Modelling the mesoscale variability in the greater Agulhas Current system using hybrid coordinate Ocean model

The ocean circulation dynamics in the greater Agulhas Current system are dominated by mesoscale variability, which is highly non-linear, and therefore difficult to measure and simulate accurately. Moreover, the shedding of Agulhas rings from the retroflection south of Africa, which is the dominant mechanism by which warm and saline water flows from the Indian into the Atlantic Ocean, is thought to be a crucial component of the thermohaline circulation. With the goal of providing an accurate simulation of the greater Agulhas Current system, and in particular its mesoscale variability, a high resolution Hybrid Coordinate Ocean Model is set up in a nested configuration. In two 11 year simulation experiments, the effect of a higher order momentum advection scheme on the simulated ocean dynamics is tested and evaluated against available satellite observations and in-situ measurements. Quantitative analyses and model validation methods are developed to objectively evaluate the simulation experiments. The resultant skewness analyses and spatial variograms are objective measures for assessing the model simulation and additionally provide new insights on the mesoscale dynamics of the greater Agulhas Current system. A 4th order momentum advection scheme is shown to significantly improve the simulation of the region, in particular the dynamics of the southern Agulhas Current and the retroflection are greatly improved. From the analyses of the two model simulations in conjunction with satellite observations and in-situ measurements, it is found that the Indo-Atlantic inter-ocean exchange, and the shedding of Agulhas rings from the retroflection, is sensitive to the strength of the Agulhas Current, which in turn is influenced by the flow dynamics in the Mozambique Channel and south of Madagascar. Mesoscale eddies drifting from these source regions to the Agulhas Current play an important role, and the connection between the Agulhas Current and the respective source regions provides a link to large-scale variability in the Indian Ocean, which in turn is related to interannual modes of variability such as the Indian Ocean Dipole and El NiÑo Southern Oscillation

Agulhas current variability determined from space : a multi-sensor approach

Includes bibliographical references (p. 119-132).Satellite remote sensing datasets including more than 6 years of high frequency Sea Surface Temperature (SST) imagery as well as surface current observations derived from 18 years of merged-altimetry and over 2 years of Advanced Synthetic Aperture Radar (ASAR) observations are combined to study the variability of the Agulhas Current. The newly available rangedirected surface currents velocities from ASAR, which rely on the careful analysis of the measured Doppler shift, show strong promise for monitoring the meso to sub-mesoscale features of the surface circulation. While the accuracy of ASAR surface current velocities suffers from occasional bias due to our current inability to systematically account for the wind-induced contribution to the Doppler shift signal, the ASAR surface current velocities are able to consistently highlight regions of strong current and shear. The synaptic nature and relatively high resolution of ASAR acquisitions make the ASAR derived current velocities a good complement to altimetry for the study of sub-mesoscale processes and western boundary current dynamics. Time-averaged range-directed surface currents derived from ASAR provide an improved map of the mean Agulhas Current flow, clearly showing the location of the Agulhas Current core over the 1000 m isobath and identifying the region at the shelf edge of the north-eastern Agulhas Bank as one of the most variable within the Agulhas Current. To determine the variability of the Agulhas Current, an algorithm to track the position of the current is developed and applied to the longer merged-altimetry and SST records. Limitations associated with altimetry near the coast favour the use of the SST dataset to track the position of the Agulhas Current in its northern region. In the southern Agulhas, where the current lies further from the coast, altimetry is suited to monitoring the position of the Agulhas Current. The front detection analysis conducted on the SST dataset in the northern Agulhas reveals the complex nature of Natal Pulses. The downstream passage of the Natal Pulses is associated with the generation of secondary offshore meanders at the inshore edge of the current. Perturbations formed during the passage of Natal Pulses evolve rapidly to either dissipate, re-merge with the initial Natal Pulse or in some rare occasion, detach from the Agulhas Current

