Contribution of sea-ice loss to Arctic amplification is regulated by Pacific Ocean decadal variability

The pace of Arctic warming is about double that at lower latitudes – a robust phenomenon known as Arctic amplification (AA)1. Many diverse climate processes and feedbacks cause AA2-7, including positive feedbacks associated with diminished sea ice6,7. However, the precise contribution of sea-ice loss to AA remains uncertain7,8. Through analyses of both observations and model simulations, we show that the contribution of sea-ice loss to wintertime AA appears dependent on the phase of the Pacific Decadal Oscillation (PDO). Our results suggest that for the same pattern and amount of sea-ice loss, consequent Arctic warming is larger during the negative PDO phase, relative to the positive phase, leading to larger reductions in the poleward gradient of tropospheric thickness and to more pronounced reductions in the upper-level westerlies. Given the oscillatory nature of the PDO, this relationship has the potential to increase skill in decadal-scale predictability of Arctic and sub-Arctic climate. Our results indicate that Arctic warming in response to the ongoing long-term sea-ice decline9,10 is greater (reduced) during periods of negative (positive) PDO phase. We speculate that the observed recent shift to the positive PDO phase, if maintained and all other factors being equal, could act to temporarily reduce the pace of wintertime Arctic warming in the near future.J.A.S. was funded by a UK Natural Environment Research Council (NERC) grants NE/J019585/1 and NE/M006123/1. J.A.F. was supported by an NSF/ARCSS grant (1304097) and NASA grant (NNX14AH896). The model simulations were performed on the ARCHER UK National Supercomputing Service. We thank the NOAA ESRL and Met Office Hadley Centre for provision of observational and reanalysis data sets. We also thank D. Ackerley for helping to diagnose the cause of model crashes, C. Deser for commenting on the manuscript prior to submission, and two anonymous reviewers for constructive criticism

Climate simulations for 1880-2003 with GISS modelE

We carry out climate simulations for 1880-2003 with GISS modelE driven by ten measured or estimated climate forcings. An ensemble of climate model runs is carried out for each forcing acting individually and for all forcing mechanisms acting together. We compare side-by-side simulated climate change for each forcing, all forcings, observations, unforced variability among model ensemble members, and, if available, observed variability. Discrepancies between observations and simulations with all forcings are due to model deficiencies, inaccurate or incomplete forcings, and imperfect observations. Although there are notable discrepancies between model and observations, the fidelity is sufficient to encourage use of the model for simulations of future climate change. By using a fixed well-documented model and accurately defining the 1880-2003 forcings, we aim to provide a benchmark against which the effect of improvements in the model, climate forcings, and observations can be tested. Principal model deficiencies include unrealistically weak tropical El Nino-like variability and a poor distribution of sea ice, with too much sea ice in the Northern Hemisphere and too little in the Southern Hemisphere. The greatest uncertainties in the forcings are the temporal and spatial variations of anthropogenic aerosols and their indirect effects on clouds.Comment: 44 pages; 19 figures; Final text accepted by Climate Dynamic

Nonlinear response of mid-latitude weather to the changing Arctic

Are continuing changes in the Arctic influencing wind patterns and the occurrence of extreme weather events in northern mid-latitudes? The chaotic nature of atmospheric circulation precludes easy answers. The topic is a major science challenge, as continued Arctic temperature increases are an inevitable aspect of anthropogenic climate change. We propose a perspective that rejects simple cause-and-effect pathways and notes diagnostic challenges in interpreting atmospheric dynamics. We present a way forward based on understanding multiple processes that lead to uncertainties in Arctic and mid-latitude weather and climate linkages. We emphasize community coordination for both scientific progress and communication to a broader public