Wind Dependencies of Deep Cycle Turbulence in the Equatorial Cold Tongues

Type Article
Date 2023-08
Language English
Author(s) Moum James N.1, Smyth William D.1, Hughes Kenneth G.1, Cherian DeepakORCID2, Warner Sally J.3, Bourles Bernard4, Brandt PeterORCID, Dengler MarcusORCID
Affiliation(s) 1 : College of Earth, Ocean, and Atmospheric Sciences, Oregon State University, Corvallis, Oregon, USA.
2 : National Center for Atmospheric Research, Boulder, Colorado, USA.
3 : Departments of Physics and Environmental Studies, Brandeis University,Waltham,Massachusetts, USA.
4 : Institut de Recherche pour le Développement, Brest, France
5 : GEOMAR Helmholtz Centre for Ocean Research Kiel, Kiel, Germany f Kiel University, Kiel, Germany
Source Journal Of Physical Oceanography (0022-3670) (Amer Meteorological Soc), 2023-08 , Vol. 53 , N. 8 , P. 1979-1995
DOI 10.1175/JPO-D-22-0203.1
WOS© Times Cited 3
Keyword(s) Tropics, Atmosphere-ocean interaction, Diapycnal mixing, Diurnal effects, Mixed layer, Mixing
Abstract

Several years of moored turbulence measurements from xpods at three sites in the equatorial cold tongues of Atlantic and Pacific Oceans yield new insights into proxy estimates of turbulence that specifically target the cold tongues. They also reveal previously unknown wind dependencies of diurnally varying turbulence in the near-critical stratified shear layers beneath the mixed layer and above the core of the Equatorial Undercurrent that we have come to understand as deep cycle (DC) turbulence. Isolated by the mixed layer above, the DC layer is only indirectly linked to surface forcing. Yet, it varies diurnally in concert with daily changes in heating/cooling. Diurnal composites computed from 10-min averaged data at fixed xpod depths show that transitions from daytime to nighttime mixing regimes are increasingly delayed with weakening wind stress t. These transitions are also delayed with respect to depth such that they follow a descent rate of roughly 6 m h-1, independent of t. We hypothesize that this wind-dependent delay is a direct result of wind-dependent diurnal warm layer deepening, which acts as the trigger to DC layer instability by bringing shear from the surface downward but at rates much slower than 6 m h-1. This delay in initiation of DC layer instability contributes to a reduction in daily averaged values of turbulence dissipation. Both the absence of descending turbulence in the sheared DC layer prior to arrival of the diurnal warm layer shear and the magnitude of the subsequent descent rate after arrival are roughly predicted by laboratory experiments on entrainment in stratified shear flows. SIGNIFICANCE STATEMENT: Only recently have long time series measurements of ocean turbulence been available anywhere. Important sites for these measurements are the equatorial cold tongues where the nature of upper ocean turbulence differs from that in most of the world's oceans and where heat uptake from the atmosphere is concentrated. Critical to heat transported downward from the mixed layer is the diurnally varying deep cycle of turbulence below the mixed layer and above the core of the Equatorial Undercurrent. Even though this layer does not directly contact the surface, here we show the influence of the surface winds on both the magnitude of the deep cycle turbulence and the timing of its descent into the depths below.

Full Text
File Pages Size Access
Publisher's official version 17 3 MB Open access
Top of the page

How to cite 

Moum James N., Smyth William D., Hughes Kenneth G., Cherian Deepak, Warner Sally J., Bourles Bernard, Brandt Peter, Dengler Marcus (2023). Wind Dependencies of Deep Cycle Turbulence in the Equatorial Cold Tongues. Journal Of Physical Oceanography, 53(8), 1979-1995. Publisher's official version : https://doi.org/10.1175/JPO-D-22-0203.1 , Open Access version : https://archimer.ifremer.fr/doc/00882/99402/