Abstract
Abstract. The “paleo calendar effect” is a common expression for the impact that changes in the length of months or seasons over time, related to changes in the eccentricity of Earth's orbit and precession, have on the analysis or summarization of climate-model output. This effect can have significant implications for paleoclimate analyses. In particular, using a “fixed-length” definition of months (i.e., defined by a fixed number of days), as opposed to a “fixed-angular” definition (i.e., defined by a fixed number of degrees of the Earth's orbit), leads to comparisons of data from different positions along the Earth's orbit when comparing paleo with modern simulations. This effect can impart characteristic spatial patterns or signals in comparisons of time-slice simulations that otherwise might be interpreted in terms of specific paleoclimatic mechanisms, and we provide examples for 6, 97, 116, and 127 ka. The calendar effect is exacerbated in transient climate simulations in which, in addition to spatial or map-pattern effects, it can influence the apparent timing of extrema in individual time series and the characterization of phase relationships among series. We outline an approach for adjusting paleo simulations that have been summarized using a modern fixed-length definition of months and that can also be used for summarizing and comparing data archived as daily data. We describe the implementation of this approach in a set of Fortran 90 programs and modules (PaleoCalAdjust v1.0).
Highlights
In paleoclimate analyses, there are generally two ways of defining months or seasons: (1) a “fixed-length” definition, wherein, for example, months are defined by a fixed number of days, and (2) a “fixed-angular” definition, wherein, again for example, months are defined by a fixed number of degrees of the Earth’s orbit
The average insolation during a fixedlength month will include the effects of the orbital variations on insolation and the changing month length
The approach we describe here for adjusting model output reported either as monthly data or as daily data to reflect the calendar effect has two fundamental steps: (1) pseudo-daily interpolation of the monthly data on a fixed-month-length calendar, followed by (2) aggregation of those daily data to fixed-angular months defined for the particular time of the simulations
Summary
There are generally two ways of defining months or seasons (or any other portion of the year): (1) a “fixed-length” definition, wherein, for example, months are defined by a fixed number of days (typically the number of days in the months of the modern Gregorian calendar), and (2) a “fixed-angular” definition, wherein, again for example, months are defined by a fixed number of degrees of the Earth’s orbit. The issue for paleoclimate analyses is that, using a fixed-length definition of months, comparisons of paleo simulations for different time periods may incorporate data from different positions along the Earth’s orbit for a particular month, which can produce patterns in data–model and model–model comparisons that mimic observed paleoclimatic changes This paleo calendar effect arises from a consequence of Kepler’s (1609) second law of planetary motion: Earth moves faster along its elliptical orbit near perihelion and slower near aphelion. We describe (a) the calendar effect on month lengths and their beginning, middle, and ending days over the past 150 kyr; (b) the spatial patterns of the calendar effect on temperature and precipitation rate for several key times (6, 97, 116, and 127 ka); and (c) the methods that can be used to calculate month lengths (on various calendars) and to “calendar adjust” monthly or daily paleo model output to an appropriate paleo calendar
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