How Do Source Water Depths Respond to Wind and Stratification Variability in the Northwest African Canary Upwelling System?

The Canary Upwelling System is among the most productive marine systems in the world. Alongshore winds drive offshore surface Ekman transport and the upward transport of nutrient rich waters from depth, sustaining major fisheries, offshore carbon export, and air–sea CO₂ exchange. Ongoing changes associated with marine heatwaves and potential shifts in wind strength and change in event duration are expected to modify upper ocean stratification, with direct consequences for the depth of source waters supplying coastal upwelling. Previous studies have explored changes in upwelling intensity and source water depth in the Northwest African region using climate model projections and indirect observational indices, including sea surface temperature, wind trends, and Ekman transport-based indices. These approaches have yielded conflicting results, often depending on the datasets, indices, and time scales used. Moreover, empirical metrics commonly used to characterize upwelling do not explicitly resolve how changes in stratification and circulation jointly control the depth of source waters feeding coastal upwelling. 

Here, we examine the response of source water depth to variability in wind forcing and upper ocean stratification in the Canary Upwelling System. The analysis focuses on 27-28°N, where the RAPID mooring array provides high time resolution observations from 2015 to 2022. Upper ocean stratification is characterized using satellite and in situ mooring observations, while wind forcing is quantified from atmospheric reanalysis products. Upwelling variability is assessed using a mooring based vertical upwelling index (MUVI) derived from bottom pressure and density measurements, and an Ekman-based upwelling index, the Coastal Upwelling Transport Index (CUTI).  Source water depth variability is quantified using density fields and isopycnal displacements from mooring observations. Spectral analyses are applied to wind, stratification, and source water depth time series to identify dominant time scales and assess the relative influence of atmospheric forcing and stratification. This framework also enables the identification of episodic intrusions of South Atlantic Central Water and their association with specific wind and stratification regimes. 

By isolating the physical controls on source water depth across weekly to seasonal time scales, this study provides a physical basis for interpreting variability in nutrient supply and ecosystem response in the Canary Upwelling System and informs future assessments of climate driven changes in coastal upwelling dynamics.