Browsing by Author "Richardson, Anthony"

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    Effects of global climate change on the recruitment of Anchovy in the Southern Benguela upwelling system
    (2001) Young, Shona Linda; Hewitson, Bruce; Richardson, Anthony
    Changes to global climate patterns, as predicted by many climatologists, will impact on ecosystems in numerous ways. The nutrient-rich waters of marine upwelling environments enable prolific plankton growth, which in turn supports vast shoals of pelagic fish. The nutrient supply is dependent on the strength and direction of winds, which govern the upwelling process as well as turbulence. A change in climate may thus affect the food supply and feeding conditions required by pelagic fish populations. This thesis investigates predicted changes in wind patterns in the southern Benguela system and assesses how these changes may impact on the recruitment of the Cape anchovy, Engraulis capensis. A general circulation model (NCAR Climate System Model) is used to compare a future simulation under double C02 conditions with a simulation of the present day wind regime. Climate change effects on anchovy in the other main upwelling systems, off the coasts of California, Peru and Morocco are also examined. Wind speeds and turbulence off the Agulhas Bank in the southern Benguela system are expected to remain suitable and potentially become even more conducive to spawning in the future. The results show similar mean wind speeds to present day values, a decrease in mean turbulence, a decrease in the frequency of extreme wind speeds and a decrease in the frequency of extreme turbulence during the anchovy spawning season (i.e. September-February). An increase in Lasker events is expected at the Eastern Bank Grid Cell, which suggests that this area may become the preferred spawning habitat in the future. The Cape Town Grid Cell also shows suitable conditions in the future simulation for anchovy spawning and may thus become an alternative spawning location. The West Coast shows an increase in alongshore wind stress and thus an increase in upwelling in the future simulation from November - February. This is likely to increase planktonic food availability and ultimately anchovy recruitment. Future simulations from the Climate System Model show that the upwelling systems off the coasts of California, Peru and Morocco are likely to continue supporting anchovy spawning and may become even more suitable in terms of wind and turbulence regimes.
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    Wind-stress variability over the Benguela upwelling system
    (2002) Risien, Craig Miller; Reason, Chris; Shillington, Frank; Richardson, Anthony
    Regional wind-stress variability over the Benguela Upwelling System is described using 16 months (01 August 1999 29 November 2000) of satellite derived QuikSCA T wind data. The QuikSCA T data are compared to the climatologies presented by Kamstra (1985) and Bakun and Nelson (1991), as well as the long-term climatology (1968-1996) of the surface vector wind speed field off the coast of southern Africa, as derived from the 2.5° resolution NCEPINCAR reanalysis dataset. Broad scale similaritie"s are found between the QuikSCA T and the long-term NCEPINCAR climatology (1968-1996) data sets. This allows one to have confidence in using this scatterometer data to investigate details of spatial and temporal variability over the Benguela System. During summer, wind-stress maxima are found at approximately 17, 29 and 34°S. These maxima strengthen in late summer. The seasonal northward migration of the South Atlantic Anticyclone becomes apparent in late autumn, when the strongest wind-stress occurs north of 28°S. A significant wind-stress minimum is observed to develop slightly north of Cape Columbine (33°S) during autumn. To the north (10-23.5°S) the Benguela is characterised by relatively strong south-easterly wind-stress during winter. To the south (24-35°S) the Benguela is characterised by relatively weak westerly to south-westerly wind-stress during winter. A southward migration of southeasterly wind-stress is observed during early spring. By November the entire Benguela Upwelling System is once again characterised by southerly to south-easterly wind stress. Wind-stress variability is investigated using both a type of artificial neural network, known as the Kohonen Self Organising Map (SOM), as well as a wavelet analysis. Two independent SOM studies are conducted. The first study produced a 6x4 SOM output array, which is used to examine seasonal variability as well as the temporal evolution of two synoptic-scale wind events. For the second study both a SOM and a wavelet analysis are applied to an extracted data set to find that the system can be divided into six discrete wind regimes, 10-15°S; 15.5-18.5°S; 19-23.5°S; 24-28.5°S; 29-32.5°S; and 33-35°S. The wavelet power spectra for these wind cells span a range of frequencies from 4 to 64 days, with each region appearing to contain distinct periodicities. To the north, 10-23.5°S, the majority of the power occurs during winter, with a 6-16 day periodicity. Further south, 24-35°S, the majority of the power occurs in the summer. Here a bi-modal distribution occurs, with peaks of 6-16 and 35-40 days. Lastly a case study sequence of the spatial distribution of wind-stress, windstress curl and SST, at a location off the west coast of southern Africa (25-300S and 12-17°E), is discussed in relation to an intense, upwelling favourable, wind event that occurred from 11-20 February 2000.