The effect of composition on the density of Indian Ocean waters

The conductivity and density was measured on samples collected in the Indian Ocean during the I9 Climate Variability and Predictability (CLIVAR) cruise. The relative densities ρ−ρ0 were compared to those determined from the equation of state of seawater. The excess densities Δρ=ρMeas−ρCalc ranged from 0 in surface waters to 0.025 kg m−3 in deep waters due to additions of nutrients and carbonates. The values Δρ were found to be a linear function of the increases of TA, TCO2, Si(OH)4 and NO3 in the deep waters (σ=0.004−0.005 kg m−3). The correlations with changes in Si(OH)4 gave the best estimates since the concentrations do not have a large effect on the conductivity.By combining the Indian and earlier Pacific measurements (N=249), we have103Δρ/kg m−3=0.12+104ΔSi(OH)4(σ=0.0041 kg m−3).These results indicate that the effect of changes in the composition of seawater for most of the world oceans can be estimated from the combined results in the Pacific and Indian oceans. The excess densities can also be accounted for by determining the change in the absolute salinity of seawater, defined as SA=S+ΔS, where S is the conductivity salinity and ΔS is equal to the mass of added nutrients and carbonates. SA can then be used in the equation of state to calculate the density (σ=0.0041×10−6 kg m−3). Measurements made on artificial seawater (S=35) agree with the values determined from the equations of state, indicating that the dissolved organic compounds in seawater do not affect the conductivity to density relationships for real seawater. We recommended either SA or ΔSi(OH)4 be used to estimate the changes in the density of seawater due to the addition of nutrients and carbonates to ocean waters.