Diurnal restratification and turbulence in the oceanic surface mixed layer: 2. Modeling

Abstract

We find that daytime restratification of the remnant layer is important to modeling both the decay of convective turbulence during the day and convective deepening the following night. Penetrating solar insolation accounted for about 60% of the observed restratification of the remnant layer. Of the other processes supplying the remaining restratification, we believe lateral advection is the most important and sets a limit on the capabilities of one-dimensional mixed-layer models. In the morning, with the end of convective forcing, there was an initial period lasting nearly an hour, similar to the convective time scale, during which ½ in the remnant layer remained nearly constant. After that, turbulence in the remnant layer could be modeled in accordance with a balance between e and the storage term for turbulent kinetic energy (TKE). Energy storage is computed using q2 = (el/C)2/3, where I = 0.84Lo, the Ozmidov scale, matches observed overturning scales during most of the decay. Calculating Lo for the observed linearly increasing stratification gives a modified exponential form for the decay. A value of C = 0.04 gives the best fit. This decay lasted about 5 hours, until (e) reached • 5 x 10 -lø W kg -1. Then for the remainder of the restratification period, dissipation and production of TKE due to vertical shear appeared to be approaching a balance.

Turbulence and stratification on the Tropical Ocean‐Global Atmosphere‐Coupled Ocean‐Atmosphere Response Experiment microstructure pilot cruise

“…[1986] to a site 800 km west of Point Conception, California, used the much shorter attenuation lengths characteristic of Jerlov Type IA water [Brainerd and Gregg, 1993b]. It also appears that strong variability in surface forcing on length scales of a few kilometers due to squalls contributes significantly to restratification through the relaxation of lateral density variations into stratification.…”

“…To explore the effect of rainfall, we applied the PriceWeller-Pinkel mixed layer model (PWP) [Price et al, 1986] to the observations at 0øN in much the same way we earlier applied it to the diurnal cycle at midlatitudes [Brainerd and Gregg, 1993b]. Initializing with a nighttime profile during deep convection, we forced the model for 12 days with the cruiseaveraged daily meteorological cycle (with rainfall set to zero).…”

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

“…However, TKE dissipation values observed with salt fingers are orders of magnitude below the observed values of the SPURS-I field campaign within DWLs. Brainerd and Gregg (1993b) discuss that because Weller and Price (1988) did not find a minimum wind speed necessary for the development of Langmuir circulation, they may be responsible for some mixing in the SBL. By examining spatial scales less than a kilometer, Farrar et al (2007) have also shown that internal waves can modulate the mixing of the DWL and impact diurnal warming.…”

Similarity scaling of turbulence in a temperate lake during fall cooling

“…Provided there is adequate depth between the newly formed diurnal thermocline and the seasonal stratification, an initially well-mixed intermediate region is present that is isolated from wind shear. Brainerd and Gregg [1993] referred to this region as the remnant layer and found that, in the ocean, dissipation within this region decayed gradually over a period of 4 h from night time values. When wind speed increases near midday, z MO deepens below z AML .…”

“…Depending on the strength of the diurnal stratification, the intensity of night time cooling and wind stress, z AML , may or may not reach the top of the seasonal thermocline. Limnological studies quantifying the diel variability of turbulence and thermal stratification within the upper water column include Imberger [1985], Jonas et al [2003], Anis and Singhal [2006], and Pernica et al [2014], and oceanographic studies include Shay and Gregg [1986], Brainerd and Gregg [1993], and Anis and Moum [1994].…”