Reconstruction of subsurface DIC and alkalinity fields in the North Pacific using assimilation of upper ocean data

Abstract

Abstract The subsurface water beneath the summer mixed layer is important to air–sea carbon flux, while its geochemical properties are not frequently observed. A data assimilation method is applied to determine the geochemical fields in the subsurface (i.e., 100 m) from the data collected at the surface in the North Pacific. This method, in the family of the inverse methods, is constructed on a one-dimensional bulk mixed layer model. In addition to temperature and salinity, dissolved inorganic carbon (DIC) and alkalinity are also considered as model variables, whereas biological productivity is omitted. The geochemical properties increase from the fall to the winter, which is the period simulated by the model, as the mixed layer develops and entrains subsurface water rich in DIC and alkalinity. Consequently, the geochemical fields in the mixed layer must have extremely sharp north–south gradients in the western region of the North Pacific and can be reproduced only by enhancing the north–south gradients in the subsurface. The fields reconstructed by the data assimilation provide useful information about the biogeochemical cycles. It is suggested that the large difference in DIC between the surface and the subsurface in the northwestern region is produced by transporting DIC from the mixed layer to the subsurface in the summer, implying extremely high biological productivity. Furthermore, it is suggested that high DIC in the ambient water is maintained by the upwelling of lower layer water.