ALBERT

Description: Besides the zonal flow that dominates the seasonal and long-term variability in the equatorial Atlantic, energetic intraseasonal meridional velocity fluctuations are observed in large parts of the water column. 15 years of full-depth velocity data from an equatorial mooring at 23°W are used to investigate intraseasonal variability and specifically the downward propagation of intraseasonal energy from the surface into the deep ocean. Near the surface (20 to 50 m), intraseasonal variability at 23°W peaks at periods between 30 to 40 days. It is associated with westward propagating Tropical Instability Waves, which undergo an annual intensification in August. Enhanced energy levels of equatorial intraseasonal variability are observed down to about 2000 m. A frequency-vertical mode decomposition shows that meridional velocity fluctuations are more energetic than the zonal ones for periods 〈 50 days. The energy peak at 30 to 40 days and vertical modes 2 to 5 excludes equatorial Rossby or gravity waves and suggests Yanai waves to be associated with the observed intraseasonal energy. Yanai waves that are considered to be generated by Tropical Instability Waves propagate their energy from near the surface west of 23°W down- and eastward to eventually reach the mooring location. The distribution of intraseasonal energy depends largely on the dominant frequency and the time, depth, and longitude of excitation with the dominant vertical mode of the Yanai waves playing only a minor role. Observations also indicate the presence of weaker intraseasonal variability at 23°W below 2000 m that is not associated with Tropical Instability Waves.