The missing Northern European winter cooling response to Arctic sea ice loss

This is the author accepted manuscript. The final version is available from the publisher via the DOI in this record.Reductions in Arctic sea ice may promote the negative phase of the North Atlantic Oscillation (NAO-). It has been argued that NAO-related variability can be used an as analogue to predict the effects of Arctic sea ice loss on mid-latitude weather. Since NAO- events are associated with colder winters over Northern Europe, a negatively-shifted NAO has been proposed as a dynamical pathway for Arctic sea ice loss to cause Northern European cooling. This study uses large-ensemble atmospheric simulations with prescribed ocean surface conditions to examine how seasonal-scale NAO- events are affected by Arctic sea ice loss. Despite an intensification of NAO- events, reflected by more prevalent easterly flow, sea ice loss doesn’t lead to Northern European winter cooling, and daily cold extremes actually decrease. The dynamical cooling from the changed NAO is “missing” because it is offset (or exceeded) by a thermodynamical effect owing to advection of warmer air masses.J.A.S. was funded by UK Natural Environment Research Council grants NE/J019585/1, NE/M006123/1 and NE/P006760/1

Simulated Atmospheric Response to Regional and Pan-Arctic Sea-Ice Loss

This is the final version of the article. Available from American Meteorological Society via the DOI in this record.The loss of Arctic sea-ice is already having profound environmental, societal and ecological impacts locally. A highly uncertain area of scientific research, however, is whether such Arctic change has a tangible effect on weather and climate at lower latitudes. There is emerging evidence that the geographical location of sea-ice loss is critically important in determining the large-scale atmospheric circulation response and associated mid-latitude impacts. However, such regional dependencies have not been explored in a thorough and systematic manner. To make progress on this issue, this study analyses ensemble simulations with an atmospheric general circulation model prescribed with sea-ice loss separately in nine regions of the Arctic, to elucidate the distinct responses to regional sea-ice loss. The results suggest that in some regions sea-ice loss triggers large-scale dynamical responses whereas in other regions sea-ice loss induces only local thermodynamical changes. Sea-ice loss in the Barents- Kara Sea is unique in driving a weakening of the stratospheric polar vortex, followed in time by a tropospheric circulation response that resembles the North Atlantic Oscillation. For October-to-March, the largest spatial-scale responses are driven by sea-ice loss in the Barents-Kara Sea and Sea of Okhotsk; however, different regions assume greater importance in other seasons. The atmosphere responds very differently to regional sea-ice losses than to pan-Arctic sea-ice loss, and the latter cannot be obtained by linear addition of the responses to regional sea-ice losses. The results imply that diversity in past studies of the simulated response to Arctic sea-ice loss can be partly explained by the different spatial patterns of sea- ice loss imposed.This work was supported by Natural Environment Research Council grants NE/J019585/1 and NE/M006123/1

Nonstationary Relationship Between Autumn Arctic Sea Ice and the Winter North Atlantic Oscillation

This is the final version. Available from American Geophysical Union (AGU) via the DOI in this record.The North Atlantic Oscillation (NAO) has a dominating influence on wintertime weather in the North Atlantic region, and therefore, it is of great interest to predict the NAO several months ahead. While state-of-the-art dynamical forecast models appear to be increasingly skillful in predicting the NAO, statistical methods with comparable or higher predictive skill are still often used. An inherent problem with statistical methods is that any empirical relationship between predictors and the NAO may be valid for some periods but subject to change over time. Here we use a set of new centennial reanalyses and large-ensemble simulations with multiple climate models to discover clear evidence of nonstationarity in the lagged correlation between autumn Barents-Kara sea ice and the winter NAO. This nonstationarity leads us to question the causality and/or robustness of the ice-NAO link. We caution against indiscriminately using Barents-Kara sea ice to predict the NAO.Horizon 2020 Framework ProgrammeLeverhulme TrustNatural Environment Research Council (NERC)Research Council of Norwa

Influence of Arctic Sea-Ice Loss in Autumn Compared to that in Winter on the Atmospheric Circulation (article)

