The Interannual Variability of the Onset of the Maize Growing Season over South Africa and Zimbabwe

Abstract

Subsistence farmers within southern Africa have identified the onset of the maize growing season as an important seasonal characteristic, advance knowledge of which would aid preparations for the planting of rain-fed maize. Onset over South Africa and Zimbabwe is calculated using rainfall data from the Climate Prediction Center (CPC) Merged Analysis of Precipitation (CMAP) and the Computing Center for Water Research (CCWR). The two datasets present similar estimates of the mean, standard deviation, and trend of onset for the common period (1979–97) over South Africa. During this period, onset has been tending to occur later in the season, in particular over the coastal regions and the Limpopo valley. However, the CCWR data (1950–97) indicate that this is part of long-term (decadal) variability. Characteristic rainfall patterns associated with late and early onset are estimated using a self-organizing map (SOM). Late onset is associated with heavier rainfall over the subcontinent. When onset is early over Zimbabwe, there is an increased frequency of more intense rainfall over northeast Madagascar during the preceding August. Accompanying these intense events is an increased frequency of positive 500-hPa geopotential height anomalies to the southeast of the continent. Similar positive height anomalies are also frequently present during early onset. The study indicates that onset variability is partly forced by synoptic conditions, and the successful use of general circulation models to estimate onset will depend on their simulation of the zonally asymmetric component of the westerly circulation.

Interannual variability in rainy season characteristics over the Limpopo region of southern Africa

“…The 500 hPa height ( Figure 6) and MSLP (not shown) anomalies imply that early onset may be associated with a strengthening and southward shift of the South Atlantic and South Indian anticyclones and increased ridging southeast of South Africa during the September-November prior to the main rainy season. Using a self-organising map approach, Tadross et al (2005) also found ridging southeast of South Africa for early onset seasons calculated for Zimbabwe. Relative convergence (divergence) (Figure 7) is seen over the central interior, the eastern seaboard of South Africa and Limpopo (large areas of the South West Indian Ocean) consistent with increased ridging southeast of South Africa.…”

Effects of temperature changes on maize production in Mozambique

“…Diurnal temperature ranges vary regionally between 10 and 14°C (Griffiths 1972). Rainfall during the major cropping season starts in southern Mozambique with the advection of moisture onshore by mid-latitude ridging systems (Tadross et al 2005). This can occur as early as September, but the systems can be sporadic, often leading to long early season dry spells.…”

Developing seasonal rainfall scenarios for food security early warning

“…The RFE2 drives a number of monitoring products used in the food security analysis such as the standardized precipitation index (SPI) and water requirement satisfaction index (WRSI). The ability to use RFE2 for quantitatively monitoring food security in Africa is affirmed in literature (Tadross et al 2005;Verdin et al 2005;Sawunyama and Hughes 2008;Tadesse et al 2008).…”

“…This research uses the technique developed in West Africa (AGRHYMET 1996) and reinforced in WRSI literature (Verdin and Klaver 2002;Senay and Verdin 2003), defining SOS as the first dekad receiving 25 mm of rainfall, followed by two dekads summing to 20 mm. While there may be local deviations from this as a viable SOS, it is a widely supported approximation (Tadross et al 2005;Tadesse et al 2008;Funk and Budde 2009;Crespo et al 2011;Harrison et al 2011).…”