting phases that continue to support implementation success. In the context of on-going conservation action, planning and evaluation need to become part of a cyclical conservation process geared towards improved practices. I suggest that significantly greater investment in planning and evaluation research is essential to move conservation science forward in fulfilling its fundamental goal of strategically guiding where, when and how to invest optimally in conservation interventions. This will be exceptionally beneficial for undertaking effective conservation interventions and will help to clearly demonstrate the value of the research for conservation practice.
- ItemOpen AccessA qualitative and physical analysis of processes around the Mascarene plateau(2015) Vianello, Patrick Angus; Rouault, Mathieu; Ansorge, Isabelle JaneThe Mascarene Plateau is a submerged volcanic plateau to the east of Madagascar which extends over 2200 km between the Seychelles and Mauritius. It is a complex feature, which is composed of 4 banks separated by 3 channels - namely the Seychelles, Saya de Malha, Nazareth and Cargados-Carajos Banks. The main objective of this thesis is to analyze cruise data obtained during the October/November 2008 ASCLME (Agulhas Somali Current Large Marine Ecosystem) cruise on board the Dr Fridtjof Nansen which surveyed the region around the Mascarene Plateau. Due to the paucity of data in the region, the cruise was a routine cruise with no specific scientific questions to be answered. A comparison is also made between sea surface temperature (SST) and Acoustic Doppler Current Profiler (ADCP) derived currents on board to satellite estimate to possibly extend cruise results in space and time. Although the resolution of satellite estimate is low compared to cruise measurements, satellite estimate of geostrophic velocities and sea surface temperatures compare well with cruise data and can therefore be used in the region with confidence. This is invaluable since the Mascarene Plateau is relatively poorly understood and it allows us to link cruise data with Rossby waves and currents impacting the region. This is the second cruise in the region as there was a research cruise on board the RRS Charles Darwin during June/July 2002. However, the June/July 2002 cruise did not sample the banks of the Mascarene Plateau.
- ItemOpen AccessResolving cross-shelf dynamics in the Agulhas Current from GlobCurrent and glider observations(2019) Maja, Tumelo; Krug, Marjolaine; Rouault, Mathieu; Johannessen, Johnny AThe Agulhas Current is the strongest Western Boundary Current of the Southern Hemisphere and it plays a significant role in the circulation of the shelf and coastal waters, whereby mesoscale (50- 500 km) and submesoscale (1 -10 km) instabilities in the Agulhas Current impact the local oceanography of the shelf region. The main objective of this study is to evaluate the ability of a gap-free and merged gridded satellite ocean current dataset, GlobCurrent, to resolve and monitor the variability of the Agulhas Current’s cross-shelf dynamics. In this study, GlobCurrent is compared to in-situ observations collected from underwater gliders through mapping and correlation analysis to assess the product’s accuracy in different subdomains and water depths of the Agulhas Current’s main area domain. We also investigate the value of using a higher resolution satellite and gap-free Sea Surface Temperature (SST) dataset to complement the GlobCurrent dataset in observing the Agulhas Current’s flow processes and features. The results show that GlobCurrent is adequate for describing large mesoscale features and deep water flows but the product has limitations in capturing fast-evolving and small mesoscale features, particularly the Durban Eddy in the KZN bight region. GlobCurrent also exhibits, at times, directional errors in addition to the current speed discrepancies. This research study demonstrates the limitation of the GlobCurrent product for monitoring ocean current variability in shallow, coastal waters and regions dominated by small mesoscale variability. This study also provides new insights on the joint use of other merged satellite products i.e. merged ODYSSEA SST, which may compensate for some of the GlobCurrent product’s shortfalls. Future studies should consider complementing altimetry-based satellite products like GlobCurrent with other merged satellite observation products such as ODYSSEA SST for better imaging of small mesoscale processes and features in shallow coastal waters.