Matthias Gröger

Atmospheric controlled freshwater release at the Laptev Sea continental margin

Considerable interannual differences were observed in river water and sea-ice meltwater inventory values derived from δ18O and salinity data in the Eurasian Basin along the continental margin of the Laptev Sea in the summers of 1993 and 1995, and in the summers of 2005 and 2006 during Nansen and Amundsen Basins Observational system (NABOS) expeditions. The annually different pattern in river and sea-ice meltwater inventories remain closely linked for all of the years studied, which indicates that source regions and transport mechanisms for both river water and sea-ice formation are largely similar over the relatively shallow Laptev Sea Shelf. A simple Ekman trajectory model for surface Lagrangian particles based solely on wind forcing can explain the main features observed between years with significantly different wind patterns and vorticities, and can also explain differences in river water distributions observed for years with a generally similar offshore wind setting. An index based on this simplified trajectory model is rather similar to the vorticity index, but reflects the hydrology on the shelf better for distinctive years. This index is not correlated with the Arctic Oscillation, but rather with a local mode of oscillation, which controls the outflow and distribution of the Eurasian Basin major freshwater source on an annual timescale.

Regionally coupled atmosphere‐ocean‐sea ice‐marine biogeochemistry model ROM: 1. Description and validation

The general circulation models used to simulate global climate typically feature resolution too coarse to reproduce many smaller-scale processes, which are crucial to determining the regional responses to climate change. A novel approach to downscale climate change scenarios is presented which includes the interactions between the North Atlantic Ocean and the European shelves as well as their impact on the North Atlantic and European climate. The goal of this paper is to introduce the global ocean-regional atmosphere coupling concept and to show the potential benefits of this model system to simulate present-day climate. A global ocean-sea ice-marine biogeochemistry model (MPIOM/HAMOCC) with regionally high horizontal resolution is coupled to an atmospheric regional model (REMO) and global terrestrial hydrology model (HD) via the OASIS coupler. Moreover, results obtained with ROM using NCEP/NCAR reanalysis and ECHAM5/MPIOM CMIP3 historical simulations as boundary conditions are presented and discussed for the North Atlantic and North European region. The validation of all the model components, i.e., ocean, atmosphere, terrestrial hydrology, and ocean biogeochemistry is performed and discussed. The careful and detailed validation of ROM provides evidence that the proposed model system improves the simulation of many aspects of the regional climate, remarkably the ocean, even though some biases persist in other model components, thus leaving potential for future improvement. We conclude that ROM is a powerful tool to estimate possible impacts of climate change on the regional scale.

Changes in distribution of brine waters on the Laptev Sea shelf in 2007

Combined salinity and δ18O data from summer 2007 reveal a significant change in brine production in the Laptev Sea relative to summer 1994. The distribution of river water and brine enriched waters on the Laptev Sea shelf is derived based on mass balance calculations using salinity and δ18O data. While in 1994 maximal influence of brines is seen within bottom waters [Bauch et al., 2009a], in 2007 the influence of brines is highest within the surface layer and only a moderate influence of brines is observed in the bottom layer. In contrast to 2007, salinity and δ18O data from summer 1994 clearly identify a locally formed brine enriched bottom water mass as mixing endmember between surface layer and inner shelf waters on one side and with higher salinity water from the outer Laptev Sea on the other side. In 2007 the brine enriched waters are predominantly part of the surface regime and the mixing endmember between surface layer and outer shelf waters is replaced by a relatively salty bottom water mass. This relatively salty bottom water probably originates from the western Laptev Sea. The inverted distribution of brines in the water column in 2007 relative to 1994 suggests a less effective winter sea-ice formation in winter 2006/2007 combined with advection of more saline waters from the western Laptev Sea or the outer shelf precedent to 1 the climatically extreme summer 2007. The observed changes result in an altered export of waters from the Laptev Sea to the Arctic Ocean halocline.

Vegetation-climate feedbacks in transient simulations over the last interglacial (128 000 - 113 000 yr BP)

Abstract The presently developed MPI/UW 3D — Earth system model for long-term integrations is applied to simulate the climate of the last interglacial. The model consists of an atmospheric and oceanic general circulation model, a dynamical terrestrial vegetation model and a marine carbon cycle model. The model was forced with time-varying insolation from 129000 to 113000 years before present (yr BP), revealing substantial feedbacks from the land biosphere on climate. These turned out to be important both for the simulated temperature in high northern latitudes and for the precipitation in the northern hemisphere monsoon belt. During the Eemian warm period, the simulated boreal forest extends in many places till the Arctic Ocean, and during the following cold period the transition between tundra and taiga migrated further south. Furthermore, associated albedo changes strongly amplify the simulated temperature changes. The intensified summer insolation during the Eemian leads to a higher precipitation over continents in the northern hemisphere. The strongest response is seen in the tropics and the African-Asian monsoon belt due to increased land-sea temperature contrasts. Vegetation is established in the Western Sahara desert. Compared to simulations with land vegetation prescribed at presentday pattern, the amount of precipitation in the Sahara is more than twice as large. The simulations show strong impacts on the time-transient climate response by triggering nonlinear delays and accelerations seen in various atmospheric and oceanic temperature time series. The simulated storage of carbon in the terrestrial biosphere is relatively large. The carbon storage in land vegetation is increased by more than 10% during the Eemian compared to the following cold period. The associated changes in storage in soil and litter account for more than 100 GtC.