Browsing by Author "Lutjeharms, Johann R E"
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- 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 AccessBuilding the next generation of South African scientists(2006) Lutjeharms, Johann R EA recent issue carried a most interesting suite of articles, demonstrating the success of the Royal Society/National Research Foundation programme in helping to build a new generation of South African scientists, ‘as ageing researchers retire and retention and new blood become increasingly important’. The NRF’s overview of the programme (‘Reaching out to the world’1 ) correctly points out that ‘South Africa needs excellent science and far more scientists, drawing on the research potential of the entire population.’
- ItemOpen AccessCyclonic Eddies in the Cape Basin(2008) Hall, C; Lutjeharms, Johann R EA great deal of attention has been paid to the inter-ocean exchange of thermohalineproperties in the Agulhas Retroflection region. Recent observations have shown thatthe highly energetic field of the southern half of the Cape Basin consists of bothcyclonic and anticyclonic eddies. These eddies interact with each other, resulting inthe vigorous stirring of these water mass. Investigations have shown that the cycloniceddies tend to be smaller and outnumber the anticyclonic rings. Nonetheless, verylittle is known of their characteristics and the nature in which they are formed. Usingremote sensing data, confirmed with hydrographic data, this study determines thelocation, frequency and seasonality of cyclonic eddy formation; their size, trajectoriesand lifespan; physical components and associations with Agulhas Rings.Cyclonic eddies were seen to split, merge and link with other cyclonic eddies, withsplitting events creating child cyclonic eddies. The 105 parent and 157 child cycloniceddies identified during this study show that an average of II parent and 17 childcyclonic eddies were formed annually. 31.58 % follow an overall west-southwestdirection, with 27.37 % translocating west north-westward. Poleward translocationspeeds average at 0.3 kIn/day, whereas translocation speeds obtained from alldirectional components averages at 2.153 kIn/day for parent and 2.975 km/day forchild cyclonic eddies. Parent cyclonic eddies lived for approximately 254 days,whereas child cyclonic eddies survived for a mean of 188 days. Of note was asignificant variation of lifespan between parent and child cyclonic eddies formed inboth the north and south of the study area. 77 % of northern and 93 % of southerncyclonic eddies were formed directly adjacent to positive sea level anomalies orAgulhas Rings, resulting in an total overall association of 82.93 % parent and 89.63 %child cyclonic eddies. Cyclonic eddy groups were seen to merge at a rate of 16.38parent and 14 child cyclonic eddies per year, whereas topography appeared to affectthe demise of 17.00 % of the investigated cyclonic eddies.Therefore this study may form a basis for further investigations into the influenceCape Basin cyclonic eddies have on the meridional transfer of heat, salt, nutrients,oxygen and carbon concentrations in the South-East Atlantic Ocean. A more in-depthstudy using model outputs and targeted in situ hydrographical data would againenhance cyclonic eddy knowledge.
- ItemOpen AccessDecay of eddies at the South-West Indian Ridge(2011) Durgadoo, Jonathan V; Ansorge, Isabelle J; de Cuevas, Beverly A; Lutjeharms, Johann R E; Coward, Andrew CThe South-West Indian Ridge in the Indian sector of the Southern Ocean is a region recognised for the creation of particularly intense eddy disturbances in the mean flow of the Antarctic Circumpolar Current. Eddies formed at this ridge have been extensively studied over the past decade using hydrographic, satellite, drifter and float data and it is hypothesised that they could provide a vehicle for localised meridional heat and salt exchange. The effectiveness of this process is dependent on the rate of decay of the eddies. However, in order to investigate eddy decay, logistically difficult hydrographic monitoring is required. This study presents the decay of cold eddies at the South-West Indian Ridge, using outputs from a highresolution ocean model. The model's representation of the dynamic nature of this region is fully characteristic of observations. On average, 3-4 intense and well-defined cold eddies are generated per year; these eddies have mean longevities of 5.0±2.2 months with average advection speeds of 5±2 km/day. Most simulated eddies reach their peak intensity within 1.5-2.5 months after genesis and have depths of 2000 m - 3000 m. Thereafter they dissipate within approximately 3 months. The decay of eddies is generally characterised by a decrease in their sea surface height signature, a weakening in their rotation rates and a modification in their temperature-salinity characteristics. Subantarctic top predators are suspected to forage preferentially along the edges of eddies. The process of eddy dissipation may thus influence their feeding behaviour.
- ItemOpen AccessA descriptive physical analysis of water movement in the South West African Indian Ocean during the Northeast monsoon season(1971) Lutjeharms, Johann R E; Harris, T F WThe Agulhas Current is unique as a western boundary current in having as its source of surface water currents that are linked to the variable current regions of the North Indian Ocean. In which way the Agulhas Current derives its supply from these currents and how these currents interrelate, is very poorly understood. In an attempt to make a contribution to the understanding of this flow an isentropic analysis in depth on seven σt-surfaces for the whole South West Indian Ocean during the Northeast Monsoon season was carried out. This analysis was augmented by the calculation of the velocities according to the Witte-Margules equation on each σt-surface at ten selected vertical sections. In addition the detailed volume transport and the velocity structure for each vertical section were calculated. The results pointed to a significant variation in transport and velocity structure from year to year. The large scale circulation in depth of the ocean is described. It is found that the Agulhas Current derives its supply of water from different sources at different depths and that at depth a large measure of recirculation of Agulhas Current Water takes place. It is not thought that this has been reported before and these findings might be of importance to the understanding of the current system as a whole. A few innovations in the standard oceanographic methods are described and two computer programs to aid analyses are presented.