Variability of coastal upwelling south of Madagascar

Madagascar’s southern coastal marine zone is a region of high biological productivity which supports a wide range of marine ecosystems, including fisheries. This high biological productivity is attributed to coastal upwelling. The thesis seeks to characterise the variability of the coastal upwelling south of Madagascar. The first part of the thesis provides new insights on the structure, variability and drivers of the coastal upwelling south of Madagascar. Satellite remote sensing is used to characterize the spatial extent and strength of the coastal upwelling. A front detection algorithm is applied to thirteen years of Multi-scale Ultra-high Resolution (MUR) Sea Surface Temperatures (SST) and an upwelling index is calculated. The influence of winds and ocean currents as drivers of the upwelling are investigated using satellite, in-situ observations, and a numerical model. Results reveal the presence of two well-defined upwelling cells. The first cell (Core 1) is located in the southeastern corner of Madagascar, and the second cell (Core 2) is west of the southern tip of Madagascar. These two cores are characterized by different seasonal variability, different intensities, different upwelled water mass origins, and distinct forcing mechanisms. Core 1 is associated with a dynamical upwelling forced by the detachment of the East Madagascar Current (EMC), which is reinforced by upwelling favourable winds. Core 2 which appears to be primarily forced by upwelling favourable winds, is also influenced by a poleward eastern boundary flow coming from the Mozambique Channel. This intrusion of Mozambique Channel warm waters could result in an asynchronicity in seasonality between upwelling surface signature and upwelling favourables winds. The second part of the thesis focuses on the interaction between the intrusion of warm water from Mozambique channel and the upwelling cell in Core 2. Cruise datasets, satellite remote sensing observations and model data analyses are combined to highlight the existence of a coastal surface poleward flow in the south-west of Madagascar: the South-west MAdagascar iv Coastal Current (SMACC). The SMACC is a relatively shallow (Coastal Current (SMACC). The SMACC is a relatively shallow (<300 m) and narrow (<100km wide) warm and salty coastal surface current, which flows along the south western coast of Madagascar toward the south, opposite to the dominant winds. The warm water surface signature of the SMACC extends from 22◦S (upstream) to 26.4◦S (downstream). The SMACC exhibits a seasonal variability: more intense in summer and reduced in winter. The average volume transport of its core is about 1.3 Sv with a mean summer maximum of 2.1 Sv. It is forced by a strong cyclonic wind stress curl associated with the bending of the trade winds along the southern tip of Madagascar. The SMACC directly influences the coastal upwelling regions south of Madagascar. Its existence is likely to influence local fisheries and larval transportpatterns, as well as the connectivity with the Agulhas Current, affecting the returning branch of the global overturning circulation. The last part of the thesis provides a holistic understanding of the inter-annual variability of the upwelling cells associated with the multiple forcing mechanisms defined in the first two parts of this work. Results reveal that the upwelling cells, Core 1 and Core 2, have different inter-annual variabilities. Inter-annual variability of Core 1 is associated with the East Madagascar Current (EMC) while Core 2 is linked with the South-west MAdagascar Coastal Current (SMACC). Inter-annual changes in the EMC occur as a result of oscillations in the South Equatorial Current (SEC) bifurcation off Madagascar, while the inter-annual variability in the SMACC is influenced by the cyclonic wind stress curl inter-annual variability. The upwelling is also linked with global/regional climate modes. Both Cores are highly correlated with the Subtropical Indian Ocean Dipole (SIOD). Core 2 is also correlated to the Indian Ocean Dipole (IOD). Both cores are significantly correlated with the El Ni˜no-Southern Oscillation (ENSO) after 12 months lag

Influence of the Indian Ocean Subtropical Dipole on the Agulhas current

Includes bibliographical references.Modern studies have successfully linked Subtropical Dipole (SIOD) events to southern Africa’s austral summer precipitation patterns, however, none have investigated the SIOD’s influence on the Greater Agulhas Current System. Here, the SIOD climatology was developed using a Regional Ocean Modeling System (ROMS) configured with GFDL-CORE v.2b reanalysis winds and heat fluxes for the 1958-2007 period. This configuration allows for a relatively accurate spatial and temporal account of the Sea Surface Temperature (SST) and Sea Surface Height (SSH) variability in the Subtropical Indian Ocean (SIO). Simulation and evaluation of SIOD events was achieved through the application of the Empirical Orthogonal Function (EOF), Wavelet Analysis and Composite Map Analysis. The EOF applied to monthly SST anomalies for the months January to December during the years 1958-2007 in the SIO resulted in the SIOD phenomenon emerging as the second EOF mode and explaining 8.93 of the total variance of the SIO. Moreover, the EOF applied only to the austral summer (JFM) months emerges the SIOD as the first EOF mode and explaining 20.84 of the total variance in the SIO. ROMS model results and statistical correlation results suggest that SIOD SST variability is neither linked to the El Nino-Southern Oscillation (ENSO) nor the Tropical Indian Ocean Dipole (IOD) phenomena, notwithstanding that SIOD events have in the past, coincided with some El Nino and La Nina events. Composite map analysis results suggest no significant influence of SIOD events on anomalous Agulhas Current SST and SSH during positive and negative SIOD years. Examination of lagged statistical correlations also showed no significant relationship between the anomalous SIOD index and the satellite derived geostrophic velocity at the core of the Agulhas Current for the period 1993-2007

