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Downwelling radiation in six regional models from the Arctic Regional Climate Model Intercomparison (ARCMIP) project is systematically biased negative in comparison with observations from the Surface Heat Budget of the Arctic Ocean (SHEBA) experiment, although the correlations with observations are relatively good. In this paper, links between model errors and the representation of clouds in these models are investigated. Although some modeled cloud properties, such as the cloud water paths, are reasonable in a climatological sense, the temporal correlation of model cloud properties with observations is poor. The vertical distribution of cloud water is distinctly different among the different models; some common features also appear. Most models underestimate the presence of high clouds, and, although the observed preference for low clouds in the Arctic is present in most of the models, the modeled low clouds are too thin and are displaced downward. Practically all models show a preference to locate the lowest cloud base at the lowest model grid point. In some models this happens also to be where the observations show the highest occurrence of the lowest cloud base; it is not possible to determine if this result is just a coincidence. Different factors contribute to model surface radiation errors. For longwave radiation in summer, a negative bias is present both for cloudy and clear conditions, and intermodel differences are smaller when clouds are present. There is a clear relationship between errors in cloud-base temperature and radiation errors. In winter, in contrast, clear-sky cases are modeled reasonably well, but cloudy cases show a very large intermodel scatter with a significant bias in all models. This bias likely results from a complete failure in all of the models to retain liquid water in cold winter clouds. All models overestimate the cloud attenuation of summer solar radiation for thin and intermediate clouds, and some models maintain this behavior also for thick clouds. [ABSTRACT FROM AUTHOR]

This paper analyzes regional sea level changes in a climate change simulation using the Max Planck Institute for Meteorology (MPI) coupled atmosphere–ocean general circulation model ECHAM5/MPI-OM. The climate change scenario builds on observed atmospheric greenhouse gas (GHG) concentrations from 1860 to 2000, followed by the International Panel on Climate Change (IPCC) A1B climate change scenario until 2100; from 2100 to 2199, GHG concentrations are fixed at the 2100 level. As compared with the unperturbed control climate, global sea level rises 0.26 m by 2100, and 0.56 m by 2199 through steric expansion; eustatic changes are not included in this simulation. The model’s sea level evolves substantially differently among ocean basins. Sea level rise is strongest in the Arctic Ocean, from enhanced freshwater input from precipitation and continental runoff, and weakest in the Southern Ocean, because of compensation of steric changes through dynamic sea surface height (SSH) adjustments. In the North Atlantic Ocean (NA), a complex tripole SSH pattern across the subtropical to subpolar gyre front evolves, which is consistent with a northward shift of the NA current. On interannual to decadal time scales, the SSH difference between Bermuda and the Labrador Sea correlates highly with the combined baroclinic gyre transport in the NA but only weakly with the meridional overturning circulation (MOC) and, thus, does not allow for estimates of the MOC on these time scales. Bottom pressure increases over shelf areas by up to 0.45 m (water column equivalent) and decreases over the Atlantic section in the Southern Ocean by up to 0.20 m. The separate evaluation of thermosteric and halosteric sea level changes shows that thermosteric anomalies are positive over most of the World Ocean. Because of increased atmospheric moisture transport from low to high latitudes, halosteric anomalies are negative in the subtropical NA and partly compensate thermosteric anomalies, but are positive in the Arctic Ocean and add to thermosteric anomalies. The vertical distribution of thermosteric and halosteric anomalies is highly nonuniform among ocean basins, reaching deeper than 3000 m in the Southern Ocean, down to 2200 m in the North Atlantic, and only to depths of 500 m in the Pacific Ocean by the end of the twenty-first century. [ABSTRACT FROM AUTHOR]

