Description: The transient response of the Atlantic Meridional Overturning Circulation (AMOC) to a deglacial ice-sheet retreat is studied using the Community Climate System Model version 3 (CCSM3), with a focus on orographic effects rather than meltwater discharge. It is found that the AMOC weakens significantly (41%) in response to the deglacial ice-sheet retreat. The AMOC weakening follows the decrease of the Northern Hemisphere ice-sheet volume linearly, with no evidence of abrupt thresholds. A wind-driven mechanism is proposed to explain the weakening of the AMOC: lowering the Northern Hemisphere ice sheets induces a northward shift of the westerlies, which causes a rapid eastward sea-ice transport and expanded sea-ice cover over the subpolar North Atlantic; this expanded sea ice insulates the ocean from heat loss and leads to suppressed deep convection and a weakened AMOC. A sea ice-ocean positive feedback could be further established between the AMOC decrease and sea-ice expansion.

Description: A significant influence of changes in the westerly winds over the Southern Ocean was proposed as a mechanism to explain a large portion of the glacial atmospheric pCO2 drawdown (Toggweiler et al., 2006). However, additional modelling studies with Earth System Models of Intermediate Complexity do not confirm the size and sometimes even the sign of the impact of southern hemispheric winds on the glacial pCO2 as suggested by Toggweiler (Men- viel et al., 2008; Tschumi et al., 2008, d’Orgeville et al., 2010). We here add to this discussion and explore the potential contribution of changes in the latitudinal position of the winds on Southern Ocean physics and the carbon cycle by using a state-of-the-art ocean general circulation model (MITgcm) in a spatial resolution increasing in the Southern Ocean (2◦ longitude; northern hemisphere: 2◦ latitude; southern hemisphere: 2◦cos(α)). We discuss how the change in carbon cycling is related to the upwelling strength and pattern in the Southern Ocean and how they depend on the changing wind fields and/or the sea ice coverage. While the previous studies explored the impact of the westlies starting from present day or pre-industrial back- ground conditions, we here perform simulations from LGM background climate. Ocean surface conditions are for reasons of consistency taken from output of the COSMOS Earth System model for a pre-industrial control and two LGM runs (Zhang et al., in preparation). Additionally, a northwards shift (by 10◦) of the westerly wind belt as proposed by Toggweiler is investigated.

Description: During the past two decades, several atmospheric and oceanic general circulation models (GCMs) have been enhanced by the capability to explicitly simulate the hydrological cycle of the two stable water isotopes H218O and HDO. They have provided a wealth of understanding regarding changes of the water isotope signals in various archives under different past climate conditions. However, so far the number of fully coupled atmosphere-ocean GCMs with explicit water isotope diagnostics is very limited. Such coupled models are required for a more comprehensive simulation of both past climates as well as related isotope changes in the Earth’s hydrological cycle. Here, we report first results of a newly developed isotope diagnostics within the Earth system model ECHAM5-JSBACH/MPIMOM. Both H218O and HDO and their relevant fractionation processes are included in all compartments and branches of the water cycle within this model. First equilibrium simulations have been performed for both pre-industrial (PI) and Last Glacial Maximum (LGM) boundary conditions. Evaluation of the PI simulation reveals a good overall model performance in accordance with available modern isotope data from vapor measurements, precipitation samples as well as marine records. For precipitation, root-mean-square error (RMSE) between model results and GNIP δ18O data is approx. 3‰. For ocean surface water, model results and GISS δ18O observational data deviate by 1‰ RMSE or less, with strongest differences in the Arctic Ocean. The LGM experiment results in spatially varying isotope depletion in precipitation between -20‰ and 0‰ in agreement with data from various isotope records. The isotope data clearly mirrors a temperature change of similar range. For the ocean surface waters, the simulated isotopic composition shows a strong glacial enrichment in the North Atlantic of more than +0.5‰. In combination with glacial SST changes a LGM calcite δ18O enrichment of +2.5‰ is simulated. Analyses of the simulated Deuterium excess changes with Antarctic ice core data reveal a good model-data agreement and support the hypothesis of rather cool tropical SST during the last glacial maximum.

