Alexandre Belleflamme

Current and future atmospheric circulation at 500 hPa over Greenland simulated by the CMIP3 and CMIP5 global models

The Greenland ice sheet is projected to be strongly affected by global warming. These projections are either issued from downscaling methods (such as Regional Climate Models) or they come directly from General Circulation Models (GCMs). In this context, it is necessary to evaluate the accuracy of the daily atmospheric circulation simulated by the GCMs, since it is used as forcing for downscaling methods. Thus, we use an automatic circulation type classification based on two indices (Euclidean distance and Spearman rank correlation using the daily 500 hPa geopotential height) to evaluate the ability of the GCMs from both CMIP3 and CMIP5 databases to simulate the main circulation types over Greenland during summer. For each circulation type, the GCMs are compared to three reanalysis datasets on the basis of their frequency and persistence differences. For the current climate (1961–1990), we show that most of the GCMs do not reproduce the expected frequency and the persistence of the circulation types and that they simulate poorly the observed daily variability of the general circulation. Only a few GCMs can be used as reliable forcings for downscaling methods over Greenland. Finally, when applying the same approach to the future projections of the GCMs, no significant change in the atmospheric circulation over Greenland is detected, besides a generalised increase of the geopotential height due to a uniform warming of the atmosphere.

Estimation of the Sea Level Rise by 2100 Resulting from Changes in the Surface Mass Balance of the Greenland Ice Sheet

The Surface Mass Balance (SMB) can be seen, in first approximation, as the water mass gained by the winter snowfall accumulation minus the mass lost by the meltwater run-off in summer. The mass gain from rainfall as well as the mass loss from erosion from the net water fluxes (the sum of the evaporation, sublimation, deposition and condensation) and from the wind (blowing snow) are negligible in the SMB equation of the Greenland Ice Sheet (GrIS) compared to the snowfall and the melt (Box et al., 2004). The ice sheet mass balance takes also into account the mass loss from iceberg calving. Consequences of a warmer climate on the Greenland ice sheet SMB will be a thickening inland, due to increased solid precipitation, and a thinning at the Greenland ice sheet periphery, due to an increasing surface melt. A climatic warming increases the snow and ice melting in summer but it enhances also evaporation above the ocean. This leads to higher moisture transport inland and, consequently, higher precipitation. The response of the iceberg calving to the climate change could be an acceleration of the glacier flow (Nick et al., 2009; Zwally et al., 2002) but these projections are very uncertain (Sundal et al., 2011) and a lot of developments are still needed in the glaciology models for improving our knowledge and modelling of the Greenland ice sheet dynamics. That is why we will focus our study only on the SMB of the Greenland ice sheet. The IPCC (Intergovernmental Panel on Climate Change) projects, in response to global warming induced by human activities, that the run-off increase will exceed the precipitation increase and therefore that the currently observed surface melting of the Greenland ice sheet (Fettweis et al., 2011b; Tedesco et al., 2011; Van den Broeke et al., 2009) will continue and intensify during the next decades (IPCC, 2007). An increasing freshwater flux from the Greenland ice sheet melting could perturb the thermohaline circulation (by reducing the density contrast driving this last one) in the North Atlantic including the drift which tempers the European climate. In addition, an enduring Greenland ice sheet melting, combined with the thermal expansion of the oceans and the melt of continental glaciers, will raise the sea level with well-known consequences for countries such as the Netherlands, Bangladesh,... The contribution of the Greenland ice sheet SMB decrease to the sea level rise is currently evaluated to be 5-10 cm by 2100 (Gregory and Huybrechts, 2006; Fettweis et al., Estimation of the Sea Level Rise by 2100 Resulting from Changes in the Surface Mass Balance of the Greenland Ice Sheet 25