Coastal Upwelling Transport at Cape Sainte-Marie, Southern Madagascar: Contribution of The CUTI Index

Abstract

This study aims to characterize upwelling transport south of Madagascar around Cape Sainte-Marie (25°S-27°S, 42°E-46°E), while distinguishing the contribution of wind forcing from that of regional ocean circulation. An annual CROCO simulation forced by ERA5 is analysed in four boxes representing the western upstream area, the coastal Core 2 cell, the Cape Sainte-Marie sector, and the export pathway toward the southern gyre. Two diagnostics are computed from simulated velocities: CUTIsouth, which measures outward transport across the southern boundary, and CUTIbox, which summarizes the mean exchange balance across the four boundaries. The results are compared with the Bakun index.

Surface transport is positive in boxes B2, B3, and B4, with a maximum at Cape Sainte-Marie. The correlation with the Bakun index confirms a clear wind-driven component in the Core 2 cell. At the cape, the difference between CUTIsouth and CUTIbox reveals compensation by lateral exchanges. In the export box, positive meridional transport and a negative mean balance highlight a connection with the southern gyre and dynamics that cannot be explained by local wind forcing alone. The box-based CUTI analysis complements the Bakun index by distinguishing meridional transport, lateral exchanges, and export toward the southern gyre. It therefore describes Core 2 as an open cell connected to Cape Sainte-Marie and the regional circulation.