Transpacific hydrographic sections at Lats. 43°S and 28°S: the SCORPIO expedition—III. Upper water and a note on southward flow at mid-depth

Abstract The characteristics of the upper 2000 m on the SCORPIO plates are dominated by the subtropical anticyclonic gyre which brings shallower waters from higher latitudes equatorward along the eastern boundary. These incoming waters are cold, of low salinity, and high in oxygen and nutrient concentrations. Within the gyre they are warmed, made more saline, and the oxygen and nutrient concentrations modified. Exchange with the inter-tropical waters also modifies the characteristics, and the waters that return poleward near the western boundary are substantially different from those that entered in the east. Along the SCORPIO sections, temperature decreases monotonically downward to great depths, but the other water characteristics have various maxima and minima, the consequence of the different layers that make up the entering water and of the several sorts of modifications and exchanges that take place within the gyre. The principal salinity feature is the immense body of fresher water that enters from the south in the east, flowing at various depths according to the density field. A major subsurface salinity minimum, the Intermediate Water, is obvious on the greater part of both plates, and a shallower but similar minimum lies above it in the east, separated by a more saline layer extending poleward from low latitudes, as part of the eastern-boundary poleward countercurrent. The higher salinities from the great central south Pacific evaporation cell, centered north of the SCORPIO sections, also occur as weak subsurface maxima returning poleward near New Zealand. The oxygen distribution has a slight maximum near 50m to 100m depth, a relic in these (mostly) summer data of the higher concentrations of the winter mixed layer. Beneath this maximum is the decrease in oxygen that is usual in the upper part of the pycnocline. In these sections this decrease creates a shallow (150m to 400m) minimum because the underlying Intermediate Water, which dominates most of the 500-m to 1500-m depth range, is higher in oxygen concentration and accounts for the oxygen maximum immediately above the deepest oxygen minimum. The pycnocline oxygen minimum is intensified in the east by the poleward subsurface flow of water from lower latitudes where the oxygen concentration is very low. The deepest oxygen minimum has its extreme concentrations in two separate cells, in the eastern area and the central area, separated by a slightly greater minimum in the water overlying the East Pacific Rise. The nutrient distributions reflect principally the major oxygen features in the upper layers. The pycnocline oxygen minimum in the east is also a nutrient maximum, so great that the underlying Intermediate Water, even at high concentrations, exists as a nutrient minimum in the eastern parts. Farther west the nutrient concentrations increase monotonically downward through the Intermediate Water oxygen maximum: phosphate and nitrate reach maximum concentrations corresponding closely to the two cells of the deepest oxygen minimum. Silicate, however, has a deep maximum only in the central Pacific Ocean, somewhat deeper than the maximum in the other nutrients. In the east it increases monotonically from the level of the Intermediate Water all the way to the bottom. A comparison of the SCORPIO sections with earlier observations in both the South and North Pacific leads to the suggestion that the separate vertical minimum in oxygen and the associated maximum in phosphate east of the Kermadec Ridge on the SCORPIO sections are a consequence of southward flow at these depths from the North Pacific. There is evidence of an anticyclonic circulation that extends to more than 2000 m depth in these latitudes, confined at depths below 1500 m to the central ocean by the Tonga and Kermadec ridges. The southward-flowing western limb of this anticyclone extends from the intertropical zone to the Antarctic Circumpolar Current. At these depths it may carry water that has entered the North Pacific as dense abyssal water, moderate in oxygen and phosphate, has become nutrient-rich and oxygen-poor as it rises in the North Pacific and mixes with the less dense water, and returns at mid-depth.