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Mira Berdahl, Asa Rennermalm, Arno Hammann, John Mioduszweski, Sultan Hameed, Marco Tedesco, Julienne Stroeve, Thomas Mote, Tomoko Koyama, and Joseph R. McConnell

Abstract

Greenland’s largest precipitation flux occurs in its southeast (SE) region during the winter, controlled primarily by easterly winds and frequent cyclogenesis in the North Atlantic. Several studies have attempted to link SE Greenland precipitation to the North Atlantic Oscillation (NAO) but results are inconsistent. This work uses reanalysis, automatic weather station data, and regional climate model output to show that the east–west position of the Icelandic low is a better predictor of SE Greenland precipitation (average correlation of r = −0.48 in DJF) than climate indices such as the NAO (r = −0.06 in DJF). In years when the Icelandic low is positioned extremely west, moisture transport increases up to ~40% (or up to 40 kg m−1 s−1) off the SE Greenland coast compared to when the low is in an extreme east position. Furthermore, in years when the Icelandic low is positioned extremely west, storm track density and intensity increase just off the SE coast of Greenland. Thus, the Icelandic low’s longitudinal position dominates SE Greenland ice sheet’s wintertime precipitation, a positive term in the ice sheet mass balance. Given SE Greenland’s importance in the overall ice sheet mass balance, the position of the Icelandic low is therefore important for making projections of future sea level.

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Alison S. Criscitiello, Sarah B. Das, Kristopher B. Karnauskas, Matthew J. Evans, Karen E. Frey, Ian Joughin, Eric J. Steig, Joseph R. McConnell, and Brooke Medley

Abstract

The climate of West Antarctica is strongly influenced by remote forcing from the tropical Pacific. For example, recent surface warming over West Antarctica reflects atmospheric circulation changes over the Amundsen Sea, driven by an atmospheric Rossby wave response to tropical sea surface temperature (SST) anomalies. Here, it is demonstrated that tropical Pacific SST anomalies also influence the source and transport of marine-derived aerosols to the West Antarctic Ice Sheet. Using records from four firn cores collected along the Amundsen coast of West Antarctica, the relationship between sea ice–modulated chemical species and large-scale atmospheric variability in the tropical Pacific from 1979 to 2010 is investigated. Significant correlations are found between marine biogenic aerosols and sea salts, and SST and sea level pressure in the tropical Pacific. In particular, La Niña–like conditions generate an atmospheric Rossby wave response that influences atmospheric circulation over Pine Island Bay. Seasonal regression of atmospheric fields on methanesulfonic acid (MSA) reveals a reduction in onshore wind velocities in summer at Pine Island Bay, consistent with enhanced katabatic flow, polynya opening, and coastal dimethyl sulfide production. Seasonal regression of atmospheric fields on chloride (Cl) reveals an intensification in onshore wind velocities in winter, consistent with sea salt transport from offshore source regions. Both the source and transport of marine aerosols to West Antarctica are found to be modulated by similar atmospheric dynamics in response to remote forcing. Finally, the regional ice-core array suggests that there is both a temporally and a spatially varying response to remote tropical forcing.

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Dorothy Koch, Susanne E. Bauer, Anthony Del Genio, Greg Faluvegi, Joseph R. McConnell, Surabi Menon, Ronald L. Miller, David Rind, Reto Ruedy, Gavin A. Schmidt, and Drew Shindell

Abstract

The authors simulate transient twentieth-century climate in the Goddard Institute for Space Studies (GISS) GCM, with aerosol and ozone chemistry fully coupled to one another and to climate including a full dynamic ocean. Aerosols include sulfate, black carbon (BC), organic carbon, nitrate, sea salt, and dust. Direct and BC-snow-albedo radiative effects are included. Model BC and sulfur trends agree fairly well with records from Greenland and European ice cores and with sulfur deposition in North America; however, the model underestimates the sulfur decline at the end of the century in Greenland. Global BC effects peak early in the century (1940s); afterward the BC effects decrease at high latitudes of the Northern Hemisphere but continue to increase at lower latitudes. The largest increase in aerosol optical depth occurs in the middle of the century (1940s–80s) when sulfate forcing peaks and causes global dimming. After this, aerosols decrease in eastern North America and northern Eurasia leading to regional positive forcing changes and brightening. These surface forcing changes have the correct trend but are too weak. Over the century, the net aerosol direct effect is −0.41 W m−2, the BC-albedo effect is −0.02 W m−2, and the net ozone forcing is +0.24 W m−2. The model polar stratospheric ozone depletion develops, beginning in the 1970s. Concurrently, the sea salt load and negative radiative flux increase over the oceans around Antarctica. Net warming over the century is modeled fairly well; however, the model fails to capture the dynamics of the observed midcentury cooling followed by the late century warming. Over the century, 20% of Arctic warming and snow–ice cover loss is attributed to the BC-albedo effect. However, the decrease in this effect at the end of the century contributes to Arctic cooling.

To test the climate responses to sulfate and BC pollution, two experiments were branched from 1970 that removed all pollution sulfate or BC. Averaged over 1970–2000, the respective radiative forcings relative to the full experiment were +0.3 and −0.3 W m−2; the average surface air temperature changes were +0.2° and −0.03°C. The small impact of BC reduction on surface temperature resulted from reduced stability and loss of low-level clouds.

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Jason E. Box, Noel Cressie, David H. Bromwich, Ji-Hoon Jung, Michiel van den Broeke, J. H. van Angelen, Richard R. Forster, Clement Miège, Ellen Mosley-Thompson, Bo Vinther, and Joseph R. McConnell

Abstract

Ice core data are combined with Regional Atmospheric Climate Model version 2 (RACMO2) output (1958–2010) to develop a reconstruction of Greenland ice sheet net snow accumulation rate, Ât(G), spanning the years 1600–2009. Regression parameters from regional climate model (RCM) output regressed on 86 ice cores are used with available cores in a given year resulting in the reconstructed values. Each core site’s residual variance is used to inversely weight the cores’ respective contributions. The interannual amplitude of the reconstructed accumulation rate is damped by the regressions and is thus calibrated to match that of the RCM data. Uncertainty and significance of changes is measured using statistical models.

A 12% or 86 Gt yr−1 increase in ice sheet accumulation rate is found from the end of the Little Ice Age in ~1840 to the last decade of the reconstruction. This 1840–1996 trend is 30% higher than that of 1600–2009, suggesting an accelerating accumulation rate. The correlation of Ât(G) with the average surface air temperature in the Northern Hemisphere (SATNHt) remains positive through time, while the correlation of Ât(G) with local near-surface air temperatures or North Atlantic sea surface temperatures is inconsistent, suggesting a hemispheric-scale climate connection. An annual sensitivity of Ât(G) to SATNHt of 6.8% K−1 or 51 Gt K−1 is found.

The reconstuction, Ât(G), correlates consistently highly with the North Atlantic Oscillation index. However, at the 11-yr time scale, the sign of this correlation flips four times in the 1870–2005 period.

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