Browsing by Author "Jackson-Veitch, Jennifer"

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    Dynamics and variability of the Subantarctic mixed-layer as determined from a high resolution glider dataset
    (2015) Du Plessis, Marcel David; Ansorge, Isabelle Jane; Jackson-Veitch, Jennifer; Swart, Sebastiaan; Mahadevan, Amala
    Traditional understanding of mixed-layer (ML) dynamics in the African sector of the Southern Ocean suggests that seasonal summer stratification and subsequent reduction in ML depth (MLD) is determined by the onset of a positive net heat ux. The impact of physical forcing mechanisms on the intra-seasonal variability of the ML is still relatively unknown. Recent research in the North Atlantic has highlighted the role that sub-mesoscale ML eddy dynamics has on ML stratification. It is now understood that large horizontal density gradients drive sub-mesoscale eddy formation which have been shown to result in the early onset of spring phytoplankton blooms at high latitudes. To date these ML eddies have been researched primarily in models with few observational studies available. To test the ML eddy hypothesis in the Subantarctic Zone (SAZ) we use high-resolution (~3km, 4-hourly) glider measurements between austral spring to late summer.
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    Simulating the characteristics of tropical cyclones over the South West Indian Ocean using an adaptive Stretched-Grid Global Climate Model
    (2015) Maoyi, Molulaqhooa Linda; Abiodun, Babatunde Joseph; Jackson-Veitch, Jennifer
    Tropical Cyclones are one of the most devastating natural phenomena. Previous attempts to simulate Tropical Cyclones (TCs) over the South West Indian Ocean (SWIO) have used Global Circulation Models (GCMs) with uniform grid. This study examines the capability of a GCM with adaptive grid stretching (CAM-EULAG, hereafter CEU) in simulating the characteristics of TCs over the SWIO. In the study, the CEU model is applied with a fine horizontal grid resolution (0.5°x0.5°) over the SWIO and coarser resolution (1°x1° - 2°x2.25°) over the rest of the globe. The model simulation is performed for 11 years (1999-2010) and validated against the Joint Typhoon Warning Centre (JTWC) best track observation, Global Precipitation Climatology Project (GPCP) satellite data, and ERA-Interim (ERAINT) reanalysis data. The study also considers the impact of El Niño Southern Oscillation (ENSO), Indian Ocean Dipole (IOD) and the South Atlantic Subtropical Dipole (SASD) on TC counts over the SWIO.
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    The variability of retention in St Helena Bay
    (University of Cape Town, 2020) Manyakanyaka, Anathi; Jackson-Veitch, Jennifer; Rouault, Mathieu
    The circulation in St Helena Bay and the variability of the retention of the Bay are investigated using seasonal climatologies of the Regional Ocean Modelling System (ROMS). While retention has been studied biologically, the seasonality of the hydrodynamics contributing to the retention have received less attention. In this study we explore how the sea temperature, atmospheric forcing and currents contribute to the seasonal recirculation dynamics in St Helena Bay. Ichthyop, a lagrangian particle tracking method is used to study the spatial variations of local retention rates, with the particles released from the Bay. The circulation on the shelf of the west coast is dominated by upwelling dynamics with the equatorward boundary current, the Benguela Current located just off the shelf. St Helena Bay is protected from the direct impact of the Benguela current by coastal geographical features. A cyclonic circulation pattern is observed in the bay especially in autumn and winter. However, the results suggest that the recirculation patterns are prominent in summer and spring due to the intensification of the Benguela Jet and the nearshore southward current flows along the coast. Similar cyclonic features are observed at 100 m depth in the water column. An analysis of the particle tracking reveals that more drifters are retained in winter than in summer, supported by what is observed in the circulation patterns. Moreover, more drifters are retained in the surface waters than the deep waters.