Investigating the currents structure in the northern Mozambique Channel, Cabo - Delgado

Abstract

The Northern Mozambique Channel has been an area of interest for several marine operations in the prospecting and exploiting of offshore hydrocarbons. Thus, a detailed understanding of the local circulation and dynamics is crucial for safety and environmental monitoring in case of an oil spill event. In the present study, a combination of different data set products was used to infer the structure of the currents and the related processes that affect their variability in the Northern Mozambique Channel. We used CTD and ADCP measurements data collected during the research cruise ship “Dr Fridtjof Nansen” in March 2018, and the satellite altimetry-derived geostrophic currents. A longitudinal transect was considered off Cabo-Delgado, Quissanga district - 12 o S with almost 100 km distance, we concentrated on the upper 200 m depth where we have ADCP measurements. Our results presented three different modes of the current structure from a horizontal perspective: on the coastal tip, the currents are southwards orientated with moderate velocities of up to 0.6 m/s and vertically homogeneous, intensifying towards the sea at 1000 m isobaths with strong velocities of up to 1 m/s south-westwards orientated, while on the eastern part of the transect at around 2000 m isobaths, the near-surface currents (above 50 m depth) are completely weak (< 0.2 m/s) and are north-westwards orientated. The weak north-westwards currents at the near-surface (above 50 m depth) observed off 2000 m isobaths are wind-driven, with less influence from the Sea Surface Height gradient. The strong south-westerly currents are found to be a result of the Sea Surface Height gradient, and their intensity is affected by the rugged steering topography on the west coast of the Channel. Moreover, the Channel is dominated by anticyclonic eddies propagating southwards with current speeds (> 1 m/s). These results suggest the need for delicate and suitable materials and methods for offshore operations, as the floating platform installations will have to withstand very strong currents up to a depth of more than 1000 m. The current speeds are fast enough to move the spilt oil to the coast quickly, affecting the coastal ecosystems and consequently human health.