Deep cycle turbulence in Atlantic and Pacific cold tongues
2022
Moum, James N. | Hughes, Kenneth G. | Shroyer, Emily L. | Smyth, William D. | Cherian, Deepak | Warner, Sally J. | Bourlès, Bernard | Brandt, Peter | Dengler, Marcus
Multiyear turbulence measurements from oceanographic moorings in equatorial Atlantic and Pacific cold tongues reveal similarities in deep cycle turbulence (DCT) beneath the mixed layer (ML) and above the Equatorial Undercurrent (EUC) core. Diurnal composites of turbulence kinetic energy dissipation rate, ϵ, clearly show the diurnal cycles of turbulence beneath the ML in both cold tongues. Despite differences in surface forcing, EUC strength and core depth DCT occurs, and is consistent in amplitude and timing, at all three sites. Time-mean values of ϵ at 30 m depth are nearly identical at all three sites. Variations of averaged values of ϵ in the deep cycle layer below 30 m range to a factor of 10 between sites. A proposed scaling in depth that isolates the deep cycle layers and of ϵ by the product of wind stress and current shear collapses vertical profiles at all sites to within a factor of 2. Plain Language Summary The equatorial cold tongues are large areas of the ocean that extract a globally disproportionate amount of heat from the atmosphere, and where that heat is mixed downward to the deeper ocean, a critical process in climate regulation. This mixing is dominated by deep cycle (DC) turbulence, a well-documented feature of the central equatorial Pacific cold tongue. Poleward of the Equatorial Undercurrent, nighttime cooling of the sea surface causes increases in turbulence to the depth of the ML, typically a few tens of meters, below which turbulence is much reduced. On the equator, in contrast, opposing currents at the surface and roughly 100 m below the surface create a dynamic environment in which nightly increases in turbulence occur over many tens of meters below the ML base. This has been termed DC turbulence. Here, using massive data sets from both Pacific and Atlantic cold tongues, we show that DC turbulence is present at each location and its main characteristics are consistent between them.
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