A meteorological analysis of extreme flood events in the southern parts of the Eastern Cape, South Africa

Abstract

Extreme flood events have been found to occur in the Eastern Cape Province of South Africa. The southern and south-eastern coastal regions are particularly susceptible to floods with daily rainfall records of up to 490 mm having been recorded here. In order to gain a better understanding of these floods, historical floods in the southern parts of the Eastern Cape have been analysed in this study. This study aimed to investigate the most extreme flood events in the history of the area and to analyse the surface observational data during the heavy rainfall events found, to contribute to the understanding of these heavy rainfall events. Seven case studies were investigated using NCEP/NCAR Reanalysis 1 data and surface observational data, to analysis the synoptic circulation and surface data tendencies during heavy rainfall events. All of the case studies that had synoptic data available showed to have a COL system present with the upper air low lying westward of the flood area. The surface synoptic situation presented an intense high pressure system lying south west or south of the country in the Atlantic Ocean between 35-45°S. Another key factor was a surface trough over the interior of the country, due to the COL, extending southwards to protrude off of the south or south eastern coastline east of the flood areas. The combination of these three systems colloquially referred to as a "Black South-Easter" , produced dangerous rainfall intensities when the winds were perpendicular to the coast. The results of the surface observations revealed temperatures increasing at night (against diurnal cycles) with increasing relative humidity preceding the heavy rainfall events. The pressure showed both increasing and decreasing trends during heavy rainfall events however in five of six cases a tight pressure gradient was present producing an influx of moist air into the flood area. Onshore winds were found to be of high importance in heavy rainfall events to bring moist air into the area. Wind speeds greater than 5m/s occurred during times of high rainfall intensities.