Changes in Arctic sea ice cover have the potential to impact midlatitude climate. A previous sensitivity study utilizing the National Center for Atmospheric Research¿s (NCAR) atmospheric general circulation model [AGCM; Community Climate Model, version 3 (CCM3)] to explore climate sensitivity to declining Arctic sea ice cover suggested that, as Arctic sea ice cover is reduced, precipitation patterns over western North America will shift toward dryer conditions in southwestern North America and wetter conditions in northwestern North America. Here, three complementary lines of research validate and explore the robustness of this possible climate change impact: 1) repetition of the previous sensitivity study (specified constant Arctic sea ice cover and atmospheric CO2) with an updated version of the NCAR AGCM [third Community Atmosphere Model (CAM3)], 2) investigation of the climate response to dynamically reduced Arctic sea ice cover (driven by a quadrupling of atmospheric CO2) in the coupled NCAR Community Climate System Model (CCSMv3), and 3) analysis of similar results from six other coupled climate system models. Results from the CAM3 sensitivity study are similar to those from the original study with declining Arctic sea ice cover driving up to 25% less mean annual precipitation (MAP) over southwestern North America and up to an 8% increase in MAP over northwestern North America. The seven coupled models also reproduce this same general pattern. At the time of CO2 quadrupling, Arctic sea ice cover is reduced (up to 90% in boreal winter) and MAP over southwestern North America decreases by up to 30% while MAP in northwestern North America increases by up to 40%. These results represent a significant shift in the precipitation pattern over western North America and support the findings of the original sensitivity study in suggesting that, as future reductions in Arctic sea ice cover take place, there will be a substantial impact on water resources in western North America. [ABSTRACT FROM AUTHOR]

The authors provide a detailed climatology and evaluation of recent climate change in the Kola Peninsula, Arctic Russia, a region influenced by both the North Atlantic and Arctic Oceans. The analysis is based on 50 years of monthly surface air temperature (SAT), precipitation (PPN), and sea level pressure (SLP) data from 10 meteorological stations for 1966-2015. Regional mean annual SAT is ~0°C: the moderating effect of the ocean is such that coastal (inland) stations have a positive (negative) value. Examined mean annual PPN totals rise from ~430 mm in the northeast of the region to ~600 mm in the west. Annual SAT in the Kola Peninsula has increased by 2.3° ± 1.0°C over the past 50 years. Seasonally, statistically significant warming has taken place in spring and fall, although the largest trend has occurred in winter. Although there has been no significant change in annual PPN, spring has become significantly wetter and fall drier. The former is associated with the only significant seasonal SLP trend (decrease). A positive winter North Atlantic Oscillation (NAO) index is generally associated with a warmer and wetter Kola Peninsula whereas a positive Siberian high (SH) index has the opposite impact. The relationship between both the NAO and SH and the SAT is broadly coherent across the region whereas their relationship with PPN varies markedly, although none of the relationships is temporally invariant. Reduced sea ice in the Barents and White Seas and associated circulation changes are likely to be the principal drivers behind the observed changes. [ABSTRACT FROM AUTHOR]

Arctic sea ice (ASI) and its potential climatic impacts have received increasing attention during the past decades, yet the relevant mechanisms are far from being understood, particularly how anomalous ASI affects climate in midlatitudes. The spring precipitation takes up as much as 30% of the annual total and significantly influences agriculture in East Asia. Here, observed evidence and numerical experiment results show that the ASI variability in the Norwegian Sea and the Barents Sea in the preceding winter is intimately connected with interannual variations of the East Asian spring precipitation (EAP). The former can explain about 14% of the total variance of the latter. The ASI anomalies persist from winter through the ensuing spring and excite downstream teleconnections of a distinct Rossby wave train prevailing over the Eurasian continent. For the reduced ASI, such a wave train pattern is usually associated with an anomalous low pressure center over the Mongolian plateau, which accelerates the East Asian subtropical westerly jet. The intensified subtropical westerly jet, concurrent with lower-level convergence and upper-level divergence, enhances the local convection and consequently favors rich spring precipitation over East Asia. For the excessive ASI, the situation tends to be opposite. Given that seasonal prediction of the EAP remains a challenging issue, the winter ASI variability may provide another potential predictability source besides El Niño-Southern Oscillation. [ABSTRACT FROM AUTHOR]

