Observed and projected climate change in the European region during the twentieth and twenty-first centuries according to Feddema

Climate change in the European region during the twentieth and twenty-first centuries is analyzed according to Feddema’s method. Precipitation and air temperature data from the twentieth century are taken from the Climatic Research Unit, while data for the twenty-first century are taken from the ENSEMBLES climate change project. The latter were bias-corrected to ensure homogeneity across the twentieth and twenty-first centuries. Climate classes based on monthly and annual values of potential evapotranspiration, precipitation and their ratio, are defined for 30-year averages, from which trend and spatial agreement analysis are calculated. There are separate classes for annual values and for intra-annual variation. The results indicate that the change of annual climate characteristics will be much more intense in the twenty-first than it was in the twentieth century. The dominant process in the projections is warming, mostly via cold to cool (about 45% of grid points) in north Europe and cool to warm (about 8% of grid points) transformations. The second most important process is the drying of moderately moist classes affecting about 10% of the grid points in south Europe. Changes in intra-annual variability classes are more common than changes in the annual ones during the twentieth century. The chance of increase in intra-annual temperature variation from high to extreme is about 5% during the course of the twentieth century, and about 10% in the following century.     

Ocean heat transport into the Arctic in the twentieth and twenty-first century in EC-Earth

The ocean heat transport into the Arctic and the heat budget of the Barents Sea are analyzed in an ensemble of historical and future climate simulations performed with the global coupled climate model EC-Earth. The zonally integrated northward heat flux in the ocean at 70°N is strongly enhanced and compensates for a reduction of its atmospheric counterpart in the twenty first century. Although an increase in the northward heat transport occurs through all of Fram Strait, Canadian Archipelago, Bering Strait and Barents Sea Opening, it is the latter which dominates the increase in ocean heat transport into the Arctic. Increased temperature of the northward transported Atlantic water masses are the main reason for the enhancement of the ocean heat transport. The natural variability in the heat transport into the Barents Sea is caused to the same extent by variations in temperature and volume transport. Large ocean heat transports lead to reduced ice and higher atmospheric temperature in the Barents Sea area and are related to the positive phase of the North Atlantic Oscillation. The net ocean heat transport into the Barents Sea grows until about year 2050. Thereafter, both heat and volume fluxes out of the Barents Sea through the section between Franz Josef Land and Novaya Zemlya are strongly enhanced and compensate for all further increase in the inflow through the Barents Sea Opening. Most of the heat transported by the ocean into the Barents Sea is passed to the atmosphere and contributes to warming of the atmosphere and Arctic temperature amplification. Latent and sensible heat fluxes are enhanced. Net surface long-wave and solar radiation are enhanced upward and downward, respectively and are almost compensating each other. We find that the changes in the surface heat fluxes are mainly caused by the vanishing sea ice in the twenty first century. The increasing ocean heat transport leads to enhanced bottom ice melt and to an extension of the area with bottom ice melt further northward. However, no indication for a substantial impact of the increased heat transport on ice melt in the Central Arctic is found. Most of the heat that is not passed to the atmosphere in the Barents Sea is stored in the Arctic intermediate layer of Atlantic water, which is increasingly pronounced in the twenty first century.     

Simulated Antarctic precipitation and surface mass balance at the end of the twentieth and twenty-first centuries

The aim of this work is to assess potential future Antarctic surface mass balance changes, the underlying mechanisms, and the impact of these changes on global sea level. To this end, this paper presents simulations of the Antarctic climate for the end of the twentieth and twenty-first centuries. The simulations were carried out with a stretched-grid atmospheric general circulation model, allowing for high horizontal resolution (60 km) over Antarctica. It is found that the simulated present-day surface mass balance is skilful on continental scales. Errors on regional scales are moderate when observed sea surface conditions are used; more significant regional biases appear when sea surface conditions from a coupled model run are prescribed. The simulated Antarctic surface mass balance increases by 32 mm water equivalent per year in the next century, corresponding to a sea level decrease of 1.2 mm year⁻¹ by the end of the twenty-first century. This surface mass balance increase is largely due to precipitation changes, while changes in snow melt and turbulent latent surface fluxes are weak. The temperature increase leads to an increased moisture transport towards the interior of the continent because of the higher moisture holding capacity of warmer air, but changes in atmospheric dynamics, in particular off the Antarctic coast, regionally modulate this signal.     

