Cecile Cabanes

Recent sea level change in the Mediterranean Sea revealed by Topex/Poseidon satellite altimetry

Using altimetry data of the Topex/Poseidon satellite available since early 1993, we show that the eastern Mediterranean sea level has been continuously rising during 1993–1999, at a rate up to 20 mm/yr southeast of Crete. Sea level rise is also observed in the Algerian-Provencal basin as well as in the Tyrrhenian and Adriatic seas. The north Ionian sea, on the other hand, shows an opposite trend, i.e., a sea level drop during the past seven years. Sea surface temperature trends are strongly correlated to sea level trends, indicating that at least part of the observed sea level change has a thermal origin. The recent Mediterranean sea level rise observed by Topex/Poseidon may be related to the warming trends reported from hydrographic cruises in the intermediate and deep waters of the eastern basin since the early 1990s, and of the western basin since the 1960s.

Sea level change from Topex-Poseidon altimetry for 1993–1999 and possible warming of the southern oceans

In this study we investigate the contribution of individual oceanic regions to the global mean sea level change (mean trend and interannual variations) observed by the satellite Topex-Poseidon from 1993 through 1999. This allows us to analyse sea level change in regions not primary influenced by short-term climate fluctuations, in particular the 1997–1998 ENSO event. We report that the sea level in the southern oceans (here defined as the region between 60°S and 30°S) has been rising between January 1993 and December 1999 at a rate of 1.0±0.1 mm/yr and 1.7±0.1 mm/yr between December 1994 and April 1999. This rise is positively correlated with an increase of sea surface temperature and thus could reflect a warming of the southern oceans waters.

Present-day sea level change: Observations and causes

We investigate climate-related processes causing variations of the global mean sea level on interannual to decadal time scale. We focus on thermal expansion of the oceans and continental water mass balance. We show that during the 1990s where global mean sea level change has been measured by Topex/Poseidon satellite altimetry, thermal expansion is the dominant contribution to the observed 2.5 mm/yr sea level rise. For the past decades, exchange of water between continental reservoirs and oceans had a small, but not totally negligible contribution (about 0.2 mm/yr) to sea level rise. For the last four decades, thermal contribution is estimated to about 0.5 mm/yr, with a possible accelerated rate of thermosteric rise during the 1990s. Topex/Poseidon shows an increase in mean sea level of 2.5 mm/yr over the last decade, a value about two times larger than reported by historical tide gauges. This would suggest that there has been significant acceleration of sea level rise in the recent past, possibly related to ocean warming.

Monitoring ocean heat content from the current generation of global ocean observing systems

Variations in the worlds ocean heat storage and its associated volume changes are a key factor to gauge global warming and to assess the Earths energy budget. It is also directly link to sea level change, which has a direct impact on coastal populations. Understanding and monitoring heat and sea level change is therefore one of the major legacies of current global ocean observing systems. In this study, we present an inter-comparison of the three of these global ocean observing systems: the ocean temperature/salinity network Argo, the gravimeter GRACE and the satellite altimeters. Their consistency is investigated at global and regional scale during the period 2005-2010 of overlapping time window of re-qualified data. These three datasets allow closing the recent global ocean sea level budget within uncertainties. However, sampling inconsistencies need to be corrected for an accurate budget at global scale. The Argo network allows estimating global ocean heat content and global sea level and reveals a positive change of 0.5 ± 0.1W m-2 and 0.5 ± 0.1 mm yr-1 over the last 8 yr (2005-2012). Regional inter-comparison of the global observing systems highlights the importance of specific ocean basins for the global estimates. Specifically, the Indonesian Archipelago appears as a key region for the global ocean variability. Both the large regional variability and the uncertainties in the current observing systems, prevent us to shed light, from the global sea level perspective, on the climatically important deep ocean changes. This emphasises, once more, the importance of continuing sustained effort in measuring the deep ocean from ship platforms and by setting up a much needed automated deep-Argo network.