Modelling dispersal and connectivity of broadcast spawning corals in the Western Indian Ocean

Coral reef degradation is happening at an alarming rate all over the world due to multiple stressors with elevated sea surface temperature being the root cause. Using the Regional Ocean Modelling System and an individual-based model for the western Indian Ocean, this thesis explored the general circulation patterns (both large and mesoscale) important to dispersal and connectivity of broadcast corals while identifying regions that act as a source of larvae and those that receive larvae. Because habitat destruction and fragmentation through severe bleaching and mortality threaten coral reef health, projected thermal stress from Global Climate Models was explored to quantify future bleaching scenarios that might impact the reproductive timing and larval dispersal. Evaluation of the ROMS configuration for the western Indian Ocean shows that the basin-scale circulation patterns of the region are appropriately captured with the mean volume transports consistent with those derived from observation. Using the eddy detection algorithm, a description of the Southern Gyre as a key aspect of the Somali Current system was identified. The Southern Gyre is associated with barotropic instabilities associated with the northward flowing Somali Current. Rossby waves arriving at the East African coast and the strength of the monsoon winds are also responsible for the evolution and intensification of the gyre. The aggregated trajectories from the Lagrangian model highlight the dominant dispersal pathways and barriers to dispersal following release. The general circulation plays an important role in the dispersal of reef larvae over the study region. At a short pelagic larval duration, most of the released larvae settle back to or near natal reefs, but as the pelagic duration increases, the number of isolated reefs and islands decreases. Even with increased pelagic duration, some reefs (e.g., Agalega and Tromelin) are completely isolated. The mean dispersal distance from release to settlement varied across the region with larvae released along the East African coast dispersed an average of 405 km before settling while those in the Seychelles archipelago dispersed about 101 km. Different blocks of clusters were observed with 16 clusters observed when the pelagic duration is shorter (5 days), compared to seven clusters when the pelagic duration is longer (60 days). The warming trends and bleaching thermal stress shows that among the 636 reef pixels in the study region, about 56% showed positive sea surface temperature trends during the study period (1985- 2016). The frequency of bleaching level thermal stress has also increased over the same period, a tendency that climate models project to continue. Even under optimistic scenarios (such as the Representative Concentration Pathway RCP 4.5), most coral reefs are projected to experience severe bleaching and possible mortality by the 2050s. Low to moderate thermal stress are projected over reefs along the East African coast and near the northwest tip of Madagascar and thus these regions may act as potential climate refugia while increasing the potential of reefs to cope with climate change

Quantifying the SST biases in data assimilative ocean simulations of the Benguela Upwelling System

The Benguela Upwelling System (BUS) on the west coast of southern Africa is one of the global ocean’s most productive upwelling systems supporting a large fishing industry, a fledgling aquaculture sector and offshore mining interests. Despite intensive monitoring and modelling studies, there is no regionally tailored ocean forecasting system that is explicitly developed to deal with the unique ocean dynamics of the Benguela. In this study, the Hybrid Coordinate Ocean Model (HYCOM) is used in conjunction with the Ensemble Optimal Interpolation (EnOI) assimilation scheme to study the impact of assimilating sea surface temperature (SST) and along-track sea level anomalies (SLA) observations on predicted upwelling dynamics in the Benguela. In order to evaluate the predictive skill and impact of data assimilation, three experiments with HYCOMEnOI are evaluated: (1) with no assimilation (HYCOMFREE), (2) only assimilating along-track SLA (HYCOMSLA) and (3) assimilating both SLA and SST (HYCOMSLA+SST). Using MODIS Terra SST as reference, the model SST outputs are evaluated. HYCOMFREE is found to exhibit a warm bias along the coast, HYCOMSLA shows an even greater warm bias while HYCOMSLA+SST conversely shows a much improved SST forecast skill. It is hypothesised that the warm biases could be due to errors in boundary conditions and/or the ERA-interim wind product used to force the model. Furthermore, a comparison of the assimilated SST product (the Operational Sea Surface Temperature and Sea Ice Analysis; OSTIA) with MODIS SST reveals biases in OSTIA up to ±1 ◦C, raising questions over its suitability for assimilation in upwelling regions. Studying the effect of assimilation on SSH, SST and surface currents before and after the assimilation suggests that an increase in SSH from assimilated SLA leads to increased warm SST biases in HYCOMSLA. This is due to an incorrect relationship between SSH and SST in the free-running HYCOM, from which the static ensemble is derived for the EnOI. HYCOMSLA+SST exhibits slightly enhanced SSH increments but the associated increase in SST is significantly reduced by the assimilated SST, resulting in a reduction of the bias with very little impact on the current dynamics. This is reflected in the surface velocitiy increments, which are similar to or worse than that of HYCOMSLA. Investigating the potential of HYCOM-EnOI as an operational forecasting system has revealed that the assimilation of SST and along-track SLA vastly improves modelled SST for the BUS upwelling. Errors in the free-running model, which constitutes the static ensemble, need addressing and comparisons between MODIS and OSTIA SSTs suggests that OSTIA may not be ideally suited for assimilation in the case of coastal upwelling, due to limitations in capturing the dynamics correctly

