DRIFTING IN THE WIND - LEEWAY ERROR IN SHIPDRIFT DATA

Much of our knowledge of the large scale pattern of surface velocity i n the oceans comes from historical shipdrift velocity data. One concer n about these data is the possibility of a substantial systematic erro r due to leeway, the downwind sideslip of ships through the water norm al to their steered courses. The leeway error is investigated here bot h theoretically and observationally and its magnitude is estimated for the tradewind region of the North Atlantic. The goal is to correct an d improve the shipdrift velocity data set. Leeway was calculated from the difference between surface currents measured on two intersecting t racklines, one nearly aligned with the mean wind direction, the other nearly normal to the wind. The residual current is ascribed to leeway. The estimated magnitude of leeway was found to be 3.5 +/- 0.4 cm/sec in a region of 5.9 m/s winds. Expressed as a percentage of the mean wi nd velocity this leeway is 0.60 +/- 0.09%. For the general case of shi p lines oriented randomly with respect to the wind, leeway would be ar ound half of the above values. Errors of this size are usually much sm aller than the mean surface currents, except in regions of slow curren ts and fast winds like the North Equatorial Current-Trade Wind region of the North Atlantic. (C) 1998 Published by Elsevier Science Ltd. All rights reserved.