Description: The Last Glacial Maximum (LGM, ~21k BC) was characterized by a cold and well-stratified ocean as documented by many proxy data from marine sediment cores, representing a benchmark test-bed for climate models. Even though much effort was made to reproduce the glacial ocean structure and associated large-scale ocean circulation, it remains difficult to reconcile the spread between models and mismatches to data. Here we employ a fully comprehensive climate model to explore the role of the ocean stratification on the Atlantic meridional overturning circulation (AMOC). Using glacial boundary conditions we find two states for the glacial ocean structure, depending on differences in the initial salinity stratification. Only one of the two ocean states is in agreement with the available proxy record. However, this state cannot be generated with LGM boundary conditions, implying a quasi steady nature of the glacial ocean water mass configuration. Furthermore, we show that the salinity stratification represents a key control on the spatial configuration and the strength of the AMOC and therefore bears the potential to reconcile the apparent differences among models and data. In combination these findings represent a new dynamical framework for AMOC changes on glacial-interglacial timescales that challenges the conventional evaluation of glacial and deglacial AMOC changes based on an ocean state derived from LGM boundary conditions.

Description: The last deglaciation is one of the best constrained global-scale climate changes documented by climate archives. Nevertheless, understanding of the underlying dynamics is still limited, especially with respect to abrupt climate shifts and associated changes in the Atlantic meridional overturning circulation (AMOC) during glacial and deglacial periods. A fundamental issue is how to obtain an appropriate climate state at the Last Glacial Maximum (LGM, 21 000 yr before present, 21 ka BP) that can be used as an initial condition for deglaciation. With the aid of a comprehensive climate model, we found that initial ocean states play an important role on the equilibrium timescale of the simulated glacial ocean. Independent of the initialization, the climatological surface characteristics are similar and quasi-stationary, even when trends in the deep ocean are still significant, which provides an explanation for the large spread of simulated LGM ocean states among the Paleoclimate Modeling Intercomparison Project phase 2 (PMIP2) models. Accordingly, we emphasize that caution must be taken when alleged quasi-stationary states, inferred on the basis of surface properties, are used as a reference for both model inter-comparison and data model comparison. The simulated ocean state with the most realistic AMOC is characterized by a pronounced vertical stratification, in line with reconstructions. Hosing experiments further suggest that the response of the glacial ocean is dependent on the ocean background state, i.e. only the state with robust stratification shows an overshoot behavior in the North Atlantic. We propose that the salinity stratification represents a key control on the AMOC pattern and its transient response to perturbations. Furthermore, additional experiments suggest that the stratified deep ocean formed prior to the LGM during a time of minimum obliquity (~ 27 ka BP). This indicates that changes in the glacial deep ocean already occur before the last deglaciation. In combination, these findings represent a new paradigm for the LGM and the last deglaciation, which challenges the conventional evaluation of glacial and deglacial AMOC changes based on an ocean state derived from 21 ka BP boundary conditions.

Description: Abrupt decadal climate changes during the last glacial-interglacial cycle are less pronounced during maximum glacial conditions and absent during the Holocene. To further understand the underlying dynamics, we conduct hosing experiments for three climate states: Pre-industrial (PI), 32 kilo years before present (ka BP) and Last Glacial Maximum (LGM). Our simulations show that a stronger temperature inversion between the surface and intermediate layer in the South Labrador Sea induces a faster restart of convective processes (32 ka BP 〉 LGM 〉 PI) during the initial resumption of the Atlantic meridional overturning circulation (AMOC). A few decades later, an AMOC overshoot is mainly linked to the advection of warmer and saltier intermediate-layer water from the tropical Atlantic into the South Labrador Sea, which causes a stronger deep-water formation than that before the freshwater perturbation. This mechanism is most pronounced during the 32 ka BP, weaker during the LGM and absent during the PI.

