Identification of Zones of High Geostrophic Current Acceleration in Western Boundary Current Extensions: Possible Mechanisms and Climate Impact

Six geostrophic current velocity anomalies (GCVAs) are highlighted at mid-latitudes of the three oceans, attributed to the geostrophic adjustment, i.e. the relaxation of the dy-namic system represented by the superposition of multifrequency Rossby waves leading to its geostrophic balance. They develop where the stratification is strong along the warm waters of tropical origin of the western boundary currents (WBCs) as they leave the conti-nents to re-enter the five subtropical gyres. Considering these quasi-stationary Rossby waves as coupled oscillators with inertia governed by the Caldirola–Kanai (CK) equations, they are resonantly forced in harmonic modes by the solar declination. Confined between the lateral boundaries of the WBC, the Rossby wave system can only evolve towards geo-strophic balance if the width of the WBC is a multiple of the wavelengths, regardless of their baroclinic mode. A hyper-resonance phenomenon occurs as soon as the widening of the WBC away from the continent enables it, resulting in an acceleration of geostrophic currents due to the restoring forces of the oscillators. These restoring forces are all the stronger as the meridional and zonal current velocities of the multifrequency oscillators, taken two by two, differ. As with the fundamental annual period wave, the geostrophic currents of the different harmonics are warm or cold depending on their direction of propagation, within geostrophic current acceleration zones (GCAZs). Cross-wavelet analysis of geostrophic current velocity at each GCAZ and geopotential height at the same location reveals plausible causal relationships. When applied to the 1993–2022 observa-tion period, this method shows that the fraction of geopotential height perturbations within the GHAs attributed to the GCAZ ranges from 10% to 47% within the period band 22.8-45.7 days and 36% to 48% within the period band 11.4-22.8 days.