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

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 t he North Pacific. This method, in the family of the inverse methods, is con structed on a one-dimensional bulk mixed layer model. In addition to temper ature and salinity, dissolved inorganic carbon (DIC) and alkalinity are als o 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 entrain s subsurface water rich in DIC and alkalinity. Consequently, the geochemica l 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 e nhancing the north-south gradients in the subsurface. The fields reconstruc ted by the data assimilation provide useful information about the biogeoche mical cycles. It is suggested that the large difference in DIC between the surface and the subsurface in the northwestern region is produced by transp orting DIC from the mixed layer to the subsurface in the summer, implying e xtremely high biological productivity. Furthermore, it is suggested that hi gh DIC in the ambient water is maintained by the upwelling of lower layer w ater. (C) 2000 Elsevier Science B.V. All rights reserved.