On the definition of seasons in paleoclimate simulations with orbital forcing

Abstract

Orbital forcing is a major driver of climate variability on timescales of 10,000 to 100,000 years. Changes in the orbital parameters cause variations in the length of the seasons by several days. Consequently, models using a fixed present-day calendar result in biased paleoseasons, especially in boreal autumn when the vernal equinox is used as an anchor point. The bias is estimated for temperatures and precipitation in a transient model simulation over the last 21,000 years and an accelerated simulation over the last 129,000 years. The largest differences of up to 4 K occur over the continents in high latitudes. Precipitation estimates are mostly affected in the low latitudes. The time-dependent bias is large enough to modify the temporal characteristics of temperature and precipitation indices. It is discussed to what extent the bias in one season is distorting comparisons between models and paleoproxies. The bias has minor implications for proxy-model comparisons in general. However, proxies of monsoon activity should be compared with fixed angular seasons. For process studies and climate sensitivity studies the use of fixed angular seasons is imperative. Copyright 2008 by the American Geophysical Union.