SENSITIVITY OF PALEOCLIMATE SIMULATION RESULTS TO SEASON DEFINITIONS

According to the Milankovitch theory, slow variations of the Earth's o rbital parameters change the amplitude of the seasonal cycle of insola tion and are considered to be the main forcing mechanism of glacial-in terglacial cycles. Because of the precession and changes in eccentrici ty the length of seasons also varies. No absolute phasing is then poss ible between the insolation curves of two different periods. Various s olutions to compare different periods have been given either for astro nomical computations [e.g., Berger and Loutre, 1991; Laskar et al., 19 93] or for model simulations [e.g., Kutzbach and Otto-Bliesner, 1982; Mitchell et al., 1988], but the sensitivity of model results to the di fferent possible solutions has never been quantified. Our results, bas ed on simulations of the last interglacial climate, 126 kyr B.P., wher e changes in the length of the seasons are large, clearly show that ph ase leads or lags between the various solutions used introduce biases in the analysis of insolation and climate change of the same order of magnitude as the Milankovitch forcing. Our main conclusions are that ( 1) when comparing various model simulations, the date of the vernal eq uinox (i.e., the phasing of the seasonal cycle of insolation) as well as the definition of seasons must be the same for all models in order to avoid artificial differences; (2) seasons based on astronomical pos itions are preferred to seasons defined with the same lengths as today , since they better account for the phasing of insolation curves. Howe ver, insolation is not the only forcing in most atmospheric general ci rculation model simulations. We also discuss the impact of the calenda r hidden behind the definition of the seasonal cycle of the other boun dary conditions, such as sea ice or sea surface temperatures.