FY 1994

Bottom water variability in the Samoa Passage

Johnson, G.C., D.L. Rudnick, and B.A. Taft

J. Mar. Res., 52(2), doi: 10.1357/0022240943077118, 177–196 (1994)

The Samoa Passage (near 10S, 170W) is the channel through which the coldest, saltiest, densest bottom water approaches the North Pacific Ocean from its southern source. Over the past 25 years, three hydrographic sections have been made across the passage. A section occupied in 1968 shows little sign of modified North Atlantic Deep Water (NADW) within the northward flowing Lower Circumpolar Water (LCPW). In contrast, a section occupied in 1987 shows a strong negative curvature in θ-S (potential temperature- salinity) and a local maximum in salinity characteristic of NADW. A third section occupied in 1992 reveals a marginal NADW signature. The three sections are objectively mapped and very fine-scale bivariate areal θ-S censuses are made for a quantitative comparison of differences in water-mass structure. The strength of the NADW signature could fluctuate over a wide range of time-scales. However, these data are consistent with decadal variability, with no NADW signal in the passage in 1968, a strong signal in 1987, and a weak one in 1992. The geostrophic volume transport through the passage is 1.0 ± 0.2, 5.6 ± 1.3, and 4.8 ± 0.6 × 10⁶ m³ s^-1 below a zero-velocity surface (ZVS) of θ = 1.2°C for the 1968, 1987, and 1992 sections respectively. The transport estimates, made for comparison with those from velocity data presently being collected by a current meter array in the passage, are sensitive to variations in the choice of ZVS.