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

[摘要] In the Gulf of Guinea, intraseasonal variability is large at the equator andalong the coast. Current data on the continental slope near 7.5° Sshow very energetic biweekly oscillations at 1300 m depth. A high resolutionprimitive equation numerical model demonstrates that this deep variability isforced by equatorial winds, through the generation of equatorial Yanai wavesthat propagate eastward and at depth, and then poleward as coastally-trappedwaves upon reaching the coast of Africa. Intraseasonal variability isintensified along the coast of the Gulf of Guinea, especially in the 10–20 dayperiod range and at depths between 500 and 1500 m. The kinetic energydistribution is well explained at first order by linear theory. Along theequator, eastward intensification of energy and bottom intensification are inqualitative agreement with vertically propagating Yanai waves, although thesignal is influenced by the details of the bathymetry. Along the coast,baroclinic modes 3 to 5 are important close to the equator, and the signal isdominated by lower vertical modes farther south. Additional current meterdata on the continental slope near 3° N display an energy profile inthe 10–20 day period band that is strikingly different from the one at7.5° S, with surface intensification rather than bottomintensification and a secondary maximum near 800 m. The model reproducesthese features and explains them: the surface intensification in the north isdue to the regional wind forcing, and the north-south asymmetry of the deepsignal is due to the presence of the zonal African coast near 5° N. A4 years time series of current measurements at 7.5° S shows that thebiweekly oscillations are intermittent and vary from year to year. Thisintermittency is not well correlated with fluctuations of the equatorialwinds and does not seem to be a simple linear response to the wind forcing.

[学科分类] 海洋学与技术