Simulating climate conditions in Europe at the time of the Last Glacial Maximum with RCA3

In a recently finished project (1) the Rossby Centre contributed with regional climate model simulations of past climates including a cold glacial climate corresponding to that of the Last Glacial Maximum (LGM, 20.000 years BP). Here, we summarize the work on investigating the regional climate at LGM. A manuscript describing this in more detail has been submitted to Tellus (2).

Lateral boundary conditions for the downscaling with RCA3 were taken from a steady-state integration with the CCSM3 global coupled atmosphere-ocean general circulation model (3,4). Forcing conditions in the global and regional models were taken to be representative of conditions prevailing at the time of the LGM whenever possible. This includes astronomical forcing, greenhouse gas concentrations, ice sheet extent and altitude, land-sea distribution, orography etc (Figure 1). The regional climate model simulation was performed at 50-km horizontal resolution for a 50-year time period.

Land areas and elevation in RCA3, Rossby Centre newsletter nov 2009
Fig 1. The distribution of land areas and elevation (m) in RCA3 for the LGM simulations. Grid boxes with less than 20% land cover are not filled.
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The results of the simulations show a very cold and dry climate in Europe with annual mean temperatures below 0 degree C in all of Europe north of about 50 degree N and also in high-altitude regions in southern Europe (Figure 2). In winter the situation is even more striking with the 0 degree C line encompassing basically all of continental Europe and monthly mean temperatures below -40 degrees C over the northern parts of the ice sheet. During summer, the area with the lowest temperatures is more confined to the ice sheet, the extent of which is readily visible in Fig. 2 The very large differences compared to the recent past are partly due to the perennial snow/ice cover in parts of northern Europe but also a consequence of the high elevation of the ice sheet.

Mean temperatures, Rossby Centre Newsletter nov 2009
Fig 2. Difference in mean temperatures of the warmest (left) and coldest (middle) month and annual mean (right) compared to the recent past climate. Corresponding anomalies as given by proxy based reconstructions are denoted in the filled circles. Units are degree C.
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The results from regions outside the ice sheet are compared with pollen-based proxy data of temperature differences with respect to the present climate (Figure 2). We note that the model results are within the uncertainty ranges for most proxy records. Given the very large uncertainty margins of the proxy data including problematic chronologies and the sparseness of data the conclusion is that the agreement between model and proxies is relatively good. This conclusion is corroborated by a comparison of simulated sea-surface temperatures in CCSM3 to proxy data for the North Atlantic showing a fair agreement albeit with a possible cold bias during summer.

Some general conclusions on the simulated regional climate at LGM that can be drawn from the studies include: i) The climate is warm enough in southern Europe to prevent the ice sheet expanding in this direction. ii) The climate in northern Europe, over the ice sheet surface, is cold enough to sustain the ice sheet. Monthly mean temperature seldom rises above 0 degree C, and over most parts of the ice sheet at least 70% of the precipitation falls as snow. If 60-100% of the precipitation accumulates on the ice sheet it would grow with 0.3-0.5 m/year. iii) The precipitation in the north-eastern part of the Fennoscandian ice sheet is very low, indicating that only limited ice sheet growth was possible in that region.

References:

1) Kjellström, E., Brandefelt, J., Näslund, J.O., Smith, B., Strandberg, G. and Wohlfarth, B, 2009: Climate conditions in Sweden in a 100,000 year time perspective. Svensk Kärnbränslehantering AB, report TR-09-04, Swedish Nuclear Fuel and Waste Management, SE-10124 Stockholm, Sweden, 139 pp.
2) Strandberg, G., Brandefelt, J., Kjellström E., Smith, B., 2009. High resolution simulation of Last Glacial Maximum climate and vegetation in Europe. Tellus, submitted.
3) Collins W D, Bitz C M, Blackmon M L, Bonan G B, Bretherton C S, Carton J A, Chang S, Doney C, Hack J J, Henderson T B, Kiehl J T, Large W G, McKenna D S, Santer B D, Smith R D, 2006. The Community Climate System Model (CCSM3), Journal of Climate 19: 2122–2143.
4) Brandefelt, J. & Otto-Bliesner, B. L. 2009: Equilibration and variability in a last glacial maximum climate simulation with CCSM3. Geophysical Research Letters 36, L19712, doi:10.1029/2009GL040364