Browsing by Author "Hermes, Juliet C"

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    Annual cycle of the Benguela Jet
    (2014) Kamwi, Blessing K; Veitch, Jennifer Anne; Hermes, Juliet C; Krug, Marjolaine
    The Benguela Jet is a north-westward flowing current in the southern Benguela region. It is known to have an important influence on the fish recruitment yet little is known about the physical properties of the jet. In this study the ability of satellite data (SST and altimetry) to resolve the Jet was investigated. Following this, the annual cycle of the Benguela Jet was investigated using monthly climatological means computed from both remotely sensed and model output data (from the Regional Ocean Modelling System). Two altimeter tracks were identified as best suited to study the Benguela Jet: the Topex-A / Jason-1A Track number 209 which crosses the Benguela Jet current in its northern region off the Cape Columbine and the Topex-B / Jason-1B track number 31, which crosses the Benguela Jet current in the southern region. The month of January and July were chosen due to the fact that they represent the peaks of summer and winter. The surface geostrophic currents derived from both the model and satellite data reproduced the existence of the jet current off Cape Columbine and the Cape Peninsula. The jet was narrow and strong in January off Cape Columbine and off the Cape Peninsula and was situated farther offshore based on the model, relative to the altimeter data. Outputs from the numerical model showed that in July the jet was confined to the coast and was stronger off the Cape Peninsula (0.5 m.s-1) compared to Cape Columbine (0.4 m.s-1). A comparison between the regions of strong velocity gradient and the position of the upwelling front were in agreement in depicting the position of the jet. Altimetry, which suffers from imitation in coastal regions, could not reveal the jet in July due to its proximity to the shore at this season. The offshore boundary of the jet is resolved by altimetry in January. The interannual variability of the Benguela Jet has been identified.
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    The dynamics and physical processes of the Comoros Basin
    (2013) Collins, Charine; Reason, Chris; Hermes, Juliet C
    The main objective of this thesis was to investigate the circulation in the ComorosBasin using observed and model datasets. These data were used to establish whether or not a Comoros Gyre exists and to investigate the nature of the eddy variability in the basin.The water masses in the Comoros Basin emulate those found further south in the Mozambique Channel. The presence of AAIW north of Madagascar confirmed that this water mass enters the Comoros Basin from the east while the presence of North Atlantic Deep Water showed that this water mass is capable of spreading northward over the Davie Ridge. The main currents in the Comoros Basin, the westward flowing NEMC and a poleward current along the western boundary, are under the influence of the monsoon winds. The NEMC intensifies during the Southwest monsoon in response to a strong wind jet which develops off the northern tip of Madagascar, whereas the poleward current weakens due to the opposing force imposed by the southwesterlies. Additionally, the circulation in the basin consist of meso-scale eddies of both polarities. Anti-cyclonic eddies, with lifespans of
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    Evaluating global ocean reanalysis systems for the greater Agulhas Current System
    (2014) Cooper, Kyle Francis; Backeberg, Bjorn Christoph; Hermes, Juliet C; Deshayes, Julie; Veitch, Jennifer Anne
    Operational oceanography aims to accurately hindcast and forecast the state of the ocean. An international initiative, the Global Ocean Data Assimilation Experiment (GODAE), developed and increased the capacity for global operational oceanography. However, the products from the global initiatives were regionally inapplicable due to low spatial resolutions, and have recently improved through GODAE OceanView. A number of local operational oceanographic initiatives have been setup over the southern African regional ocean, but proved to be unsustainable and ended. Recently, the aim to develop a regional ocean prediction system has arisen, and initial steps have been taken. This thesis aims to address the lack of local capacity in operational oceanography, and contribute to a crucial process in developing a regional ocean prediction system. Here, we validate and investigate the differences between three global reanalysis products, namely MyOcean (GLORYS2V1), HYCOM (U.S Naval Research Laboratory) and BlueLINK (OFAM3). These reanalysis products are validated and investigated over the greater Agulhas Current System, which is a crucial system in Southern African regional ocean. The salient oceanographic features represented in the reanalysis products are initially compared to historical literature of the region and followed by available unassimilated observations (i.e. independent). The results show that the reanalysis products from MyOcean, and the U.S Naval Research Laboratory satisfactorily simulate the major salient oceanographic features of the Agulhas Current System. Bluelink does not correctly portray the structure of the source and retroflection regions, and therefore has limited use over the Agulhas Current System. The differences between the three products indicates that the data assimilate does not sufficiently constrain the models in order for their solutions over the Agulhas System to converge. The evaluation of these global ocean reanalysis products is a critical step toward a regional ocean prediction system over Southern Africa, and building toward the local capacity to accomplish this goal.
