Detection/Attribution Studies from Observational Data and Historical CMIP5 Coupled Climate Models on the Regional Sea Level Variability during the 20th and 21st Century in the Pacific Ocean.

Abstract:

From recent published studies, it is not yet clear whether the sea level spatial trend patterns of the Pacific Ocean from satellite altimetry over 20 years are mostly due to internal climate variability or if some anthropogenic fingerprint is already detectable. The objective of this study is to investigate this issue in a more detailed manner. For that purpose, we remove the signal corresponding to the Pacific Decadal Oscillation (PDO) from the observed altimetry sea level trend over 1993-2012 and find some significant residual pattern in the western tropical Pacific. When the same is performed on the thermocline attributed steric sea level trend (since most of the observed sea level trend in the tropics is explained by the vertical thermal structure of the ocean), essentially the same residual pattern remains. This suggests that the internal climate variability alone cannot account for all of the observed trend patterns in the western tropical Pacific and that there is likely a fingerprint of anthropogenic forcing. In addition, by making use of different sets of CMIP5 coupled climate models (with and without external forcing), we try to separate the contribution of anthropogenic forcing and internal climate variability to the Pacific Ocean regional sea level variability between 1900 and 2005. As in the CMIP5 models, the timing of the internal climate variability is poorly reproduced, performing an ensemble mean of several such models will therefore compensate this internal variability thereby giving an estimate of regional sea level variability driven by external (natural and/or anthropogenic) forcing only. We show that the sea level trend pattern from the ensemble climate models in the tropical Pacific is very similar to the observed sea level trend patterns from satellite altimetry after removing the PDO signal. This suggests that the presence of anthropogenic fingerprint is already detectable in satellite altimetry based sea level.