The effect of composition on the density of South Pacific Ocean waters

The conductivities and densities were measured on 331 samples collected in the South Pacific Ocean during the P18 CLIVAR (Climate Variability and Predictability) cruise. The relative densities (ρ - ρ⁰) were compared to those determined from the equation of state of seawater. The excess densities (Δρ = ρ_Meas - ρ_Calc) ranged from ± 0.003 kg m^{- 3} in surface waters to 0.015 kg m^{- 3} in deep waters due to additions of nutrients and carbonates. The values of Δρ were found to be a linear function of the increases of ΔNTA, ΔSi(OH)₄ and ΔNO₃ in the deep waters (σ = 0.002 to 0.005 kg m^{- 3}). The best correlations of Δρ with changes in concentration of ΔSi(OH)₄ are (N = 331) given byΔ ρ / kg m^{- 3} = - 1.7 · 10^{- 3} + 9.4 · 10^{- 5} Δ Si (OH)₄ (σ = 0.0043 kg m^{- 3}) . Correlation of the South Pacific data with earlier measurements made in the North Pacific and Indian oceans (N = 596) are given byΔ ρ / (kg m^{- 3}) = - 1.7 · 10^{- 3} + 9.44 · 10^{- 5} Δ Si (OH)₄ (σ = 0.0043 kg m^{- 3}) . The excess densities can also be accounted for by determining the change in the absolute salinity (S_A) of seawater. The values of ΔS have been estimated fromΔ S = 50 Δ NTA + 68 Δ SiO₂ + 63 Δ HNO₃ + 82 Δ H₃ PO₄ . The values of Δρ/kg m^{- 3} calculated from the estimated values of S_A agreed with the measured densities with a standard error of ± 0.004 kg m^{- 3}. Values of S_A determined from density measurements also show a correlation with ΔSi(OH)₄. The Indian and South Pacific values have been fitted to (σ = 0.0044)Δ S = - 2.4 · 10^{- 3} + 1.06 · 10^{- 4} Δ Si (OH)₄ . An error of ± 0.0044 g kg^{- 1} in S_A is equivalent to and error of ± 0.003 kg m^{- 3} in the density (which is the precision of the density measurements).