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    The inter-annual variability in the onset of the enhanced chlorophyll-a east of Madagascar
    (2015) Oozeeraully, Yuneeda Bibi Naheed; Reason, Chris; Hermes, Juliet C; Collins, Charine
    Chlorophyll-a concentration is a measure of phytoplankton biomass and is therefore used as a proxy for primary production. The chlorophyll bloom occurring in the South western Indian Ocean is one of the major blooms in the open ocean and is subject to controversies about its formation, propagation and termination mechanisms. In this study, the region was divided into two sub-regions, the South East region (48°E-66°E, 24°S-30°S) and the East region (70°E-88°E, 24°S-30°S) and satellite datasets of chlorophyll-a, sea surface temperature, wind speed and direction and sea surface height were analysed. A bloom was characterized by twice the mean level of the sub-region (~0.08mg/m3) and three bloom years were identified: 2004, 2006 and 2008. The timing, propagation and termination of the bloom showed spatial and temporal variability. The wavelet analysis revealed a semi-annual signal for 2006 and 2008 and an annual one in 2004 due to the gradual decline in the concentration of chlorophyll-a. The transport of the bloom is affected by the passage of eddies where the concentration of chlorophyll-a is higher in the core and around the edges in cyclonic and anticyclonic eddies respectively. The bloom could be present at the sub-surface level throughout the years but is only apparent at the surface during specific years.
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    Investigating the relationship between volume transport and sea surface height in the Agulhas Current System
    (2018) Vermeulen, Estee; Hermes, Juliet C; Backeberg, Björn; Elipot, Shane; Vichi, Marcello
    The relationship between the volume transport of the Agulhas Current at 34°S (the position of the Agulhas Current Time-series array) and the gradient of sea surface height across the current was investigated using a regional Hybrid Coordinate Ocean Model. Previous studies have suggested a high correlation between SSH slope and Agulhas Current transport and, based on 3 years of in situ measurements, a transport proxy between along-track satellite data and in situ data was developed. The purpose of this modelling study was to re-create the Agulhas Current transport proxy in a virtual modelling environment, to test the validity of the underlying assumption on which the satellite-altimeter proxy was based. The Agulhas transport proxy assumed nine, constant linear relationships between SSH slope and integrated transport per unit distance over the 22-year transport time-series, based on the 3-year sampling period and a constant vertical stratification. The 34-year regional-hindcast from HYCOM provided the means to test the sensitivity of the transport proxy to vertical changes in the current and the length scale of observations used to build a constant, linear relationship between transport and SSH slope. During the investigation it was found that HYCOM contained exaggerated levels of offshore variability. This resulted in stronger correlations for the inshore linear regression models with a decreasing trend moving offshore. Based on the overall performance of the 34-year transport proxies it was concluded that the proxy was more capable of estimating the net transport of the Agulhas Current across the array instead of only the southwest transport component. Therefore, transport estimates inshore were more accurate than the transport estimates offshore, when the current is in a meandering state, and the poorer performance of the southwest transport proxy, specifically developed to capture the transport during offshore meander events, was less capable of estimating an accurate transport estimate. Results showed that calculating the proxy over longer time periods did not significantly improve the skill of the Agulhas transport proxy, suggesting the 3-years was a sufficient time-period used to develop the transport proxy in HYCOM. This study motivates the need to improve long-term monitoring methods, where the usage of numerical ocean models could help understand the sensitivities and limitations involved in the development of transport proxies in future.
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    Links between the Seychelles-Chagos thermocline ridge and large scale climate modes and primary productivity; and the annual cycle of chlorophyll-a
    (2014) Dilmahamod, Ahmad Fehmi; Reason, Chris; Hermes, Juliet C
    The Seychelles-Chagos Thermocline Ridge (SCTR) is a region of upwelling present at 55°E- 90°E and 5°S-12°S in the southwest tropical Indian Ocean. It is a region of strong ocean-atmosphere interactions due to the high variability of the thermocline depth caused by the local Ekman pumping. Sea-viewing Wide Field-of-view Sensor (SeaWiFS) has shown high variability of surface chlorophyll-a (SChl-a) in the SCTR region. The Indian Ocean Dipole (IOD) and El Niño Southern Oscillation (ENSO) have also driven significant interannual variation of the depth of 20°C isotherm (D20) and SChl-a in the southern tropical Indian Ocean. A 50-years hindcast (RUN58-07) from a coupled bio-physical model was used to study the SChl-a concentration on an annual time scale and the interannual variability of D20 and SChla in the SCTR in response to IOD and ENSO events. Initial analysis revealed a high overestimation of SChl-a in the 50-year run. Therefore, a 44-years hindcast (RUN58-01) of the same coupled model was taken into consideration. Comparisons with observations show that the RUN58-07 reproduces the D20 and SSH better than the RUN58-01 but the RUN58-01 shows better agreement with SeaWiFS. Results reveal that the SCTR exhibits an annual cycle of SChl-a concentration, with a peak in austral winter (June-August) due to the strong southeasterlies, increasing wind stirring and induced upwelling. Vertical sections of the SCTR also indicate that an increase in surface concentration in austral winter is compensated by a decrease in subsurface phytoplankton blooms. Composite figures show that IOD events exhibit a greater influence on the subsurface and surface variability in the SCTR region. The IOD deepens and shoals the D20 in the SCTR and eastern Indian Ocean respectively whereas ENSO displays a weaker and less-extensive influence on the D20. The spatial distribution of SChl-a in the Indian Ocean is completely disrupted by IOD during which the SCTR becomes oligotrophic whereas the eastern Indian Ocean becomes highly productive. ENSO, however, does not display any significant biogeochemical signature in the SCTR. This study should improve our understanding of the interannual variability of the thermocline depth and chlorophyll-a in the SCTR region; and for the optimization of the management of fishery resources and marine ecosystems.
