Role of "effective density" in improving the accuracy of bottom-pressure based sea level measurements - a case study from gulf of guinea

Seasonal observations of a set of bottom pressures and concurrent tide-staff measurements from Takoradi harbour located in the Gulf of Guinea indicated that the effective depth-mean density value, p eff of this clear water body is less than (= 0.25% - 1.84%) its bulk density, p b obtained from precision density measurements made on discrete water samples. Thus, use of p b to estimate sea level elevation yields an underestimation of this order in sea level measurements. Although, this effect was observed in association with both wind and rainfall, the comparative reduction of p eff value was more prominent during the period when rainfall was strong. Presence of microbubbles introduced into the upper layers of the sea water, as a result of turbulence induced by wind forcing and impact of rain drops, is suspected to be responsible for the observed effective reduction in the in situ density of this shallow water body. The accuracy of bottom-pressure based sea level measurements can be improved by introducing p eff value into sea level estimation. It is shown that p eff value can be estimated by using a statistically derived simple linear model, which is constructed from a set of bottom pressures and concurrent tide-staff measurements.