As the Arctic sea ice thins and ultimately disappears in a warming climate, its insulating power decreases. This causes the surface air temperature to approach the temperature of the relatively warm ocean water below the ice. The resulting increases in air temperature, water vapor, and cloudiness lead to an increase in the surface downwelling longwave radiation (DLR), which enables a further thinning of the ice. This positive ice-insulation feedback operates mainly in the autumn and winter. A climate change simulation with the Community Earth System Model shows that, averaged over the year, the increase in Arctic DLR is 3 times stronger than the increase in Arctic absorbed solar radiation at the surface. The warming of the surface air over the Arctic Ocean during fall and winter creates a strong thermal contrast with the colder surrounding continents. Sea level pressure falls over the Arctic Ocean, and the high-latitude circulation reorganizes into a shallow 'winter monsoon.' The resulting increase in surface wind speed promotes stronger surface evaporation and higher humidity over portions of the Arctic Ocean, thus reinforcing the ice-insulation feedback. [ABSTRACT FROM AUTHOR]

The authors explore possible temperature modifications of the Atlantic Water Layer (AWL) induced by climate change, performing simulations for 1970 to 2099 with a coupled ice-ocean Arctic model (CIOM). Surface fields to drive the CIOM were provided by the Canadian Regional Climate Model (CRCM), driven by outputs from the Canadian Centre for Climate Modelling and Analysis (CCCma) Coupled Global Climate Model, version 3 (CGCM3) following the A1B climate change scenario. In the present climate, represented as 1990-2009, the CIOM can reliably reproduce the AWL compared to Polar Science Center Hydrographic Climatology (PHC) data. For the future climate, assuming the A1B climate change scenario, there is a significant increase in water volume transport into the central Arctic Ocean through Fram Strait due to the weakened atmospheric high pressure system over the western Arctic and an intensified atmospheric low pressure system over the Nordic seas. The AWL temperature tends to decrease from 0.36°C in the 2010s to 0.26°C in the 2060s. In the vertical, the warm Atlantic water core slightly expands before the 2030s, significantly shrinks after the 2050s, and essentially disappears by 2070-99, in the southern Beaufort Sea. The temperature decrease after 2030 is mainly due to the reduced heat fluxes in the Kara and Barents Seas. In the northeastern Barents and Kara Seas, the loss of sea ice increases the heat loss from the Atlantic water and reduces the water temperature near the bottom, contributing to decreased heat fluxes into the central Arctic Ocean, as well as decreased AWL temperature at central Arctic Ocean intermediate layers. In addition, the vertically integrated heat loss also plays an important role in the AWL cooling process. [ABSTRACT FROM AUTHOR]

Wintertime Arctic sea ice extent has been declining since the late twentieth century, particularly over the Atlantic sector that encompasses the Barents-Kara Seas and Baffin Bay. This sea ice decline is attributable to various Arctic environmental changes, such as enhanced downward infrared (IR) radiation, preseason sea ice reduction, enhanced inflow of warm Atlantic water into the Arctic Ocean, and sea ice export. However, their relative contributions are uncertain. Utilizing ERA-Interim and satellite-based data, it is shown here that a positive trend of downward IR radiation accounts for nearly half of the sea ice concentration (SIC) decline during the 1979-2011 winter over the Atlantic sector. Furthermore, the study shows that the Arctic downward IR radiation increase is driven by horizontal atmospheric water flux and warm air advection into the Arctic, not by evaporation from the Arctic Ocean. These findings suggest that most of the winter SIC trends can be attributed to changes in the large-scale atmospheric circulations. [ABSTRACT FROM AUTHOR]

