[1] Climate model simulations consistently show that in response to greenhouse gas forcing surface temperatures over land increase more rapidly than over sea. The enhanced warming over land is not simply a transient effect, since it is also present in equilibrium conditions. We examine 20 models from the IPCC AR4 database. The global land/sea warming ratio varies in the range 1.36 -1.84, independent of global mean temperature change. In the presence of increasing radiative forcing, the warming ratio for a single model is fairly constant in time, implying that the land/sea temperature difference increases with time. The warming ratio varies with latitude, with a minimum in equatorial latitudes, and maxima in the subtropics. A simple explanation for these findings is provided, and comparisons are made with observations. For the low-latitude (40°S-40°N) mean, the models suggest a warming ratio of 1.51 ± 0.13, while recent observations suggest a ratio of 1.54 ± 0.09. Citation: Sutton, R. T., B. Dong, and J. M. Gregory

European climate exhibits variability on a wide range of timescales. Understanding the nature and drivers of this variability is an essential step in developing robust climate predictions and risk assessments. The Atlantic Ocean has been suggested as an important driver of variability in European climate on decadal timescales 1-6 , but the importance of this influence in recent decades has been unclear, partly because of difficulties in separating the influence of the Atlantic Ocean from other contributions, for example, from the tropical Pacific Ocean and the stratosphere 7-12 . Here we analyse four data sets derived from observations to show that, during the 1990s, there was a substantial shift in European climate towards a pattern characterized by anomalously wet summers in northern Europe, and hot, dry, summers in southern Europe, with related shifts in spring and autumn. These changes in climate coincided with a substantial warming of the North Atlantic Ocean 13 , towards a state last seen in the 1950s. The patterns of European climate change in the 1990s are consistent with earlier changes attributed to the influence of the North Atlantic Ocean 4,6 and provide compelling evidence that the Atlantic Ocean was the key driver. Our results suggest that the recent pattern of anomalies in European climate will persist as long as the North Atlantic Ocean remains anomalously warm.The observed pattern of multidecadal variation in North Atlantic sea surface temperatures (SSTs) has become known as the Atlantic Multidecadal Oscillation 4,5,14 (AMO; Fig. 1). The instrumental record shows that, relative to the average temperature of the rest of the world's oceans, the temperature of the North Atlantic Ocean has fluctuated between anomalously warm and anomalously cool phases, each lasting several decades at a time. Palaeoclimate records suggest that similar variations extend much further back in time 3,15 . In climate model simulations, AMO-like variations in SST are closely related to variations in the Atlantic Meridional Overturning Circulation 3,5 (AMOC). These variations can arise in the absence of any external forcings 5 , but can also be influenced by such forcings 16 . The extent to which recent variations in the AMO have been influenced by external forcings is a subject of active research 16,17 . Here, however, we focus on the impacts of the AMO rather than its drivers.Climate models suggest that variations in the AMO can cause significant climate impacts in many regions, including North Africa, North and South America, South and East Asia and Europe 1-6,18-21 . For Europe, previous research has shown that the warm state of the North Atlantic Ocean in the period 1931-1960, relative to the period 1961-1990, forced anomalous low pressure and increased precipitation over western Europe in summer 4 . These impacts are consistent with the observed correlation on decadal timescales

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A new coupled general circulation climate model developed at the Met Office's Hadley Centre is presented, and aspects of its performance in climate simulations run for the Intergovernmental Panel on Climate Change Fourth Assessment Report (IPCC AR4) documented with reference to previous models. The Hadley Centre Global Environmental Model version 1 (HadGEM1) is built around a new atmospheric dynamical core; uses higher resolution than the previous Hadley Centre model, HadCM3; and contains several improvements in its formulation including interactive atmospheric aerosols (sulphate, black carbon, biomass burning, and sea salt) plus their direct and indirect effects. The ocean component also has higher resolution and incorporates a sea ice component more advanced than HadCM3 in terms of both dynamics and thermodynamics. HadGEM1 thus permits experiments including some interactive processes not feasible with HadCM3. The simulation of present-day mean climate in HadGEM1 is significantly better overall in comparison to HadCM3, although some deficiencies exist in the simulation of tropical climate and El Niño variability. We quantify the overall improvement using a quasi-objective climate index encompassing a range of atmospheric, oceanic, and sea ice variables. It arises partly from higher resolution but also from greater fidelity in modeling dynamical and physical processes, for example, in the representation of clouds and sea ice. HadGEM1 has a similar effective climate sensitivity (2.8 K) to a CO2 doubling as HadCM3 (3.1 K), although there are significant regional differences in their response patterns, especially in the Tropics. HadGEM1 is anticipated to be used as the basis both for higher-resolution and higher-complexity Earth System studies in the near future.