Description: We explore the impact of a latitudinal shift in the we- sterly wind belt over the Southern Ocean (SO) on the Atlantic meridional overturning circulation (AMOC) and on the carbon cycle for Last Glacial Maximum background conditions using a state-of-the-art ocean general circulation model. For this “westerly wind hypothesis” (Toggweiler et al. 2006) we find that a southward shift in the westerly winds leads to an intensification of the AMOC (northward shift to a weakening). This agrees with other studies (Sijp & England 2009) starting from pre-industrial background, but the responsible processes are different. During deglaciation a gradual shift in westerly winds might thus be responsible for a part of the AMOC enhancement, which is indicated by various studies. The net effects of the changes in ocean circulation lead to a rise in atmospheric pCO2 of less than 10 μatm for both a northward and a southward shift in the winds. For northward shifted winds the zone of upwelling of carbon and nutrient rich waters in the Southern Ocean is expanded, leading to more CO2 out-gassing to the atmosphere but also to an enhanced biological pump in the subpolar region. For southward shifted winds the upwelling region contracts around Antarctica leading to less nutrient export northwards and thus a weakening of the biological pump. A shift in the southern hemisphere westerly wind belt is probably not the domi- nant process which tightly couples atmospheric CO2 rise and Antarctic temperature during deglaciation which is suggested by the ice core data.

Description: A significant influence of changes in the westerly winds over the Southern Ocean was proposed as a mechanism to explain a large portion of the glacial atmospheric pCO2 draw down (Toggweiler et al., 2006).However, additional modelling studies with Earth System Models of Intermediate Complexity do not confirm the size and sometimes even the sign of the impact of southern hemispheric winds on the glacial pCO2 as suggested by Toggweiler (Menviel et al., 2008; Tschumi et al., 2008, dOrgeville et al., 2010). We here add to this discussion and explore the potential contribution of changes in the latitudinal position of the winds on Southern Ocean physics and the carbon cycle by using a state-of-the-art ocean general circulation model (MITgcm) in a spatial resolution increasing in the Southern Ocean. We discuss how the change in carbon cycling is related to the upwelling strength and pattern in the Southern Ocean and how they depend on the changing wind fields and/or the sea ice coverage. While the previous studies explored the impact of the westerlies starting from present day or pre- industrial background conditions, we here perform simulations from Last Glacial Maximum (LGM, about 20,000 year before present) background climate. Ocean surface conditions are taken from output of the COSMOS Earth System model for the LGM run (Zhang et al., submitted). Both a northwards and southwards shift of the westerly wind belt by 10° is investigated. Our results show, that the background climate has a significant influence on the simulations. A northwards shift of the Southern Ocean wind belt leads in both cases (LGM and present day) to a reduction of the Agulhas leakage and thus to a fresher Atlantic sea surface and consequently to reduced Atlantic meridional overturning circulation. A southwards shift in the wind belt leads during LGM background climate to an intensification of the ACC. Patterns how temperature and biogeochemistry (DIC) changes over depth are nearly opposite to those of d’Orgeville et al. (2010). Atmospheric CO2 rises by 5 ppmv for both cases, the northwards or southwards wind shift. In conclusion, we propose that the investigation of changes in the westerly wind belt in the Southern Ocean for present day is not comparable with LGM and simulation studies without LGM background climate are of limited use. References dOrgeville, M.; Sijp, W. P.; England, M. H. Meissner, K. J. (2010) On the control of glacial- interglacial atmospheric CO2 variations by the Southern Hemisphere westerlies Geophysical Research Letters, 37, L21703; doi: 10.1029/2010GL045261. Menviel, L., A. Timmermann, A. Mouchet, and O. Timm (2008), Climate and marine carbon cycle response to changes in the strength of the southern hemispheric westerlies, Paleoceanography, 23, PA4201; doi:10.1029/2008PA001604. Toggweiler, J. R, J. Russell, and S. R. Carson (2006), Midlatitude westerlies, atmospheric CO2, and climate change during the ice ages, Paleoceanography, 21, PA2005; doi: 10.1029/2005PA001154. Tschumi, T., F. Joos, and P. Parekh (2008), How important are Southern Hemisphere wind changes for low glacial carbon dioxide? A model study, Paleoceanography, 23, PA4208, doi: 10.1029/2008PA001592. Zhang, X., G. Lohmann, G. Knorr, X. Xu, Two Ocean States at the Last Glacial Maximum, Climate of the Past, submitted.