Wednesday, 17 December 2014: 3:10 PM
William D Neff and Gilbert P Compo, Cooperative Institute for Research in Environmental Sciences, Boulder, CO, United States
A recent paper [Neff et al., 2014]examined the factors underlying the 2012 melt episode that covered the Greenland ice sheet and compared it with the same factors identified for the last episode in 1889, using the Twentieth Century Reanalysis (20CR: [Compo et al., 2011]). A key factor was the presence of an Atmospheric River (AR) that transported warm air from a mid-continent heat wave over the Atlantic Ocean and thence to the west coast of Greenland and then over the ice sheet. The 20CR proved quite effective in defining the structure and transport paths for both events. Although these events with wide spread melting of the ice sheet surface are extremely rare, a question remains as to the frequency of AR events and the ancillary conditions required for extensive melting of the ice sheet. Although the 20CR was effective in capturing the structure of AR events in 1889, an analysis of the northward transport of moisture off the west coast of Greenland at 850 hPa shows weaker transport prior to 1921 than after. In this study, we use time series of meridional velocity and specific humidity at 850 hPa during boreal summer months as a screening tool for high transport events. We attribute the muted representation of synoptic features to be an artifact of of sparse available stations reporting pressure along the northeast coast of Canada prior to 1921. For this reason we use different thresholds before and after 1921 to identify potential AR events. For each potential event we then examine maps of integrated water vapor between 240oW to 340oW and 20oN to 80oN to identify those with the IWV pattern characteristic of an AR. In our earlier study, we used the only station, Ilulissat, recording daily data on the west coast of Greenland corresponding to the entire1871-2012 period of the 20CR for verification of events in 1889 and 2012. In that analysis, temperatures maximized prior to each event together with light precipitation on the coast. In this study, we have used this station to further verify our results.

Compo, G. P., et al. (2011), The Twentieth Century Reanalysis Project, Quarterly Journal of the Royal Meteorological Society, 137(654), 1-28.

Neff, W., G. Compo, F. M. Ralph, and M. D. Shupe (2014), Continental heat anomalies and the extreme melting of the Greenland ice surface in 2012 and 1889, Journal of Geophysical Research: Atmospheres.