A transpacific hydrographic section along latitude 24°N: the distribution of properties in the subtropical gyre

Abstract

An intensively sampled transpacific hydrographic section along 24°N was completed in the spring of 1985. The data are described here in terms of the spatial distribution of properties, the distribution along isopycnal surfaces, and, where possible, the relationship of these distributions to the large-scale circulation of the Pacific Ocean. Near-surface waters of subtropical origin display a salinity maximum in mid-ocean, with lower salinity to the west due to greater rainfall and lower salinity in the east due to advection of water from the north. In the next layers down, containing waters of subpolar origin, the low salinity and high dissolved oxygen concentrations of those waters are most pronounced in the eastern ocean where the subpolar water is swept clockwise into the subtropical gyre. Differences between patterns of dissolved oxygen concentration and salinity indicate that both horizontal advection and upwelling contribute to observed distributions near the eastern boundary and that the two tracers contain independent information. In the upper kilometer, the eastern Pacific is richer in tracer signals and has steeper property gradients than the west.The deep Pacific has long been recognized to be the most uniform of the oceans. Although property gradients are small, they are significant, and it is found that on all isopycnal surfaces below the upper kilometer salinity increases and dissolved oxygen concentration decreases towards the east on basin-wide scales. These zonal gradients are weakest in the abyss, where there is a substantial net input of southern water, and strongest at mid-depth. Vertical diffusion is the likely cause of the uniformity in this pattern over so much of the deep North Pacific, with oxygen consumption in waters of greater age in the east also being a plausible contributor.With a highly sampled data set such as the 24°N transpacific section it is appropriate to ask how many stations are required to define property distributions and to estimate large-scale circulation and transport. Estimation of geostrophic transport requires high spatial resolution to detect flow near sloping topography at all depths. A 50% decimation of the 24°N station pattern yields a severe degradation in the estimation of transport.