The Beaufort Sea high (BSH) plays an important role in forcing Arctic sea ice and the Beaufort Gyre. This study examines the variability and long-term trends of atmospheric circulation over the Chukchi and Beaufort Seas using the ECMWF Interim Re-Analysis (ERA-Interim) for the period 1979-2012. Because of the mobility of the BSH through the year, EOF analysis is applied to the sea level pressure (SLP) field in order to investigate the principal patterns of BSH variability. In each season, the three leading EOF modes explain nearly 90% of the total variance and reflect a strengthened or weakened BSH centered over the western Arctic Ocean (EOF1), a north-south dipole-like SLP anomaly (EOF2), and a west-east dipole-like SLP anomaly (EOF3), respectively. These three EOF modes offer distinct influences on local climate in each season and have different connections with the large-scale climate variability modes in winter. In particular, the second principal component (PC2) associated with EOF2 in the autumn exhibits a tendency toward high-index polarity significant at the 5% level, and is related to strongly reduced sea ice extent. Further, the authors have detected significant anticyclonic trends among surface wind fields associated with a strengthened BSH during summer and autumn, but significant cyclonic trends associated with a weakened BSH during early midwinter, consistent with significant trends in SLP gradients between western Arctic Ocean and the adjoining landmass. Comparison with forced trends of surface winds from various simulations from the IPCC Fifth Assessement Report (AR5) indicates that summertime changes in atmospheric circulation cannot be explained by natural external forcing or lower boundary forcings and may instead be attributable to external anthropogenic forcing. [ABSTRACT FROM AUTHOR]

Analysis of weekly sea ice charts produced by the U.S. National Ice Center from 1976 to 2007 indicates large interannual variations in the averaged winter landfast ice extent around the Arctic Basin. During the 32-yr period of the record, landfast ice cover was relatively extensive from the early to mid-1980s but since then has declined in many coastal regions of the Arctic, particularly after the early 1990s. While the Barents, Baltic, and Bering Seas show increases in landfast ice area, the overall change for the Northern Hemisphere is negative, about −12.27 (±2.8) × 103 km2 yr−1, or −7 (±1.5)% decade−1 relative to the long-term mean. Except in a few coastal regions, the seasonal duration of landfast ice is shorter overall, particularly in the Laptev, East Siberian, and Chukchi Seas. The decreased winter landfast ice extent is associated with some notable changes in ice growth and melt patterns, in particular the slowed landfast ice expansion during fall and early winter since 1990. The observed changes in Arctic landfast ice could have profound impacts on the Arctic coasts. The challenge is to understand and project the responses of the whole coastal ecosystem to changing ice cover and Arctic warming. [ABSTRACT FROM AUTHOR]

The Community Climate System Model, version 4 has revisions across all components. For sea ice, the most notable improvements are the incorporation of a new shortwave radiative transfer scheme and the capabilities that this enables. This scheme uses inherent optical properties to define scattering and absorption characteristics of snow, ice, and included shortwave absorbers and explicitly allows for melt ponds and aerosols. The deposition and cycling of aerosols in sea ice is now included, and a new parameterization derives ponded water from the surface meltwater flux. Taken together, this provides a more sophisticated, accurate, and complete treatment of sea ice radiative transfer. In preindustrial CO2 simulations, the radiative impact of ponds and aerosols on Arctic sea ice is 1.1 W m−2 annually, with aerosols accounting for up to 8 W m−2 of enhanced June shortwave absorption in the Barents and Kara Seas and with ponds accounting for over 10 W m−2 in shelf regions in July. In double CO2 (2XCO2) simulations with the same aerosol deposition, ponds have a larger effect, whereas aerosol effects are reduced, thereby modifying the surface albedo feedback. Although the direct forcing is modest, because aerosols and ponds influence the albedo, the response is amplified. In simulations with no ponds or aerosols in sea ice, the Arctic ice is over 1 m thicker and retains more summer ice cover. Diagnosis of a twentieth-century simulation indicates an increased radiative forcing from aerosols and melt ponds, which could play a role in twentieth-century Arctic sea ice reductions. In contrast, ponds and aerosol deposition have little effect on Antarctic sea ice for all climates considered. [ABSTRACT FROM AUTHOR]

