Hydroclimatic Variation Over Southern Africa at Intra-Annual and Inter-Annual Time Scales with Special Reference to the Role of the Oceans
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HYDROCLIMATIC VARIATION OVER SOUTHERN AFRICA AT INTRA-ANNUAL AND INTER-ANNUAL TIME SCALES WITH SPECIAL REFERENCE TO THE ROLE OF THE OCEANS Report to the Water Research Commission by M Rouault, CJR Reason, N Vigaud, A Mavume & N Fauchereau Department of Oceanography, University of Cape Town WRC Report No 1476/1/08 ISBN 978-1-77005-654-1 APRIL 2008 DISCLAIMER This report has been reviewed by the Water Research Commission (WRC) and approved for publication. Approval does not signify that the contents necessarily reflect the views and policies of the WRC, nor does mention of trade names or commercial products constitute endorsement or recommendation for use. ii Executive summary Over the last century, South Africa has suffered from dramatic interannual changes in rainfall, characterized by severe droughts and wet spells. Such variability of rainfall affects water resources, the agricultural industry and thus the Gross National Product. It has a particularly detrimental effect on rural subsistence farmers, the health of people in rural areas and the sustainable management of the natural environment. There is a need to enable resource managers (water and agriculture) to optimally exploit best current climatological knowledge in dealing with hydroclimatic variability, thereby enhancing their decision-making ability for the short, medium and long term Over the last 15 years much knowledge has been gained on how the oceans can modify the global climate and affect the rainfall of southern Africa, from the decadal to interannual scales. Previous research has shown that many droughts occur in southern Africa during the mature phase of El Niño, when the central and eastern Pacific and the Indian Ocean are warmer. Conversely, during La Niña conditions, wet spells are more likely to happen. The equatorial and tropical oceans have a profound effect on the world’s weather patterns due to high sea surface temperatures, elevated heat contents of the first hundred metres below the surface and associated high rainfall rates. Southern African interannual climate variability is also affected, but in a manner that offers some predictability depending on how well ocean-atmosphere processes are understood. Although the El Niño Southern Oscillation (ENSO) phenomenon is the dominant regulator of Southern Africa summer rainfall, the Indian and Atlantic Oceans also have a role to play. In fact it seems that the warming of the Indian Ocean since the 1950’s has made ocean variability more important for southern Africa and this has increased the spatial extent and intensity of drought in Southern Africa since the mid- seventies. On top of it are long-term trends; for instance, the ocean’s temperature has risen globally during the last 50 years due to the anthropogenic increase in CO2. Given the high correlation between ocean surface temperature and southern African rainfall, this increase must be taken seriously. This project set out to enhance the understanding of hydroclimatic variability in southern Africa with special reference to the role of the oceans and thereby address some of the needs of resource managers in this connection. Specific objectives were to: Assess the suitability of indices used to represent hydroclimatic variation over southern Africa from a joint ocean/atmosphere system and water-resource management perspective. Select, assess and apply the most promising of advanced remote sensing and modelling products which would assist in achieving the objectives of this project. Test hypotheses concerning the importance of the heat content of upper ocean layers and evaporation from the oceans in influencing the weather and climate of countries in southern Africa. Familiarise water resource managers with advances in, capabilities of and potential benefits of using improved prediction tools. iii As a first step in pursuing these objectives, the benefits of the standardised precipitation index (SPI) for characterising and monitoring droughts were examined. The SPI allows the intensity and spatial extent of droughts to be monitored at different time scales (3, 6, 12 and 24 months). It is linked to the probability of occurrence of dry or wet events and can be used for operational drought monitoring at any location, where the rainfall time series extends over at least 30 years. In this study, the SPI was employed to do a retrospective analysis of the spatial extent and intensity of droughts in South Africa from 1921 to 2000, using data from the 93 Rainfall Districts of the South African Weather Service. According to this index, the 8 most severe droughts at the 6-month time scale for the summer rainfall region of South Africa happened in 1926, 1933, 1945, 1949, 1952, 1970, 1983 and 1992. There is also considerable decadal variability and it was only for the average number of dry districts per year that a cyclic