Browsing by Author "Hewitson, Bruce C"

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    Late quaternary environmental reconstruction and climate modelling in the winter rainfall region of the Western Cape, South Africa
    (2001) Mulock-Houwer, Anne; Hewitson, Bruce C; Meadows, Michael E
    The southwestern Cape of South Africa is a floristically diverse region experiencing winter rainfall, and is important as it represents a significant southern hemisphere example of a Mediterranean climate regime. Acquiring palaeoenvironmental data from this region is imperative in understanding the climatic changes that have occurred during the Late Quaternary, with a view to improved palaeclimatic modelling. The spatial distribution of studies for the Late Quternary in the southwestern Cape is uneven and palaeoenvironmental reconstructions have been largely restricted to sites, which are easilty accessible, and appropriate only for the methods being used (e.g. palynology for wetlands). Moreover, many of the palaeoenvironmental reconstructions implemented thus far fall outside the winter rainfall region sensu stricto. Hence spatial differentiation in climatic response may have remained obscured due to the 'selection' of sites used.
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    North Atlantic climate variability from a self-organizing map perspective
    (2007) Reusch, David B; Alley, Richard B; Hewitson, Bruce C
    [1] North Atlantic variability in general, and the North Atlantic Oscillation (NAO) in particular, is a long-studied, very important but still not well-understood problem in climatology. The recent trend to a higher wintertime NAO index was accompanied by an additional increase in the Azores High not coupled to changes in the Icelandic Low, as shown by a self-organizing maps (SOMs) analysis of monthly mean DJF mean sea level pressure data from 1957 to 2002. SOMs are a nonlinear tool to optimally extract a user-specified number of patterns or icons from an input data set and to uniquely relate any input data field to an icon, allowing analyses of occurrence frequencies and transitions complementary to principal component analysis (PCA). SOMs analysis of ERA-40 data finds a North Atlantic monopole roughly colocated with the mean position of the Azores High, as well as the well-known NAO dipole involving the Icelandic Low and the subtropical high. Little trend is shown in December, but the Azores High increased along with the NAO in January and February over the study interval, with implications for storminess in northwestern Europe. In short, our SOM-based analyses of winter MSLP have both confirmed prior knowledge and expanded it through the relative ease of use and power with nonlinear systems of the SOM-based approach to climatological analysis.