Winds vs' Currents An Analysis of Wind Forcing

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Winds vs. Currents An Analysis of Wind
Forcing

• Presented by
• ENS Rebecca Wolf
• 16 March 2007

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Project Objective

• -Real time analysis of wind forcing on current
  circulation

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Presentation Guidelines

• Experiment
• Background
• Analysis
• Conclusion

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Experiment

• Adaptive Sampling and Prediction System
• 2 ADCP moorings
• Aug 2006

ADCP 1 (56m)
ADCP 2 (92m)
meters
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Hardware
http//www.mbari.org/aosn/MontereyBay2003/Images/a
osn_assets/m1_close_400.jpg
www.rdinstruments.com/datasheets/wh_sentinel.pdf
MBARI M2
Workhorse Sentinel ADCP in mooring
Underwater ADCP
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Wind Forcing

• Wind forcing in terms of upwelling and relaxation
  events appears to be responsible for coastal
  circulation within the Monterey Bay.

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Upwelling Events

• 2-D Process
• The first dimension is the winds.
• Equatorward winds blow warm surface water
  offshore
• Colder bottom water starts to rise to the surface
  near the coast ? relocates the warmer surface
  water offshore.
• The second dimension is the winds effects on the
  circulation.
• Cross shelf temperature and density gradients at
  surface/ sloped upwards to the coast

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Relaxation Events

• A relaxation event is defined as a phenomenon
  when there is no significant wind.
• During typical relaxation events described in
  previous studies warm water remains farther off
  shore and does not provide significant surface
  heating.
• In the northern California shelf region
  relaxation events occur most frequently
  throughout the spring-summer upwelling season.

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What is causing the action?
is it the wind?
Poleward flow
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Visual Analysis

• How to interpret the following plots
• Winds
• Relaxation events are positive
• Upwelling events are negative
• ADCP (current flow)
• Poleward current flow is positive
• Equatorward current flow is negative
• Relaxation events ?poleward current
• Upwelling events?equatorward current

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Indications of strong equatorward and poleward
flow.
Poleward flow
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R
R
R
Strong flows _at_ depth
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Observations

• 4 day Relaxation period
• Poleward flow (north )? Evident with no wind
• Stronger correlations in beginning than end of
  time series

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Do the numbers work?
Correlation Values
Autospectra
Coherence Phase
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Cross Correlation Values for Winds and ADCP
data
Short Time Series (1 month) ? Small DOF? High
Significance Level?Less Correlation
-No evident correlation -As expected evidence of
Ekman turning (50m)
Better analysis look at spectra
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Spectra Analysis

• Autospectra / Coherence Phase
• Detrend method? Linear detrend
• 5 Overlapping pieces ?1/3 total points
• Hourly Sample Interval
• Confidence Interval of 95

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ADCP 2
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Observations

• During rotation energy was forced into the along
  shelf component
• Across Shelf is smaller by order of magnitude
  (104 vs 103)
• Low pass energy points up in Along Shore/down in
  Across Shore?more low freq energy in along shelf
• Dirunal and semi diurnal tidal peaks
• Along shelf/Near bottom currents?distinct
  inertial peaks
• Wind generated peak primary freq _at_which wind
  pumps energy into ocean 20hrs

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Wind Spectra
Sea breeze
-More energy in along shelf ? result of
rotation -Diurnal sea breeze peak present
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ADCP 2
2 coherent points
consistent phase coherence
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• Across Shelf
• No coherence
• Period to small need 5-10 days for sufficient
  current spin up from wind
• Positive phase indicates wind leads current
• Along Shelf
• Coherence observed at all three depths
• 5-10 day period with expected lag times
• Current follows the wind
• Similar phase angles and lag times

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No coherence
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One more consideration Along Shelf Wind
Across Shelf Current

• No coherence in ADCP 1
• Along Shelf
• Coherence observed surface and near bottom
  currents
• No coherence in mid depth due to Eckman dynamics

Current
45º
Wind
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Conclusions

• Correlations do not show concrete local wind
  forcing
• Coherence found in along shelf components not
  across shelf components
• Currents appear to be forced by non local events
  (eddies, meanders, along shore pressure gradient)

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Future Research/
Military Applications

• Data comparison with model outputs
• Regional Ocean Modeling System
• Harvard Ocean Prediction System
• Navy Coastal Ocean Model
• Decipher the differences in model accuracy and
  prediction.
• Accurate current prediction by ocean models will
  benefit amphibious operations, special warfare
  operations, and mine warfare in the littoral zone.

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Resources

• MATLAB
• Dr. Steve Ramp
• Fred Bahr
• Korso, P.M., 1987 Structure of the coastal
  current field off northern California during thr
  coastal ocean dynamics experiment, Journal of
  Geophysical Research,92,1637-1654.
• Ramp, S.R. , J.D. Paduan, I. Shulman, J. Kindle,
  F.L. Bahr, and F. Chavez ,2005 Observations of
  upwelling and relaxation events in the northern
  Monterey Bay during August 2000, Journal of
  Geophysical Research, 111, C07013,
  doi10.1029/2004JC002538.
• Send, U., R.C. Beardsley, C.D. Winant, 1987
  Relaxation from upwelling in the coastal ocean
  dynamics experiments, Journal of Geophysical
  Research, 92,1683-1698.

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Questions ?