Dynamics of the Mozambique channel using surface drifters and satellite-derived geostrophic and Ekman currents

Abstract

This study examined the surface current dynamics around Madagascar and the Mozambique Channel. Dividing these regions into the Eastern Madagascar Coast, Northern, Central, and Southern Mozambique Channel subregions allowed for the assessment of the role of geostrophic-only and combined geostrophic and Ekman currents in the dynamics. Moreover, calculated virtual drifters were compared with 151 hourly Surface Velocity Program drifters (SVP drifters) from the Global Drifter Program spanning from 2000 to 2019. In addition, while some SVP drifters retained their drogues, others lost them; this status was considered in the analysis. The virtual drifter’s trajectories were calculated using the basic Lagrangian modelling equation, which requires velocity fields. The velocity fields employed are based on the CNES-CLS18 mean dynamic topography model, which comprises satellite altimeter observations, wind from numerical models and in-situ observations from SVP drifters. These velocity fields are compiled at four levels of processing by the European Union-Copernicus Marine Service. The virtual drifters from the two velocity fields were designated as Virtual Geostrophic Drifter (VGD) and Virtual Geostrophic-Ekman Drifter (VGED), while the SVP drifters were classified as real drifters. The results reveal substantial underestimations, averaging 29% with VGDs and improving to 21% when Ekman drift is incorporated in the VGEDs compared to real drifters. This shows improvement in the CNES-CLS18 mean dynamic topography model. The Eastern Madagascar Coast exhibited the highest velocity discrepancies due to the formed dipole in the southern tip of Madagascar, which prevented the movement of specific VGD to the channel. However, the Northern Mozambique Channel showed lower underestimation due to strong western boundary currents, local winds, and tropical cyclones, highlighting the region’s dominance of wind seasonality. Due to the eddy’s linearity and non-linearity, virtual drifters have substantial position errors in the Central and Southern Mozambique Channel. Moreover, virtual drifters failed to identify key submesoscale structures because the velocity field’s 25 km resolution was a significant issue. The dispersion of virtual drifters in the offshore Cabo Delgado coast of the Northern Mozambique Channel was predominantly influenced by seasonal Ekman-driven currents. This highlights the necessity of incorporating these dynamics into future simulations as winds modulate this region. A considerable part of these virtual drifters ran aground shortly after being released due to the strong winds from the south that influence the ocean surface, displacing them 45° to the left in relation to the wind’s direction. This phenomenon was observed predominantly in winter.