- ItemOpen AccessThe general hydrography of the Mozambique Channel(2003) Jamaloodien, Shaheen; Lutjeharms, Johann R EThe greater Agulhas Current system is believed to form a key link in the global ocean circulation since it is the inter-ocean conduit for warm Indian Ocean water to the Atlantic Ocean. This system has been thought to derive its water from the South Equatorial Current via two sources: the Mozambique Current, and secondly, the East Madagascar Current. In spite of their global significance surprisingly few observations have been made in these source regions. In March 2000 a multidisciplinary cruise, the first one in 25 years, the Agulhas Current Sources Experiment (ACSEX-1) was carried out in the Mozambique Channel. The main aim of the ACSEX-1 cruise was to establish the existence, trajectory and hydrographic structure of the Mozambique Current. The use of satellite altimetry and numerical modeling revealed high mososcale activity in the Mozambique Channel. Thus guided by real-time altimetric data, the cruise sections intersected the main regions of high mesoscale activity in the centre of the Channel. From this dataset we are now able to determine whether the Mozambique Current is a continuous current or whether it exists merely of a train of eddies, as the altimetric data suggest.
- ItemOpen AccessThe influence of ocean ridges on the circulation to the south of the Mozambique channel and Madagascar(2008) Halo, Issufo Ferrão Mário; Ansorge, Isabelle Jane; Penven, Pierrick; Lutjeharms, Johann R EThe Mozambique Channel and the region south of Madagascar are dominated by high variability of the oceanic flow due to the ubiquitous presence of mesoscale eddies. The bottom topography of this region has several shallow ridges. The water flowing through the Mozambique Channel propagates southwards predominantly as a train of anti-cyclonic eddies, moving towards the Agulhas Current. South of Madagascar, dipolar vortices regularly propagate in a south-westward direction. Their deep extent favours interaction with the shallow bathymetry of the Davie, Mozambique and Madagascar Ridges. The role of the Madagascar Ridge on the mesoscale circulation is investigated using altimetric observations and model simulations.
- ItemOpen AccessOcean model diagnosis of variability in the South Indian Ocean(2005) Hermes, Juliet C; Reason, Chris; Lutjeharms, Johann R EEvidence 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).
- ItemOpen AccessOn the discontinuous nature of the Mozambique Current(2012) Lutjeharms, Johann R E; Biastoch, Arne; van der Werf, Petra M; Ridderinkhof, Herman; de Ruijter, Wilhelmus P MThe concept of a spatially continuous western boundary current in the Mozambique Channel has historically been based on erroneous interpretations of ships' drift. Recent observations have demonstrated that the circulation in the Channel is instead dominated by anti-cyclonic eddies drifting poleward. It has therefore been suggested that no coherent Mozambique Current exists at any time. However, satellite and other observations indicate that a continuous current - not necessarily an inherent part of Mozambique Eddies - may at times be found along the full Mozambican shelf break. Using a high-resolution, numerical model we have demonstrated how such a feature may come about. In the model, a continuous current is a highly irregularly occurring event, occurring about once per year, with an average duration of only 9 days and with a vertical extent of about 800 m. Surface speeds may vary from 0.5 m/s to 1.5 m/s and the volume flux involved is about 10 Sv. The continuous current may occasionally be important for the transport of biota along the continental shelf and slope.
- ItemOpen AccessThe structure and transport of the Agulhas Return Current(1996) Ansorge, Isabelle Jane; Lutjeharms, Johann R EThe Agulhas Current flows along the eastern coast of southern Africa as one of the largest western boundary currents in the world's ocean. On overshooting the southern tip of Africa at approximately 20°E, a retroflection loop is formed that causes the current to double back on itself and to form the easterly flowing Agulhas Return Current. No focussed investigation to establish the hydrography of this important component of global ocean circulation has to date been carried out.
- ItemOpen AccessThe structure of the upper ocean, atmosphere and heat fluxes - Tropical Indian Ocean(1998) Majodina, Mark; Jury, Mark R; Lutjeharms, Johann R E; Rouault, MathieuSea surface temperatures in the tropical Indian Ocean have been shown to be inversely related to South African summer rainfall, making advanced predictions of this rainfall practicable. Such predictability has enormous potential economic benefits. However, these predictions have been purely statistical; very little is currently known about the marine-atmospheric processes in the Indian Ocean tropics. To address this lack of information, the structure of the upper ocean and lower atmosphere as well as the surface heat fluxes in the tropical Indian Ocean have been investigated. This was done by a special measurement programme on a research cruise in the region. Global gridded meteorological data have been used to complement the shipboard observations. Heat fluxes have been computed from the cruise observations and related to the main atmospheric patterns at the time. These patterns were identified from principal components analysis. Air-sea interaction could thus be estimated over the full tropical Indian Ocean. It is found that the thermocline depth is linked to the cyclonic ocean current shear and to the overlying distribution of wind stress curl. The meridional advection of air into the central Indian Ocean region is shown to modulate the characteristics of the atmospheric boundary layer in the tropical Indian Ocean. The maximum turbulent heat and moisture transports to the atmosphere are found near cyclonic atmospheric disturbances. These are the first reliable observations of the heat and moisture fluxes in that part of the tropical Indian Ocean implicated in South African rainfall. It is clear from this investigation that the synoptic atmospheric systems and the meridional flow of air are critical to enhanced atmospheric convection in the region.
- ItemOpen AccessTropical cyclones in the South-West Indian Ocean : intensity changes, oceanic interaction and impacts(2008) Mavume, Alberto Francisco; Rouault, Mathieu; Lutjeharms, Johann R E; Brundrit, GeoffThis study investigates the climatology, intensification and ocean atmosphere interaction in relation to the passage of tropical cyclones (TCs) in the South-West Indian Ocean (SWIO). A Climatology of TCs in the SWIO including landfall in the area of Mozambique and Madagascar was developed for the 1952-2007 and 1980-2007 periods.