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Conference by Floriane Sudre – Spatio-temporal variability of ocean fronts in the Mozambique Channel and influence on marine ecosystems

May 9th, 2022, R/V Marion Dufresne, Mozambique Channel

Authors: Luis Chomienne & Illona Ribot

Floriane Sudre is a PhD student at the Mediterranean Institute of Oceanography working on ocean fronts. Her thesis is part of the multidisciplinary Ocean Front Changes project. The project aims to find out how fronts affect marine life and what’s the influence of climate change on fronts, all by keeping ties with the fishermen, various stakeholders and NGOs.

So, what is a front? Fronts are located at the border between two water masses with different properties, such as temperature, salinity or density. For example, sea surface temperature (SST) fronts lie at the frontier between colder and warmer waters. Front zones are ephemeral ecosystems where biological production is high which attracts many predators such as seabirds, sharks or marine mammals. Thus, fronts attract many fishermen, explaining the importance of keeping in contact with them.

Figure 1 : Map of Mozambique Channel with the different zones (Islands region; NMC: North Mozambique Channel; SMC: South Mozambique Channel; Agulhas: Agulhas Current formation region) and main currents (black).

The first part of the RESILIENCE oceanographic cruise was taking place in the Mozambique Channel. In this narrow region, the South Equatorial Current (SEC, figure 1), comes in from the north of Madagascar and reaches the Comoros Islands, creating many instabilities and generating eddies which continue their way south until they join the Agulhas Current.

Figure 2 : Carte de la modélisation des gradients de température de surface, mettant en évidence les fronts.

Floriane studies the seasonal variability of surface temperature fronts by using an ocean model, covering 1993 to 2014. She considers fronts as those who have a gradient higher than 0.05°C per km, some of them can go up to 2.3°C per km! Permanent fronts can be found along the continental shelves.

The Mozambique Channel was separated in 4 distinct regions (figure 1) that have different seasonal variability (figure 3). First, there is the Islands region, influenced by the topography of the Comoros Islands, and North Mozambique Channel (NMC) region which is influenced by the strangulation effect of the channel. These two northern regions are highly influenced by the monsoon. The third region, corresponds to the South Mozambique Channel (SMC) and is too further south to be influenced by the monsoon. The last region, the Agulhas region, is characterized by the strong current coming from the Indian Ocean.

Figure 3 : Variations saisonnières de l’intensité des fronts en fonction de la zone.
Figure 4 : Bouée placée dans le canal du Mozambique (SBE39 correspond aux instruments CTD)

On board, we obtained in situ data that will complement model output and satellite data in our understanding of front variability and their use by marine life. The mooring, placed in the channel during the cruise and inspired by H. Glotin’s BOMBYX (translated acronym: Multimodal buoy for biodiversity and physical oceanography, figure 4) will be able to gather data on a vertical profile from 1056 meters of depth up to the surface with several instruments, including:

  • 10 CTDs for salinity, temperature and density measurements which will, when all gathered together provide a vertical profile of hydrological data;
  • 2 ADCPs used to measure the water velocity using acoustics (with one aiming upward and the other one downward in order to obtain data on the whole water column);
  • 1 fluorometer to measure chlorophyll concentration near the surface;
  • 2 hydrophones to record marine mammal positions.

All these tools will thus give a holistic view of the bio-physical interactions in the marine environment.

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