Inverse Modeling of Intra-annual Variability in the Subtropical North Pacific

Abstract

Climatological data on the oceanic and atmospheric variability are inverted to study seasonal variation of the Kuroshio Extension (KE) and the recirculation gyre to the south. The processed datasets include climatological fluxes of heat, salt, and momentum at the ocean surface; Levitus hydrography; TOPEX/Poseidon altimetry; and surface drifter data. A variational data assimilation technique is used to retrieve variability in the open ocean region (18°?42°N, 142°E?160°W) bounded at 1000 m from below. By optimizing the open boundary values of oceanic fields in combination with initial conditions and atmospheric forcing, model solutions that are consistent with various climatological datasets within limits of observational errors were found. Using this technique, the mean geographical positions of the Subtropical Mode Water (STMW) and Central Mode Water (CMW) formation sites were estimated, the structures were analyzed, and estimates of the mode water production rates were obtained. Computations indicate that CMW formation is likely to occur 15°?20° west of the location diagnosed formerly without taking salinity data into the account. The optimized seasonal cycle is characterized by the STMW and CMW production rates of 3.8 ± 0.6 and 3.1 ± 0.5 Sv (1 Sv ≡ 106 m3 s?1), respectively. The KE annual mean transport in the upper 1000 m is diagnosed as 68 ± 7 Sv with a maximum of 79 ± 8 Sv in June?July and a minimum of 56 ± 6 Sv in December?January. Analysis of the heat and salt budgets in the region has shown that atmospheric fluxes are counterbalanced by the horizontal divergence of the advective temperature and salinity transports. In the annual mean, horizontal diffusion plays a minor role in the budgets.