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This study investigates the contribution of external forcings on global and regional ocean wave height change during 1961–2020. Historical significant wave height (Hs) produced for different CMIP6 external forcings and preindustrial control conditions following the Detection and Attribution Model Intercomparison Project (DAMIP) are employed. The internal variability ranges are compared with different external forcing scenario. Statistically significant linear trends in Hs computed over regional ocean basins are found to be mostly associated with anthropogenic forcings: greenhouse gas‐only (GHG) and aerosol‐only (AER) forcing. For Hs, GHG signals are robustly detected and dominant for most of the global ocean, except over North pacific and South Atlantic, where AER signals are dominant. These results are supported by multi‐model analysis for wind speed. The remarkable increase in Hs over the Arctic (22.3%) and Southern (8.2%) Ocean can be attributed to GHG induced sea‐ice depletion and larger effective fetch along with wind speed increase. Plain Language Summary: We quantify the influence of anthropogenic forcings (greenhouse gas‐only and aerosol‐only forcing) and natural forcing to the significant wave height trends during 1961–2020 using CMIP6 individual forcing experiments. It is shown that anthropogenic influence is majorly responsible for the significant wave height changes and natural (solar and volcanic activities) forcings show limited influence. The human‐induced greenhouse gas increases are found to be the dominating factor for most of the global ocean, whereas anthropogenic aerosols are the dominating forcing for a few ocean basins, such as North Pacific and South Atlantic. The multimodel analysis for wind speed corroborates the relative dominance of signals in wave height change. In the polar ocean (Arctic and Southern Ocean), we see exceptional wave height increase compared to other regions. Sea‐ice decline associated with greenhouse gas forcing provides larger fetch for the waves to grow in polar region. Moreover, the contrasting influence of greenhouse gas and aerosol forcing to sea‐ice area and wind speed changes are shown to drive the total wave height changes. Key Points: CMIP6/DAMIP simulations show that anthropogenic signals are robustly detected for the significant wave height (Hs) trends during 1961–2020Greenhouse gases are the major contributor for Hs trends over the global ocean, but aerosols dominance is seen for a few regional basinsHigh increase in Hs over the Polar oceans is due to greenhouse gas induced sea‐ice decline, fetch enlargement and wind speed increase [ABSTRACT FROM AUTHOR]

Arctic hydrological cycle has important global implications through modifying deep ocean circulation via fresh water fluxes. However, attribution of the observed changes in Arctic moisture budget remains challenging due to the lack of reliable observations. Here, as a first step, past changes in hydrological cycle over the Arctic were evaluated using CMIP6 historical simulations. To examine possible influences of individual external forcings, CMIP6 multi-model simulations performed under natural-plus-anthropogenic (ALL), greenhouse gas (GHG), natural (NAT), and aerosol (AER) forcings were compared. Results indicate that Arctic precipitation and evaporation increase in ALL and GHG but decrease in AER. In ALL and GHG, Arctic precipitation increases in summer mainly due to enhanced poleward moisture transport whereas Arctic moistening during winter is affected more by increased surface evaporation over sea-ice retreat areas. In AER, Arctic precipitation tends to decrease due to reduced evaporation over sea-ice advance areas during cold months. Poleward meridional moisture flux (MMF) across 70°N is the strongest in summer due to the highest moisture. GHG has stronger MMF than other forcing simulations due to larger increase in moisture in line with stronger warming. The MMF response is found to be largely determined by variations in transient eddies with distinct seasonal and regional contributions. [ABSTRACT FROM AUTHOR]

The sixth assessment report of the IPCC assessed that the Arctic is projected to be on average practically ice-free in September near mid-century under intermediate and high greenhouse gas emissions scenarios, though not under low emissions scenarios, based on simulations from the latest generation Coupled Model Intercomparison Project Phase 6 (CMIP6) models. Here we show, using an attribution analysis approach, that a dominant influence of greenhouse gas increases on Arctic sea ice area is detectable in three observational datasets in all months of the year, but is on average underestimated by CMIP6 models. By scaling models' sea ice response to greenhouse gases to best match the observed trend in an approach validated in an imperfect model test, we project an ice-free Arctic in September under all scenarios considered. These results emphasize the profound impacts of greenhouse gas emissions on the Arctic, and demonstrate the importance of planning for and adapting to a seasonally ice-free Arctic in the near future. A dominant influence of anthropogenic greenhouse gas increases on Arctic sea ice area is detectable in all months. By scaling climate models' sea ice response to best match observed trends, an ice-free Arctic in September is projected under all scenarios. [ABSTRACT FROM AUTHOR]

Arctic warming has sparked a growing interest because of its possible impacts on mid-latitude climate. A number of unusually harsh cold winters have occurred in many parts of East Asia and North America in the past few years, and observational and modelling studies have suggested that atmospheric variability linked to Arctic warming might have played a central role. Here we identify two distinct influences of Arctic warming which may lead to cold winters over East Asia or North America, based on observational analyses and extensive climate model results. We find that severe winters across East Asia are associated with anomalous warmth in the Barents-Kara Sea region, whereas severe winters over North America are related to anomalous warmth in the East Siberian-Chukchi Sea region. Each regional warming over the Arctic Ocean is accompanied by the local development of an anomalous anticyclone and the downstream development of a mid-latitude trough. The resulting northerly flow of cold air provides favourable conditions for severe winters in East Asia or North America. These links between Arctic and mid-latitude weather are also robustly found in idealized climate model experiments and CMIP5 multi-model simulations. We suggest that our results may help improve seasonal prediction of winter weather and extreme events in these regions. [ABSTRACT FROM AUTHOR]

We present a study that suggests greening in the circumpolar high-latitude regions amplifies surface warming in the growing season (May-September) under enhanced greenhouse conditions. The investigation used a series of climate simulations with the Community Atmospheric Model version 3-which incorporates a coupled, dynamic global vegetation model-with and without vegetation feedback, under both present and doubled CO concentrations. Results indicate that climate warming and associated changes promote circumpolar greening with northward expansion and enhanced greenness of both the Arctic tundra and boreal forest regions. This leads to additional surface warming in the high-latitudes in the growing season, primarily through more absorption of incoming solar radiation. The resulting surface and tropospheric warming in the high-latitude and Arctic regions weakens prevailing tropospheric westerlies over 45-70N, leading to the formation of anticyclonic pressure anomalies in the Arctic regions. These pressure anomalies resemble the anomalous circulation pattern during the negative phase of winter Arctic Oscillation. It is suggested that these circulation anomalies reinforce the high-latitude and Arctic warming in the growing season. [ABSTRACT FROM AUTHOR]