Abstract
Abstract. Changes in the Earth's orbit lead to changes in the seasonal and meridional distribution of insolation. We quantify the influence of orbitally induced changes on the seasonal temperature cycle in a transient simulation of the last 6000 years – from the mid-Holocene to today – using a coupled atmosphere-ocean general circulation model (ECHAM5/MPI-OM) including a land surface model (JSBACH). The seasonal temperature cycle responds directly to the insolation changes almost everywhere. In the Northern Hemisphere, its amplitude decreases according to an increase in winter insolation and a decrease in summer insolation. In the Southern Hemisphere, the opposite is true. Over the Arctic Ocean, decreasing summer insolation leads to an increase in sea-ice cover. The insulating effect of sea ice between the ocean and the atmosphere leads to decreasing heat flux and favors more "continental" conditions over the Arctic Ocean in winter, resulting in strongly decreasing temperatures. Consequently, there are two competing effects: the direct response to insolation changes and a sea-ice insulation effect. The sea-ice insulation effect is stronger, and thus an increase in the amplitude of the seasonal temperature cycle over the Arctic Ocean occurs. This increase is strongest over the Barents Shelf and influences the temperature response over northern Europe. We compare our modeled seasonal temperatures over Europe to paleo reconstructions. We find better agreements in winter temperatures than in summer temperatures and better agreements in northern Europe than in southern Europe, since the model does not reproduce the southern European Holocene summer cooling inferred from the paleo reconstructions. The temperature reconstructions for northern Europe support the notion of the influence of the sea-ice insulation effect on the evolution of the seasonal temperature cycle.
Highlights
The amplitude of the seasonal temperature cycle depends on changes in the Earth’s orbital parameters that alter the seasonal and the meridional distribution of insolation (Milankovic, 1941; Berger, 1978)
It has been assumed that this increase in insolation seasonality has lead to increased surface temperature seasonality. If this is the case for all latitudes, or if responses of the climate system to insolation changes have to be taken into account is subject of the present study
The authors find that during abrupt cold climate events, the annual temperature anomalies in high northern latitudes are dominated by the winter temperature anomalies that are caused by increased winter seaice cover in the Northern Hemisphere
Summary
The amplitude of the seasonal temperature cycle depends on changes in the Earth’s orbital parameters that alter the seasonal and the meridional distribution of insolation (Milankovic, 1941; Berger, 1978). The authors find that during abrupt cold climate events, the annual temperature anomalies in high northern latitudes are dominated by the winter temperature anomalies that are caused by increased winter seaice cover in the Northern Hemisphere. This leads to “continental” conditions over large parts of the northern high latitudes and to an increase in the amplitude of the seasonal temperature cycle. Time-slice simulations of the mid-Holocene with coupled AO-GCMs have been part of the PMIP2-initiative (e.g., Braconnot et al, 2007a,b) that covered various aspects of the climate system but did not investigate the response of the seasonal temperature cycle to changes in insolation. We compare the simulation results to Holocene temperature reconstructions obtained from pollen archives
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