Interaction of land surface processes and the atmosphere in the Arctic - sensitivities and extremes

For several years, the Arctic has now been in the focus of scientific debate on climate
\nchange. It is a region of high spatio-temporal climate variability and additionally a region
\nof high climate sensitivity due to strong feedback processes like the ice-albedo feedback. As
\nthere is a strong linkage to the global climate system, changes in the Arctic impact onto the
\nglobal climate. In modeling the Arctic climate, regional climate models are an important tool
\nbecause with their high resolution they provide the possibility to account for the horizontally
\nheterogeneous soil and surface characteristics. Here, the regional climate model HIRHAM is
\nused with 25km resolution for the analysis of spatial patterns, variability and trends of the
\nArctic climate and temperature-derived indices describing climate extremes.
\nInter-annual temperature variability (ITV) for present-day conditions (1958 to 2008) is
\nexamined from station data, the ERA40 re-analysis and HIRHAM results. It shows a pronounced
\ndecadal variability and specific regional and seasonal characteristics. Seasonal temperatures
\nin general show warming trends, though they are mostly not statistically significant.
\nIntra-seasonal extreme temperature range (ETR) trends were of mixed sign and only
\nsignificant from station data over the eastern Russian Arctic. In general, the spatial pattern
\nand magnitude of the Arctic temperature variability, both of seasonal temperature and
\nintra-seasonal ETR, are well reproduced by HIRHAM.
\nThe large variability of the Arctic temperature, which is inherent in the analysis period,
\ndemonstrates that natural variability is an important factor in the Arctic climate. This
\nvariability is not restricted to climate means but also appears in temperature extremes. An
\nanalysis of station-derived an re-analysis-based climate indices shows complex behavior, some
\nmeasures like frost days show consistent decrease (i.e. warming), while others like cold spell
\ndays provide a more diverse picture. As with seasonal temperatures, only few trends are found
\nstatistically significant. The indices examined exhibit strong inter-annual and decadal-scale
\nvariability and heterogeneous spatial patterns.
\nThese climate indices are then employed in the validation of the HIRHAM model. The
\nmodel well reproduces trends and variability of most indices while there is an offset in some
\nabsolute values (e.g. frost days, growing degree days). Other measures like cold and warm
\nspells are calculated with non-systematic biases; deviations in trends and variability occur in
\nsummer for cold spells and in spring and summer for warm spells due to an earlier spring
\nwarming and a too low variability of the maximum temperature over sea ice in HIRHAM.
\nHIRHAM is furthermore used as a downscaling tool for future projections (ECHAM5/MPIOM
\noutput under the IPCC scenario SRES A1B). The strong increase in mean annual air
\ntemperature (5–8 K) is expected to increase active layer thickness and permafrost boundaries
\nwill move northwards. On top of this general warming trend, the additional analysis of
\nfuture changes, using the mean conditions for the warmer climate, highlights some particularly
\nvulnerable regions (West Siberian Plain, Laptev Sea coast, Canadian Archipelago), which are
\nprojected to be warmer, to experience increased warm spells and to be wetter in summer; all
\nthis contributes to amplify the permafrost degradation initiated by the general warming.
\nDifferent realizations of HIRHAM are run for a sensitivity study looking into the importance
\nof land-surface-conditions for climate model projections. The different model setups are:
\n(1) the incorporation of freezing/thawing of soil moisture, (2) the inclusion of top organic soil
\nhorizons typical for the Arctic and (3) a vegetation shift due to a changing climate. Direct
\nthermal responses in 2m air temperature and turbulent heat fluxes over land lead to changes
\nin mean sea level pressure and geopotential height throughout the Arctic. This points to
\nthe importance of dynamical feedbacks within the atmosphere-land system. Land and soil
\nprocesses have a distinct remote influence on large scale circulation patterns in addition to
\ntheir direct, regional effects. The projected changes are clearly afflicted with uncertainties
\ndue to the different setups for land-surface-conditions; the highest temperature uncertainties
\nare found over tundra regions. This demonstrates that for an improvement of the land-surface
\nscheme of the HIRHAM model, all three representations of land-surface-processes have to be
\nincorporated.

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