Spatial and temporal variability of coastal upwelling activity in Northwest Africa (1994–2022)

Abstract

• This study represents the first detailed analysis of upwelling activity variability along the Moroccan Atlantic coast, using a 28-year series of in-situ data (1994–2022) from 106 oceanographic campaigns. • Significant trends in the average seasonal and latitudinal variability of upwelling were observed in the water column down to 500 m depth, with marked differences between the northern and southern regions, particularly during summer and autumn. • Strong correlations between temperature and salinity were observed, while an inverse correlation was found with phosphate concentration. • The study highlights the relationship between upwelling activity and regional climatic phenomena, such as the surface water warming in 2015 and the post-2019 warming, which coincided with the Atlantic El Niño. • This study serves as a reference for upwelling activity in the Canary Current Large Marine Ecosystem, particularly along the Moroccan Atlantic coast from 32°3 N to 21°N. Influenced by the Canary Current, the northwest African coast is one of the world's four major upwelling systems. This study is the first to investigate the latitudinal variability of upwelling activity along the Moroccan Atlantic coast, using a 28-year time series of in-situ data (1994–2022) analyzed on a seasonal scale. Five distinct zones were identified, each with specific oceanographic conditions. The Cape Cantin (32°30 N) and Cape Draa (28°30 N) regions show strong upwelling seasonality, with a peak in summer followed by relaxation in autumn. The North Atlantic Central Waters (NACW) strongly influences these areas. The Cape Boujdour (25°30 N) region exhibits the most active upwelling center, with near-permanent activity. Dakhla (23°30 N) is a water retention area influenced by the Cape Boujdour filament in autumn, while the Cape Blanc region (21°N) exhibits permanent upwelling activity throughout the year, acting as a frontal zone influenced by saltier NACW and South Atlantic Central Waters (SACW), which is less saline, phosphate-rich, and oxygen-depleted. Principal component analysis (PCA) confirmed the latitudinal and seasonal variability of upwelling. Cold, less saline, and phosphate-rich waters were more pronounced in summer across all zones. A relaxation of upwelling was observed in autumn, while the winter season showed an absence of upwelling, particularly in the northern zones (32°30N–28°30 N). Additionally, this study revealed a significant warming of surface waters in autumn 2015, and another warming event after 2019, coinciding with the Atlantic Niño phenomenon. These findings provide a valuable reference for understanding upwelling activity trends in this region.