The dynamics and physical processes of the Comoros Basin

Includes abstract.Includes bibliographical references.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 o

Impact of the Agulhas Current on Southern Africa Precipitation: A Modeling Study

Postponed access: the file will be available after 2022-05-22The Agulhas Current (AC) creates a sharp temperature gradient with the surrounding ocean, leading to a large turbulent flux of moisture from ocean to atmosphere. We use two simulations of the Weather Research and Forecasting (WRF) Model to show the seasonal impact of the warm core of the AC on southern Africa precipitation. In one simulation the sea surface temperature (SST) of the AC is similar to satellite observations, while the second uses satellite SST observations spatially smoothed to reduce the temperature of the core of the AC by ~1.5°C. We show that decreasing the SST of the AC reduces the precipitation of the wettest seasons (austral summer and autumn) inland. Over the ocean, reducing the SST reduces precipitation, low-level wind convergence, SST, and SLP Laplacians above the AC in all seasons, consistent with the pressure adjustment mechanism. Moreover, winter precipitation above the AC may also be related to increased latent flux. In summer and autumn, the AC SST reduction is also associated with decreased precipitation farther inland (more than 1.5 mm day−1), caused by an atmospheric circulation that decreases the horizontal moisture flux from the AC to South Africa. The reduction is also associated with higher geopotential height extending from the surface east and over the AC to the midtroposphere over southeastern Africa. The westward tilted geopotential height is consistent with the linear response to shallow diabatic heating in midlatitudes. An identical mechanism occurs in spring but is weaker. Winter rainfall response is confined above the AC.publishedVersio

Simulation of variability in the tropical Western Indian Ocean

Includes bibliographical references.The oceanic circulation and properties in the Tanzanian shelf region in the tropical western Indian Ocean have been studied in this thesis using a regional ocean model. The study investigated the influences of the Northeast Madagascar Current (NEMC) in the Tanzanian shelf waters at the annual cycle. Furthermore, the thesis examined the interannual variability of the sea surface temperature (SST) in the Tanzanian shelf region, and compares it with that offshore or with subsurface temperature. At the annual cycle, the westward-flowing NEMC advects relatively warm and fresh waters from the north of Madagascar towards the Tanzanian shelf region by interrupting the upwelled water from the Seychelles-Chagos ridge. At interannual timescales, the weakest interannual SST variations, which lie over the weak subsurface waters variations, occur in the coastal waters off Tanzania, where its variance is shared with waters to the north of Madagascar. Such SST variations are dominated by variability at about five year periods. The strongest interannual SST variations, which lie over the strongest subsurface temperature variations, occur offshore, being dominated by two periods, one at about 2.7 and the other near five years. The interannual variability of the region seems to be linked to El Niño- Southern Oscillation (ENSO) and Indian Ocean Dipole (IOD) events, which induce changes in the thermocline and surface forcing in the region. Local surface heat flux exchanges driven by the anomalous shortwave radiation dominate the weakest interannual SST variability in the Tanzanian shelf region, with some contribution by the advection of heat anomalies from the NEMC. Further offshore, the strongest interannual variability of the SST is dominated by the thermocline variations induced by local Ekman pumping from local wind stress curl and by remote forcing from large-scale climate modes.