Hydrological variability in a comprehensive Earth System Model simulation of the past 2,000 years

Abstract:

The focus of climate reconstructions at large-scales has been so far placed on temperature (cf. PAGES2k, PMIP3) and particularly over the last millennium. Here, we present new global simulations with an Earth System Model covering the past 2,000 years and more specifically investigate the hydrological changes over southwestern North America (sNADA), and the European continent.

On a global scale, changes in soil wetness are negatively correlated to changes in (local) temperature with the strongest correlations over the tropical and subtropical non-desert covered areas. Long term-trends over the full simulation period indicate increases for NH summer soil wetness over central NA, central Europe, whereas southern NA and southwestern Europe show drying tendencies.

The evolution of the modelled and the reconstructed sNADA and soil wetness, respectively, over southwestern North America show only very little coherence, even on multi-decadal time scales. One explanation may be the high amount of internal variability and deficiencies in both model and reconstruction. An interesting second analysis pertains to the hydrological changes over the European continent in comparison with southwestern North America. Here, results indicate that on multi-decadal time scale those regions, especially the European Mediterranean, share common variance at lower frequencies on top of the millennial-scale trends.

The prominent volcanic eruption in 528 AD produces an immediate increase of soil wetness over southwestern Europe and western North America. However, on a global scale this pattern is not robust, as in the 2^nd ensemble member the same eruption produces a different pattern, especially in tropical areas pointing to the high degree of internal variability involved despite pronounced changes in volcanic activity.