Seasonal sea level variations in the gulf of Cadiz continental shelf from in-situ measurements and satellite altimetry

Abstract

Long-term time series (1997–2008) of gridded multi-mission near shore altimeter data and in-situ tide gauge records have been analyzed to investigate the seasonal variability of sea level along the Gulf of Cadiz continental shelf and the Strait of Gibraltar. The contribution of different forcing mechanisms including atmospheric pressure, wind, steric anomalies and river runoff was also addressed. Comparison between altimeter and ground-truth data showed that the former can be a valuable tool to study the sea level seasonal cycle near the coast as well as to detect anomalous data from neighboring tide gauges. Overall, both sets of time series presented a similar mean seasonal cycle along the coast, dominated by the annual component and, to a lesser extent, by the semiannual one. Low-frequency atmospheric pressure and steric anomalies were the main forcing factors affecting the sea level seasonal cycle, their effect diminishing toward the Strait of Gibraltar. The combined effect of both forcing factors accounted for 55–61%/62–63% of the observed sea level variance at the tide gauge stations/ altimeter points located outside the Strait of Gibraltar and for only 27%/48% at those placed within the Strait. The influence of local wind was small in all stations except for the westernmost tide gauges. The effect of large river discharges was notable at the tide gauge located within the Guadalquivir river mouth, with 18% of the pressure-adjusted sea level variance explained, and negligible (< 0.1%) at the one placed within the Tinto–Odiel rivers mouth, indicating that such effects only need to be taken into account when dealing with tide gauges located within major river estuaries. Moreover, the effect of river runoff seems to be confined within a small area near the river mouth, thus not affecting the altimeter measurements. A small but significant influence of the wintertime North Atlantic Oscillation on sea level was observed, its effect being mainly reflected through changes in the atmospheric pressure, wind and river discharge.