Comparison of satellite altimetry sea level anomalies and hydrographic observations in the Mediterranean Sea

Abstract

Steric sea level (SSL) computed from hydrographic observations in the Mediterranean Sea is compared against altimetric sea level anomalies (SLA) at seasonal and inter-annual time scales for the period 1993-2008. SSL (referenced to 300 m) is computed using two data sets: in situ profiles and gridded products obtained from interpolated observations. The impact of expendable/mechanical bathythermograph (XBT/MBT) biases affecting some of the in situ profiles is investigated by comparing both corrected and uncorrected data. For the period 2003-2008 the mass component is estimated from GRACE observations and subtracted from SLA. The analysis of the spatio-temporal distribution of profiles shows that the number of profiles with data below 300 m is a small percentage of the total and that their spatial coverage of the Mediterranean basin is very limited. This is an important handicap for regions where the contribution of the deep layers to SSL is significant. Overall, SSL and SLA are shown to be consistent in the Mediterranean at seasonal time scales, although the annual amplitude of the SSL from in situ profiles and interpolated data is considerably smaller than that of the SLA. The agreement at inter-annual time scales is less good. At some particular locations SSL computed from individual profiles is more correlated with SLA than the gridded products. At basin and sub-basin scales, however, interpolated and in situ observations provide similar results in terms of their correlation with observed SLA. The XBT/MBT bias corrections have little effect on the SSL at the time scales considered in this study.