Recent observations of Arctic sea ice show that the decrease in summer ice cover over the last few decades has occurred in conjunction with a significant loss of multiyear ice. The transition to an Arctic that is populated by thinner, first-year sea ice has important implications for future trends in area and volume. Here, a reduced model for Arctic sea ice is developed. This model is used to investigate how the survivability of first-year and multiyear ice controls the mean state, variability, and trends in ice area and volume. A hindcast with a global dynamic--thermodynamic sea ice model that traces first-year and multiyear ice is used to estimate the survivability of each ice type. These estimates of survivability, in concert with the reduced model, yield persistence time scales of September area and volume anomalies and the characteristics of the sensitivity of sea ice to climate forcing that compare well with a fully coupled climate model. The September area is found to be nearly in equilibrium with climate forcing at all times, and therefore the observed decline in summer sea ice cover is a clear indication of a changing climate. Keeping an account of first-year and multiyear ice area within global climate models offers a powerful way to evaluate those models with observations, and could help to constrain projections of sea ice decline in a warming climate. [ABSTRACT FROM AUTHOR]

The issue of multidecadal variability in the North Atlantic has been an important topic of late. It is clear that there are multidecadal variations in several climate variables in the North Atlantic, such as sea surface temperature and sea level height. The details of this variability, in particular the dominant patterns and time scales, are confusing from both an observational as well as a theoretical point of view. After analyzing results from observational datasets and a 500-yr simulation of an Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4) climate model, two dominant time scales (20–30 and 50–70 yr) of multidecadal variability in the North Atlantic are proposed. The 20–30-yr variability is characterized by the westward propagation of subsurface temperature anomalies. The hypothesis is that the 20–30-yr variability is caused by internal variability of the Atlantic Meridional Overturning Circulation (MOC) while the 50–70-yr variability is related to atmospheric forcing over the Atlantic Ocean and exchange processes between the Atlantic and Arctic Oceans. [ABSTRACT FROM AUTHOR]

A high-resolution sea ice–ocean numerical model of the North Water polynya has been developed to study the wind-driven circulation during polynya events. An idealized three-layer stratified ocean is used to initialize the model to characterize the baroclinic response to realistic wind and ice conditions. The model general circulation pattern is mainly forced by an along-channel sea level gradient between the Arctic Ocean and Baffin Bay, which determines the magnitude of the southward Baffin Current, and by an across-channel sea level gradient in Baffin Bay, which drives the northward West Greenland Current (WGC). These two currents are found to be anticorrelated to each other in the Smith Sound area. During strong northerly wind events, occurring quite frequently in the winter–spring period in the polynya, nutrient-rich Baffin Bay waters transported by the WGC are forced toward the Greenland shelf, coinciding with upwelling events along the Greenland coast. Whenever an ice bridge is present (i.e., the polynya exists and is substantially open), upwelling also occurs at the landfast ice edge. In such cases, the total upwelling area is increased by an amount that depends on the form of the ice bridge but could easily double during certain years. The baroclinic circulation associated with the upwelling response includes the formation of a cyclonic eddy attached to the ice edge that is generated during strong northerly wind events. Primary production estimations reveal that upwelling during polynya events plays a significant role in the early spring phytoplankton bloom, suggesting that the disappearance of the polynya as a result of climate change may have profound implications for the entire ecosystem. [ABSTRACT FROM AUTHOR]

As a contribution to a detailed evaluation of Intergovernmental Panel on Climate Change (IPCC)-type coupled climate models against observations, this study analyzes Arctic sea ice parameters simulated by the Max-Planck-Institute for Meteorology (MPI-M) fully coupled climate model ECHAM5/Max-Planck-Institute for Meteorology Hamburg Primitive Equation Ocean Model (MPI-OM) for the period from 1980 to 1999 and compares them with observations collected during field programs and by satellites. Results of the coupled run forced by twentieth-century CO2 concentrations show significant discrepancies during summer months with respect to observations of the spatial distribution of the ice concentration and ice thickness. Equally important, the coupled run lacks interannual variability in all ice and Arctic Ocean parameters. Causes for such big discrepancies arise from errors in the ECHAM5/MPI-OM atmosphere and associated errors in surface forcing fields (especially wind stress). This includes mean bias pattern caused by an artificial circulation around the geometric North Pole in its atmosphere, as well as insufficient atmospheric variability in the ECHAM5/MPI-OM model, for example, associated with Arctic Oscillation/North Atlantic Oscillation (AO/NAO). In contrast, the identical coupled ocean–ice model, when driven by NCEP–NCAR reanalysis fields, shows much increased skill in its ice and ocean circulation parameters. However, common to both model runs is too strong an ice export through the Fram Strait and a substantially biased heat content in the interior of the Arctic Ocean, both of which may affect sea ice budgets in centennial projections of the Arctic climate system. [ABSTRACT FROM AUTHOR]

