Paleo calendar-effect adjustments in time-slice and transient climate-model simulations (PaleoCalAdjust v1.0): impact and strategies for data analysis

摘要

The “paleo calendar effect” is a common expression forthe impact that changes in the length of months or seasons over time,related to changes in the eccentricity of Earth's orbit and precession, haveon the analysis or summarization of climate-model output. This effect canhave significant implications for paleoclimate analyses. In particular,using a “fixed-length” definition of months (i.e., defined by a fixednumber of days), as opposed to a “fixed-angular” definition (i.e., definedby a fixed number of degrees of the Earth's orbit), leads to comparisons ofdata from different positions along the Earth's orbit when comparing paleowith modern simulations. This effect can impart characteristic spatialpatterns or signals in comparisons of time-slice simulations that otherwisemight be interpreted in terms of specific paleoclimatic mechanisms, and weprovide examples for 6, 97, 116, and 127 ka. The calendar effect isexacerbated in transient climate simulations in which, in addition to spatialor map-pattern effects, it can influence the apparent timing of extrema inindividual time series and the characterization of phase relationships amongseries. We outline an approach for adjusting paleo simulations that havebeen summarized using a modern fixed-length definition of months and thatcan also be used for summarizing and comparing data archived as daily data.We describe the implementation of this approach in a set of Fortran 90programs and modules (PaleoCalAdjust v1.0).