This is the author accepted manuscript. The final version is available from Wiley/American Geophysical Union via the DOI in this record.The dataset associated with this article is located in ORE at: https://doi.org/10.24378/exe.963There is growing evidence that Arctic sea ice loss affects the large-scale atmospheric circulation. Some studies suggest that reduced autumn sea ice may be a precursor to severe midlatitude winters. Here we use coupled ocean–atmosphere model experiments to investigate the extent to which the winter atmospheric circulation response to Arctic sea ice loss is driven by sea ice loss in preceding months. We impose different seasonal cycles of sea ice by using various combinations of sea ice albedo parameters. Year-round sea ice loss causes an equatorward migration of the eddy-driven jet and a shift toward the negative phase of the North Atlantic Oscillation in winter. However, these circulation changes are not found when sea ice is reduced only in late summer and autumn, despite high-latitude warming persisting into the winter. Our results imply that the winter atmospheric circulation response to sea ice loss is primarily driven by sea ice loss in winter rather than in autumn.NER

Pacific Ocean Variability Influences the Time of Emergence of a Seasonally Ice-Free Arctic Ocean

This is the final version. Available from Wiley / American Geophysical Union via the DOI in this record.Data are freely available at the following repositories: CESM1 Large Ensemble, http://www.cesm.ucar.edu/projects/community-projects/LENS/data-sets.html; NOAA ERSST version 5, https://www1.ncdc.noaa.gov/pub/data/cmb/ersst/v5/netcdf/; NSIDC Sea Ice Index, ftp://sidads.colorado.edu/DATASETS/NOAA/G02135/.The Arctic Ocean is projected to become seasonally ice-free before mid-century unless greenhouse gas emissions are rapidly reduced, but exactly when this could occur depends considerably on internal climate variability. Here we show that trajectories to an ice-free Arctic are modulated by concomitant shifts in the Interdecadal Pacific Oscillation (IPO). Trajectories starting in the negative IPO phase become ice-free 7 years sooner than those starting in the positive IPO phase. Trajectories starting in the negative IPO phase subsequently transition towards the positive IPO phase, on average, with an associated strengthening of the Aleutian Low, increased poleward energy transport and faster sea-ice loss. The observed IPO began to transition away from its negative phase in the past few years. If this shift continues, our results suggest increased likelihood of accelerated sea-ice loss over the coming decades, and an increased risk of an ice-free Arctic within the next 20-30 years.Leverhulme TrustNatural Environment Research Council (NERC

How Robust is the Atmospheric Response to Projected Arctic Sea Ice Loss Across Climate Models? (article)

This is the final version. Available from the American Geophysical Union via the DOI in this recordThe dataset associated with this article is available in ORE at https://doi.org/10.24378/exe.1823We assess the reliability of an indirect method of inferring the atmospheric response to projected Arctic sea ice loss from CMIP5 simulations, by comparing the response inferred from the indirect method to that explicitly simulated in sea ice perturbation experiments. We find that the indirect approach works well in winter, but has limited utility in the other seasons. We then apply a modified version of the indirect method to 11 CMIP5 models to reveal the robust and non-robust aspects of the wintertime atmospheric response to projected Arctic sea ice loss. Despite limitations of the indirect method, we identify a robust enhancement of the Siberian High, weakening of the Icelandic Low, weakening of the westerly wind on the poleward flank of the eddy-driven jet, strengthening of the subtropical jet, and weakening of the stratospheric polar vortex. The surface air temperature response to projected Arctic sea ice loss over mid-latitude continents is non-robust across the models.Natural Environment Research Council (NERC)Leverhulme Trus

Ocean-atmospheric state dependence of the atmospheric response to Arctic sea ice loss

This is the final version of the article. Available from American Meteorological Society via the DOI in this record.The Arctic is warming faster than the global average. This disproportionate warming – known as Arctic amplification – has caused significant local changes to the Arctic system and more uncertain remote changes across the Northern Hemisphere midlatitudes. Here, we use an atmospheric general circulation model (AGCM) to test the sensitivity of the atmospheric and surface response to Arctic sea ice loss to the phase of the Atlantic Multidecadal Oscillation (AMO), which varies on (multi-)decadal timescales. Four experiments are performed, combining low and high sea ice states with global sea surface temperature (SST) anomalies associated with opposite phases of the AMO. A trough-ridge-trough response to wintertime sea ice loss is seen in the PacificNorth America sector in the negative phase of the AMO. We propose that this is a consequence of an increased meridional temperature gradient in response to sea ice loss, just south of the climatological maximum, in the central midlatitude North Pacific. This causes a southward shift in the North Pacific storm track, which strengthens the Aleutian Low with circulation anomalies propagating into North America. While the climate response to sea ice loss is sensitive to AMO-related SST anomalies in the North Pacific, there is little sensitivity to larger magnitude SST anomalies in the North Atlantic. With background ocean-atmospheric states persisting for a number of years, there is the potential to improve predictions of the impacts of Arctic sea ice loss on decadal timescalesThis work was supported by the Natural Environment Research Council grants NE/M006123/1 and NE/J019585/1. The HadGAM2 simulations were performed on the ARCHER UK National Supercomputing Service. For the provision of observed and reanalysis data the Met Office Hadley Centre and NOAA ESRL are thanked. Model data are available from the authors upon request

