Subsurface structure of the Pacific Decadal Oscillation and its relation to the mode waters

We investigated the three-dimensional structure of monthly temperature anomalies related to the Pacific Decadal Oscillation (PDO) and its relation to the mode waters in the North Pacific, by using a 55-year long ocean state estimation data set, ESTOC, which was produced applying the four-dimensional variational (4D-VAR) data-assimilation approach based on the strong constraint formalism, and thus is dynamically self-consistent. It is shown that ESTOC better represents both the PDO structure at the sea surface and the characteristics of the mode waters in the North Pacific, when compared with its first guess field. Subsurface temperature anomalies are also modified through data assimilation procedure, and it is likely that the resultant decadal variations are consistent with observation. Further investigation suggests that this improvement in the subsurface layer is at least partly related to the decadal variation of the mode waters.