Abstract
Abstract Advanced warning of delayed onset or early cessation of the rainy seasons would be extremely valuable information for farmers in east Africa and is a common request from regional stakeholders. Such warnings are beginning to be provided, however forecast skill for these metrics has not been demonstrated. Here the forecast skill of the ECMWF seasonal hindcasts is evaluated for onset and cessation forecasts over east Africa. Correlations of forecasted and observed onset and cessation anomalies are only above a 95% statistical significance level for a small part of the domain during the long rains, whilst they are significant for short rains onset and cessation over a large part of the region. The added value of updating the forecast outlook with the extended range 46 day forecast is assessed and this gives a small improvement. For the short rains detection of early onset is better near the coast, and late onset detection is better over northwestern Kenya. During exceptionally dry years the method to detect onset and cessation fails. Using this as a definition of a failed season, the model shows significant skill at anticipating long rains season failure in the northwest of Kenya, and short rains failure in Somalia and northeast Kenya. In addition the strength of the correlation between long rains cessation and seasonal total is shown to be particularly weak in observations but too strong in the hindcasts. Predictability of onset and cessation for both seasons appears to arise primarily from the link with seasonal total and it is unclear that the model represents variability in onset and cessation beyond this. This has important implications for operational forecasting: any forecast of season timing which is ‘inconsistent’ with seasonal total (e.g. an early onset but low total rainfall) must be treated with caution. Finally links with zonal winds are investigated. Late onset is correlated with easterly (westerly) anomalies during the long (short) rains, though the strength and spatial pattern of the relationship is not well represented in the model. Early cessation is correlated with easterly anomalies in both seasons for most of the region in both observations and hindcasts. However for the long rains the sign of the correlation is reversed along the coast in observations but not in the hindcasts. These dynamical inconsistencies may have a negative impact on forecast skill and have the potential to inform process-based development of climate modelling in the region.
Highlights
The population of largely semi-arid East Africa is exposed to climate variability and climate change impacts are uncertain (Rowell et al, 2015), risks can be partially mitigated with shorter term forecasts (Washington et al, 2006)
Though the skill of onset forecasts has been evaluated for west Africa (Vellinga et al, 2013) and it has been demonstrated using an atmospheric model forced by sea surface temperature (SST) that onset of the short rains is more reproducible than the long rains (Philippon et al, 2015), the level of forecast skill for east Africa has not been described in the scientific literature
For most of the region, the bias is less than seven days (Fig. 2), though the mean S5 onset on the Kenya/Tanzania border is several weeks too early, and over the Ethiopian highlands is several weeks too late
Summary
The population of largely semi-arid East Africa is exposed to climate variability and climate change impacts are uncertain (Rowell et al, 2015), risks can be partially mitigated with shorter term forecasts (Washington et al, 2006). (Owusu et al, 2017), as it would allow farmers to better manage potential risks to their planting and harvesting activity To this end, the IGAD Climate Predictions and Applications Centre has started to disseminate onset and cessation forecasts at the Greater Horn of Africa Regional Climate Outlook Forum (GHACOF), though no evaluation of expected skill is provided. Mean dates and variability of the start and end of the rains over Kenya/Tanzania has been determined for the two rainfall seasons (the March-May long rains, MAM, and October-December short rains, OND) (Camberlin and Okoola, 2003; Camberlin et al, 2009; Philippon et al, 2015). Though the skill of onset forecasts has been evaluated for west Africa (Vellinga et al, 2013) and it has been demonstrated using an atmospheric model forced by sea surface temperature (SST) that onset of the short rains is more reproducible than the long rains (Philippon et al, 2015), the level of forecast skill for east Africa has not been described in the scientific literature
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