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Deep currents in the Gulf of Guinea: along slope propagation of intraseasonal waves

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Abstract:

In the Gulf of Guinea, intraseasonal variability is large at the equator and along the coast. Current data on the continental slope near 7.5° S show very energetic biweekly oscillations at 1300 m depth. A high resolution numerical model demonstrates that this deep variability is forced by equatorial winds, through the generation of equatorial Yanai waves that propagate eastward and at depth, and then poleward as coastal-trapped waves upon reaching the coast of Africa. Intraseasonal variability is intensified along the coast, especially in the 500–1500 m depth range, with the largest intensification in the 10–20 day period range. The structure of kinetic energy is well explained at first order by a linear model with six baroclinic modes. Along the equator, eastward intensification of energy and bottom intensification are in qualitative agreement with vertically propagating Yanai waves, although the signal is clearly influenced by the details of the bathymetry. Along the coast, vertical modes 3 to 5 are important close to the equator, and the signal is dominated by lower modes farther south. Additional current meter data on the continental slope near 3° N display an energy profile in the 10–20 day period band that is strikingly different from the one at 7.5° S, with surface intensification rather than bottom intensification and a secondary maximum near 800 m. The model reproduces these features and explains them: the surface intensification in the north is due to the regional wind forcing, and the north-south dissymetry of the deep signal is due to the presence of the zonal African coast near 5° N. A 4 years time series at 7.5° S displays intermittencies of the 10–20 day signal near the bottom. This intermittency is not correlated with fluctuations of the equatorial winds and does not seem to be a simple linear response to the wind forcing.

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