Caution needed when linking weather extremes to amplified planetary waves

Copyright © 2013 National Academy of Science

Increasing fall-winter energy loss from the Arctic Ocean and its role in Arctic temperature amplification

Copyright © 2010 American Geophysical UnionArctic surface temperatures have risen faster than the global average in recent decades, in part due to positive feedbacks associated with the rapidly diminishing sea ice cover. Counter-intuitively, the Arctic warming has been strongest in late fall and early winter whilst sea ice reductions and the direct ice-albedo feedback have been greatest in summer and early fall. To reconcile this, previous studies have hypothesized that fall/winter Arctic warming has been enhanced by increased oceanic heat loss but have not presented quantitative evidence. Here we show increases in heat transfer from the Arctic Ocean to the overlying atmosphere during October–January, 1989–2009. The trends in surface air temperature, sea ice concentration and the surface heat fluxes display remarkable spatial correspondence. The increased oceanic heat loss is likely a combination of the direct response to fall/winter sea ice loss, and the indirect response to summer sea ice loss and increased summer ocean heating

The central role of diminishing sea ice in recent Arctic temperature amplification

Copyright © 2010 Nature Publishing GroupThe rise in Arctic near-surface air temperatures has been almost twice as large as the global average in recent decades-a feature known as 'Arctic amplification'. Increased concentrations of atmospheric greenhouse gases have driven Arctic and global average warming; however, the underlying causes of Arctic amplification remain uncertain. The roles of reductions in snow and sea ice cover and changes in atmospheric and oceanic circulation, cloud cover and water vapour are still matters of debate. A better understanding of the processes responsible for the recent amplified warming is essential for assessing the likelihood, and impacts, of future rapid Arctic warming and sea ice loss. Here we show that the Arctic warming is strongest at the surface during most of the year and is primarily consistent with reductions in sea ice cover. Changes in cloud cover, in contrast, have not contributed strongly to recent warming. Increases in atmospheric water vapour content, partly in response to reduced sea ice cover, may have enhanced warming in the lower part of the atmosphere during summer and early autumn. We conclude that diminishing sea ice has had a leading role in recent Arctic temperature amplification. The findings reinforce suggestions that strong positive ice-temperature feedbacks have emerged in the Arctic, increasing the chances of further rapid warming and sea ice loss, and will probably affect polar ecosystems, ice-sheet mass balance and human activities in the Arctic

Half-century air temperature change above Antarctica: Observed trends and spatial reconstructions

Copyright © 2012 American Geophysical UnionThis study provides a comprehensive analysis of observed 50-year (1961–2010) seasonal air temperature trends from radiosonde ascents above Antarctica. Comparisons between multiple radiosonde data sets (homogenized in different ways) at each of eight Antarctic stations reveals substantial differences in the upper-air temperature trend magnitudes and their statistical significance between data sets. However, when considering the average of these data sets at each station, or averaging across all stations, a robust vertical profile of half-century temperature change emerges, characterized by mid-tropospheric warming and stratospheric cooling. Statistically significant Multistation-mean 500 hPa warming (0.1 to 0.2°C decade−1) is found in all seasons, whereas the lower stratospheric cooling has been manifest primarily in austral spring and summer, but with larger magnitudes (−1.0 to −2.0°C decade−1). We undertake the first spatial reconstructions of pan-Antarctic upper-air temperature trends. They strongly suggest that both the year-round mid-tropospheric warming and spring and summer lower stratospheric cooling have occurred above the entire continent, although their magnitudes and significance vary regionally. The reconstructed 500 hPa warming trends in winter and spring are largest over West Antarctica, the Ross Ice shelf, Victoria Land and Oates Land, and show close resemblance to those found in previously published surface temperature trend reconstructions, suggesting coupling between the surface and trends aloft. We speculate that the winter and spring mid-tropospheric warming may, in part, be driven by tropical ocean warming, analogous to proposed mechanisms for the co-located surface warming. The spring and summer lower stratospheric cooling is entirely consistent with the temperature response to ozone depletion

Declining summer snowfall in the Arctic: Causes, impacts and feedbacks

Copyright © 2012 Springer Verlag. The final publication is available at link.springer.comRecent changes in the Arctic hydrological cycle are explored using in situ observations and an improved atmospheric reanalysis data set, ERA-Interim. We document a pronounced decline in summer snowfall over the Arctic Ocean and Canadian Archipelago. The snowfall decline is diagnosed as being almost entirely caused by changes in precipitation form (snow turning to rain) with very little influence of decreases in total precipitation. The proportion of precipitation falling as snow has decreased as a result of lower-atmospheric warming. Statistically, over 99% of the summer snowfall decline is linked to Arctic warming over the past two decades. Based on the reanalysis snowfall data over the ice-covered Arctic Ocean, we derive an estimate for the amount of snow-covered ice. It is estimated that the area of snow-covered ice, and the proportion of sea ice covered by snow, have decreased significantly. We perform a series of sensitivity experiments in which inter-annual changes in snow-covered ice are either unaccounted for, or are parameterized. In the parameterized case, the loss of snow-on-ice results in a substantial decrease in the surface albedo over the Arctic Ocean, that is of comparable magnitude to the decrease in albedo due to the decline in sea ice cover. Accordingly, the solar input to the Arctic Ocean is increased, causing additional surface ice melt. We conclude that the decline in summer snowfall has likely contributed to the thinning of sea ice over recent decades. The results presented provide support for the existence of a positive feedback in association with warming-induced reductions in summer snowfall

Atmospheric impacts of Arctic sea-ice loss, 1979-2009: separating forced change from atmospheric internal variability

Copyright © 2013 Springer Verlag. The final publication is available at Springer via http://dx.doi.org/10.1007/s00382-013-1830-9The ongoing loss of Arctic sea-ice cover has implications for the wider climate system. The detection and importance of the atmospheric impacts of sea-ice loss depends, in part, on the relative magnitudes of the sea-ice forced change compared to natural atmospheric internal variability (AIV). This study analyses large ensembles of two independent atmospheric general circulation models in order to separate the forced response to historical Arctic sea-ice loss (1979–2009) from AIV, and to quantify signal-to-noise ratios. We also present results from a simulation with the sea-ice forcing roughly doubled in magnitude. In proximity to regions of sea-ice loss, we identify statistically significant near-surface atmospheric warming and precipitation increases, in autumn and winter in both models. In winter, both models exhibit a significant lowering of sea level pressure and geopotential height over the Arctic. All of these responses are broadly similar, but strengthened and/or more geographically extensive, when the sea-ice forcing is doubled in magnitude. Signal-to-noise ratios differ considerably between variables and locations. The temperature and precipitation responses are significantly easier to detect (higher signal-to-noise ratio) than the sea level pressure or geopotential height responses. Equally, the local response (i.e., in the vicinity of sea-ice loss) is easier to detect than the mid-latitude or upper-level responses. Based on our estimates of signal-to-noise, we conjecture that the local near-surface temperature and precipitation responses to past Arctic sea-ice loss exceed AIV and are detectable in observed records, but that the potential atmospheric circulation, upper-level and remote responses may be partially or wholly masked by AIV.Australian Research CouncilMerit Allocation Scheme on the Australian National Computational InfrastructureUS National Science Foundation (NSF) Office of Polar ProgramsUK Natural Environment Research Council (NERC