Multidecadal variability in the Nile River basin hydroclimate controlled by ENSO and Indian Ocean dipole

Shereif H Mahmoud; Thian Yew Gan

doi:10.1016/j.scitotenv.2020.141529

Multidecadal variability in the Nile River basin hydroclimate controlled by ENSO and Indian Ocean dipole

Sci Total Environ. 2020 Dec 15:748:141529. doi: 10.1016/j.scitotenv.2020.141529. Epub 2020 Aug 10.

Authors

Shereif H Mahmoud¹, Thian Yew Gan²

Affiliations

¹ Department of Civil and Environment Engineering, University of Alberta, Edmonton, Alberta T6G 2G7, Canada.
² Department of Civil and Environment Engineering, University of Alberta, Edmonton, Alberta T6G 2G7, Canada. Electronic address: tgan@ualberta.ca.

PMID: 32827894
DOI: 10.1016/j.scitotenv.2020.141529

Abstract

Climate change impacts on the hydroclimate of the Nile River Basin (NRB) tend to be analyzed mostly based on short-term data and confined to a specific hydroclimate variable at sub-basin level. This study provides a better understanding of the hydrological cycle and the hydroclimate variability of NRB and aim to find the origin of the driving forces. Firstly, eight change point detection methods were used to investigate the abrupt changes in the NRB hydroclimate. Next, we used wavelet transform coherence (WTC), spatial correlation, and detrended cross-correlation (DCCA) to analyze the inter-annual to multidecadal variabilities of the hydroclimate of NRB because they are effective in capturing the temporal variability at multiple scales. Our results show significant hydroclimatic changes and trends attributed to climate change impact after the 1970s. For instance, precipitation and relative humidity (RH) decreasing at 16.2 mm/decade and 0.3 5%/decade, respectively. In contrast, geopotential height (GPH), climate warming, wind speed and zonal wind stress increasing at 3.1 m/decade, 0.19 °C/decade, 0.02 m/decade and 1.51 m²/s²/decade, respectively. These observed changes are strongly linked to El Niño and Indian Ocean Dipole (IOD). Our results also indicate that the largely strengthened IOD and El Niño amplitudes since the 1970s controlled the multidecadal variability of NRB's hydroclimate. In addition to ENSO-induced warming in NRB, El Niño exhibited a strong negative (positive) influence on precipitation and RH (GPH, surface temperature, wind speed, AET) over lowlands of Ethiopia, Kenya, Uganda, Sudan, Eritrea, Rwanda, and Burundi over the past 70 years. Our analysis revealed that IOD can either intensify or decrease the impacts of El Niño on the NRB's hydroclimate. For instance, IOD have a greater negative influence on the precipitation variability over Sudan, Ethiopia, Congo, Egypt, and Eritrea. These results were further confirmed by the changes in atmospheric circulation patterns in NRB during active El Niño and La Niño episodes. The increase in GPH anomalies associated with El Niño warming indicates a greater saturation vapor pressure, which at lower levels cause a lower dew point and a higher surface temperature. In addition, El Niño-driven changes to vector and meridional wind patterns created a strong anti-cyclonic wave of dry air that keeps moving dry air into the NRB. These waves propagate southward towards the NRB, controlling the circulation of air mass, heat, and moisture fluxes and affect the surface weather patterns of NRB.

Keywords: Climate change; ENSO; Hydroclimate; Indian Ocean Dipole; Multidecadal variability; Nile river.