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We simulated Arctic climate using a high‐resolution implementation of WRF driven by HadGEM‐ES2 climate model outputs, following IPCC5 warming scenarios RCP 2.6, RCP 4.5 and RCP 8.5. The downscaled results indicate that, by the end‐of‐century, there are no significant changes in the average minimum central pressures or total number of winter Arctic cyclones. However, there are significant changes in the spatial patterns. For example, the frequency and vorticity of cyclones tend to increase over the western Arctic. Due to the poleward movement of the polar frontal zone and the increased low‐level tropospheric baroclinicity, more cyclones are expected to form within the Arctic Basin and migrate into the western‐central Arctic. We expect about 39% more cyclone tracks forming and dying in the Arctic Basin under RCP8.5, compared to present climate. On the other hand, with the reduced baroclinicity in the entire troposphere, there is a reduced genesis, frequency, and vorticity of cyclones over the Atlantic Arctic. Moreover, the depth of cyclones shows a robust decrease over the Arctic Basin, suggesting weakened eddy kinetic energy. With increasing greenhouse gases, the changes in cyclone vorticities and their depths tend to be stronger. In addition, the high resolution Polar WRF results demonstrate that changes in cyclone track density and intensities consistently become more pronounced with increasing radiative forcing. Plain Language Summary: We used advanced climate models to simulate how the Arctic climate might change in the future. We focused on the scenarios outlined by the Intergovernmental Panel on Climate Change (IPCC) and their effects on Arctic cyclones. Our results show that, by the end of the 21st century, the overall characteristics of winter Arctic cyclones do not experience significant changes. However, there are interesting regional variations. The frequency and vorticity (spinning) force of cyclones are projected to decrease over the Atlantic Arctic but increase over the western Arctic. Simultaneously, fewer cyclones are expected over the Atlantic Arctic. Furthermore, the depth of cyclones over the Arctic Basin is expected to decrease, suggesting a weakening in their strength. In IPCC scenarios with higher emissions, the changes in cyclone characteristics become more pronounced. Similar trends are also observed in results from the latest climate models. Key Points: The frequency and vorticity of cyclones tend to increase (decrease) over the western‐central (Atlantic) Arctic under climate scenariosThe decreasing cyclone depths in the Arctic Basin suggest a weakened eddy kinetic energy in warming climate scenariosThe poleward shift of the frontal zone and increased baroclinicity lead to 39% more cyclones forming and dying in the Arctic Basin under the warmest scenario [ABSTRACT FROM AUTHOR]

We show that the winter Beaufort High (BH) index defined by sea level pressure (SLP) has a robust negative trend under the scenarios SSP5‐8.5 and SSP2‐4.5, with a reduction by about 5 hPa and 2 hPa, respectively, by the end of the 21st century. The negative trends in the BH SLP are associated with the changes in the background SLP over the Arctic basin. However, the vorticity of the winter BH tends to intensify under SSP5‐8.5, but shows no robust increase under SSP2‐4.5. The intensification is associated with the enhanced ridge over the Western Arctic. Therefore, it is necessary to take into account the dynamic aspects of the BH, such as vorticity. Based on this assessment, under the most likely emissions scenario, the winter BH is likely to weaken through the 21st century, in terms of SLP, but shows no robust changes in term of vorticity. Plain Language Summary: We used the latest IPCC global climate model outputs to investigate the wintertime changes in the Beaufort High (BH). Under the climate change scenarios, during winter, the sea level pressure (SLP) over the BH region is expected to decrease. The SLP reduction is up to 5 hPa by the end‐of‐the‐century. Thus, the BH seems to become weaker. However, its vorticity tends to increase under SSP5‐8.5, mostly due to the intensification of the ridge over the western North America. Key Points: The Beaufort High (BH) index defined by sea level pressure (SLP) decreases under warming climate scenariosMeanwhile, the BH vorticity tends to intensify due to the change in the ridge over the Western ArcticBesides the SLP, impact studies of the BH on the Arctic Ocean need to consider its dynamics such as vorticity [ABSTRACT FROM AUTHOR]

The provision of reliable results from numerical wave models implemented over vast ocean areas can be considered as a time-consuming process. In this regard, the estimation of areas with maximum similarity in wave climate spatial areas and the determination of associated representative point locations for these areas can play an important role in climate research and in engineering applications. To deal with this issue, we apply a state-of-the-art clustering method, Geo-SOM, to determine geographical areas with similar wave regimes, in terms of mean wave direction (MWD), mean wave period (T0), and significant wave height (Hs). Although this method has many strengths, a weakness is related to detection and accounting of the most extreme and rare events. To resolve this deficiency, an initial preprocessing method (called PG-Geo-SOM) is applied. To evaluate the performance of this method, extreme wave parameters, including Hs and T0, are calculated. We simulate the present climate, represented as 1979 to 2017, compared to the future climate, 2060–98, following the Intergovernmental Panel on Climate Change (IPCC) future scenario RCP8.5 in the northwestern Atlantic Ocean. In this approach, the wave parameter data are divided into distinct groups, or clusters, motivated by their geographical positions. For each cluster, the centroid spatial point and the time series of data are extracted, for Hs, MWD, and T0. Extreme values are estimated for 5-, 10-, 25-, 50-, and 100-yr return periods, using Gumbel, exponential, and Weibull stochastic models, for both present and future climates. Results show that for parameter T0, the impact of climate change for the study area is a decreasing trend, while for Hs, in coastal and shelf areas up to about 1000 km from the coastline, increasing trends are estimated, and in open-ocean areas, far from the coast, decreasing trends are obtained. [ABSTRACT FROM AUTHOR]

