Long-term memory in 1000 years simulated temperature records

Abstract

We study the appearance of long-term persistence in temperature records, obtained from the global coupled general circulation model ECHO-G for two runs, using detrended fluctuation analysis. The first run is a historical simulation for the years 1000–1990 (with greenhouse gas, solar, and volcanic forcing), while the second run is a 1000-year “control run” with constant external forcings. We consider daily data of all grid points as well as their biannual averages in order to suppress 2-year oscillations appearing in the model records for some sites near the equator. Our results substantially confirm earlier studies of (considerably shorter) instrumental data and extend their results from decades to centuries. In the case of the historical simulation we find that most continental sites have correlation exponents γ between 0.8 and 0.6. For the ocean sites the long-term correlations seem to vanish at the equator and become nonstationary at the Arctic and Antarctic circles. In the control run the long-term correlations are less pronounced. Compared with the historical run, the correlation exponents are increased, and show a more pronounced latitude dependence, visible also at continental sites. When analyzing biannual averages, we find stronger long-term correlations in the historical run at continental sites and a less pronounced latitude dependence. In all cases, the exponent γ does not depend on the continentality of the sites.