Current Status and Future Plan for the FRSGC Global Ocean Data Assimilation System
Toshiyuki Awaji, K. Baba, T. Niinomi, K. Horiuchi, N. Sugiura, K. Takahashi, and S. Masuda
Frontier Research System for Global Change (FRSGC)
Tokyo, Japan
Climatological state of a global ocean. This will enhance progress in the research of climate system recent satellite and in-situ observations clearly show that heat capacity of the ocean is approximately 1000 times that of the atmosphere and that meridional heat transport carried by ocean circulation is comparable to that of the atmosphere. Thus, accurate estimates of ocean circulations and their variabilities are essential to strengthen our understanding of the physical processes driving climate variations. In particular for variablities with time scales longer than a season, a better description of the ocean state becomes desirable since the ocean retains the "memory" of disturbances much longer than the atmosphere. Because of the difficulties involved in making in-situ observations covering the entire ocean and of the limitation of satellite remote sensing to depths close to the sea surface, observational data available for studies of ocean circulation processes remain very limited in space and time. For these reasons, a data assimilation approach, in which observational data are incorporated into numerical models in order to obtain a complete dataset using assimilation techniques, is as attractive prospect for modern oceanography. A considerable amount of works has already been done in the area of assimilation schemes and models. However, some important aspects of the ocean circulation, particularly in subsurface layers, are not represented well even in current sophisticated assimilation systems, and therefore need improving. The variational adjoint method is the most likely mean of making this improvement (Tziperman et al., 1992; Marotzke and Wunsh, 1993), and some experiments along these lines have already been attempted (Schiller, 1995; Yu and Malanotte-Rizzoli, 1998). However, these studies used regional models and were focussed on the Atlantic circulations, often using an approximate adjoint, mainly because of the huge computational load. Thus the important roles of throughflow at open boundaries and its relation to large-scale circulations was missed in these ealier assimilation studies. By using a free-surface ocean general circulation model and the variational adjoint method, our FRSGC group constructs a global ocean data assimilation system capable of providing a comprehensive 4-dimensional data set with high accuracy and good dynamical consistency from many of the available observational data. The assimilation system will be soon applied to an identical twin experiment to define the dynamics and in the mechanisms driving the global-scale ocean circulation.
