Revisiting interannual to decadal teleconnections influencing seasonal rainfall in the Greater Horn of Africa during the 20th century

Abstract

This paper revisits teleconnections for three major rainy seasons at the Greater Horn of Africa for the period 1901–2013. Sea surface temperature‐based climate indices known to influence Short, Kiremt, and Long Rains are used in a comprehensive statistical analysis to detect non‐stationary behaviour in teleconnections and to split them into interannual and decadal time scales. Physical mechanisms are proposed for identified significant non‐stationary teleconnections. Interannual variability in the October–December Short Rains is predominantly influenced by the Indian Ocean Dipole (IOD) with a percent variance explained (PVE) of up to 80% in recent years. However, abrupt shifts in this teleconnection occurred around 1918, 1951, 1987, and recently. The Short Rains also correlate strongly with El Niño‐Southern Oscillation (ENSO). However, ENSO influence on Short Rains is mediated by in‐phase occurrence of IOD. Decadal variations in Short Rains are more directly explained by low‐frequency variability in the Pacific Ocean (PO). A hitherto undocumented non‐stationary relation was found between Atlantic Niño 3 and June–September Kiremt Rains. The non‐stationarity seems to be related to a decadal regime shift in the West African monsoon system in the late 1960s. The variability of Kiremt Rains is also strongly associated with ENSO, although the recent increased correlation is not non‐stationary. Consistent with recent studies, the post‐1998 March–May Long Rains decline is strongly associated with decadal variability in the PO. The PVEs in the stable correlations using Pacific Decadal Oscillation and Interdecadal Pacific Oscillation indices range from 25 to 64%, mostly due to low‐frequency variability (>8 years). The results have ramifications for seasonal‐to‐decadal forecasting and can be taken to design modelling studies to gain insights into the physical mechanisms.

Investigating Long-Range Seasonal Predictability of East African Short Rains: Influence of the Mascarene High on the Indian Ocean Walker Cell

“…The dynamical pathways were further explored by Ummenhofer et al (2009), who used ensemble simulations to show that the warm western SST pole of the IOD has a larger role than the eastern cold pole in promoting stronger easterly winds from the Indian Ocean and moisture convergence over East Africa. Bahaga et al (2015) also confirmed the dominant role of the western pole of the IOD using AGCM simulations, which demonstrated that the warm western pole initiated a Gill-type response. In Hastenrath et al (2011), the important role of upper-level atmospheric motion was addressed: the summer warming and high stand of upper-tropospheric topography over South Asia enables strong uppertropospheric easterlies over the northern-central Indian Ocean in the following boreal autumn, while lack of such a mechanism in boreal spring leads to less predictability of the long rains.…”

“…A more recent study by Nicholson (2015) utilizing a 139-year rainfall record showed that the links between the Walker cell and the East African short rains were significantly weaker during certain historical periods (e.g., 1920-60) and stressed the time dependence of the links. The nonstationary correlation between Indian Ocean SSTs (e.g., IOD) and EASR was further investigated by Bahaga et al (2019), which suggests that more interannual variability of the EASR was explained by the Indian Ocean SSTs in recent years.…”

Skill of dynamical and GHACOF consensus seasonal forecasts of East African rainfall

“…The regression value achieved by the model utilising IOD and Niño 3.4 was 0.90, and demonstrates the dominance that these two modes of variability have over the region, during this period. Bahaga et al (2019) demonstrated however that the correlation between these modes can fluctuate on decadal timescales, with the period used here found to have a particularly high correlation.…”

“…They also often fail to consider the nonstationary relationship between rainfall and the predictors. Such nonstationary relationships of teleconnections to East African rainfall in particular have been highlighted by Clark et al (2003), Bahaga et al (2019). To best meet user needs, a combination of statistical and dynamical methods is often most appropriate (Doblas-Reyes et al 2013).…”

Evolutionary drought patterns over the Sahel and their teleconnections with low frequency climate oscillations

“…While the interest of relevant stakeholders in climate change is on the rise because of the need to address challenges related to water security and economic growth, the debate on the continued influence of atmospheric circulation features, warming of the tropical oceans, and indices of climate variability on hydro-meteorological processes is ongoing (e.g., 8,65,63,5,75,58,6,23,74). The catalogue of devastating impacts, including crop failure, loss of freshwater stocks, low gross domestic product (GDP), food insecurity, and loss of national income due to extreme droughts have been highlighted in some reports (see, e.g., 89,52,66,83,92,16,92,53,82,14).…”