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3rd HyMeX workshop 1-4 June 2009 Heraklion (Gournes), Crete-Greece

Sea level variability in the context of HyMeX

Damià Gomis (IMEDEA (Universitat de les Illes Balears - CSIC)); Enrique Álvarez, Samuel Somot, Mikis Tsimplis, Gabriel Jordà, Marta Marcos, Francisco M. Calafat, Roland Aznar

Sea level is an integrating parameter that can be used for closing the water budget of the Mediterranean Sea, both at basin and sub-basin scale. In the framework of the VANIMEDAT project we have described the main features of Mediterranean sea level variability during the last decades, paying particular attention to the physical processes underlying the observed changes. Tide records spanning the 2nd half of the XX century show trends of less than 1 mm/yr and exhibit a significant inter-decadal variability: between 1960 and 1994 most tide gauges show negative trends, typically ranging between -0.5 and -1.0 mm/yr, while during the last decade all Mediterranean tide gauges report marked sea level rise (between 5 and 10 mm/yr). The negative are thought to be mostly due to a regional increase in the atmospheric pressure concentrated in the winter season. The trend in the atmospheric component of sea level has been evaluated in about -0.6 mm/yr for the last half of the XX century and in -1.0 mm/yr between 1960 and 1994. The steric contribution trends are of the order of 1 mm/yr during the 2nd half of the XX century, increasing to 10 mm/yr during the exceptional decade of the 1990s. The mass contribution (the most important one for HyMeX) has only been estimated in a direct way from gravimetry data, which span the few last years and has a much lower spatial resolution than the other sea-level components. In principle the total mass component can also be derived from hydrographic and altimetry data, but it has never been attempted.

In the framework of the HyMeX project we can contribute with the evaluation of Mediterranean sea level variability on the basis of in-situ sea level measurements and altimetry data. Measured sea level can be compared with steric estimates from hydrographic data and with the output of the improved models developed under HyMeX. The combination of direct sea level measurements with water mass observations and numerical models will permit the investigation not only of total sea level, but also of its different components (atmospheric forcing, steric component and water budget variability) and their associated physical processes. In particular, the monitoring of the exchanges at the various oceanic Straits undertaken during the HyMeX intense period of observation altogether with the corresponding observations of heat and mass exchanges with the atmosphere and the land will allow to close the water budget and compare total amounts with observed mean sea level. Hence, the relation between sea level and other HyMeX tasks is clear: on one hand we need of improved water budget estimates to understand sea level variability; on the other hand, sea level can be used to validate heat and mass exchanges, then providing the modellers with an additional validation parameter.