S. Lorenz

A tropical mechanism for Northern Hemisphere deglaciation

[1] We investigate the role of the tropics in the melting and reforming of the Laurentide ice sheet on glacial timescales using an atmospheric general circulation model. It is found that warming of tropical sea surface temperatures (SSTs) from glacial boundary conditions, as observed at the end of glacial periods, causes a large increase in summer temperatures centered over the ice sheet-forming regions of Canada. This highlatitude response to tropical change is due to relatively small changes in the circulation of the extratropical atmosphere, which lead to changes in the vertical profiles of temperature and moisture in the extratropical atmosphere. The maximum perturbation in the summer radiative balance over the Laurentide ice sheet (>25 W/m 2 ) due to the changes in the local atmospheric water vapor inventory is much larger than that induced by glacial to interglacial changes in atmospheric CO2. These changes via an atmospheric bridge between the tropics and extratropics represent a mechanism for deglaciations which is consistent with timing constraints. In contrast, a cold perturbation to tropical SST for interglacial boundary conditions results in only very small changes in the delivery of water vapor to the Laurentide region, and therefore almost no cooling over the Canadian region. This implies that tropical SSTs could play a more important role in melting ice sheets in the Northern Hemisphere than in reforming them, possibly providing a mechanism which could help to explain the rapidity of deglaciation relative to glacial inception.

Northern High‐Latitude Climate Changes During the Holocene as Simulated by Circulation Models

To simulate the evolution of Holocene climate, forcing factors for the northern high-latitude climate are examined using different numerical models. A global coupled atmosphere‐ocean circulation model driven by astronomical forcing over the last 7,000 years shows a long-term sea-surface temperature decrease in the Nordic Seas region associated with changes in seasonal insolation. A continuous cooling in the northeastern Atlantic is accompanied by a persistent warming in the Labrador Sea from the mid- to late Holocene. This temperature pattern can be attributed to a progressive weakening of the Icelandic Low and altered winds over the Nordic Seas induced by changes in insolation. In the early Holocene, important forcing for the Nordic Seas in the early Holocene was probably caused by melting land ice masses and “deepening” of the Bering Strait. The effects of meltwater discharge and Bering Strait inflow are studied in a regional model of the Arctic and North Atlantic Oceans. It is suggested that a gradual increase in the flux of Pacific water through the Bering Strait during the early Holocene slowly affected the polar climate by melting ice and causing ocean circulation changes in the Nordic Seas. This high-latitude response to sea level change is shown to be different from the signature obtained by a freshwater release linked to the final outburst drainage of Laurentide lakes.