Ekman Divergence From Shipboard Wind Measurements

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Ekman Divergence From Shipboard Wind Measurements

• Tracey Delk, LT
• 16 Mar 2004

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Outline

• Objective
• Background
• Data
• Methods
• Results

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Objective

• Calculate Ekman Volume Transport within the area
  encompassed by the track for Leg 1 from R/V Point
  Sur wind measurements.

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Background Ekman Transport

• Ekman transport is a wind driven circulation
• Momentum from winds transferred to ocean by
  friction
• Yields a relationship between wind stress and
  depth-integrated flow

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Background Ekman Transport

• Wind-driven component of transport in the Ekman
  layer is directed perpendicular to the mean wind
  stress
• Function of balance of Coriolis and friction
• To the right (left) in the Northern (Southern)
  Hemisphere

Wind Stress
Ekman Transport
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Background Ekman Transport

• Flow in Ekman layer transports mass
• Ekman Transport (ME)
• Integral of Ekman velocity from surface to Ekman
  depth (kg/m/s)
• Volume Transport (Q)
• ME divided by water density and multiplied by the
  width perpendicular to the transport

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Background Ekman Transport

• Assumptions
• Steady-state flow
• Mass-transport is only a function of wind-stress
  at the surface and Coriolis

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Data

• 20 s wind speed and direction measurements
• From port and starboard anemometers
• 16.8 m above the sea surface

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Methods

• Vector-averaged port and starboard measurements
• Filtered wind data 2 ways
• Decimate
• Resamples data after low-pass filtering
• Resampled to hourly averages
• Smoothed
• Filtfilt function with Hanning window
• Zero-phase forward and backward filtering
• Sampled value for every hour

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U and V Components
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Methods

• Calculated wind stress at surface
• Used Dr. Garwoods MATLAB routine for calculating
  fluxes (Large and Pond model JPO 1992)
• Required other variables in addition to winds
• Barometric pressure
• Humidity
• Air and Sea Temp

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Averaged Winds Along Track
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Methods

• Calculated Ekman Transport components
• MEy -tx/f
• MEx ty/f
• Rotated to a local coordinate system in order to
  calculate transport relative to the box
• X-direction parallel to northern and southern
  boundaries
• Y-direction parallel to western boundary

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Methods

• Calculated Volume Transport (Q) based on the
  following sign conventions
• transport into the box (CONV)
• - transport out of the box (DIV)

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Results

• Positive Transport
• A southward transport
• B eastward transport
• C northward transport

y
x
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Results
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Results
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Results
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Results

• Decimated value
• 0.4046 Sv (106 m3 /s)
• Smoothed value
• 0.39463 Sv

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References

• Pond, S. and G. L. Pickard (1983) Introductory
  Dynamical Oceanography 2nd Ed.,
  Butterworth-Heinemen.
• Tomczak, M. and J. S. Godfrey (2003) Regional
  Oceanograpy an Introduction 2nd Ed., Pergamon
  Press.
• Stewart, R. H. Introduction to Physical
  Oceanography (2003), Open Source Text Book.