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Assimilation of Upper Ocean Data into Numerical Models

 

M. Ikeda (Graduate School of Environmental Earth Science, Hokkaido University, Sapporo, Japan, also Frontier Research System for Global Change, Tokyo, Japan)

email: mikeda@ees.hokudai.ac.jp

Y. Sasai (Graduate School of Environmental Earth Science, Hokkaido University, Sapporo, Japan)

email: sasai@ees.hokudai.ac.jp

 

INTRODUCTION

 

Oceanographic data are collected much more extensively from the surface than subsurface using satellite remote-sensing and ship of opportunity. A data assimilation method is proposed for determining the heat, salt and CO2 contents in the subsurface as well as the air-sea fluxes. The method is tested with a much simpler model, which is chosen to be a vertical 1-D model here, although the ultimata goal is to implement this method into a general circulation ocean model. The period from fall to winter and the subpolar North Pacific Ocean are chosen, in which the ocean interacts most extensively with the atmosphere via mixed layer development. Once the method is established and the results are verified, the information retrieved by this method will be useful for monitoring the global ocean, measuring air-sea heat and CO2 fluxes, and estimating the oceanic uptake of the anthropogenic carbon.

The mixed layer in the North Pacific develops from 20-30m in summer to its maximum depths around 100 to 300m in winter. The general features are deeper winter mixed layer in the Kuroshio extension, and shallower one in the eastern Pacific. The seasonal evolution of pCO2 shows a remarkable difference between the subtropical and the subpolar regions (Takahashi et al, 1993). In the subtropical region, the pCO2 is higher in summer and lower in winter, being attributable to its temperature-dependence. In the subpolar region, the seasonal evolution is opposite to that in the subtropical region. The subpolar evolution can be explained as follows: vertical convection brings highly carbonated water to the mixed layer during mixed-layer development, and then, biological productivity and settlement reduce the total carbon in the mixed layer from spring bloom through entire summer.

 

MIXED-LAYER MODEL

 

During mixed layer development, intense vertical mixing occurs due to static instability. In recent years, a supercomputer enables us to use a numerical ocean model with a very high vertical resolution. However, the high resolution model faces difficulty in the main task to be achieved in the present study; i. e., reconstruction of ocean fields using data assimilation. Since the vertical diffusion coefficient becomes extremely large in the model, it is difficult to solve the equations that are adjoint to governing equations for a continuously stratified model. A bulk (1-layer) mixed-layer model is instead used in this work. The primary properties of the mixed layer are temperature T, salinity S and a mixed-layer thinckness H. The mixed layer receives negative heat flux -Q through the sea surface.

 

 

 

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