The pH of the North Atlantic Ocean: Improvements to the global model for sound absorption in seawater

At frequencies below 1 kHz, sound absorption coefficients in the ocean are a function of pH, and at higher frequencies they are dependent upon MgSO4. The pH dependent terms are attributable to relaxation of B(OH)3 and MgCO3 species, and the ensemble effect has been approximated (Mellen et al., 1987a) as α = α1(MgSO4) + α2(B(OH)3) + α3(MgCO3), where α is the absorption coefficient in decibels per kilometer, and αn = (S/35)An[ƒ2ƒn/(ƒ2 + ƒn2)], where S is the salinity, An is the amplitude of each component and depends on pH, ƒ is the frequency, and ƒn is the relaxation frequency. Overall accuracy of ±15% in a requires that pH be known to 0.05 pH units. The presently used oceanic pH field for sound absorption models is derived from a combination of Geochemical Ocean Sections Study (GEOSECS) data and Soviet data from the 1978 Gorshkov atlas for the North Atlantic where GEOSECS data are absent. We compare the North Atlantic fields with the well‐constrained Transient Tracers in the Ocean North Atlantic CO2 data set and find large differences. We further show that sufficiently strong correlations exist between CO2 system variables and other more commonly available hydrographic properties and that improved renditions of the pH field in other regions of the ocean are possible once equivalent local correlations are established, thus leading to greatly improved estimates of the sound absorption field.