Journal of Oceanography, Vol. 59 (No. 6), pp. 931-943, 2003
Toshiyuki Awaji1,2*, Shuhei Masuda2, Yoichi Ishikawa1, Nozomi Sugiura2,Takahiro Toyoda1 and Tomohiro Nakamura1
1Department of Geophysics, Graduate School of Science, Kyoto University, Kyoto 606-8502, Japan
2Frontier Research System for Global Change, Yokohama 236-0001, Japan
(Received 15 March 2003; in revised form 18 August 2003; accepted 20 August 2003)
Abstract: A four-dimensional variational data assimilation system has been applied to an experiment to describe the dynamic state of the North Pacific Ocean. A synthesis of available observational records and a sophisticated ocean general circulation model produces a dynamically consistent dataset, which, in contrast to the nudging approach, provides realistic features of the seasonally-varying ocean circulation with no artificial sources/sinks for temperature and salinity fields. This new dataset enables us to estimate heat and water mass transports in addition to the qualification of water mass formation and movement processes. A sensitivity experiment on our assimilation system reveals that the origin of the North Pacific Intermediate Water can be traced back to the Sea of Okhotsk and the Bering Sea in the subarctic region and to the subtropical Kuroshio region further south. These results demonstrate that our data assimilation system is a very powerful tool for the identification and characterization of ocean variabilities and for our understanding of the dynamic state of ocean circulation.