Climate-related uncertainties in projections of the twenty-first century terrestrial carbon budget: off-line model experiments using IPCC greenhouse-gas scenarios and AOGCM climate projections

A terrestrial ecosystem model (Sim-CYCLE) was driven by multiple climate projections to investigate uncertainties in predicting the interactions between global environmental change and the terrestrial carbon cycle. Sim-CYCLE has a spatial resolution of 0.5°, and mechanistically evaluates photosynthetic and respiratory CO₂ exchange. Six scenarios for atmospheric-CO₂ concentrations in the twenty-first century, proposed by the Intergovernmental Panel on Climate Change, were considered. For each scenario, climate projections by a coupled atmosphere–ocean general circulation model (AOGCM) were used to assess the uncertainty due to socio-economic predictions. Under a single CO₂ scenario, climate projections with seven AOGCMs were used to investigate the uncertainty stemming from uncertainty in the climate simulations. Increases in global photosynthesis and carbon storage differed considerably among scenarios, ranging from 23 to 37% and from 24 to 81 Pg C, respectively. Among the AOGCM projections, increases ranged from 26 to 33% and from 48 to 289 Pg C, respectively. There were regional heterogeneities in both climatic change and carbon budget response, and different carbon-cycle components often responded differently to a given environmental change. Photosynthetic CO₂ fixation was more sensitive to atmospheric CO₂, whereas soil carbon storage was more sensitive to temperature. Consequently, uncertainties in the CO₂ scenarios and climatic projections may create additional uncertainties in projecting atmospheric-CO₂ concentrations and climates through the interactive feedbacks between the atmosphere and the terrestrial ecosystem.     

Arctic aerosol and Arctic climate: Results from an energy budget model

An energy budget model is used to study the effect on Arctic climate of optically active aerosol in the Arctic atmosphere. The dependence of the change in surface temperature on the vertical distribution of the aerosol and on the radiative properties of the aerosol-free atmosphere, the Arctic surface, and the aerosol, itself, are calculated. An extensive sensitivity analysis is performed to assess the degree to which the results of the model are dependent upon the assumptions underlying it.List of Symbols Used I ₀ Solar flux at the top of the Arctic Atmosphere (Arctic here means 70° N latitude to the pole) - a _S Surface albedo of the Arctic (a _S ^c is the value of surface albedo at which the sign of the surface temperature perturbation changes) - Reflection coefficient of the aerosol-free Arctic atmosphere - Absorption coefficient of the aerosol-free Arctic atmosphere - Transmission coefficient of the aerosol-free Arctic atmosphere - RI ₀ Total flux of sunlight reflected from the Arctic - A _A I ₀ Total flux of sunlight absorbed in the Arctic atmosphere - A _S I ₀ Total flux of sunlight absorbed at the Arctic surface - A _aer I ₀ Total flux of sunlight absorbed in the Arctic aerosol - Q _A Net atmospheric flow of energy, per unit of Arctic surface area, north across 70° N latitude - Q _S Net oceanic flow of energy, per unit of Arctic surface area, north across 70° N latitude - E Convective plus latent heat fluxes from surface to atmosphere - F _A Net flow of energy to the Arctic atmosphere - F _S Net flow of energy to the Arctic surface - T _A An effective temperature of the Arctic atmosphere - T _S Surface temperature of the Arctic - w Single-scattering albedo of the aerosol - t Optical depth of the aerosol - g Fraction of incident radiation scattered forward by the aerosol - Reflection coefficient of the aerosol - Absorption coefficient of the aerosol - Transmission coefficient of the aerosol - p,q Number of atmospheric layers and the inverse of the fraction of incident IR absorbed in each layer in the energy budget model - F,G,H Measures of the amount of IR-active atmosphere above the surface, the aerosol, and the clouds     

The Arctic freshwater cycle during a naturally and an anthropogenically induced warm climate

