GENIE:QuaternaryBoundaryConditions

Bob Marsh, National Oceanography Centre, Southampton, ([mailto:rma@noc.soton.ac.uk rma@noc.soton.ac.uk])

Specifying boundary conditions for Quaternary experiments
Through two studies with the 36x36s grid, we have developed boundary conditions and experimental designs for carrying out long simulations through all or part of the last glacial cycle. This page provides information on:


 * which boundary conditions
 * interpolation in time
 * interpolation in space
 * filenames and flags

In the full glacial cycle experiments (Marsh et al., submitted), the ocean and atmosphere are initially at rest and prognostic variables are set to constant values: the ocean with temperature 20°C and salinity 34.9 psu; the atmosphere with temperature 0°C and relative humidity 0%. Each experiment has a 124,000-year duration. For the first 4000 years, boundary conditions are fixed at values for 120 ky BP in order for the thermohaline circulation and global climate to reach an equilibrium state. Lunt et al. (2006) find that this is sufficient time for a simulation to come to equilibrium, and that an equilibrated climate is a good approximation to the equivalent snapshot of a transient climate in the absence of short-term variability such as Heinrich events. The implication is that, if we had started from, say, 150 ky BP and carried out a 30-ky spin-up under transient boundary conditions for 150-120 ky BP, we would obtain a very similar initial state.

Orbital forcing
Solar insolation changes with variations in the Earth's orbit, which are caused by gravitational interaction between the sun, the planets and their satellites (Pälike, 2005). Consequently the amount of energy received at a given latitude and in a given month is dependent upon the Earth's orbital and rotational parameters defined as obliquity, eccentricity, and precession, which vary on several dominant time scales, including ~20Kyr (precession), ~40Kyr (obliquity) and ~100 and 400Kyr (eccentricity). Obliquity variations have a substantial effect on local annual-mean insolation but do not influence the global mean annual incoming solar radiation. Eccentricity variations do affect annual insolation, although the effect is relatively small. Variations in precession control the timing of the approach of the perihelion (when the Earth is closest to the Sun) and consequently the seasonality of insolation. The model uses orbital parameters to generate the insolation forcing at all latitudes. These parameters are constructed using data from Berger (1978).

Radiative trace gases
1. CO2

Ice core records show that CO2 is an important amplifier of orbital forcing (Genthon et al., 1987). We use the record of CO2 directly measured in air bubbles trapped in the Vostok ice core (Petit et al., 1999), interpolated to every 1000 years, shown in Figure 2a. The CO2 concentrations are set to the GT4 timescale (Petit et al., 1999), an extension of the original timescale (Lorius et al. 1985) including subsequent improvements (Jouzel et al. 1993). The chronology is derived from ice-flow and ice accumulation models and therefore is purely based on physics rather than climatic correlations, apart from a number of control ages. The timescale accuracy of the CO2 reconstruction over the last 120 ky is ±5 ky for the last 110 ky, which decreases to ±10 ky for the remaining 10 ky. The record indicates an approximate 100 ppmv glacial-interglacial range, with minimum values around the LGM.

Ice Sheets
The "ICE-4G" reconstruction provides terrestrial ice sheet fraction and orography for the period from 21 ky BP to the present, at a time resolution of 1 ky (Peltier, 1994). The ICE-4G method is based on an understanding of how relative sea-level records the viscous deformation of the earth, due to the presence of terrestrial ice sheets, using exponential relaxation curves for ice-covered regions. These curves contain information about the internal rheology of the planet and the space- and time-dependent thickness of the ice load when this is combined with relative sea-level data, and are deconvolved to produce the paleotopography of the terrestrial ice sheets.

In a related GENIE-1 study, Lunt et al. (2006) used the ICE-4G reconstructions every 1000 years over 0-21 ky BP, along with the Arnold et al. (2002) reconstruction of northern ice sheets at 30 ky BP, and combined the ICE-4G data over 0-21 ky BP and the Vostok delta O-18 record (Petit et al., 1999) over 0-29 ky BP to obtain ice sheet extent and orography over 22-29 ky BP. As the ICE-4G record spans the period of maximum ice cover (LGM) to the present, which can be assumed as an approximation of the minimum amount of ice cover over the glacial-interglacial cycle (neglecting more extensive melting of the Greenland ice sheet during the Eemian), this period can be likewise used as an analogue to approximate the ice volume change during the period since 120 ky BP. Marsh et al. (submitted) therefore reconstructed a millennial time series of ice sheet extent and orography over 22-120 ky BP using the low-resolution sea level reconstruction of Siddall et al. (2003), as an indirect measure of change in global ice volume. This reconstruction spans +8.1 m to -112.4 m, relative to present day sea level.

Time variation of reconstructed ice volume should be consistent with the time-varying glacial freshwater fluxes that include both global net evaporation (for ice sheet growth) and meltwater pulses (MWPs). The modelled ice sheets are therefore directly based on the smoothed and HE-adjusted global mean sea-level curve.