La Variabilité Régionale du Niveau de la Mer

Benoit Meyssignac 1
1 GOHS
LEGOS - Laboratoire d'études en Géophysique et océanographie spatiales
Abstract : Over the XXth century, tide gauge records indicate a rise in global sea level of 1.7 mm.a-1. For the past two decades, satellite altimetry data indicate a faster sea level rise of 3.2 mm.a-1 (period 1993-2011). Thanks to its global coverage, they also reveal a strong regional variability in sea level rise that is several times bigger than the global rise in many regions of the world. This regional signal, which must be added to the global sea level rise to compute the total sea level signal, is essential when assessing the potential impacts of sea level rise in coastal areas and low lying islands. In this thesis, we analyse the observed regional variability in sea level rise from satellite altimetry (since 1993), we propose a reconstruction of the past regional variability since 1950 (i.e. prior to altimetry) and we discuss its causes (thermal expansion of the ocean plus land ice loss) and origins (from natural or anthropogenic origin). First, we propose a reconstruction of the sea level variations for the past decades (before the altimetry era) by combining tide gauge records with the principal spatial structures of the ocean deduced from ocean general circulation models. This method enables to reconstruct the 2 dimensional sea level variations since 1950 with a spatial coverage and resolution similar to the satellite altimetry ones. In the second part of this thesis, the reconstruction method is applied to estimate the past regional variability in three regions which are particularly vulnerable to sea level rise: the tropical Pacific, the Mediterranean sea and the Arctic ocean. For each region, the reconstruction gives an estimation of the total (regional component plus global mean) 2-dimensional sea level rise over the past decades. For the sites where vertical crustal motion monitoring is available, we compute as well the total relative sea level (i.e. total sea level rise plus the local vertical crustal motion) since 1950. The objective is to provide estimates of the relative local sea level rise at climatic time scales to allow further studies on the coastal impacts of sea level rise. In the last part of this thesis, the question of the origins of the regional variability in sea level rise is addressed. We examine whether the regional variability in observed sea level rise since 1993 is a consequence of the anthropogenic activity or if it results essentially from the natural variability of the climate system. We focus on the Tropical Pacific where the regional variability in sea level rise is particularly strong since 1993. On the basis of the reconstruction of the sea level variations since 1950, we show that the recent regional variability in sea level rise observed by satellite (over the last 17 years) in this region is not stationnary. It fluctuates with time, following some low frequency of the ENSO climate mode of variability. With the CMIP3 climate models, we show that this regional variability is dominated by the natural variability of the climate system (essentially by the internal variability of the climate system) and that the signature of the anthropogenic activity is still too weak in this region to be detected.
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Benoit Meyssignac. La Variabilité Régionale du Niveau de la Mer. Océan, Atmosphère. Université Paul Sabatier - Toulouse III, 2012. Français. ⟨tel-00779038⟩

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