Research Article

# Ocean Surface Winds Drive Dynamics of Transoceanic Aerial Movements

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• jmunoz@rjb.csic.es

Affiliation: Real Jardín Botánico (CSIC), Madrid, Spain

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• Affiliation: Departament de Biologia Animal (Vertebrats), Universitat de Barcelona, Barcelona, Spain

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• Published: August 13, 2008
• DOI: 10.1371/journal.pone.0002928

### Wind support calculation

#### Posted by felix1 on 25 Feb 2009 at 10:33 GMT

As far as I can understand the formula for COST does not properly distinguish between head- and tailwind. The quotient (1/S) goes towards zero with increasing wind speed. Therefore COST will get smaller regardless of the angle (HRMF), hence COST will decrease with increasing wind speed under tailwind AND headwind conditions. I wonder if I overlooked a detail in the formula, othewise the formula is not suited for the purpose mentioned.

### RE: Wind support calculation

#### jmunoz replied to felix1 on 08 Apr 2009 at 11:33 GMT

Sorry for our delay in replaying.

To calculate the cost of travelling over the ocean surface in relation to wind contidions we used an Anisotropic Cost Analysis to create a friction surface. This surface is a grid where the cost to move over each pixel depends on the angle between the movement heading and the azimuth wind as well as the wind speed.
To calculate the cost to move over a pixel, speed is not considered as an isolate element but in conjunction with the angle between wind and movement vectors: tail winds are considered as a help and head winds as an obstacle to the movement, both proportionally to absolute speed in m/s, see figure at

http://www.esnips.com/doc...

In the absence of wind, the cost of moving through the friction surface is set to 31 (arbitrary units) and it is constant with respect to movement heading (i.e. wind do not help neither obstacle the movement). In the case of movement following the exact wind vector azimuth (tail wind, corresponding to 0º between vector flight and vector wind) and maximum wind speed (30 m/s as defined by the sensitivity of the QuikSCAT), we assigned the minimum resistance value of 1 (arbitrary units). Deviations from this minimum cost angle and/or decrease in wind speed are increasingly costly until reching a maximum of 61 (arbitrary units) at an angle of 180º (head wind, opposite to bird flight) and a wind speed of 30 m/s (see above figure). This is the interpretation of the function on PLoS ONE paper; we avoided changes of sign as the ArcInfo “pathdistance” module only works with positive values.

No competing interests declared.

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