Dynamical Origin Of Coastal Upwelling At Cape Sainte-Marie, Southern Madagascar

Abstract

This study investigates the origin of coastal upwelling at Cape Sainte-Marie in southern Madagascar, using a one-year CROCO simulation representative of 2010, referred to as Core 2 by Ramanantsoa et al. [1]. The objective is to characterize the life cycle of this cell, from its formation at the cape to its export toward the southern gyre and to clarify the respective roles of ocean dynamics and wind forcing in its onset. Joint analysis of surface currents, normalized relative vorticity, sea level, kinetic energy, thermohaline structure, and instability diagnostics shows that Core 2 is associated with current-separation upwelling. Between days 40 and 90 (mid-February to early April), a pre-existing boundary flow intensifies and separates at Cape Sainte-Marie, generating a cyclonic cell in geostrophic balance that uplifts the isopycnals and produces a cold, saline anomaly. At the cape, the eddy is primarily energized by baroclinic instability, which clearly dominates the other energy sources and for which the necessary conditions for development are satisfied. The cell subsequently detaches from the cape and migrates westward toward the southern gyre by mid-April. The process is recurrent, with a second eddy generation developing in early September. Core 2 therefore emerges not simply as wind-driven upwelling, but as a current-separation upwelling system: wind modulates its intensity, whereas its origin and evolution are controlled by current separation at Cape Sainte-Marie, baroclinic amplification, and export toward the southern gyre.