Sudden changes of the Atlantic meridional overturning circulation (AMOC) are believed to have caused large, abrupt climate changes over many parts of the globe during the last glacial and deglacial period. This study investigates the mechanisms by which a large freshwater input to the subarctic North Atlantic and an attendant rapid weakening of the AMOC influence North Pacific climate by analyzing four different ocean–atmosphere coupled general circulation models (GCMs) under present-day or preindustrial boundary conditions. When the coupled GCMs are forced with a 1-Sv (Sv ≡ 106 m3 s-1) freshwater flux anomaly in the subarctic North Atlantic, the AMOC nearly shuts down and the North Atlantic cools significantly. The South Atlantic warms slightly, shifting the Atlantic intertropical convergence zone southward. In addition to this Atlantic ocean–atmosphere response, all of the models exhibit cooling of the North Pacific, especially along the oceanic frontal zone, consistent with paleoclimate reconstructions. The models also show deepening of the wintertime Aleutian low. Detailed analysis of one coupled GCM identifies both oceanic and atmospheric pathways from the Atlantic to the North Pacific. The oceanic teleconnection contributes a large part of the North Pacific cooling: the freshwater input to the North Atlantic raises sea level in the Arctic Ocean and reverses the Bering Strait throughflow, transporting colder, fresher water from the Arctic Ocean into the North Pacific. When the Bering Strait is closed, the cooling is greatly reduced, while the Aleutian low response is enhanced. Tropical SST anomalies in both the Atlantic and Pacific are found to be important for the equivalent barotropic response of the Aleutian low during boreal winter. The atmospheric bridge from the tropical North Atlantic is particularly important and quite sensitive to the mean state, which is poorly simulated in many coupled GCMs. The enhanced Aleutian low, in turn, cools the North Pacific by increasing surface heat fluxes and southward Ekman transport. The closure of the Bering Strait during the last glacial period suggests that the atmospheric bridge from the tropics and air–sea interaction in the North Pacific played a crucial role in the AMOC–North Pacific teleconnection. [ABSTRACT FROM AUTHOR]

The minimum of Arctic sea ice extent in the summer of 2007 was unprecedented in the historical record. A coupled ice–ocean model is used to determine the state of the ice and ocean over the past 29 yr to investigate the causes of this ice extent minimum within a historical perspective. It is found that even though the 2007 ice extent was strongly anomalous, the loss in total ice mass was not. Rather, the 2007 ice mass loss is largely consistent with a steady decrease in ice thickness that began in 1987. Since then, the simulated mean September ice thickness within the Arctic Ocean has declined from 3.7 to 2.6 m at a rate of -0.57 m decade-1. Both the area coverage of thin ice at the beginning of the melt season and the total volume of ice lost in the summer have been steadily increasing. The combined impact of these two trends caused a large reduction in the September mean ice concentration in the Arctic Ocean. This created conditions during the summer of 2007 that allowed persistent winds to push the remaining ice from the Pacific side to the Atlantic side of the basin and more than usual into the Greenland Sea. This exposed large areas of open water, resulting in the record ice extent anomaly. [ABSTRACT FROM AUTHOR]

