North Atlantic Oscillation Drives Winter Streamflow Across Moroccan Rivers

Winter river flows across much of Morocco march to the beat of the North Atlantic Oscillation, according to a new analysis of five decades of streamflow records published in Theoretical and Applied Climatology. The study, led by Mohamed Elmehdi Saidi with colleagues, examined monthly discharge data from 23 gauging stations spread across five major watersheds: the Bouregreg, Draa, Oum Er-Rbia, Souss-Massa and Tensift basins, covering the period from 1971 to 2021.

The researchers focused on the winter months of December through March and tested how river discharge at each station relates to two large-scale atmospheric circulation patterns: the North Atlantic Oscillation (NAO) and the Western Mediterranean Oscillation (WeMO). Trends in the flow records were assessed with the Mann-Kendall test and Sen’s slope estimator, while the teleconnections were quantified through Pearson correlation analysis.

The results reveal two contrasting hydrological regimes within the country. The Bouregreg and Souss-Massa basins follow a winter-dominated Atlantic rainfall regime, with discharge peaking between December and March. In contrast, the Tensift and upper Draa basins operate under a mixed snow-rain regime, in which water stored as snow in the High Atlas Mountains delays the flow maximum until spring, in the March-to-May window.

Only one basin showed a statistically significant long-term decline in streamflow over the 51-year record. The Tensift basin exhibited a negative trend with a Mann-Kendall Z statistic of −2.18, a p-value of 0.029, and a Sen’s slope of −0.036 cubic metres per second per year. The authors note that this decline is consistent with the documented decline of the High Atlas snowpack.

The teleconnection analysis produced the study’s most striking pattern. Thirteen of the 23 stations showed significant negative correlations between winter discharge and the NAO index, meaning that when the NAO is in a positive phase, Atlantic Morocco’s rivers tend to run lower, and when it is negative, they run higher. The strongest relationships appeared in the Oum Er-Rbia basin, with a correlation coefficient of −0.784, and the Bouregreg basin, at −0.756.

Crucially, the NAO signal was not uniform across the country. The correlations weakened along a north-to-south gradient, and the Draa basin, the southernmost of the five, showed no significant NAO relationship at all. This spatial structure indicates that the large-scale circulation pattern governs winter river response mainly in the Atlantic-facing basins of northern and central Morocco, while the southernmost basin shows no detectable NAO influence in this dataset.

The WeMO index, which captures variability in the western Mediterranean, told a weaker and partly redundant story. Its correlations with streamflow were generally smaller, and the index itself correlates with the NAO at roughly 0.65, making it difficult to separate its influence. The authors conclude that the NAO is the dominant winter driver of Moroccan Atlantic streamflow, with the WeMO adding little independent signal.

For a semi-arid country, the findings carry practical weight for water resource management, which is how the authors frame the work. The strong statistical links documented here highlight the role of large-scale atmospheric circulation in shaping regional hydrological variability in North Africa. The record’s own caveats remain clear: the significant declining trend was confined to the Tensift basin, the NAO signal was absent in the Draa basin, and the analysis establishes statistical associations between circulation indices and discharge over 1971–2021 rather than projections of future change.

Subject of Research: Seasonal and interannual streamflow variability in Morocco and its teleconnections with large-scale atmospheric circulation

Article Title: Seasonal and interannual variability of streamflow in Morocco and teleconnections with large-scale atmospheric circulation

Article References: Saidi, M. E., Boughdadi, S., Rachdane, M., Moustadraf, J., & El Khalki, E. M. (2026). Seasonal and interannual variability of streamflow in Morocco and teleconnections with large-scale atmospheric circulation. Theoretical and Applied Climatology, 157(11), Article 753. https://doi.org/10.1007/s00704-026-06691-0

Image Credits: AI Generated

DOI: 10.1007/s00704-026-06691-0

Keywords: streamflow, Morocco, North Atlantic Oscillation, WeMO, teleconnections, hydrology, High Atlas, Tensift basin, Oum Er-Rbia, semi-arid, climate variability, water resources