Analysis of factors in the freshening trend of the water in the North Pacific through back-tracing using an adjoint model

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A multi-decadal freshening trend is found in the mid-depth layer in a large part of the North Pacific subtropical gyre through the analysis of synthesis data of an ocean model and observations generated by an ocean data assimilation system. In particular, the trend is large in the Kuroshio-Oyashio mixing area east of Japan. The backward tracing of water mass using an adjoint model indicates that freshened water in the mixing intrudes around 160°E into the subtropical gyre after advected by the Kuroshio, and that it results in the freshening of entire the tropical gyre.