Evolution of Turbulence in the Diurnal Warm Layer

Abstract

The daily evolution of temperature, stratification, and turbulence in the diurnal warm layer is described from time series measurements at low to moderate winds and strong insolation in the equatorial Indian Ocean. At 2.0-m depth, turbulence dissipation rates (ε) decreased by two orders of magnitude over 1–2 h immediately after sunrise, initiated by stratification caused by penetrating solar radiation prior to the change in sign of net surface heat flux from cooling to warming. Decaying turbulence preceded a period of rapid growth, in which ε increased by two orders of magnitude over a few hours, and following which ε approached a daytime period of near-steady state. Decay and growth rates predicted by a simplified turbulence model are consistent with those observed. During the daytime period of near-steady state, asymmetric temperature ramps were associated with enhanced ε, supporting the interpretation that this period represents a balance between buoyancy and shear production associated with a shear-driven response to trapping of momentum within the diurnal warm layer.

Wind Limits on Rain Layers and Diurnal Warm Layers

“…Similar to the results from previous studies, DWLs consistently formed on days when Q Net < À600 W m À2 (heating the ocean) and U 10 ≤ 7 m s À1 (Figures 5 and 6; Asher et al, 2015;Matthews et al, 2014;Moulin et al, 2018;Webster et al, 1996). DWLs were not observed when one or both of these conditions were not met, such as on 13 October in the suppressed period, on 19, 24, and 25 October, and on 22 November during disturbed and active periods, and during all WWB days ( Figure 6).…”

“…These data were interpolated to a 1‐m vertical grid. When the ship was headed into a steady mean eastward surface current, the thermistor chain provided a record of T measurements undisturbed by the ship's wake (Moulin et al, ). In comparison, Chameleon profiling from the stern as the ship remained stationary and pointed into the steady eastward current.…”

Rain and Sun Create Slippery Layers in the Eastern Pacific Fresh Pool

“…Decay of turbulence under temperature- and salinity-stratified capping layers is a known phenomenon, observed, among others, by Brainerd and Gregg (1993) , Smyth et al (1997) , Sutherland et al (2016) , and Moulin et al (2018) . Despite some differences between the heat- and rain-induced capping cases (the latter tend to be more abrupt), these previous observations produced similar e-folding timescales of turbulence decay on the order of tens of minutes.…”

“…On clear days, robust thermal stratification is created via solar heating, only to be mixed away during the period of night-time cooling and ensuing convective mixing. Therefore, the EPFP undergoes typical tropical diurnal warm layer cycling (e.g., Moulin et al, 2018 ), but with an added complication of pronounced rainfall effects. Resulting variability of upper-ocean stratification can be expected to have a profound effect on surface boundary layer turbulence and, by extension, on Ekman layer development.…”

Capturing Fresh Layers with the Surface Salinity Profiler

“…Therefore, much of our detailed knowledge about the impacts of rainfall on the upper meter of the ocean comes from laboratory experiments (Ho et al, 2004;Zappa at al., 2009;Harrison et al, 2012) and modeling (Soloviev et al, 2015;Drushka et al, 2016;Bellenger et al, 2016). These studies have been supplemented by a limited number of field observations that have characterized rain impacts on the upper few meters of the ocean surface (e.g., Price et al, 1979;Soloviev and Lukas, 1997;Wijesekera et al, 1999;Asher et al, 2014a;Walesby et al, 2015;Moulin et al, 2018;Thompson et al, 2019;ten Doeschate et al, in press). The consistent results from these laboratory, modeling, and field studies indicate that rainfall forms layers of stably stratified fresher water on the order of 1 m to 10 m thick at the ocean surface, and that these "fresh layers" can persist from minutes to hours, depending on ocean surface mixing due to wind forcing, convective overturning, and internal and surface waves.…”