Links between Barents‐Kara sea ice and the Extratropical Atmospheric Circulation explained by internal variability and tropical forcing

This is the final version. Available from American Geophysical Union (AGU) via the DOI in this record.Changes in Arctic sea ice have been proposed to affect midlatitude winter atmospheric circulation, often based on observed coincident variability. However, causality of this covariability remains unclear. Here, we address this issue using atmospheric model experiments prescribed with observed sea surface temperature variations and either constant or time‐varying sea ice variability. We show that the observed relationship between late‐autumn Barents‐Kara sea ice and the winter North Atlantic Oscillation can be reproduced by simulated atmospheric internal variability but is not simulated as a forced response to sea ice. Observations and models suggest reduced sea ice is linked to a weaker Aleutian Low. We show that simulated Aleutian Low variability is correlated with observed sea ice variability even in simulations with fixed sea ice, implying that this relationship is not incidental. Instead, we suggest that covariability between sea ice and the Aleutian Low originates from tropical sea surface temperature and rainfall variations and their teleconnections to the extratropics.Natural Environment Research Council (NERC

Observed statistical connections overestimate the causal effects of Arctic sea-ice changes on midlatitude winter climate

This is the final version. Available from the American Meteorological Society via the DOI in this recordData Availability: All data used in this study are publicly available. ERA-Interim reanalysis can be found at https://www.ecmwf.int/en/forecasts/datasets/reanalysis-datasets/era-interim, coupled model data can be found at https://www.earthsystemgrid.org/dataset/ucar.cgd.ccsm4.CLIVAR_LE.html , and AMIP data can found at https://www.esrl.noaa.gov/psd/repository/facts.Disentangling the contribution of changing Arctic sea ice to midlatitude winter climate variability remains challenging because of the large internal climate variability in midlatitudes, difficulties separating cause from effect, methodological differences, and uncertainty around whether models adequately simulate connections between Arctic sea ice and midlatitude climate. We use regression analysis to quantify the links between Arctic sea ice and midlatitude winter climate in observations and large initial-condition ensembles of multiple climate models, in both coupled configurations and so-called atmospheric model intercomparison project (AMIP) configurations, where observed sea ice and/or sea surface temperatures are prescribed. The coupled models capture the observed links in interannual variability between winter Barents-Kara sea ice and Eurasian surface temperature, and between winter Chukchi-Bering sea ice and North American surface temperature. The coupled models also capture the delayed connection between reduced November-December Barents-Kara sea ice, a weakened winter stratospheric polar vortex, and a shift towards the negative phase of the North Atlantic Oscillation in late winter, although this downward impact is weaker than observed. The strength and sign of the connections both vary considerably between individual 35-year-long ensemble members, highlighting the need for large ensembles to separate robust connections from internal variability. All the aforementioned links are either absent, or substantially weaker, in the AMIP experiments prescribed with only observed sea ice variability. We conclude that the causal effects of sea ice variability on midlatitude winter climate are much weaker than suggested by statistical associations, evident in observations and coupled models, because the statistics are inflated by the effects of atmospheric circulation variability on sea ice.Natural Environment Research Council (NERC

Insignificant effect of Arctic amplification on the amplitude of midlatitude atmospheric waves

This is the final version. Available from the American Association for the Advancement of Science via the DOI in this record. Whether Arctic amplification has contributed to a wavier circulation and more frequent extreme weather in midlatitudes remains an open question. For two to three decades starting from the mid-1980s, accelerated Arctic warming and a reduced meridional near-surface temperature gradient coincided with a wavier circulation. However, waviness remains largely unchanged in model simulations featuring strong Arctic amplification. Here, we show that the previously reported trend toward a wavier circulation during autumn and winter has reversed in recent years, despite continued Arctic amplification, resulting in negligible multidecadal trends. Models capture the observed correspondence between a reduced temperature gradient and increased waviness on interannual to decadal time scales. However, model experiments in which a reduced temperature gradient is imposed do not feature increased wave amplitude. Our results strongly suggest that the observed and simulated covariability between waviness and temperature gradients on interannual to decadal time scales does not represent a forced response to Arctic amplification.Natural Environment Research Council (NERC