New Observations of the Slippery Near-Surface Layer in the Equatorial Indian Ocean

Near-surface ocean measurements from a bow-mounted fast thermistor chain on R/V Revelle during DYNAMO show the details of the evolving structure over the upper 6 m. Spectral fitting over restricted frequency bands of the inertial subrange (to avoid surface wave contamination) permit surprisingly good estimates of turbulence quantities. At night, and during relatively low wind conditions, the turbulence dissipation rate is constant and roughly equal to the surface buoyancy flux, as predicted by Monin-Obukhov similarity scaling, thus validating our estimates of near-surface dissipation, thereby permitting a new perspective on the detailed temporal evolution of the near-surface TKE dissipation.

Suppression of nighttime turbulence occurs immediately with the rising of the sun and ensuing near-surface stratification. Under moderately weak wind conditions, the time scale to reach peak suppression in turbulence at 1.3 m depth is about 2 hours, with a depth-dependent time lag of up to 6 hours to peak suppression at 20 m depth. Thereafter, the turbulence increases, reaching a daytime maximum 3-4 hours after peak suppression. We hypothesize that the increase in daytime turbulence is associated with the buildup of a near-surface diurnal jet formed by the trapping of momentum in a stratifying shallow layer. The time scales for this momentum buildup are estimated based on simple one-dimensional scalings, from which we hope to derive a generalized result.

This daytime high in TKE dissipation persists until the onset of cooling at the surface. A secondary minimum occurs as the temperature gradient can no longer be sustained by surface heating.