Abstract
We study the interannual and seasonal variability in the Mediterranean Sea over the period 1958-2004 by comparing a numerical simulation (the 1/4o ORCA-R025 G70 model run, ‘ORCA’ hereafter) with altimetry and the MEDAR temperature and salinity database. The model is forced by the ERA40 atmospheric forcing and has a salinity restoring term applied at surface. Comparing temperature between ORCA and MEDAR shows good interannual variability agreement (correlations of ~0.8 in the western Mediterranean and ~0.5 in the eastern Mediterranean) at surface layers (0-150 m), but slightly higher mean values in the model (0.08-0.16°C). The salinity analysis shows that the surface salinity restoring term has obliterated most of the interannual variability. Mean surface salinities are slightly lower in the model (~0.3), replicated in deeper layers to a lesser degree, and could mean that the restoring term applies insufficient evaporation to compensate for a weak atmospheric forcing (ERA40) water loss flux. The sea level analysis comparing sea surface height (SSH) and steric height from ORCA and sea level anomalies from altimetry (1993-2004) shows good correlations (~0.8) in the interannual variability and annual cycle. However, the model’s SSH overestimates (~15 mm/yr) observed positive altimetric trends (~3-4 mm/yr). In an attempt to identify the source of this overestimation, a water budget calculation was performed between the horizontal and vertical water fluxes in the Mediterranean Sea. Horizontal transport through the main straits shows appropriate values when compared to observations. Thus, the cause of the exaggerated SSH trend is probably a water flux imbalance. By improving surface salinity restoring and atmospheric forcing, the ORCA simulations can provide very promising tools for studies of interannual variability in the Mediterranean Sea.
Highlights
The ocean plays a fundamental role in the slow evolution of climate on our planet (Molines et al, 2006)
SUMMARY: We study the interannual and seasonal variability in the Mediterranean Sea over the period 1958-2004 by comparing a numerical simulation with altimetry and the MEDAR temperature and salinity database
This study has focused on the interannual and seasonal variability in the Mediterranean Sea by performing a model assessment of the ORCA-R025 G70 Simulation and comparing it with altimetry and the MEDAR observational database
Summary
The ocean plays a fundamental role in the slow evolution of climate on our planet (Molines et al, 2006). Over the last few decades, the quality of oceanic observations has greatly increased, especially since the World Ocean Circulation Experiment and more recently with the advent of spatial oceanography and the ARGO programme. These datasets have highlighted the complexity and ubiquity of the oceans’ variability over a wide range of space and time scales comprised of many interlinked processes. In order to study the relative importance of each of the mechanisms playing a role in ocean variability (external forcing or internal ocean variability), it is crucial to complement all the information from observations (both in situ and remote sensing) with data from numerical modelling studies, which have been steadily improving over the last decade
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