Wednesday, March 29, 2017

Interpreting the Mixed Layer

(Today's post was written by scientist and co-Principal Investigator Eric Kunze)

As pointed out in a previous blog, in order to follow post-storm restratification of the mixed-layer, we need to simultaneously find a deep mixed-layer, horizontal density gradients which imply vertical shear (through the thermal-wind balance) and storms to homogenize the mixed-layer. At the same time, we need to avoid too much horizontal shear which would act to disperse our swarm of freely drifting profiling floats. This is a tough combination but historical measurements were favorable. We have had no trouble finding density fronts but have been wary of deploying the floats in them because of our concern of finding horizontal shear.  At our first site, stratification extended to the surface with only hints of a previous mixed layer. Furthermore, passing storms from the west tended to veer north of our latitude before reaching our longitude. Moving north, our second site had a mixed-layer thickness of 20 m (when it was supposed to be 100 m) and the storms continued to veer north. We have moved further north, though limited by the strength of the oncoming storms, to find a mixed layer 70-80 m thick at 35N, 139W. 23 floats are now deployed across a narrow density front, all but 2 that we brought along.

To get a better sense of whether the mixed layer was behaving as expected, time-series of various variables have been averaged over the mixed layer for each float depth profile (roughly once an hour). The easiest to explain are the top and bottom panels which show mixed-layer density σθ (top), east velocity u (red) and north velocity v (blue) bottom panel). Density lightens midway through 24 MAR, then undergoes weak diurnal oscillations, lightening during daylight and getting denser at night. The spread of 0.02 is a signature of horizontal variability that is needed for restratification. The velocities show that there is not a single but multiple frequencies of a day or so. The north velocity cresting before east velocity is a signature of low-frequency (1-day period) internal waves. 
Figure showing various characteristics of the mixed layer, including density, velocity, stratification, and shear. (Figure credit: Eric Kunze)
The second panel from the top shows mixed-layer stratification N (green) and vertical shear |Vz| (red).  Before 28 MAR, stratification and shear undergo daily oscillations, strengthening during the day and weakening with occasional very weak N at night. The daylight restratification could be due to the the advection of lighter water over heavier water that we are looking for or warming by the sun. After mid 27 MAR, the stratification increases by a factor of 2 and no longer exhibits nocturnal weakening, a signature of strong restratification. perhaps associated with the larger density contrasts. 

In the third panel from the top, vertical shear is repeated (red) along with horizontal density gradients normalized to mimic the thermal-wind balance that dominates much low-frequency variability in the ocean. Thermal wind balance in essence is a balance between horizontal density gradients and vertical shear.  Strictly speaking, both should be smoothed over more than a day for the balance to hold. The signals are comparable but sometimes horizontal gradients are larger than shear, sometimes small. This comparison can be confounded by wind-driven mixed-layer shear and near-inertial wave vertical shear, both of which can act with or against thermal wind shear. 

So we don't have the answer yet. 
Years of analysis will be needed to combine results from the EM floats shown here with towyo measurements from SWIMS and the air-sea flux buoy.  But we do see a signal of restratification and have enough horizontal coverage to evaluate contributions from advection vs. air-sea fluxes.

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