Numerical simulation of the annual cycle of climate during the ice ages

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Abstract
By using a hemispheric thermodynamic grid model, the annual cycle of climate for 18,000 years ago is simulated. It is shown that the difference between the position of the snow‐ice boundary of 18,000 years ago and the one of today was much larger in summer than in winter and that the annual cycle of the snow‐ice boundary for today has more variability than that for 18,000 years ago. Owing to the temperature snow‐ice feedback, the zonally averaged ground and midtropospheric temperature anomalies (with respect to today normals) are negative and their absolute values are larger in summer than for the other seasons of the year and increase from lower to higher latitudes. In lower latitudes, in some regions of Asia and Africa, the spring and early summer temperatures were higher during the ice age climate because the insolation was about 20 W m−2higher than it is today, and the lower latitudes are much more sensitive to insolation than the higher latitudes are, where the temperature snow‐ice feedback is the main mechanism which produced a dominant decrease of temperature. Comparison of the computed surface temperature values with the ‘Observed’ values shows generally good agreement. The computed average ground temperature anomaly for the northern hemisphere for July is equal to −4.7°C, in good agreement with the values −4.8°C obtained by Gates.