Global Surface Circulation Derived from Drifting Buoy Data and Its Predictability by Simultaneous Assimilation Model of Drifting Buoy and Altimetric Data
Toshiyuki AWAJI, Yoichi ISHIKAWA, and Kazunori AKITOMO
Department of Geophysics, Kyoto University, Kyoto 606-01, Japan
(awaji@stooge.kugi.kyoto-u.ac.jp)
Abstract
The historical drifting-buoy data from the Marine Environmental Data Service(MEDS) in Canada are analyzed to obtain the global surface circulation on (2° × 2°) grids ((1° × 1°) grids in specific regions). Our buoy-derived velocity map provides more detailed structures of the surface currents than the previous ones. for example, jet-like structures of the western boundary current and the equatorial current are succesfully reproduced. Next, considering the advantage that the buoy-derived velocity can give the absolute SSH, a simultaneous assimilation model of drifting buoy and altimetric data is proposed to determine the mean SSH as well as the temporal evolution of the suface circulation on synoptic scales. The result shows that realistic buoy deployment (32 in a 40° square) can effectively constrain the model variables; that is, both the absolute (mean plus time varying) velocity and SSH (interface depth in 1.5-layer models) fields are significantly improved.Successive correction of the mean SSH is made with the simultaneous assimilation of drifting buoy and altimetric data. Consequently, a better correction of the mean SSH is obtained: the initial error of the mean SSH is reduced by approximately 40% after the 1-year experiment. These results clearly show that simultaneous assimilation of drifting buoy and altimetric data into the dynamical model is a very useful tool for improving the model's realism.
1 Introduction
The velocity measurement of the ocean currents is one of the most important problems in physical oceanography. The complex ocean-atmosphere interactions and their effect on climate will not he fully understood until an accurate estimation of the velocity field of the oceans is realized (Nerem et al., 1990). For example, the heat flux at sea surface is affected significantly by heat transport due to the surface current (Hastenrath and Greischar, 1993).
For observing the surface current, the drifting buoy is one of the most attractive means because it measures the in-situ absolute velocity through tracking of the buoy movement by satellites. With many buoy data that cover the vast geographical extent, the time series of the realistic surface current field could be obtained. However, for the larger scale circulation, there was a large lean in the buoy distribution because of the unsystematic deployment so far. This has limited most studies of ocean circulations based on buoy data to some specific regions, such as the North Atlantic (Richardson, 1983), the Southern Ocean (Patterson, 1985). This problem is quite serious for obtaining the surface current in the regions accompanied by significant horizontal divergence of flow. Poor observations compelled the rather coarse maps of the surface current in most of the previous studies.
