Description: ERA-Interim reanalysis data and data of the Hadley Centre Global Environmental Model version 2 (HadGEM2) are compared with continuous meteorological observations of near-surface wind and temperature carried out for more than 30 years at Neumayer station, situated on the Ekstro¨m Ice Shelf of Antarctica. Significant temperature correlations between Neumayer climate and the climate of both the interior of the Antarctic continent and oceanic regions north of Neumayer are investigated using observational data and model data. Mean sea level pressure fluctuations at Neumayer can be connected to changes in the Southern Annular Mode (SAM). Shortcomings in the ERA-Interim reanalysis data with spurious trends of up to 7 C over 31 years are identified at several places in Antarctica. Furthermore, it is shown that katabatic winds in both the ERA-Interim reanalysis data and in the HadGEM2 climate model are underrepresented in frequency and speed, presumably due to the problems in representing topography in these relatively coarse resolution models. This may be one reason for the positive 2m air temperature bias of 3 C in the models at Neumayer station. The results of this study reemphasize that climatic trends in regions with a low station density can not be assessed solely from model data. Thus, it is absolutely necessary to maintain polar observatories such as Neumayer station to quantify climate change over the Southern Ocean and Antarctica.

Description: The surface of the world’s oceans has been warming since the beginning of industrialization. In addition to this, multidecadal sea surface temperature (SST) variations of internal origin exist. Evidence suggests that the North Atlantic Ocean exhibits the strongest multidecadal SST variations and that these variations are connected to the overturning circulation. This work investigates the extent to which these internal multidecadal variations have contributed to enhancing or diminishing the trend induced by the external radiative forcing, globally and in the North Atlantic. A model study is carried out wherein the analyses of a long control simulation with constant radiative forcing at preindustrial level and of an ensemble of simulations with historical forcing from 1850 until 2005 are combined. First, it is noted that global SST trends calculated from the different historical simulations are similar, while there is a large disagreement between the North Atlantic SST trends. Then the control simulation is analyzed, where a relationship between SST anomalies and anomalies in the Atlantic meridional overturning circulation (AMOC) for multidecadal and longer time scales is identified. This relationship enables the extraction of the AMOC-related SST variability from each individual member of the ensemble of historical simulations and then the calculation of the SST trends with the AMOC-related variability excluded. For the global SST trends this causes only a little difference while SST trends with AMOC-related variability excluded for the North Atlantic show closer agreement than with the AMOC-related variability included. From this it is concluded that AMOC variability has contributed significantly to North Atlantic SST trends since the mid nineteenth century.

Description: We analyse the ENSO-like variability in the newly established global climate model ECHAM-FESOM. This is the first global coupled model with an ocean module supporting unstructured meshes. The Finite Element Sea Ice - Ocean Model (FESOM) is a dynamical ocean model development at AWI Bremerhaven. In contrast to conventional ocean models, the spatial discretization is based on the Finite Element method. This method allows a variable spatial resolution of the triangular surface mesh with high mesh-stretching factors. FESOM has been used in numerous recent, yet uncoupled, studies. Its validation in the climate context is still ongoing activity. ECHAM is a state-of-the-art spectral atmosphere model developed at the Max-Planck-Institute for Meteorology in Hamburg for climate modelling purposes. We apply the latest generation, version 6, with a T63L47 resolution. FESOM and ECHAM are currently coupled by the OASIS3-MCT coupler and a structured exchange mesh. We analyse two simulation runs that differ in the tropical ocean mesh resolution between 15°N and 15°S. Setup 1 uses a reference mesh with a resolution of about 1° in the tropics. In contrast, Setup 2 has a higher resolution of 1/4° (in a narrow band around the equator) that gradually decreases to 1°. Outside the tropics both meshes are identical. Modelled Nino3.4 indices are compared with observations and the influence of the mesh resolution is discussed.

Description: The timing of phenological phases, such as leaf unfolding of trees, is partly driven by the weather. The impact of weather on the spring phenology of a selection of tree species in the phenological garden at Valentia Observatory, Caherciveen, County Kerry, Ireland, for the period 1967-2004 was assessed. In particular, the beginning of the growing season (BGS), as determined by the date of leaf unfolding, was considered. The impact on BGS of a range of meteorological parameters and derived parameters, including minimum, maximum and mean air temperature (°C), soil temperature (°C), rainfall (mm), sunshine (hours), number of air frost days and number of rain days was investigated. These parameters were averaged or summed over the months of February, March and April and linear regression analysis with BGS was carried out for several tree species. The results showed that the regression coefficients of BGS on minimum air and soil temperature were highest, with Populus canescens and Populus tremula showing the strongest relationships. Temperature parameters were found to be more important in driving advancing phenology than any other meteorological parameter tested.

Description: Local climate is dependent on the global climate. Here, a global picture on climate change is presented using predictions from the EC-Earth simulations for the end of the century. The results indicate a general rise in annual mean temperature everywhere: 2-4 degrees (global average), 1-6 degrees (over Europe) and 1-4 degrees (Ireland). Changes in precipitation are more varied: large increases (〉100%) at high northern latitudes and in the equatorial Pacific but decreases of more than 50% over the subtropics; winters in Europe are predicted to be up to 20% wetter and summers up to 20% drier. Changes in extremes are also presented in this chapter.

Description: Coupled climate models with increasing greenhouse gas concentrations and changing aerosol concentrations indicate an increase in the westerly airflow in mid-latitudes in winter as well as more extreme storms and precipitation events. However, declining Arctic sea ice may alter this projection. A sensitivity experiment run with the EC-Earth global model with Arctic sea ice removed shows a weakening of the westerly flow over Ireland. Such a change would increase the likelihood of cold continental air outbreaks over Ireland during winter.

Description: The evolution of snow and ice thicknesses and temperature in an Arctic lake was investigated using two models: a high-resolution, time-dependent model (HIGHTSI) and a quasi-steady two-layer model on top of a lake model (FLake). In situ observations and a Numerical Weather Prediction model (HIRLAM) were used for the forcing data. HIRLAM forecasts, after orography correction, were comparable with the in situ data. Both lake-ice models predicted the ice thickness (accuracy 5 cm), surface temperature (accuracy 2–3 °C in winter, better in spring), and ice-breakup date (accuracy better than five days) well. HIGHTSI was better for ice thickness and ice-breakup date, while FLake gave better freezing date. Snow thickness outcome was worse, in particular for the melting season. Surface temperature was highly sensitive to air temperature, stratification and albedo, and the largest errors (positively biased) resulted in strongly stable conditions.