Recent observations show dramatic changes of the Arctic atmosphere–ice–ocean system. Here the authors demonstrate, through the analysis of a vast collection of previously unsynthesized observational data, that over the twentieth century the central Arctic Ocean became increasingly saltier with a rate of freshwater loss of 239 ± 270 km3 decade-1. In contrast, long-term (1920–2003) freshwater content (FWC) trends over the Siberian shelf show a general freshening tendency with a rate of 29 ± 50 km3 decade-1. These FWC trends are modulated by strong multidecadal variability with sustained and widespread patterns. Associated with this variability, the FWC record shows two periods in the 1920s–30s and in recent decades when the central Arctic Ocean was saltier, and two periods in the earlier century and in the 1940s–70s when it was fresher. The current analysis of potential causes for the recent central Arctic Ocean salinification suggests that the FWC anomalies generated on Arctic shelves (including anomalies resulting from river discharge inputs) and those caused by net atmospheric precipitation were too small to trigger long-term FWC variations in the central Arctic Ocean; to the contrary, they tend to moderate the observed long-term central-basin FWC changes. Variability of the intermediate Atlantic Water did not have apparent impact on changes of the upper–Arctic Ocean water masses. The authors’ estimates suggest that ice production and sustained draining of freshwater from the Arctic Ocean in response to winds are the key contributors to the salinification of the upper Arctic Ocean over recent decades. Strength of the export of Arctic ice and water controls the supply of Arctic freshwater to subpolar basins while the intensity of the Arctic Ocean FWC anomalies is of less importance. Observational data demonstrate striking coherent long-term variations of the key Arctic climate parameters and strong coupling of long-term changes in the Arctic–North Atlantic climate system. Finally, since the high-latitude freshwater plays a crucial role in establishing and regulating global thermohaline circulation, the long-term variations of the freshwater content discussed here should be considered when assessing climate change and variability. [ABSTRACT FROM AUTHOR]

The Arctic Ocean freshwater budgets in climate model integrations of the twentieth and twenty-first century are examined. An ensemble of six members of the Community Climate System Model version 3 (CCSM3) is used for the analysis, allowing the anthropogenically forced trends over the integration length to be assessed. Mechanisms driving trends in the budgets are diagnosed, and the implications of changes in the Arctic–North Atlantic exchange on the Labrador Sea and Greenland–Iceland–Norwegian (GIN) Seas properties are discussed. Over the twentieth and the twenty-first centuries, the Arctic freshens as a result of increased river runoff, net precipitation, and decreased ice growth. For many of the budget terms, the maximum 50-yr trends in the time series occur from approximately 1975 to 2025, suggesting that we are currently in the midst of large Arctic change. The total freshwater exchange between the Arctic and North Atlantic increases over the twentieth and twenty-first centuries with decreases in ice export more than compensated for by an increase in the liquid freshwater export. Changes in both the liquid and solid (ice) Fram Strait freshwater fluxes are transported southward by the East Greenland Current and partially removed from the GIN Seas. Nevertheless, reductions in GIN sea ice melt do result from the reduced Fram Strait transport and account for the largest term in the changing ocean surface freshwater fluxes in this region. This counteracts the increased ocean stability due to the warming climate and helps to maintain GIN sea deep-water formation. [ABSTRACT FROM AUTHOR]

The oceanic Ekman transport and pumping are among the most important parameters in studying the ocean general circulation and its variability. Upwelling due to the Ekman transport divergence has been identified as a leading mechanism for the seasonal to interannual variability of the upper-ocean heat content in many parts of the World Ocean, especially along coasts and the equator. Meanwhile, the Ekman pumping is the primary mechanism that drives basin-scale circulations in subtropical and subpolar oceans. In those ice-free oceans, the Ekman transport and pumping rate are calculated using the surface wind stress. In the ice-covered Arctic Ocean, the surface momentum flux comes from both air–water and ice–water stresses. The data required to compute these stresses are now available from satellite and buoy observations. But no basin-scale calculation of the Ekman transport in the Arctic Ocean has been done to date. In this study, a suite of satellite and buoy observations of ice motion, ice concentration, surface wind, etc., will be used to calculate the daily Ekman transport over the whole Arctic Ocean from 1978 to 2003 on a 25-km resolution. The seasonal variability and its relationship to the surface forcing fields will be examined. Meanwhile, the contribution of the Ekman transport to the seasonal fluxes of heat and salt to the Arctic Ocean mixed layer will be discussed. It was found that the greatest seasonal variations of Ekman transports of heat and salt occur in the southern Beaufort Sea in the fall and early winter when a strong anticyclonic wind and ice motion are present. The Ekman pumping velocity in the interior Beaufort Sea reaches as high as 10 cm day-1 in November while coastal upwelling is even stronger. The contributions of the Ekman transport to the heat and salt flux in the mixed layer are also considerable in the region. [ABSTRACT FROM AUTHOR]

