Abstract
Abstract. In this study we analyse the role of internal variability in regional climate simulations through a comparison of two regional paleoclimate simulations for the last millennium. They share the same external forcings and model configuration, differing only in the initial condition used to run the driving global model simulation. A comparison of these simulations allows us to study the role of internal variability in climate models at regional scales, and how it affects the long-term evolution of climate variables such as temperature and precipitation. The results indicate that, although temperature is homogeneously sensitive to the effect of external forcings, the evolution of precipitation is more strongly governed by random unpredictable internal dynamics. There are, however, some areas where the role of internal variability is lower than expected, allowing precipitation to respond to the external forcings. In this respect, we explore the underlying physical mechanisms responsible for it. This study identifies areas, depending on the season, in which a direct comparison between model simulations of precipitation and climate reconstructions would be meaningful, but also other areas where good agreement between them should not be expected even if both are perfect.
Highlights
The climate system fluctuates naturally over a large frequency range, from days to millions of years (Huybers and Curry, 2006)
In this study we have compared the evolution of surface air temperature (SAT) and PRE in two millennial paleoclimate simulations performed with a Regional Circulation Models (RCMs) with a spatial resolution of 30 km for the Iberian Peninsula (IP)
The results indicate that the long-term evolution of SAT is strongly affected by the external forcings driving the simulation
Summary
The climate system fluctuates naturally over a large frequency range, from days to millions of years (Huybers and Curry, 2006). In the context of anthropogenic climate change, it is important to have available reliable estimations of the amplitude of natural variability on multidecadal timescales and at regional spatial scales, since this variability may hinder the attribution of trends observed to the anthropogenic forcing In this respect, recent detection and attribution studies (Hegerl et al, 2011) have shown the fingerprint of external forcings in the temperature evolution of climate at continental scale during the last millennium. The only difference lies in the initial condition used to run the two simulations in the global model, and these experiments allow us to investigate the role of external forcing in the evolution of several climate variables, temperature, compared to the magnitude of the internal variability of the model at regional scale. We focus on the evolution of near-surface air temperature (SAT) and precipitation (PRE) in winter (mean of December–January–February) and summer (mean of June–July–August)
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