Diurnal cycles of current, temperature, and turbulent dissipation in a model of the equatorial upper ocean

Abstract

We have used a simple, one‐dimensional model to simulate the diurnal cycle of the equatorial upper ocean. The model is initialized with the stratification and shear of the Equatorial Undercurrent (EUC) and is driven with heating and wind stress. A surface mixed layer is determined by bulk stability requirements, and a transition layer below the mixed layer is simulated by requiring that the gradient Richardson number be no less than ¼. A principal result is that the nighttime phase of the diurnal cycle is strongly affected by the EUC, resulting in deep mixing and large dissipation at night consistent with observations of the equatorial upper ocean during the 1984 Tropic Heat experiment. The day time (heating) phase of the simulated diurnal cycle is very similar to that seen at mid‐latitudes. Solar heating produces a stably stratified surface layer roughly 10 m thick within which there is little, O(3 × 10−8 W kg−1), turbulent dissipation. For the typical range of conditions at the equator, diurnal warming of the sea surface is 0.2°–0.5°C, and the diurnal variation of surface current (diurnal jet) is 0.1–0.2 m s−1, consistent with observations. The nighttime (cooling) phase of the simulated diurnal cycle is quite different from that seen at mid‐latitudes. As cooling removes the warm, stable surface layer, the wind stress can work directly against the shear of the EUC. This produces a transition layer that can reach to 80 m depth, or nearly to the core of the EUC. Within this layer the turbulent dissipation is quite large, O(2 × 10−7 W kg−1). Thus the simulated dissipation has a diurnal range of more than a factor of 5, as observed in Tropic Heat, though the diumal cycle of stratification and current are fairly modest.

Diurnal Shear Instability, the Descent of the Surface Shear Layer, and the Deep Cycle of Equatorial Turbulence

“…Results from the previous section reinforce the idea (e.g., Price et al 1986;Schudlich and Price 1992;Pham and Sarkar 2010) that the diurnal descent of the wind-driven mixed layer is driven by shear instability. In what follows, we will test that hypothesis further by means of an explicit linear stability analysis of the observed flows.…”

“…It has long been suspected that the development of the nocturnal mixing layer can be initiated by shear instability such as would result from the descending shear layer (e.g., Price et al 1986). That idea has been applied in simulations of nearsurface equatorial mixing (Schudlich and Price 1992), but has not been tested via explicit stability analysis. That analysis is now possible thanks to two new developments: 1) the aforementioned high-resolution velocity profiles and 2) a new instability theory that takes account of the role of turbulence.…”

Temperature Advection by Tropical Instability Waves

“…Thus, shear instability, buoyancy forcing, and wind stirring mutually affect each other, and because of this nonlinearity (especially near the equator) it is not possible to single out the contributions of the individual processes. This is discussed in detail in Schudlich and Price (1992). Chen et al (1994) estimated the empirical parameters of the mixed layer model from local synoptic data, but for the present context of TIWs, it is not clear whether these parameter values are still appropriate.…”

Physics of diurnal warm layers : turbulence, internal waves, and lateral mixing

“…PWP however, does not explicitly solve for turbulent kinetic energy dissipation. Additional calculations are done following Schudlich and Price (1992) regions such as the Sargasso Sea (Jerlov, 1976). A 9-band extinction parameterization was also used for the model runs (Paulson and Simpson, 1981) and it does change the structure of the near-surface temperature.…”