Coupled Atmospheric and Oceanic Effects on Mixed Layer Depth Variability

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Coupled Atmospheric and Oceanic Effects on Mixed
Layer Depth Variability

• Operational Oceanography
• OC/MR3570
• William Swick

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Project Goals

• To further understanding of the role of air-sea
  influences on the oceanic mixed layer.
• Navy Impacts
• USW
• Sound Velocity Profile

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Description of Data

• Station S2 (PC Tides Model Run and CTD Timeseries
  Location
• Rain Gauge Stations

• Conductivity, temperature and depth (CTD)
• 29 hr timeseries (02 03 Feb)
• UDAS
• Wind Speed
• Air Temp
• Humidity
• Solar Radiation
• Sea Surface Temperature
• PC Tides
• 0900L 02 Feb - 1400L 03 Feb
• Rain Gauge
• Six Stations

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Coupled Atmosphere-Ocean Effects

• Surface stress provided by wind forcing
• Buoyancy forcing related to the thermal exchanges
  between atmosphere and ocean
• Salinity Fluxes related to precipitation and
  evaporation.
• Vertical motion at the Mixed Layer Depth (-h)
• Internal Waves

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Ocean

• Large Contributor to Variability
• Amplitude increases with depth
• Internal Wave 20m Amplitude
• Tidally Forced Internal Wave
• Phase Lag
• Internal wave Experiment
• ITEX1 (Apr 94)

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Are the atmospheric effects negligible compared
to such a significant internal wave???
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Atmospheric Effects

• Heating
• Qo () cooling
• Qo (-) heating
• Wind
• Rain

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Entrainment

• we () means mixing of water from below deeping
  the mixed layer
• we (-) is not physical. Water cannot be unmixed

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Tale of two regimes

• Deepening regime
• Only positive or zero
• Krauss and Turner Mixed Layer Depth Equation only
  holds for deepening regimes.
• Atm-Ocean coupling
• Salinity and Temperature Jumps
• As h decreases. Entrainment increases

• Shallowing Regime
• How it works physically

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.04
.07
Negative effect
Positive effect
Which is dominant?
Salinity dominates Temperature
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• Mixed Layer is shallowing due to
  precipitation!!!
• Atmospheric conditions predict the mixed layer
  depth

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Shallowing Regime
Using ships winds, calculated cooling rate, and
observed mixed layer depth we can calculate the
estimated rain rate.
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Rain rate comparision

• Wind 5 m/s
• Cooling rate 25 W/m/m
• Observed mixed layer depth 16m
• Calculated rain rate per hour 0.37 cm/hr

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Is that the answer?

• Dynamic Problem
• Advection/River Input
• Function of (Wind, Total Surface Heat Flux, Mixed
  Layer Depth)
• Rain rate variability
• Spatial/Temporal
• What happened after 1900L?

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• Precipitation rate has slowed allowing
  entrainment.
• Mixed Layer is in a deepening regime
• Ocean inputs provide for a rapid Mixed Layer
  Deepening

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Conclusion

• Mixed Layer Depth Variability
• Two Regimes
• Shallowing
• Atmospheric Inputs determine shallowing
• Underlying Ocean T-S profile is not felt
• Vertical Motion at h is still important
• Deepening
• Atm-Ocean are strongly coupled
• Highly variable
• Atm (Wind/Heat/Rain)
• Ocean (Salinity and Temperature Jumps, Vertical
  Motion at h)
• In a deepening regime, we(), in an area like
  Monterey Bay (large internal waves) the vertical
  motion will tend to dominant.
• During a shallowing regime the atmospheric inputs
  determine mixed layer depth along with the
  internal wave component.

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Questions?
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References

• Garwood, R.W., Jr. An oceanic mixed layer model
  capable of simulating cyclic states. J. Phys.
  Oceanogr., 7, 455-471, 1977.
• Garwood, R.W., Jr. Enhancements to deep turbulent
  entrainment. In Deep Convection and Deep Water
  Formation in the Ocean, Ed. by P. C. Chu and J.
  C. Gascard, Elsevier, 189-205, 1991.
• Kraus, E.B. and J.S. Turner. A dynamic
  thermodynamic sea ice model. Tellus, 19, 98-105,
  1967.
• Rosenfeld, L.K., J.D. Paduan, E.T. Petruncio, and
  J.E. Goncalves, 1999 Numerical simulations and
  observations of the internal tide in a submarine
  canyon. Proceedings, 'Aha Huliko'a Hawaiian
  Winter Workshop, University of Hawaii at Manoa,
  January 19-22.