Journal of Oceanography, Vol. 53 (No. 5), pp. 489-516, 1997
Yoichi Ishikawa, Toshiyuki Awaji and Kazunori Akitomo
Department of Geophysics, Kyoto University, Kyoto 606-01, Japan
(Received 25 March 1996; in revised form 28 June 1997; accepted 7 July 1997)
Abstract: A dataset derived from 1864 drifting buoys archived at the MEDS (Marine Environmental Data Service) in Canada and the JODC (Japan Oceanographic Data Center) are analyzed to obtain the global surface circulation and its kinetic energy distribution on a 2° × 2° grid system (1° × 1° grids in specific regions). The current distribution presents more realistic features of the global surface circulation at meso- to large scales than previously obtained. For example, jet-like structures within the western boundary current and the equatorial current, which have never been resolved in the previous studies with coarser grids, are successfully reproduced. Moreover, the conspicuous seasonal cycle of surface current, such as drastic flow reversal, is captured in the Indian Ocean and the equatorial regions. A large mean kinetic energy (>1000 cm2s-2) appears in the equatorial current, the western boundary current and its extension, and in the Antarctic Circumpolar Current region. In these regions, the eddy kinetic energy is comparable to or larger than the mean kinetic energy, suggesting strong interactions between mean flow and eddies. When the energy distribution is compared with that from eddy resolving general circulation models, both synoptic views of the mean fields are found to be in fairly good agreement. In the eddy field, however, the energy level in the models is much smaller (by a factor of 5) than the buoy-derived energy level. This result is basically consistent with the previous studies using altimeter data, but the energy level obtained from drifting buoy data is larger than that from altimeter data. The difference here is due to the relatively long sampling intervals of altimeter observations and disregard of the ageostrophic component in calculating altimeter-derived velocities. These results show that the surface circulation field derived from drifting buoys provides useful information which can be used to make models more realistic.