A relation between geostrophic current speed and drifting speed of surface drifter.

Abstract

Drifting speed (Udrm) of surface drifters are compared with geostrophic current speed (UG) at the sea surface referred to 1000db, 2000db and 3000db surface in the Kuroshio area south of Japan. Udrltt and UG are in the positive correlation and the gradient of regression line are 0. 66 (1000db), 0. 68 (2000db) and 0. 80 (3000db), respectively. Udrlft is about 20cm/s larger than UG on the average, and differences between them seem not to depend on the differences of observed period nor on the drifting duration near the stations for Nansen or CTD casts. Centrifugal force’s effect caused by the curvature of the drifter movement is checked to be negligibly small. Exclusion of five extreme cases where the difference between Udr1rt and UG referred to 1000db are greater than 40cm/s makes an even better correlation ; the gradient of regression being 0. 96 (10000db), 0. 99 (2000db) and 1. 00 (3000db), respectively. This fact strongly suggests that mean current field in the Kuroshio area is under the geostrophic balance.