variation (18 to 20 year period) could be found. The average number of wet districts seems to have increased in the 1960’s. Drought lasting for 3 years has not been uncommon for each of the 8 South African rainfall regions defined by the South African Weather Service. The SPI was also used in a retrospective analysis of the spatial extent of droughts in Southern Africa (south of 10°S) from 1901 to 1999. In terms of spatial extent, the 8 most severe droughts at the 6-month scale (October-April) for the summer rainfall region of Southern Africa ended in 1916, 1924, 1933, 1949, 1970, 1983, 1992 and 1995. At the 2-year scale, they ended in 1906, 1933, 1983, 1984, 1992, 1993, 1995 and 1996. Areas affected by those droughts ranged from 3.4 to 2 million km2. Eight of those 12 years are El Niño years. Preliminary data indicate that 2001/2002, 2002/2003 (El Niño year) and 2003/2004 experienced severe droughts at a number of scales and that 2005/2006, a La Niña year, was the wettest year since the mid 70’s. This confirms the increase in the spatial extent of drought in Southern Africa since the 1970’s due to the stronger relationship between ENSO and southern African rainfall. It is clear that combining retrospective analysis with real time monitoring could be extremely beneficial in the development of drought response strategies and awareness plans. Indeed, following these results, the South African Weather Service is now using the SPI for monitoring drought in South Africa. With regard to the role of the ocean in regulating southern African climate, better understanding of the mechanisms leading to warm and cold sea surface temperatures and variations of the heat content of the upper ocean is of prime importance. These mechanisms have been investigated in ocean areas that correlate to Southern African rainfall. A study of the behaviour of tropical cyclones over the Mozambique Channel using remote sensing was carried out, with specific focus on the interaction with the water masses in the channel of two cyclones that moved into the region from the east (Hudah, Eline) and one that was generated over the channel itself (Japhet). Movement over warm ocean eddies in the Mozambique Channel was concomitant with increases in wind speeds of all cyclones from 30 m s-1 to 50 m s-1. There was an association between the increase in tropical cyclone intensity and the anomalously high heat content of warm eddies traversed under the track of each cyclone. Cooling of surface waters in the wake of cyclones was manifested by temperature decreases of between 2 and 7ºC indicating mixing to depths of 80 m or more. At that depth, the temperature difference in an eddy and outside is about 5ºC, potentially making these eddies an extra source of energy for cyclones. Evaporation at the surface of the ocean leads to cooling at the surface and turbulent flux of moisture from the ocean to the atmosphere (also called turbulent latent heat flux). Estimates of turbulent latent heat fluxes were obtained in various studies. In one such study, in the Agulhas Current Retroflection region south of Africa, it was iv found that by not accounting for sharp gradients of SST and wind speed, the commonly used NCEP reanalysis data underestimates the monthly mean of latent and sensible fluxes over Agulhas eddies by a factor of three. Due to the high heat content of eddies, the impact of eddy-associated SST gradients on the overlying atmosphere is durable and can last for a few months. The implication of this finding is that models that rely on NCEP data to simulate atmospheric behaviour may overlook a potentially important source of climate variability such as the Agulhas current region. Another mechanism leading to cooling or warming of the ocean is advection by currents. Use of satellite remote sensing has enabled this project to discover a never- before documented current, now named the South Indian Ocean Countercurrent (SICC). The structure of the subtropical SICC was revealed by altimeter-derived, absolute geostrophic surface velocities. It is a narrow, eastward-flowing current between 22° and 26°S. Multi-year averaging identifies it as a well-defined current between Madagascar and 80°E, continuing with lower intensity between 90° and 100°E. Volume transports down to 800 dbar are of the order of 107 m3s-1. Evidence has been found of a narrow branch of the South Equatorial Current (SEC) approaching Madagascar near 18°S and feeding the East Madagascar Current (EMC), which appears to continue westward around the southern tip of Madagascar. It then partially retroflects and nourishes the SICC. The deepening or shoaling of the thermocline is also a key process controlling the sea surface temperature of the ocean, and propagation of Kelvin and Rossby waves is known to control the development of warm and cold anomalies in the upper ocean, especially at the equator. These effects have been investigated in a study of the variability of the tropical southeast Atlantic, a sector to date neglected by the international research community.