The Arctic freshwater cycle plays an important role in regulating regional and global climate. Current observations suggest that an intensification of the high-northern latitude hydrological cycle has caused a freshening of the Arctic and sub-Arctic seas, increasing the potential of weakening overturning strength in the Nordic seas, and reducing temperatures. It is not known if this freshening is a manifestation of the current anthropogenic warming and if the Arctic freshwater cycle has exhibited similar changes in the past, in particular as a response to naturally induced periods of warming, for example during the mid-Holocene hypsithermal. Thus, we have used an earth model of intermediate complexity, LOVECLIM, to investigate the response of the Arctic freshwater cycle, during two warm periods that evolved under different sets of forcings, the mid-Holocene and the twenty-first century. A combination of proxy reconstructions and modelling studies have shown these two periods to exhibit similar surface temperature anomalies, compared to the pre-industrial period, however, it has yet to be determined if the Arctic freshwater cycle and thus, the transport and redistribution of freshwater to the Arctic and the sub-Arctic seas, during these two warm periods, is comparable. Here we provide an overview that shows that the response of the Arctic freshwater cycle during the first half of the twenty-first century can be interpreted as an ‘extreme’ mid-Holocene hydrological cycle. Whilst for the remainder of the twenty-first century, the Arctic freshwater cycle and the majority of its components will likely transition into what can only be described as truly anthropogenic in nature.     

Variability and trends of sub-continental scale surface climate in the twentieth century. Part II: AOGCM simulations

This work presents an analysis of simulated temperature and precipitation variability and trends throughout the twentieth century over 22 land regions of sub-continental scale in the HADCM3 and HADCM2 (two realizations) coupled models. Regional temperature biases in the HADCM3 and HADCM2 are mostly in the range of -5 K to +3 K for the seasonal averages and -3 K to +2 K for the annual average. Seasonal precipitation biases are mostly in the range of -50% to 75% of present day precipitation, with a tendency in both models to overpredict cold season precipitation. Except for cold season temperature in mid- and high-latitude Northern Hemisphere regions, the average climatology of the HADCM2 and HADCM3 is of comparable quality despite the lack of an ocean flux adjustment in the HADCM3. Both models show warming trends of magnitude in line with observations, although the observed inter-regional patterns of warming trend are not well reproduced. Measures of temperature and precipitation interannual to interdecadal variability in the models are in general agreement with observations except for Northern Hemisphere summer temperature variability, which is overestimated. The models somewhat underestimate the inter-decadal variations in interannual variability measures observed during the century and overestimate the range of anomalies. Both models tend to overpredict the occurrences of short persistences (1-3 years) and underpredict the occurrence and maximum length of long persistences (greater than three years), which is an indication of a deficiency in the simulation of long-lived anomaly regimes. Compared to observations, the models produce a higher magnitude of temporal anomaly correlation across regions and correlation between temperature and precipitation anomalies for a given region. This suggests that local processes that may be effective in decoupling the observed regional anomalies are not captured well. Overall, the variability measures in the HADCM2 and HADCM3 are of similar quality, indicating that the use of a flux correction in the HADCM2 does not strongly affect the regional variability characteristics of the model.     

Systemic impacts of climate change on an eroding coastal region over the twenty-first century

A numerical model detailing the functioning and emergent behaviour of an eroding coastal system is described. Model output from a 50-km study region centred on the soft-rock shore of northeast Norfolk was verified through comparison with cliff recession rates that were extracted from historical maps spanning more than a century. Predictions were then made for the period 2000 to 2100 under combined climatic change and management scenarios. For the scenarios evaluated, the model was relatively insensitive to increases in offshore wave height and moderately sensitive to changes in wave direction, but the most important effects were associated with accelerated sea-level rise (SLR). In contrast to predictions made using a modified version of the Bruun rule, the systems model predicted rather complex responses to SLR. For instance, on some sectors of coast, whereas the Bruun rule predicted increased recession under accelerated SLR, the systems model actually predicted progradation owing to the delivery of sediment from eroding coasts up-drift. By contrast, on coasts where beaches are underlain by shore platforms, both the Bruun rule and the systems model predicted accelerated recession rates. However, explicit consideration of the interaction between beach and shore platform within the systems model indicates that these coasts have a broader range of responses and lower overall vulnerability to SLR than predicted by the Bruun rule.     

