The determination of pHT in the equatorial Atlantic Ocean and its role in the sound absorption modeling in seawater

The observations carried out during the CITHER 1 cruise (January–March 1993), part of the WOCE Hydrographic Programme, enable a detailed description (3600 analyzed water samples) of the distribution of the pHT in the Atlantic equatorial belt with a precision of ±0.003 pHT units. They show a minimum of pHT<7.75 at 400–500 dbar associated with the vertical oxygen minimum characteristic of the equatorial zone, and a maximum (pHT>8.00) centered at 1900–2000 dbar, i.e. at the salinity and oxygen maxima level in the North Atlantic Deep Water (NADW). The results of pHT are used to propose a multiple linear regression directly connecting the pHT to the hydrological parameters and nutrients, which are more largely available than pH measurements. This is in order to extend the description of the pHT field to the whole tropical Atlantic zone (30°N–30°S), and thus, to provide a model of sound absorption at low frequencies (<1 kHz) in the ocean, which depends on the pH with an uncertainty of about ±5% in the absorption coefficient for this oceanic area.