Beschreibung: Here we establish a spatio-temporal evolution of the sea-surface temperatures in the North Atlantic over Dansgaard-Oeschger (DO) events 5-8 (c.30-40ka) using the proxy surrogate reconstruction method. Proxy data suggest a large variability in North Atlantic sea-surface temperatures during the DO-events of the last glacial period. However, proxy data availability is limited and cannot provide a full spatial picture of the oceanic changes. Therefore, we combine fully coupled, general circulation model simulations with planktic foraminifera based sea-surface temperature reconstructions to obtain a broader spatial picture of the ocean state during DO-events 5-8. The resulting spatial sea-surface temperature patterns agree over a number of different general circulation models and simulations. We find that sea-surface temperature variability over the DO-events is characterized by colder conditions in the subpolar North Atlantic during stadials than during interstadials, and the variability is linked to changes in the Atlantic Meridional Overturning circulation, and in the sea-ice cover. Forced simulations are needed to capture the strength of the temperature variability and to reconstruct the variability in other climatic records not directly linked to the sea-surface temperature reconstructions. This is the first time the proxy surrogate reconstruction method has been applied to oceanic variability during MIS3. Our results remain robust, even when age uncertainties of proxy data, the number of available temperature reconstructions, and different climate models, are considered. However, we also highlight shortcomings of the methodology that should be addressed in future implementations.

Beschreibung: Here we establish a spatio-temporal evolution of the sea-surface temperature anomalies in the North Atlantic over Dansgaard Oeschger (DO) events 5-8 (30-40ka) using the proxy surrogate reconstruction method. We combine fully coupled, general circulation model simulations with planktic foraminifera based sea-surface temperature reconstructions to obtain a broader spatial picture of the ocean state during DO-events 5-8. Here, the model simulations consists of 10 simulations with HadCM3, 8 runs without freshwater forcing and 2 runs with freshwater forcing. These simulations were run by Paul J. Valdes, Joy S. Singarayer and William H.G. Roberts. A description of the simulations can be found in Singarayer and Valdes (2010). All model simulations were run using the University of Bristol's ACRC facilities, (www.acrc.bris.ac.uk). This is the first time the proxy surrogate reconstruction method has been applied to oceanic variability during MIS3. The sea-surface temperature reconstructions should be used with care.

Beschreibung: Here, we present datasets of diatom abundance and a diatom-based sea surface temperature reconstruction from the Iceland Basin. The 419.5 cm long marine sediment core DA12-11/2-GC01 was recovered in September 2012 by the Danish R/V Dana using a gravity corer. The core was taken from a location at the northern edge of the Gardar Drift and the eastern edge of the Björn Drift in the Iceland Basin (61.36.536 °N, 20.42.164 °W) at a water depth of 2120 m. Diatoms were analysed at 5 cm intervals over the upper 2 m of core, representing the period 6100-0 cal years BP, with chronological constraints provided by 6 radiocarbon dates. Diatoms were analysed using standard procedures, including the identification and counting of a minimum of 300 diatom per sample, which allowed calculation of the percentage abundances of different species. These abundances were converted into estimates of sea surface temperature using the weight averaging partial least squares transfer function approach.

Beschreibung: Here, we have calculated new sea-surface temperature estimates over Dansgaard-Oeschger (DO) events 5-8 (c. 30-40 ka) from 14 sites in the North Atlantic, based on previously published planktic foraminifera relative abundance datasets. These proxy records suggest a large variability in North Atlantic sea-surface temperatures during DO-events of the last glacial period. However, proxy data availability is limited and cannot provide a full spatial picture of the oceanic changes. Therefore, we combine fully coupled, general circulation model simulations with the planktic foraminifera based sea-surface temperature reconstructions to obtain a broader spatial picture of the ocean state during DO-events 5-8. The resulting spatial sea-surface temperature patterns agree over a number of different general circulation models and simulations. We find that sea-surface temperature variability over the DO-events is characterized by colder conditions in the subpolar North Atlantic during stadials than during interstadials, and the variability is linked to changes in the AMOC, and in the sea-ice cover. Forced simulations are needed to capture the strength of the temperature variability and to reconstruct the variability in other climate records not directly linked to the sea-surface temperature reconstructions.

Beschreibung: The Greenland Ice Sheet (GrIS) mass loss has been accelerating at a rate of about 20 ± 10 Gt/yr2 since the end of the 1990's, with around 60 % of this mass loss directly attributed to enhanced surface meltwater runoff. However, in the climate and glaciology communities, different approaches exist on how to model the different surface mass balance (SMB) components using: (1) complex physically-based climate models which are computationally expensive; (2) intermediate complexity energy balance models; (3) simple and fast positive degree day models which base their inferences on statistical principles and are computationally highly efficient. Additionally, many of these models compute the SMB components based on different spatial and temporal resolutions, with different forcing fields as well as different ice sheet topographies and extents, making inter-comparison difficult. In the GrIS SMB model intercomparison project (GrSMBMIP) we address these issues by forcing each model with the same data (i.e., the ERA-Interim reanalysis) except for two global models for which this forcing is limited to the oceanic conditions, and at the same time by interpolating all modelled results onto a common ice sheet mask at 1 km horizontal resolution for the common period 1980–2012. The SMB outputs from 13 models are then compared over the GrIS to (1) SMB estimates using a combination of gravimetric remote sensing data from GRACE and measured ice discharge, (2) ice cores, snow pits, in-situ SMB observations, and (3) remotely sensed bare ice extent from MODerate-resolution Imaging Spectroradiometer (MODIS). Our results reveal that the mean GrIS SMB of all 13 models has been positive between 1980 and 2012 with an average of 340 ± Gt/yr, but has decreased at an average rate of −7.3 Gt/yr2 (with a significance of 96 %), mainly driven by an increase of 8.0 Gt/yr2 (with a significance of 98 %) in meltwater runoff. Spatially, the largest spread among models can be found around the margins of the ice sheet, highlighting the need for accurate representation of the GrIS ablation zone extent and processes driving the surface melt. In addition, a higher density of in-situ SMB observations is required, especially in the south-east accumulation zone, where the model spread can reach 2 mWE/yr due to large discrepancies in modelled snowfall accumulation. Overall, polar regional climate models (RCMs) perform the best compared to observations, in particular for simulating precipitation patterns. However, other simpler and faster models have biases of same order than RCMs with observations and remain then useful tools for long-term simulations. Finally, it is interesting to note that the ensemble mean of the 13 models produces the best estimate of the present day SMB relative to observations, suggesting that biases are not systematic among models.