绍兴市近49 a降水和旱涝变化特征分析

利用1961—2015年夏季(5—8月)湖南89个台站的逐月降水资料和NCEP/NCAR再分析资料、海温资料,计算了湖南近55 a的旱涝急转指数(LDFAI),挑选出湖南夏季旱涝急转(旱转涝和涝转旱两种类型)异常年,分析了异常年的同期大尺度环流和前期海温的基本特征,结果表明：(1)旱转涝年,旱期对流层中层鄂霍次克海有阻塞高压,副高偏西偏南,湖南受中纬度偏西气流控制,南亚高压较常年整体偏北偏强,湖南上空伴随着下沉运动加强,水汽辐散,致使湖南少雨干旱;涝期副高较同期偏南,湖南受中纬度低槽和副高共同影响,南亚高压北移,东伸脊点位于川渝交界附近,且高压中心呈青藏高压模态,湖南上空伴随着强烈的上升运动和水汽汇合,导致湖南降水增多。(2)涝转旱年,涝期副高较常年偏东,冷暖空气交汇在湖南地区,南亚高压整体较常年偏南偏弱,湖南上空伴随着上升运动和水汽汇合,湖南偏涝;旱期副高较常年偏西,湖南受副高控制,此时南亚高压主体偏强偏东,东伸脊点位于湖北一带,高压中心呈伊朗高压模态,加上湖南上空下沉运动和水汽输送辐散异常偏强,干旱少雨。(3) LDFAI指数与前期(前一年夏、秋、冬季和当年春季)太平洋相关海区海温存在显著相关性,这为湖南夏季旱涝急转类型的预测提供了参考信号。

     

极端降水事件变化的观测研究

利用湖北1961—2014年站点逐日降水资料及人口和人口城乡构成数据,通过平均值分离、最小二乘法拟合、趋势系数和气候倾向率分析、M-K检验和累积距平检验等方法,从时空分布、趋势变化、局地特征等方面,分析了近54 a湖北极端降水的变化特征及其与城市化的关系,结果表明：(1)对于第95百分位的极端降水事件,湖北极端降水阈值的范围为43.5~85.1 mm,大部分站的阈值在暴雨雨量范围,高阈值区位于江汉平原和鄂东,低阈值区位于鄂西北,最高阈值出现在武汉站,最低阈值出现在竹山站和房县站。(2)近54 a来湖北多年平均的极端降水日(D_ep)、极端降水量(P_ep)、极端降水强度(I_ep)、最大5 d降水(R_{5 d})和雨量比均存在明显的区域特征,但I_ep、R_{5 d}的地域差异不如D_ep、P_ep明显。鄂西南南部以及鄂东南东部和南部是极端降水事件的高发区,鄂西北北部是极端降水事件的低发区。(3)极端降水指数(R)能反映极端降水的强弱,其大尺度存在明显的年际差异,而长期变化趋势不显著,P_ep、I_ep和雨量比呈弱增加趋势,R_{5 d}和极端降水频数呈弱减少趋势。(4)城市化发展速度会改变R及其局地距平百分比DR_ij、趋势系数和气候倾向率的空间分布。随着城市化发展速度加快,湖北城市“雨岛效应”的格局发生了变化,D_ep、P_ep、I_ep、R_{5 d}及其DR_ij从南北差异明显变为东西差异明显,江汉平原和鄂东的D_ep、P_ep、I_ep、R_{5 d}增加,而鄂西南的减少,且四者趋势系数通过显著性水平检验的站点数更多,气候倾向率绝对值也普遍增大,但大部分站点的变化趋势为负值。(5)湖北极端降水具有明显的城市效应,城市化发展速度较快的大城市代表站的极端降水阈值大于配对的小城市代表站,两种代表站平均的D_ep、P_ep、I_ep、R_{5 d}的年际变化较一致,但大城市代表站的I_ep、R_{5 d}普遍较大,极端降水的变化趋势更明显。

     

A transitioning Arctic surface energy budget: the impacts of solar zenith angle, surface albedo and cloud radiative forcing

Snow surface and sea-ice energy budgets were measured near 87.5°N during the Arctic Summer Cloud Ocean Study (ASCOS), from August to early September 2008. Surface temperature indicated four distinct temperature regimes, characterized by varying cloud, thermodynamic and solar properties. An initial warm, melt-season regime was interrupted by a 3-day cold regime where temperatures dropped from near zero to ?7°C. Subsequently mean energy budget residuals remained small and near zero for 1 week until once again temperatures dropped rapidly and the energy budget residuals became negative. Energy budget transitions were dominated by the net radiative fluxes, largely controlled by the cloudiness. Variable heat, moisture and cloud distributions were associated with changing air-masses. Surface cloud radiative forcing, the net radiative effect of clouds on the surface relative to clear skies, is estimated. Shortwave cloud forcing ranged between ?50 W m^?2 and zero and varied significantly with surface albedo, solar zenith angle and cloud liquid water. Longwave cloud forcing was larger and generally ranged between 65 and 85 W m^?2, except when the cloud fraction was tenuous or contained little liquid water; thus the net effect of the clouds was to warm the surface. Both cold periods occurred under tenuous, or altogether absent, low-level clouds containing little liquid water, effectively reducing the cloud greenhouse effect. Freeze-up progression was enhanced by a combination of increasing solar zenith angles and surface albedo, while inhibited by a large, positive surface cloud forcing until a new air-mass with considerably less cloudiness advected over the experiment area.     

