Changes in hydro-meteorological conditions over tropical West Africa (1980–2015) and links to global climate

Abstract

The role of global sea surface temperature (SST) anomalies in modulating rainfall in the African region has been widely studied and is now less debated. However, their impacts and links to terrestrial water storage (TWS) in general, have not been studied. This study presents the pioneer results of canonical correlation analysis (CCA) of TWS derived from both global reanalysis data (1980−2015) and GRACE (Gravity Recovery and Climate Experiment) (2002 − 2014) with SST fields. The main issues discussed include, (i) oceanic hot spots that impact on TWS over tropical West Africa (TWA) based on CCA, (ii) long term changes in model and global reanalysis data (soil moisture, TWS, and groundwater) and the influence of climate variability on these hydrological indicators, and (iii) the hydrological characteristics of the Equatorial region of Africa (i.e., the Congo basin) based on GRACE-derived TWS, river discharge, and precipitation. Results of the CCA diagnostics show that El-Niño Southern Oscillation related equatorial Pacific SST fluctuations is a major index of climate variability identified in the main portion of the CCA procedure that indicates a significant association with long term TWS reanalysis data over TWA (r = 0.50, ρ < 0.05). Based on Man-Kendall's statistics, the study found fairly large long term declines (ρ < 0.05) in TWS and soil moisture (1982 − 2015), mostly over the Congo basin, which coincided with warming of the land surface and the surrounding oceans. Meanwhile, some parts of the Sahel show significant wetting (rainfall, soil moisture, groundwater, and TWS) trends during the same period (1982 − 2015) and aligns with the ongoing narratives of rainfall recovery in the region. Results of singular spectral analysis and regression confirm that multi-annual changes in the Congo river discharge explained a considerable proportion of variability in GRACEhydrological signal over the Congo basin (r = 0.86 and R 2 = 0.70, ρ < 0.05). Finally, leading orthogonal modes of MERRA and GRACE-TWS over TWA show significant association with global SST anomalies.

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).…”

“…Arguably, the interactions of such influence with circulation features and mesoscale convective systems, among other climate phenomena (e.g., 25,26,46) have direct repercussions on hydrological processes. Indeed, the long history of unfavourable hydrological conditions of the West African Sahel provides a basis to diagnose the contributions of these oceanic factors on drought evolutions in the region (65).…”

“…SPI also shows linked to a sequence of oceanic phenomenon and plethora of multi-scale climate teleconnections, including quasi-periodic phenomena (e.g., ENSO, sunspot activity, Quasi-Biennial Oscillation, etc. ), and other low frequency oscillations (e.g.,65,24,50,71). Consequently, all of the climate modes described in Section 2 where integrated in the PLSR scheme as predictors to model the temporal series of SPEI and SPI (predictands obtained from the PCA scheme) over the Sahel.…”

The water resources of tropical West Africa: problems, progress, and prospects

“…A typical example is the Volta Basin where human water management is considered a significant driver in the dynamics of surface water (e.g., Ndehedehe et al 2017a;Ferreira and Asiah 2015). Further, several scientific reports (e.g., Ndehedehe et al 2018b;Andam-Akorful et al 2017;Panthou et al 2014;Todd et al 2011;Sheffield and Wood 2008) have outlined exhaustively the influence of global climate and some oceanic hot spots on precipitation changes in West Africa. The influence of climate variability and low-frequency variability that are connected to slow oceanic and climate oscillations from global SST anomaly on the region's TWS has only been recently reported (e.g., Ndehedehe et al 2018b;Andam-Akorful et al 2017).…”

“…Further, several scientific reports (e.g., Ndehedehe et al 2018b;Andam-Akorful et al 2017;Panthou et al 2014;Todd et al 2011;Sheffield and Wood 2008) have outlined exhaustively the influence of global climate and some oceanic hot spots on precipitation changes in West Africa. The influence of climate variability and low-frequency variability that are connected to slow oceanic and climate oscillations from global SST anomaly on the region's TWS has only been recently reported (e.g., Ndehedehe et al 2018b;Andam-Akorful et al 2017). Such influence modulates hydrological processes and provides significant control on the spatial and temporal distributions of regional changes in TWS and other water budget quantities (e.g., runoff), resulting in increased acceleration of the water cycle.…”