Wind Stress and Ekman Mass Transport along CalCOFI lines: 67,70 and 77 by Lora Egley

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Wind Stress and Ekman Mass Transport along
CalCOFI lines 67,70 and 77 by Lora Egley
http//www.mlml.calstate.edu/marinops/marinops.htm
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Scientific Objective

• Analyze shipboard wind data for Leg1
• Compute Surface wind stress and Ekman Mass
  Transport.
• Ekman had hypothesized a momentum balance in the
  ocean surface layer between wind stress and
  Coriolis acceleration and a velocity spiral he
  had never directly observed.

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Forcing
Total Current
Geostrophic flow
Frictional, wind driven (Ekman) Flow
Boundary Conditions
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FORCING
Transport
Wind Stress
Coriolis Force
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The theory
http//www.sp.uconn.edu/geo101vc/Lecture20/sld014
.htm
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Indirect and repeated confirmation of Ekman
layers in the open ocean

• Has only occurred recently
• 1981 Davis et. al. showed a mixed layer
  response to the winds in the northeastern Pacific
  which was Ekman like
• 1987 Price et. al. demonstrated the spiral
  Ekman layer structure in the subtropical North
  Atlantic
• 1995 Chereskin found direct evidence for Ekman
  balance in the California Current

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Ekmans assumptions

• Assumptions
• Steady State
• Closure assumption (Eddy Viscosity constant)
• Vertical Homogeneous ocean
• Away from horizontal boundaries

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Data consisted of

• The SAIL data Wind Direction, Wind Speed,
  Position, Time, Air Temperature, Relative
  humidity, Sea surface temperature, Pressure
• Measurements taken from 2-5 AUG 2001

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Methods

• Utilized Bulk Formulas
• Most difficult part was to get spatially oriented
  so that I could calculate the pertinent component
  along each leg
• Stress calculations based on every data point
• Time averaged stress x and y components hourly
  (30min average at corners of the box)
• Then solved for the hourly mass transport
• Determined distance traveled each hour
• Then solved for the Volume transport

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Calculated Mass Transport using Bulk Formulas
Reference Smith(1988)
Base equations to begin indirect calculations
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Transport relation predicts a net westward
transport for our southward wind stress
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2AUG01 1945-2045 taux.03km/ms2 My0.26m2/s

Hourly average (CalCOFI Line 67)
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Data showing a transport out of box.
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Data showing a transport out of box.
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• Net Volume into the box
• CalCOFI line 670.0027381Sv.
• Net Volume out of the box
• CalCOFI line 70
• -0.16214Sv.
• CalCOFI line 77
• -0.03698Sv.

Total
-0.19912Sv.
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Southwest Transport
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Central Coast of California Northwest Pacific

• Eastern Boundary Current California Current
• Coastal box 225km x 170km

Taux Mass Transport y
0.0027Sv
-0.1621Sv
-0.0370Sv.
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Concerns

• Biggest concern was the time and spatial
  variability
• Wind Direction is highly variable. (Fortunate
  enough to have steady winds towards the Southeast
  for the 4 days)

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Time Series

• Hanning Window
• Filtfilt
• From an autocorrelation function Calculated
  integral time scale 20 hours

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Summary of Results

• Net mass transport predominately to the Southwest
  supporting the Ekman theory.
• Net loss out of the box. (Indicating Upwelling)