Arctic cyclone activity is investigated in the context of climate change and variability by using a modified automated cyclone identification and tracking algorithm, which differs from previously used algorithms by single counting each cyclone. The investigation extends earlier studies by lengthening the time period to 55 yr (1948– 2002) with a 6-hourly time resolution, by documenting the seasonality and the dominant temporal modes of variability of cyclone activity, and by diagnosing regional activity as quantified by the cyclone activity index (CAI). The CAI integrates information on cyclone intensity, frequency, and duration into a comprehensive index of cyclone activity. Arctic cyclone activity has increased during the second half of the twentieth century, while midlatitude activity generally decreased from 1960 to the early 1990s, in agreement with previous studies. New findings include the following. 1) The number and intensity of cyclones entering the Arctic from the midlatitudes has increased, suggesting a shift of storm tracks into the Arctic, particularly in summer. 2) Positive tendencies of midlatitude cyclone activity before and after the 1960–93 period of decreasing activity correlate most strongly with variations of cyclone activity in the North Atlantic and Eurasian sectors. 3) Synchronized phase and amplitude variations in cyclone activity over the Arctic Ocean (70°–90°N) and the Arctic marginal zone (60°– 70°N) play a critical role in determining the variations of cyclone activity in the Arctic as a whole. 4) Arctic cyclone activity displays significant low-frequency variability, with a negative phase in the 1960s and a positive phase in the 1990s, upon which 7.8- and 4.1-yr oscillations are superimposed. The 7.8-yr signal generally corresponds to the alternation of the cyclonic and anticyclonic regimes of the Arctic sea ice and ocean motions. [ABSTRACT FROM AUTHOR]

Melt onset dates, freeze onset dates, and melt season duration were estimated over Arctic sea ice, 1979–2001, using passive microwave satellite imagery and surface air temperature data. Sea ice melt duration for the entire Northern Hemisphere varied from a 104-day minimum in 1983 and 1996 to a 124-day maximum in 1989. Ranges in melt duration were highest in peripheral seas, numbering 32, 42, 44, and 51 days in the Laptev, Barents-Kara, East Siberian, and Chukchi Seas, respectively. In the Arctic Ocean, average melt duration varied from a 75-day minimum in 1987 to a 103-day maximum in 1989. On average, melt onset in annual ice began 10.6 days earlier than perennial ice, and freeze onset in perennial ice commenced 18.4 days earlier than annual ice. Average annual melt dates, freeze dates, and melt durations in annual ice were significantly correlated with seasonal strength of the Arctic Oscillation (AO). Following high-index AO winters (January–March), spring melt tended to be earlier and autumn freeze later, leading to longer melt season durations. The largest increases in melt duration were observed in the eastern Siberian Arctic, coincident with cyclonic low pressure and ice motion anomalies associated with high-index AO phases. Following a positive AO shift in 1989, mean annual melt duration increased 2–3 weeks in the northern East Siberian and Chukchi Seas. Decreasing correlations between consecutive-year maps of melt onset in annual ice during 1979–2001 indicated increasing spatial variability and unpredictability in melt distributions from one year to the next. Despite recent declines in the winter AO index, recent melt distributions did not show evidence of reestablishing spatial patterns similar to those observed during the 1979–88 low-index AO period. Recent freeze distributions have become increasingly similar to those observed during 1979–88, suggesting a recurrent spatial pattern of freeze chronology under low-index AO conditions. [ABSTRACT FROM AUTHOR]