The Southern Westerlies during the last glacial maximum in PMIP2 simulations |
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| Authors: | Maisa Rojas Patricio Moreno Masa Kageyama Michel Crucifix Chris Hewitt Ayako Abe-Ouchi Rumi Ohgaito Esther C Brady Pandora Hope |
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| Institution: | 1.Department of Geophysics,University of Chile Blanco Encalada,Santiago,Chile;2.Institute of Ecology and Biodiversity,Santiago,Chile;3.Department of Ecological Sciences,University of Chile,Santiago,Chile;4.LSCE/IPSL,UMR CEA-CNRS-UVSQ 1572, CE Saclay,Gif-sur-Yvette Cedex,France;5.Institut d’Astronomie et de Géophysique G. Lemaitre,Université catholique de Louvain,Louvain-la-Neuve,Belgium;6.Met Office,Exeter,UK;7.Center for Climate System Research,The University of Tokyo,Kashiwa,Japan;8.Frontier Research Center for Global Change,Japan Agency for Marine-Earth Science and Technology,Yokohama,Japan;9.Climate Change Research National Center for Atmospheric Research,Boulder,USA;10.Bureau of Meteorology Research Centre,Melbourne,Australia |
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| Abstract: | The Southern Hemisphere westerly winds are an important component of the climate system at hemispheric and global scales.
Variations in their intensity and latitudinal position through an ice-age cycle have been proposed as important drivers of
global climate change due to their influence on deep-ocean circulation and changes in atmospheric CO2. The position, intensity, and associated climatology of the southern westerlies during the last glacial maximum (LGM), however,
is still poorly understood from empirical and modelling standpoints. Here we analyse the behaviour of the southern westerlies
during the LGM using four coupled ocean-atmosphere simulations carried out by the Palaeoclimate Modelling Intercomparison
Project Phase 2 (PMIP2). We analysed the atmospheric circulation by direct inspection of the winds and by using a cyclone
tracking software to indicate storm tracks. The models suggest that changes were most significant during winter and over the
Pacific ocean. For this season and region, three out four models indicate decreased wind intensities at the near surface as
well as in the upper troposphere. Although the LGM atmosphere is colder and the equator to pole surface temperature gradient
generally increases, the tropospheric temperature gradients actually decrease, explaining the weaker circulation. We evaluated
the atmospheric influence on the Southern Ocean by examining the effect of wind stress on the Ekman pumping. Again, three
of the models indicate decreased upwelling in a latitudinal band over the Southern Ocean. All models indicate a drier LGM
than at present with a clear decrease in precipitation south of 40°S over the oceans. We identify important differences in
precipitation anomalies over the land masses at regional scale, including a drier climate over New Zealand and wetter over
NW Patagonia. |
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