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A buoy spiraling at Burdwood Bank

Image of the Month - September 2021

Magellan AB track from Argos overlaid on tidal currents and bathymetry (credits Cnes/Aviso)

Oceans have an average depth of nearly 3800 m. However, Mariana trench is more than 11000 m deep, and at the reverse some areas are shallow, including some in the middle of an ocean basin. Some of those seamounts or banks are just showing at the surface, other are deep enough so as not to threaten navigation. But, even in these cases, they have an impact on the ocean dynamics, also on climate though ocean mixing. Tides, in particular, are notoriously of more amplitude over the continental shelves (see Image of the Month, Feb. 2017), but it is true of any shallow area. 

During last Vendée Globe around the world alone sail race, as during all the editions of this race since 2000, some skippers launched buoys (undragged ones) for the Argonautica educational project (CNES). One was launched after crossing the Drake passage, north-east when going back. The buoy, named Magellan AB, began spiraling with a period of about 12 h and continued doing so for nearly one month. It was in fact released over a undersea bank, the Burdwood (or Namuncurá) Bank. This is a 300-km long, 60-km wide bank south of Malvinas islands. It is 200-m deep with peaks as high as 50 m deep, and tidal currents there are noticeably stronger than in the surrounding deep ocean,  influencing the ocean dynamic overt the bank.

Altimetry helped to improve the tide models by providing water height in the middle of the ocean where no tide gauge can be installed, and is still helping. In turn, tide models are used to remove tidal heights from the altimetry data - when the study is not focused on this phenomenon. The next challenge will be improving even more the tide models to be used for the SWOT mission, with new small scales effects visible like internal tides thanks to the 2D-mapping of this satellite, and to its unprecedent accuracy.

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