Water following characteristics of Global Drifter Program drifters with and without subsurface float

In the Northeast Pacific (40-50°N, 135-180°W), the angle between the wind friction velocity (u*) and the current observed by drifters with subsurface float (Global Drifter Program Version 1; GDP-V1) was found to be consistently 10-15° larger at all frequencies than the angle observed by drifters without subsurface float (Global Drifter Program Version 2; GDP-V2). To investigate the cause, cross-spectral analysis and vector regression between the wind and current were performed after carefully screening drifter tracks to study the wind-driven currents observed by drifters with different configurations. Vector regression analysis between the wind and current revealed that the angle of wind-driven current observed by GDP-V1 drifters was 10-13° larger for u* < 1.5 cm s−1 compared to that observed by GDP-V2 drifters. One possible explanation for a smaller angle between wind and current from drifters without subsurface float is the shallowing of observed depth due to the shrinking of the holey sock drogue induced by surface wave action. The depth of the current observed by GDP-V2 drifters during the winter was estimated using the observed angle and the e-folding depth calculated from the angle at 15 m by GDP-V1 drifters. In the winter, the mean depth of the wind-driven current observed by GDP-V2 drifters, which have been deployed since the early 2000s, was approximately in the range of 8-10 m depending on the estimation of the e-folding depth either from the angle change or from the amplitude decay in the Ekman layer. Except for the friction velocity exceeding 1.5 cm s−1, a nearly constant amplitude between surface current and friction velocity at all friction velocity ranges is another finding in our study.