Analysis of the projected regional sea-ice changes in the Southern Ocean during the twenty-first century

Using the set of simulations performed with atmosphere-ocean general circulation models (AOGCMs) for the Fourth Assessment Report of the Intergovernmental Panel on Climate Change (IPCC AR4), the projected regional distribution of sea ice for the twenty-first century has been investigated. Averaged over all those model simulations, the current climate is reasonably well reproduced. However, this averaging procedure hides the errors from individual models. Over the twentieth century, the multimodel average simulates a larger sea-ice concentration decrease around the Antarctic Peninsula compared to other regions, which is in qualitative agreement with observations. This is likely related to the positive trend in the Southern Annular Mode (SAM) index over the twentieth century, in both observations and in the multimodel average. Despite the simulated positive future trend in SAM, such a regional feature around the Antarctic Peninsula is absent in the projected sea-ice change for the end of the twenty-first century. The maximum decrease is indeed located over the central Weddell Sea and the Amundsen–Bellingshausen Seas. In most models, changes in the oceanic currents could play a role in the regional distribution of the sea ice, especially in the Ross Sea, where stronger southward currents could be responsible for a smaller sea-ice decrease during the twenty-first century. Finally, changes in the mixed layer depth can be found in some models, inducing locally strong changes in the sea-ice concentration.

Population increase impacts the climate,using the sensitive Arctic as an example

The global population during the last 100 years has increased from 2 to 7.7 billion, causing an increase in greenhouse gases in the atmosphere. In order to see how population increase is directly related to physical variables of the climate, this Perspective article places observations and scenarios of climate change into context and puts forth a statistical modeling study on how the sensitive Arctic climate responds to the increasing population. The relationships between population, Arctic sea-ice extent (SIE), and surface air temperature (SAT) are very strong, with the increasing population explaining 96% of the decreasing SIE and about 80% of the increasing SAT in the Arctic. Our projection for the SIE using the population as a “proxy predictor” for a projected population of 10 billion people on the Earth in 2100, yields a SIE of 9.30 and 8.21 million km² for a linear and squared relationship, respectively, indicating no “tipping point” for the annual ice extent in this century. This adds another dimension to climate understanding for the public at large using population as a proxy variable, instead of the more abstract CO₂ parameter. This also indicates that it is important to attempt to limit the ongoing increase in population, which is the main cause of the greenhouse gas emissions, in addition to reducing per capita emissions by an exponential increase in implementing renewable energy, a formidable challenge in this century.     

The summer radiation and heat budget of the Arctic and Antarctic

Summary For two pairs of arctic and antarctic stations, one coastal and one mountainous, an intercomparison between the summer radiation and surface energy budgets was carried out. The station pairs were similar in both latitude and altitude. It was found that the global radiation was larger for both antarctic stations. This is the result of a smaller Earth-Sun distance and cleaner atmosphere in Antarctica. Cloudiness, and for the arctic mountainous station substantial screening of the sun, also contributed. Further, large differences were found in the albedo. In the Arctic, the summer surfaces considered were bare tundra and melting snow, with respective albedos of 20 and 59%, while in Antarctica the surfaces considered were melting and dry snow, with albedos of 67 and 83% respectively.This results in a less positive radiation balance at both antarctic stations, despite the higher incoming global radiation. In turn, less sensible heat transfer from the surface to air results in lower temperatures in the Antarctic. The reduced rate of evaporation in Antarctica results in a drier atmosphere and less cloudy conditions.

