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Krismer T.R.,Max Planck Institute for Meteorology | Krismer T.R.,International Max Planck Research School on Earth System Modelling IMPRS | Giorgetta M.A.,Max Planck Institute for Meteorology | Esch M.,Max Planck Institute for Meteorology
Journal of Advances in Modeling Earth Systems

This study investigates seasonal modulations of the quasi-biennial oscillation (QBO) of the tropical stratosphere. For this purpose, the Max Planck Institute Earth System Model (MPI-ESM), which internally generates a realistic QBO compared to the ERA-40 data set, is employed. The modeled QBO is forced with resolved and parametrized waves. At 5 hPa, the seasonal distribution of the onset of QBO westerly jets clusters in spring and fall due to the coupling of the QBO and the semiannual oscillation. This seasonal clustering of the westerly jets extends throughout the stratosphere, shifting to later months with increasing pressure. QBO westerly jets starting in the upper stratosphere in fall propagate to the middle stratosphere more slowly than westerly jets starting in spring. This is attributed to seasonal modulations of the QBO forcing and enhanced wave filtering by the QBO westerly jet in the lower stratosphere in fall and winter compared to spring and summer. The observed stalling of the QBO easterly jet in the lower stratosphere and the accompanied prolonged persistence of the QBO westerly jet in the vicinity of the tropopause are attributed equally to seasonal variations of the resolved and parameterized wave forcing and the advective forcing. ©2013. American Geophysical Union. All Rights Reserved. Source

Ziemen F.A.,Max Planck Institute for Meteorology | Ziemen F.A.,International Max Planck Research School on Earth System Modelling IMPRS | Ziemen F.A.,University of Alaska Fairbanks | Rodehacke C.B.,Max Planck Institute for Meteorology | And 2 more authors.
Climate of the Past

In the standard Paleoclimate Modelling Intercomparison Project (PMIP) experiments, the Last Glacial Maximum (LGM) is modeled in quasi-equilibrium with atmosphere-ocean-vegetation general circulation models (AOVGCMs) with prescribed ice sheets. This can lead to inconsistencies between the modeled climate and ice sheets. One way to avoid this problem would be to model the ice sheets explicitly. Here, we present the first results from coupled ice sheet-climate simulations for the pre-industrial times and the LGM. Our setup consists of the AOVGCM ECHAM5/MPIOM/LPJ bidirectionally coupled with the Parallel Ice Sheet Model (PISM) covering the Northern Hemisphere. The results of the pre-industrial and LGM simulations agree reasonably well with reconstructions and observations. This shows that the model system adequately represents large, non-linear climate perturbations. A large part of the drainage of the ice sheets occurs in ice streams. Most modeled ice stream systems show recurring surges as internal oscillations. The Hudson Strait Ice Stream surges with an ice volume equivalent to about 5 m sea level and a recurrence interval of about 7000 yr. This is in agreement with basic expectations for Heinrich events. Under LGM boundary conditions, different ice sheet configurations imply different locations of deep water formation. © 2014 Author(s). Source

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