Interannual-to-Decadal Variations of Tropical-Subtropical Exchange in the Pacific Ocean: Boundary versus Interior Pycnocline Transports

Interannual-to-decadal variations of tropical-subtropical mass exchange in the Pacific Ocean are investigated using a near-global ocean general circulation model along with satellite observations of sea level and wind and a data assimilation product. The analysis focuses on the variability of pycnocline transports through the western boundary and interior near 10°N and 10°S. In contrast to time-mean exchange, where boundary and interior pycnocline transports are both equatorward, the variations of boundary and interior pycnocline transports are found to be generally anticorrelated to each other. Moreover, the variation of the boundary pycnocline transport is smaller than that of the interior, again different from time-mean exchange, where the boundary transport at 10°N is substantially larger than that through the interior. Interannual variations of the boundary and interior transports are consistent with near-surface geostrophic flow inferred from sea level data. Interior pycnocline flow into the Tropics is weaker in the 1990s than that in the 1980s, in agreement with recent observations. However, approximately half of it is compensated by an opposite change in boundary flow at 10°N. The results indicate that the interior pathway is more important to interannual and decadal variability of tropical-subtropical exchange than the boundary pathway, despite a much larger time-mean transport of the western boundary current at 10°N. To a large extent, the counteracting tendency of the boundary and interior flow and the larger variation of the latter can be explained by the combined effect of variability in off-equatorial wind stress curl in the western Pacific and near-equatorial zonal wind stress. The former changes the strength of horizontal circulation and results in a variation of boundary pycnocline flow that is opposite in direction but comparable in magnitude to that of the interior pycnocline flow. The latter primarily affects the strength of the shallow meridional overturning circulation with net pycnocline flow (mostly in the interior) opposing the surface Ekman flow. The covariability of these two forcings leads to an enhancement of interior transport. The relative variability of boundary and interior pycnocline flow is insensitive to whether the Indonesian Throughflow is present or not.