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Zusammenfassung Für jeweils zwei arktische und antarktische Stationen, von denen jeweils eine an der Küste, eine in den Bergen liegt, wurden Vergleiche in bezug auf sommerliche Strahlungs- und Oberflächenenergiebilanz angestellt. Die Stationen liegen auf vergleichbarer Höhe und Breite. Es hat sich gezeigt, daß die Gesamt-Einstrahlung in den antarktischen Stationen größer war, aufgrund geringerer Erde-Sonnen-Distanz und der reineren Atmosphäre. Bewölkung und vor allem die für die arktischen Gebirgsstationen ausschlaggebende Sonnenabschirmung tragen auch zu dieser Differenz bei. Weiters wurden starke Albedounterschiede beobachtet. In der Arktis waren die Beobachtungsoberflächen im Sommer offene Tundra und schmelzender Schnee mit einer Albedo von 20 bzw. 59%, während die Antarktisoberflächen, nasser und trockener Schnee, eine Albedo von 67 bzw. 83% aufwiesen.Dies ergibt eine weniger positive Strahlungsbilanz für die beiden antarktischen Stationen trotz höherer Gesamteinstrahlung. Infolgedessen bewirkt die geringere Wärmeabgabe des Bodens an die Luft niedere Temperaturen in der Antarktis. Geringere Verdunstung ergibt somit geringere Bewölkung und eine trockenere Atmosphäre über der Antarktis.

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With 5 Figures     

Sources of spread in simulations of Arctic sea ice loss over the twenty-first century

We show that intermodel variations in the anthropogenically-forced evolution of September sea ice extent (SSIE) in the Arctic stem mainly from two factors: the baseline climatological sea ice thickness (SIT) distribution, and the local climate feedback parameter. The roles of these two factors evolve over the course of the twenty-first century. The SIT distribution is the most important factor in current trends and those of coming decades, accounting for roughly half the intermodel variations in SSIE trends. Then, its role progressively decreases, so that around the middle of the twenty-first century the local climate feedback parameter becomes the dominant factor. Through this analysis, we identify the investments in improved simulation of Arctic climate necessary to reduce uncertainties both in projections of sea ice loss over the coming decades and in the ultimate fate of the ice pack.     

The impact of Arctic sea ice on the Arctic energy budget and on the climate of the Northern mid-latitudes

The atmospheric general circulation model EC-EARTH-IFS has been applied to investigate the influence of both a reduced and a removed Arctic sea ice cover on the Arctic energy budget and on the climate of the Northern mid-latitudes. Three 40-year simulations driven by original and modified ERA-40 sea surface temperatures and sea ice concentrations have been performed at T255L62 resolution, corresponding to 79?km horizontal resolution. Simulated changes between sensitivity and reference experiments are most pronounced over the Arctic itself where the reduced or removed sea ice leads to strongly increased upward heat and longwave radiation fluxes and precipitation in winter. In summer, the most pronounced change is the stronger absorption of shortwave radiation which is enhanced by optically thinner clouds. Averaged over the year and over the area north of 70° N, the negative energy imbalance at the top of the atmosphere decreases by about 10?W/m² in both sensitivity experiments. The energy transport across 70° N is reduced. Changes are not restricted to the Arctic. Less extreme cold events and less precipitation are simulated in sub-Arctic and Northern mid-latitude regions in winter.     

Specific features of the transport of freshwater anomalies in the Arctic Ocean

The results are considered of studying inhomogeneities in the thermohaline structure of the Arctic Ocean surface layer from the data of different measurement platforms including North Pole drifting stations and ITP (Ice-Tethered Profiler) autonomous buoys. The characteristics of inhomogeneities in the thermohaline structure and of mechanisms of their transport are presented. Qualitative conclusions concerning the types of eddies revealed from the results of observations are proposed as well as the classification of dynamic systems that transport water masses.     

Heat budget of the upper Arctic Ocean under a warming climate

The heat budget of the upper Arctic Ocean is examined in an ensemble of coupled climate models under idealised increasing CO₂ scenarios. All of the experiments show a strong amplification of surface air temperatures but a smaller increase in sea surface temperature than the rest of the world as heat is lost to the atmosphere as the sea-ice cover is reduced. We carry out a heat budget analysis of the Arctic Ocean in an ensemble of model runs to understand the changes that occur as the Arctic becomes ice free in summer. We find that as sea-ice retreats heat is lost from the ocean surface to the atmosphere contributing to the amplification of Arctic surface temperatures. Furthermore, heat is mixed upwards into the mixed layer as a result of increased upper ocean mixing and there is increased advection of heat into the Arctic as the ice edge retreats. Heat lost from the upper Arctic Ocean to the atmosphere is therefore replenished by mixing of warmer water from below and by increased advection of warm water from lower latitudes. The ocean is therefore able to contribute more to Arctic amplification.