Climate drivers of variability within and across the major Eastern Boundary Upwelling Systems

The climate variability and change in Eastern Boundary Upwelling Systems (EBUS) impacts marine ecosystems services globally. We use passive tracers in an ocean model hindcast at 1/4 degree resolution to diagnose low-frequency variability within and across EBUS between 1958-2015. Seasonal modulations of alongshore wind stress, wind stress curl, and thermocline depth are the main drivers of low-frequency variability in each EBUS. We find that Atlantic and Pacific EBUS are independent. In the Pacific, the only coherent variability between EBUS (California and Humboldt) is associated with El Niño, while the remaining low-frequency variance is dominated by the North and South Pacific expressions of the Meridional Modes, which are independent in each hemisphere and characterized during the negative phases by strong cooling and high sea level pressure (SLP) offshore (e.g. upwelling winds). In the Atlantic, coherent variability between EBUS (Canary and Benguela) is associated with upwelling trends, which are not dynamically linked and represent different processes. In the Canary, a negative upwelling trend is connected to the Atlantic Multi-decadal Oscillation. In the Benguela, a positive upwelling trend is forced by the climate change trends in global SLP. Aside from the trend components, the residual variability is forced by localized offshore high SLP variability.