Hahn Lily

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Hahn
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Lily
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  • Article
    Importance of Orography for Greenland cloud and melt response to atmospheric blocking
    (American Meteorological Society, 2020-04-16) Hahn, Lily ; Storelvmo, Trude ; Hofer, Stefan ; Parfitt, Rhys ; Ummenhofer, Caroline C.
    More frequent high pressure conditions associated with atmospheric blocking episodes over Greenland in recent decades have been suggested to enhance melt through large-scale subsidence and cloud dissipation, which allows more solar radiation to reach the ice sheet surface. Here we investigate mechanisms linking high pressure circulation anomalies to Greenland cloud changes and resulting cloud radiative effects, with a focus on the previously neglected role of topography. Using reanalysis and satellite data in addition to a regional climate model, we show that anticyclonic circulation anomalies over Greenland during recent extreme blocking summers produce cloud changes dependent on orographic lift and descent. The resulting increased cloud cover over northern Greenland promotes surface longwave warming, while reduced cloud cover in southern and marginal Greenland favors surface shortwave warming. Comparison with an idealized model simulation with flattened topography reveals that orographic effects were necessary to produce area-averaged decreasing cloud cover since the mid-1990s and the extreme melt observed in the summer of 2012. This demonstrates a key role for Greenland topography in mediating the cloud and melt response to large-scale circulation variability. These results suggest that future melt will depend on the pattern of circulation anomalies as well as the shape of the Greenland Ice Sheet.
  • Article
    North Atlantic natural variability modulates emergence of widespread Greenland melt in a warming climate
    (John Wiley & Sons, 2018-09-10) Hahn, Lily ; Ummenhofer, Caroline C. ; Kwon, Young-Oh
    Record‐breaking melt over Greenland in recent decades is linked not only to climate change but also to natural variability, including persistent atmospheric high‐pressure conditions in the negative phase of the North Atlantic Oscillation and warm North Atlantic Ocean temperatures during the positive phase of the Atlantic Multidecadal Oscillation. However, the relative importance of natural variability for Greenland melt under varying degrees of greenhouse forcing is still unclear. Using reanalysis data and a large ensemble of climate model simulations, we find that a negative North Atlantic Oscillation and positive Atlantic Multidecadal Oscillation consistently promote heightened summer melt under various forcing conditions. Moreover, timing of widespread 21st century Greenland melt varies considerably between ensemble members due to different phasing of these modes of natural variability. These results indicate the importance of natural modes of variability across a range of external forcing conditions for interannual melt variability and the emergence of widespread Greenland melt.