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From synoptic to interdecadal variability in southern african rainfall: toward a unified view across time scales

Abstract : During the austral summer season (November-February), southern African rainfall, south of 208S, has been shown to vary over a range of time scales, from synoptic variability (3-7 days, mostly tropical temperate troughs) to interannual variability (2-8 years, reflecting the regional effects of El Niño-Southern Oscillation). There is also evidence for variability at quasi-decadal (8-13 years) and interdecadal (15-28 years) time scales, linked to the interdecadal Pacific oscillation and the Pacific decadal oscillation, respectively. This study aims to provide an overview of these ranges of variability and their influence on regional climate and large-scale atmospheric convection and quantify uncertainties associated with each time scale. We do this by applying k-means clustering onto long-term (1901-2011) daily outgoing longwave radiation anomalies derived from the 56 individual members of the Twentieth Century Reanalysis. Eight large-scale convective regimes are identified. Results show that 1) the seasonal occurrence of the regimes significantly varies at the lowfrequency time scales mentioned above; 2) these modulations account for a significant fraction of seasonal rainfall variability over the region; 3) significant associations are found between some of the regimes and the aforementioned modes of climate variability; and 4) associated uncertainties in the regime occurrence and convection anomalies strongly decrease with time, especially the phasing of transient variability. The shortlived synoptic anomalies and the low-frequency anomalies are shown to be approximately additive, but even if they combine their respective influence at both scales, the magnitude of short-lived perturbations remains much larger.
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Submitted on : Friday, January 14, 2022 - 11:50:49 AM
Last modification on : Thursday, April 7, 2022 - 1:58:15 PM
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Benjamin Pohl, Bastien Dieppois, Julien Crétat, Damian Lawler, Mathieu Rouault. From synoptic to interdecadal variability in southern african rainfall: toward a unified view across time scales. Journal of Climate, American Meteorological Society, 2018, 31 (15), pp.5845 - 5872. ⟨10.1175/jcli-d-17-0405.1⟩. ⟨hal-03526116⟩

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