Dynamics of climate variability over the all-year rainfall region of South Africa

Abstract

Climatologically, the Cape south coast is unique in the sense that it lacks the pronounced seasonality observed over the remainder of South Africa. Here, rainfall occurs all-year round, implying that rainfall-producing systems characteristic of both the winter and summer rainfall regions contribute to rainfall over the Cape south coast. However, the relative contributions of different rain-producing weather systems to annual rainfall have not been quantified to date. This region has also not received as much attention as the winter and summer rainfall regions of the country with regards to quantifying and understanding its interannual rainfall variability. Furthermore, seasonal forecast skill over the Cape south coast region is generally poorer and less well investigated than forecast skill over the summer rainfall region and to a lesser extent the winter rainfall region. This study addresses these issues through objective identification of the prevailing synoptic types of the Cape south coast region by application of the self-organizing map (SOM) technique, with a subsequent analysis of their interannual variability, intraseasonal variability, and predictability. The relative contribution of different rain-producing systems to annual rainfall over the Cape south coast is quantified. Ridging high pressure systems contribute most to the mean annual rainfall (46%), followed by tropical-temperate troughs (28%) and cut-off lows (COLs). COLs, co-occurring with ridging high pressure systems and tropical-temperate troughs contribute to 16% of the mean annual rainfall. When extreme rainfall is considered, COLs contribute to 29% of all extreme rainfall events along the Cape south coast. Particular configurations of ridging high pressure systems and tropical-temperate troughs that are linked to interannual variability of seasonal rainfall are identified. These systems are primarily ridging high pressure systems, in particular those ridging from far south of the subcontinent, and tropical-temperate troughs occurring during seasons with weaker zonal mid- and upper air winds. COLs are also linked to interannual variability of seasonal rainfall, despite their infrequent occurrence - highlighting the importance of COLs as high impact weather systems. The COL link with rainfall variability is particularly strong during March-April-May (MAM) and even more so for June-July-August (JJA). It is also shown that the interannual variability in the frequency distribution of the occurrence of synoptic types within a season is linked to the El Niño Southern Oscillation (ENSO) and the Southern Annular Mode (SAM), suggesting potential predictability of intraseasonal variability at the seasonal time scale. The predictability of intraseasonal characteristics over the Cape south coast at the seasonal time scale is subsequently assessed by utilizing an ensemble of simulations performed by an atmosphere-ocean coupled global circulation model administered by the UK Met Office. Hindcasts of 14 austral spring and summer seasons initialized at a 1-month lead time are used to assess the model s ability to predict the intraseasonal characteristics of weather systems. This assessment revealed that some skill exists in the predictability of the intraseasonal characteristics of synoptic types over the Cape south coast region of South Africa at the seasonal time scale. The result implies that there is potential to predict whether specific high impact weather systems (e.g. COLs) or systems associated with good rainfall (e.g. ridging highs ridging from anomalously far south of the subcontinent) will occur at anomalously high or low frequencies during the next season, with associated benefits to the agricultural and water sectors.