Ekman Divergence From Shipboard Wind Measurements

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Presentation transcript:

Ekman Divergence From Shipboard Wind Measurements Tracey Delk, LT 16 Mar 2004

Outline Objective Background Data Methods Results

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

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

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

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

Background: Ekman Transport Assumptions Steady-state flow Mass-transport is only a function of wind-stress at the surface and Coriolis

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

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

U and V Components

Methods Calculated wind stress at surface Used Dr. Garwood’s 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

Averaged Winds Along Track

Methods Calculated Ekman Transport components MEy = -τx/f MEx = τy/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

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

Results Positive Transport A: southward transport B C Positive Transport A: southward transport B: eastward transport C: northward transport y’ x’

Results

Results

Results

Results Decimated value Smoothed value 0.4046 Sv (106 m3 /s)

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.