Deep Eddy Kinetic Energy in the tropical Pacific from Lagrangian Floats

– Audrey DELPECH – LEGOS, Toulouse, France –

Abstract : At the ocean surface, satellite observations have shown evidence of a large spectrum of waves at low latitudes, responsible for a large part of the surface variability. However, very little is known on the existence and properties of the deep variability. Most of the subsurface observations rely on localized measurements, which do not allow for a global estimation. In this study, we use the velocity estimates, provided by the Argo floats drifts at 1000 m, to analyze the spatial and temporal distribution of the deep Eddy Kinetic Energy (EKE) and its spectral signature with an unprecedented time and space coverage. In the tropical Pacific, high EKE is found along the Equator, at the western boundary and poleward of 7°N. EKE meridional distribution is also found to vary at the scale of the meridionally-alternating mean zonal jets: it is higher inside eastward currents. We develop a statistical scale analysis to determine the temporal and spatial scale-dependence of this deep EKE footprint. We show the presence of periodic features whose characteristics are compatible with theoretical Equatorial waves dispersion relations. Annual and semi-annual Rossby waves are observed at the Equator, as well as 30-day Yanai waves, consistent with surface Tropical Instability Waves. The location and intensification of these waves match the downward energy propagation predicted by the ray tracing linear theory. Short-scale variability (with ~70-day periods and 500-km wavelength) has also been detected poleward of 7°N. The generation mechanisms of this variability is discussed, as well as its potential importance for the mean circulation.