Interannual variability in dry and wet spell characteristics over Zambia

Abstract

We studied the variability of dry and wet spell frequency over Zambia during the core of the rainy season (December to February, DJF), and associated circulation anomalies. Zambia's large rural population depends critically on rain-fed agriculture; in general, the southern part of Zambia is prone to a high number of dry spells during DJF, and to wet spells of short duration, while the northern regions of the country more typically have a large number of wet spells during DJF, which can lead to flooding. Good rainfall over southern Zambia is typically associated with low level convergence of moisture emanating from the tropical southeast Atlantic Ocean and the tropical western Indian Ocean. During seasons with increased numbers of dry spells over Zambia, low level easterly anomalies typically exist over Zambia, thereby reducing moisture from the tropical southeast Atlantic. Seasons with reduced dry spell frequency and more wet spells tend to be characterised by low level westerly anomalies over southern Angola and western Zambia. Furthermore, DJF seasons occurring during the mature phase of El Niño (La Niña) events tend to show above (below) average dry spell frequency. All El Niño seasons during the period of study showed reduced frequency of wet spells over Zambia, but not all La Niña events were associated with above average wet spell frequency. In fact, wet spell frequency over Zambia during several neutral seasons was greater than that observed during some La Niña events. A relatively strong relationship between anomalies in dry spell frequency and Niño 3.4 sea surface temperature (SST) was found. Relationships with Indian Ocean SST anomalies were also explored, but these were weaker than those found for Niño 3.4 SST. KEY WORDS: Dry spell frequency · Zambia · ENSO · Interannual variability Resale or republication not permitted without written consent of the publisherClim Res 32: [49][50][51][52][53][54][55][56][57][58][59][60][61][62] 2006 mer rainy seasons over each southern African country (such as Zambia) are issued based on the outputs from SST(sea surface temperature)-regional rainfall regression models and General Circulation Model (GCM) products obtained from the South African Weather Service, the UK Meteorological Office, the European Centre for Medium-Range Weather Forecasts and the International Research Institute. However, the need by user groups for information about onset dates and dry/wet spells needs to be specifically addressed, particularly for regions depending on predominantly rain-fed subsistence agriculture such as Zambia. Thus, Zambia and the southern African region urgently need more tailor-made applications-oriented seasonal forecasts (e.g. Klopper 1999, Johnston et al. 2004.However, improved forecasting relies on better understanding of the climate variability of the region and on assessing what rainy season characteristics might have some predictability. Possible relationships between large-scale climate modes or identifiable regional circulation anomalies and dry and w...

Modelling the atmospheric response over southern Africa to SST forcing in the southeast tropical Atlantic and southwest subtropical Indian Oceans

“…The issue of whether regional SST anomalies lead to changes in the frequency or intensity of dry and wet spells during the summer rainy season or in the onset and cessation dates of this season have not been considered. These factors are very important for the agriculture-driven economy of most of the countries in the region (Usman and Reason, 2004;Tadross et al, 2005;Reason et al, 2005c;Hachigonta and Reason, 2006). Therefore, there is considerable motivation for a better understanding of the relationships between southern African rainfall variability and regional SST forcing.…”

Inter- and Intra-annual precipitation variability and associated relationships to ENSO and the IOD in southern Africa

“…Better understanding how these underlying climate modes influence precipitation variability across space and over time continues to be an important area of research, and will need to be integrated with a more in-depth understanding of region-specific atmospheric processes (Cook et al, 2004). Previously published work (Hachigonta and Reason, 2006;Hoell et al, 2014) has started to highlight the importance of these interacting climate processes in modulating the frequency and distribution of precipitation in the region. Further investigation is necessary to understand how underlying climate processes may influence the landscape at the regional or transboundary-scale, which may inform regional-scale land-use and wildlife management decisions in KAZA and other transfrontier conservation efforts across southern Africa.…”

“…For example, in Zimbabwe, Manatsa and Matarira (2009) highlight the influence of the IOD+ on unusually dry periods, and conclude that ENSO might have less to do with the country's rainfall variability. However, just north during the height of the Zambian rainy season (December-January-February), Hachigonta and Reason (2006), identify the strength of importance of ENSO phases on variable dry and wet spell frequencies with less influence shown in relation to associations to Indian Ocean SST anomalies.…”

Intra‐seasonal variability over the northeastern highlands of Tanzania

“…For agricultural practices, the consistency with which the basic minimum of needed rainfall is received is more important than the total received over time. Crops are more likely to do well with uniformly spread 'light' rains than with a few 'heavy' rains interrupted by dry periods (Usman and Reason, 2004;Hachigonta and Reason, 2006). The timing of rainfall onset, duration, and cessation relative to the cropping calendar is more fundamental to crop viability than focusing on total seasonal rainfall alone.…”