Effects of Climate Change on Moroccan Coastal Upwelling: Relationships between the NAO, Upwelling Index, and Sea Surface Temperature (1978-2024)

Abstract

This study investigates climate change impacts on Moroccan Atlantic coastal upwelling (21$^{\circ}$--35$^{\circ}$N), analyzing interrelationships among the North Atlantic Oscillation (NAO), Coastal Upwelling Index (CUI), and Sea Surface Temperature (SST) from 1978--2024 using ERA5 reanalysis data. Methods include index calculations, seasonal correlations, Granger causality tests, linear trends, and breakpoint detection to evaluate spatio-temporal dynamics and causality. Results reveal seasonal variability: strong NAO--CUI coupling in winter (DJF; $r = +0.65$), local upwelling dominance in summer (JJA; $r = -0.46$ CUI--SST), and a north--south gradient with intense southern upwelling (mean Ekman transport $37.1 \pm 34.5$ m$^3$/s/100m). Trends show significant SST warming ($+0.0736$ units/decade; $+0.149^{\circ}$C/decade spatially), summer upwelling decline ($-0.0635$ units/decade in JJA), and weak NAO shifts. Granger causality indicates rapid local forcing (CUI $\to$ SST, lag 1--3 months), delayed teleconnections (NAO $\to$ SST, lag 2--4 months), and ocean feedback (SST $\to$ NAO, lag 7--11 months). Breakpoints (1992--1997) signal warming acceleration, highlighting a paradox of global SST rise amid localized upwelling-induced cooling per Bakun's hypothesis. Findings underscore implications for fisheries, aquaculture, and tourism, advocating adaptive management via seasonal forecasting. Keywords: Coastal upwelling, NAO, SST, climate change, Granger causality, Moroccan Atlantic, Ekman transport, spatio-temporal variability.