Research ArticleCLIMATOLOGY

Abrupt shift in the observed runoff from the southwestern Greenland ice sheet

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Science Advances  13 Dec 2017:
Vol. 3, no. 12, e1701169
DOI: 10.1126/sciadv.1701169
  • Fig. 1 The Tasersiaq catchment in southwest Greenland.

    Red, ice-free part of the catchment; blue, ice-covered part of the catchment; A, unnamed ice-dammed lake delivering glacial lake outburst floods (GLOFs) to the catchment (subtracted from the discharge time series); B, Amitsulôq ice cap. Inset: map location in Greenland shown in green. A more detailed map of the outlet region is provided in fig. S1.

  • Fig. 2 Seasonal discharge from the Tasersiaq catchment.

    Daily mean discharge (in blue) with GLOFs (in red) and missing data (gray boxes). The vertical scale (discharge) is similar for all plots but varies in extent according to the peak discharge of the 5-year period in question. Years with severe influence from volcanic eruptions are labeled in blue text, with the relative impact on lower stratospheric mean aerosol optical depth at 66.5°N over time shown as a dotted gray line (see Fig. 3).

  • Fig. 3 Annual ice sheet runoff from the Tasersiaq catchment.

    The annual runoff from the ice-covered part of the Tasersiaq catchment (left-side y axis). Red, data from regular years; blue, data from volcano-influenced year (only 1992 has sufficient data coverage; see Fig. 2); light blue bars, runoff from the ice-free part of the catchment derived from HIRHAM5, with uncertainties estimated from comparison to winter (September to May) surface mass balance measurements on the Amitsulôq ice cap (marked B in Fig. 1 and fig. S1) (44). Thin gray curve (right-side y axis): zonal mean aerosol optical depth at 550 nm at 66.5°N, indicating the attenuation of the sunlight from the aerosols as it passes through the atmosphere (27). The aerosol optical depth shows distinct peaks in 1983 and 1992 after major volcanic eruptions. The dashed vertical line indicates timing of the hypothesized change in runoff regime.

  • Fig. 4 Relation between runoff and the GBI.

    The standardized annual runoff anomaly as a function of the summertime [June, July, and August (JJA)] GBI anomaly (volcano-affected years 1983 and 1992 excluded), where GBI is defined as the mean 500-hPa geopotential height between 60° to 80°N and 20° to 80°W (30). Blue, 2002 and before; red, 2003 and after. Labels within plot provide the last two digits of the year. Dashed line shows linear fit of blue and red entries with r2 = 0.55.

  • Fig. 5 Atmospheric circulation change from trajectory path analysis.

    The origin (A) and change in origin (B) of summertime air masses at Tasersiaq. (A) The mean air parcel trajectory density for each summer (JJA), averaged over the entire period 1975–2014. The unit “air parcel time per area” denotes the time an air parcel eventually arriving at Tasersiaq has spent over a given area over the week before its arrival, based on modeled trajectory paths. (B) The difference between the mean air parcel trajectory density anomalies of the periods 1975–2002 and 2003–2014, illustrating a general shift toward the south in the origin and path of summertime air masses arriving in Tasersiaq.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/3/12/e1701169/DC1

    Catchment delineation

    The HIRHAM5 regional climate model experiment

    fig. S1. Outlet region of the Tasersiaq catchment.

    fig. S2. Stage-discharge relation for Tasersiaq.

    fig. S3. Signature rate of change of the discharge during a GLOF.

    fig. S4. Comparison between modeled and measured snow accumulation.

    fig. S5. Positive identification of the source lake of the GLOFs.

    fig. S6. The change in origin of summertime air masses at Tasersiaq.

    table S1. Position of measuring stations.

    References (4547)

  • Supplementary Materials

    This PDF file includes:

    • Catchment delineation
    • The HIRHAM5 regional climate model experiment
    • fig. S1. Outlet region of the Tasersiaq catchment.
    • fig. S2. Stage-discharge relation for Tasersiaq.
    • fig. S3. Signature rate of change of the discharge during a GLOF.
    • fig. S4. Comparison between modeled and measured snow accumulation.
    • fig. S5. Positive identification of the source lake of the GLOFs.
    • fig. S6. The change in origin of summertime air masses at Tasersiaq.
    • table S1. Position of measuring stations.
    • References (45–47)

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