Deep Eddy Kinetic Energy in the Tropical Pacific From Lagrangian Floats

Abstract

At the ocean surface, satellite observations have shown evidence of a large spectrum of waves at low latitudes. However, very little is known about 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 of this variability. In this study, we use velocity estimates, provided by Argo float drifts at 1,000 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 an original 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 semiannual 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 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 are discussed, as well as its potential importance for the mean circulation.

From Mixing to the Large Scale Circulation: How the Inverse Cascade Is Involved in the Formation of the Subsurface Currents in the Gulf of Guinea

“…Rossby waves, instead of nonlinear eddies, have been shown to play a significant role in the formation of mean jet-like currents in some studies (see [109] and references therein). Rossby waves can be important to explain other jet-like structures observed in the present simulation, in particular, the western region or deeper low latitude intermediate currents [23,44,45,124].…”

Intra-Annual Rossby Waves Destabilization as a Potential Driver of Low-Latitude Zonal Jets: Barotropic Dynamics

“…Intra-annual waves with large wavenumbers such as the ones described in this study are not unrealistic as they have been observed in the tropical ocean (Farrar 2011;Tuchen et al 2018;Delpech et al 2020b). Farrar (2011) evidenced barotropic intra-annual waves in the northern tropical Pacific ocean, radiating away from the Tropical Instablity Waves with a 30-day period and a 1000-km wavelength.…”

“…The containment of the wavemaker inside a localized forcing region ( ) ( ) is essential to have an unforced region where the different waves can freely propagate without interference by the forcing. It is also more realistic as most intra-annual waves observed in the ocean are found to be locally generated and vary spatially (e.g., Tuchen et al 2018;Delpech et al 2020b). The envelope of this forcing region is defined following Eq.…”

“…Tuchen et al (2018) described evidence of 50-day Yanai waves with a baroclinic structure at the equator. Delpech et al (2020b) described observations of waves at 1000 m in the northern tropical Pacific with a 70-day period and a 500-km wavelength with properties consistent with barotropic Rossby waves. It is interesting to note that the waves observed by Farrar (2011) are found to be stable to NTLI (e.g., Fig.…”

“…One of the major energy sources present in the ocean at low latitude is associated with planetary waves. It has been shown that in the Atlantic and Pacific ocean, a large source of energy in the deep is associated with annual and semi-annual Rossby waves, as well as intra-annual waves: 30-day, 1000-km mixed Rossby-gravity waves, associated with surface Tropical Instability Waves; and short-scale variability, with periods around 70 days and wavelengths around 500 km (Bunge et al 2008;Von Schuckmann et al 2008;Tuchen et al 2018;Lyman et al 2005Lyman et al , 2007Kessler and McCreary 1993;Eriksen and Richman 1988;Farrar and Weller 2006;Farrar 2011;Farrar and Durland 2012;Lee et al 2017;Delpech et al 2020b) (See Fig. 1 for an overview of the spectral characteristics of the observed variability).…”