The abrupt deceleration of accelerated Greenland Ice Sheet (GrIS) melting since 2013, after a period of acceleration previously noted, is studied here. It is shown that the deceleration of GrIS melting since 2013 is due to the reduction in short‐wave solar radiation in the presence of increasing total cloud cover, which is driven by a more persistent positive summer North Atlantic Oscillation on the decadal time scale. By presenting the coherence with the temperature variability at the weather stations in Greenland, which have century‐long records, we deduce that the acceleration of GrIS melting during the early 2000s and the subsequent deceleration since 2013 will reoccur frequently on decadal time scales, with the amplitude nearly half of the multidecadal warming trend of the GrIS melt. It can reduce the mass loss from the GrIS on short to medium time scales but is unlikely to halt mass loss related to climate change in the future. This finding highlights the importance of internal climate variability on the mass budget of the GrIS and therefore on predictions of future global sea level change and may help to assist planning for associated social and economic consequences. Plain Language Summary: The Greenland Ice Sheet (GrIS) mass change and its contribution to the global sea level variability is a topic that naturally piques our curiosity. Since the satellite first captured the Greenland‐wide ice sheet mass variability in 2002, the GrIS experienced accelerated melting, leading to global sea level acceleration. The long‐term GrIS melting depends on climate change such as greenhouse warming. However, recently, this acceleration is slowed down due to the summer North Atlantic Oscillation phase changing to positive. We use century‐long surface temperature records as index to estimate the contribution of this slowdown phenomenon and find that the amplitude of this internal variability is nearly half of the multidecadal warming trend of the GrIS; therefore, this cycle can slow down but cannot stop the loss of the Greenland Ice Sheet mass at present and in the near future. This finding highlights the role of the internal variability of the GrIS on predictions of future global sea level change and the associated social and economic consequences. Key Points: Greenland Ice Sheet exhibits decelerated melt since 2013The deceleration is due to increased cloud during positive summer NAOThe accelerated and decelerated GrIS melt will reoccur on decadal time scale [ABSTRACT FROM AUTHOR]

An analysis method proposed by Huang is improved and used to dissect the radiative forcing in the instantaneous quadrupling CO2 experiment from phase 5 of the Coupled Model Intercomparison Project (CMIP5). Multiple validation tests show that the errors in the forcing estimates are generally within 10%. The results show that quadrupling CO2, on average, induces a global-mean all-sky instantaneous top-of-the-atmosphere forcing of 5.4 W m−2, which is amended by a stratospheric adjustment of 1.9 W m−2 and a tropospheric adjustment of −0.1 W m−2. The resulting fully adjusted radiative forcing has an ensemble mean of 7.2 W m−2 and a substantial intermodel spread (maximum-minimum) of 2.4 W m−2, which results from all the forcing components, especially the instantaneous forcing and tropospheric adjustment. The fidelity of the linear decomposition of the overall radiation variation is improved when forcing is explicitly estimated for each model. A significant contribution by forcing uncertainty to the intermodel spread of the surface temperature projection is verified. The results reaffirm the importance of evaluating the radiative forcing components in climate feedback analyses. [ABSTRACT FROM AUTHOR]

An analysis method proposed by Huang is improved and used to dissect the radiative forcing in the instantaneous quadrupling CO2 experiment from phase 5 of the Coupled Model Intercomparison Project (CMIP5). Multiple validation tests show that the errors in the forcing estimates are generally within 10%. The results show that quadrupling CO2, on average, induces a global-mean all-sky instantaneous top-of-the-atmosphere forcing of 5.4 W m−2, which is amended by a stratospheric adjustment of 1.9 W m−2 and a tropospheric adjustment of −0.1 W m−2. The resulting fully adjusted radiative forcing has an ensemble mean of 7.2 W m−2 and a substantial intermodel spread (maximum-minimum) of 2.4 W m−2, which results from all the forcing components, especially the instantaneous forcing and tropospheric adjustment. The fidelity of the linear decomposition of the overall radiation variation is improved when forcing is explicitly estimated for each model. A significant contribution by forcing uncertainty to the intermodel spread of the surface temperature projection is verified. The results reaffirm the importance of evaluating the radiative forcing components in climate feedback analyses. [ABSTRACT FROM AUTHOR]

The purpose of this study is to investigate possible local changes in the wave climate for the coastal waters off eastern Canada, particularly in the Gulf of St. Lawrence (GSL) related to changes in marine winds, storm and the sea ice climate, due to climate change. These analyses are based on application of a dynamical downscaling approach whereby a regional climate model is driven by climate change estimates from the Canadian Global Climate Model (CGCM3) to provide relatively high resolution winds to drive a wave model. The CGCM3 simulation follows the A1B climate change scenario of the Special Report on Emission Scenarios from the Fourth Assessment Report (AR4) of the Intergovernmental Panel on Climate Change (IPCC AR4, 2007). The analyses of the wave climate are based on simulations of the waves from a third-generation wave model, WAVEWATCHIII™, and the downscaled winds obtained from the Canadian Regional (atmospheric) Climate Model (CRCM). We show that the significant wave heights in the Gulf of St. Lawrence (hereafter GSL) and neighboring coastal waters will slightly increase in the winter and decrease in the summer, in response to changes in storms and sea ice in the future climate (2040–2069) compared to the present wave climate, represented as 1970–1999. This time period is denoted as the “historical” wave climate in this study. In summer, the changes in significant wave heights ( Hs ) are associated with estimated decreases in the frequency of the occurrence of the cyclones. Projected changes in return values for summer extremes in the wave climate are consistent with the associated changes in the maximum Hs values. In winter, the projected increases in return values are mostly concentrated in the St. Lawrence Estuary, the northern and southwestern GSL, consistent with changes in the maximum waves in these regions. An important factor related to change in the winter wave climate is change in the sea ice. [ABSTRACT FROM AUTHOR]