The role of subtropical highs and evolution of thermal lows intensity in Eastern Boundary Upwelling Systems

The surface atmospheric flow in Eastern Boundary Upwelling Systems (EBUS) is often characterised by strong spatiotemporal heterogeneity resulting from coastal orography, land–sea drag contrast, and coastline orientation. These heterogeneities are poorly represented in most multidecadal climate studies, which rely on simulated data at a spatial resolution of about 1º. Given the known importance of EBUS for marine habitats, the fishing industry, and the atmospheric conditions in some of the most densely populated coastal regions, it is relevant to assess their recent multidecadal evolution in greater detail.

By analysing atmospheric (ERA5) and oceanic (SODA3) reanalysis data, both with about ¼º spatial resolution in the four main EBUS (Canary, California, Humboldt, and Benguela), we linked the meridional displacement of subtropical anticyclones and the evolution in intensity of thermal lows to trends in EBUS coastal wind stress over a historical 40-year period (1981–2020). We also assessed the evolution of the mixed layer depth (MLD), a key variable for regulating upper ocean heat and CO2 transfer rates at the ocean–atmosphere interface. Results show seasonal poleward shifts in the southern hemisphere subtropical anticyclones and poleward or equatorward meridional shifts in the northern hemisphere. Thermal lows exhibit strengthening trends, though with marked differences in intensity among EBUS and seasons. For coastal wind stress, a predominant strengthening is observed in the four EBUS, but with heterogeneities linked to the seasonal meridional displacement of the subtropical high in the Canary EBUS, and to the strengthening of the thermal low in California. In the upper ocean, the dominant wind stress strengthening leads to prevailing MLD deepening in Canary and Benguela, highlighting the dominant role of wind stress, while the intermittent opposing MLD trends in California and Humboldt indicate competing influences of ocean surface warming and wind stress strengthening.