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    Modelling the effects of spatio-temporal spawning variability on the transport and retention success of sardine (Sardinops sagax) eggs and larvae around South Africa
    (2018) McGrath, Alice; Moloney, Coleen L; Van der Lingen, Carl D; Hermes, Juliet C; Roy, Claude
    The transport and retention of sardine (Sardinops sagax) eggs on the west and south coasts of South Africa were investigated using a Lagrangrian, particle tracking, individual-based model (IBM), coupled with two 3-D hydrodynamic Regional Ocean Modeling System (ROMS) model configurations of different generations. The same IBM is coupled with PLUME, an older ROMS model configuration used in a previous sardine IBM for the southern Benguela, as well as BENGSAFE, a more recent, better resolved ROMS model configuration. Results from the two runs were compared to see if and how the underlying ocean model affects modelled retention and transport. An updated IBM, extended spatially and temporally to better represent sardine spawning and recruitment, was then coupled with the BENGSAFE model configuration, and assessed. The effects of spawning area, and month, year and depth of particle release, were investigated for each coupled 3D-IBM simulation using linear models. The portion of particles transported from the south coast to the west coast was a focus throughout, as this is of great importance to the management of the sardine fishery. In all simulations, three main systems were identified depending on where particles were released and recruited. Two of these systems were retention-based: particles released on the west coast and retained in the west coast recruitment area (WC-WC), and particles released on the south coast and retained in the south coast recruitment area (SC-SC). The other system is transport-based and represents particles released on the south coast and transported by ocean dynamics to recruit in the west coast recruitment area. Results were similar for the west coast when either hydro- dynamic model configuration was used; depth of release proved to be important in this system with its effect varying depending where on the west coast particles were released. Differences occurred on the south coast, with more transport to the west coast and more offshore loss in the higher resolved BENGSAFE IBM than the PLUME IBM. The difference was attributed to the positioning of the PLUME model's boundary in close proximity to the Agulhas Current, and the conditions at this boundary not properly resolving the current. The initial BENGSAFE IBM was spatially restricted to match the PLUME IBM, and therefore south coast spawning and recruitment was not fully represented. Therefore, the BENGSAFE IBM was spatially extended on the south coast and run for a longer time period. Similar results were obtained in the extended BENGSAFE IBM and the initial BENGSAFE IBM. However south coast retention increased and the average percentage of particles transported to the west coast decreased slightly. Spawning area proved to be an important determinant of retention and transport success in all recruitment systems, and the further east a particle was spawned the less likely it was to be transported to the west coast, and the more likely it was to be retained on the south coast. Transport to the west coast was most successful in late winter; a significant monthly effect on transport success was present. Correlations between standardised anomalies for stock assessment model estimated recruitment and IBM modelled retention/transport for the west and south coast were not significant, highlighting that retention and transport are not the only determinants of recruitment success. The extended BENGSAFE IBM can act as a base level to which extra layers of biological complexities can be added, such as horizontal and vertical egg distribution matching observations, incorporating gonadosomatic index (GSI) values, diurnal vertical migration and buoyancy.
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    Ocean model diagnosis of variability in the South Indian Ocean
    (2005) Hermes, Juliet C; Reason, Chris; Lutjeharms, Johann R E
    Evidence exists that sea surface temperature (SST) variability in the South Indian Ocean may significantly influence weather and climate patterns in the southern African region. SST, in tum, can be influenced by variability in ocean fluxes, observations of which are limited in the South Indian Ocean and it is necessary to augment them with estimates derived from models. Two sets of variability in this region are examined in this thesis. The first concerns the large-scale interannual variability of the oceans neighbouring South Africa and the second, inter-ocean fluxes south of Africa on meso-through to interannual timescales. In terms of the former, a global ocean model forced with 50 years of NCEP (National Centre for Environmental Prediction) re-analyses winds and heat fluxes, has been used to investigate the evolution and forcing of interannual SST variability in the South Indian Ocean and co-variability patterns in the South Atlantic. Secondly, an eddy- permitting model is used to investigate volume, heat and salt fluxes in the oceanic region south of Africa and the effect of variations in the strength of wind forcing. Interannual dipole-like SST variability in the South Indian and South Atlantic Oceans were realistically simulated using the global ocean model, ORCA2. The model results imply that there are connections between large-scale modulations of the midlatitude atmospheric circulation of the Southern Hemisphere and co-evolving SST variability in the South Atlantic and South Indian Oceans. The atmospheric variability results in an increase (decrease) in strength of the anticyclonic wind fields over each ocean during positive (negative) dipole events. The resulting wind anomalies lead to changes in surface heat fluxes, short wave radiation, meridional Ekman heat transport and upwelling, all of which contribute to the evolution of these SST dipole patterns. Evidence is found of links between these dipole patterns and the Antarctic Oscillation and ENSO (El Niño Southern Oscillation).