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Variability and Trends in Anticyclonic Circulation over the
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Greenland Ice Sheet, 1948-2013
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Jill Rajewicz and Shawn J. Marshall
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Supplementary Text and Figures
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S1. Methods
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Daily NCEP/NCAR climate reanalyses for the period May 1, 1948 to September 30, 2013 were
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downloaded for the region 5-90W and 60-90N. Belleflamme et al. [2013] demonstrate that
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these are similar to 500-hPa fields and associated circulation patterns over Greenland in the
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European Centre for Medium-Range Weather Forecasts (ECMWF) climate reanalysis. Fettweis
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et al. [2011] also note that 700-hPa temperature fields in the NCEP/NCAR and ECMWF
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Reanalyses are similar, and are highly correlated with both regional-climate model derived
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(ERA-forced) melt estimates in Greenland [Fettweis et al., 2013] and independent Greenland Ice
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Sheet melt indices such as SSM/I-derived melt area [Mote and Anderson, 1995; Abdalati and
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Steffen, 1997; Mote, 2007; Tedesco, 2007].
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We opt for NCEP/NCAR reanalyses because of its long record; it is available for more than 66
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years, from 1948 to present, whereas updated reanalyses that take advantage of satellite
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constraints (NCEP2, ERA-Interim) are only available from 1979-2013 [Kanamitsu et al., 2002;
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Dee et al., 2011]. The multidecadal perspective afforded by NCEP/NCAR is important to our
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analyses. Moreover, we are primarily interested in the synoptic-scale, 500-hPa circulation
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conditions, for which the relatively coarse-resolution (2.5) NCEP/NCAR fields have been
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shown to be sufficient [Belleflamme et al., 2013; results presented below].
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As a test of the NCEP/NCAR climatology in Greenland, we compare NCEP/NCAR fields
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against those from the NCEP2 and ERA-Interim Reanalyses for the periods of overlap, 1979-
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2012 and 1979-2013, respectively. NCEP2 and ERA-Interim surface and pressure-level fields
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are processed as per the NCEP/NCAR Reanalysis. NCEP2 and ERA-Interim take advantage of
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satellite-based observations to provide a greater array of assimilated data in climate
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reconstructions and also have refined surface grids and land surface models. NCEP2 couples the
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atmospheric model with a higher-resolution land surface model on a Gaussian grid (ca. 1.75),
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while ERA-Interim upper-air fields are available at a resolution of 0.75. Comparisons between
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NCEP/NCAR, NCEP2, and ERA-Interim fields relevant to our study are presented below.
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Melt days (Nm) and positive degree days (PDD) are summed for each Greenland grid cell based
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on daily mean 2-m temperatures for the period May 1 to Sept 30 of each summer, from 1948-
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2013. Mean summer geopotential height, vorticity, and temperature fields are based on the
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period June 1-August 31 (JJA). Relative vorticity is calculated from 500-hPa geopotential height
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assuming geostrophic flow, as the curl of the geostrophic wind velocities [Holton, 2004],
=
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𝑔
𝑓
1
[𝑅2 𝑐𝑜𝑠𝜃 (
𝜕2 𝑍500
𝜕
2
1
𝜕2 𝑍500
) + 𝑅2 (
𝜕𝜃2
)],
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where g is gravity, f is the Coriolis parameter, R is Earth’s radius, and (, ) denote longitude
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and latitude. For geostrophic flow, relative vorticity is a measure of the curvature of the
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geopotential height field; it is positive for cyclonic flow, zero for purely zonal flow, and negative
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for anticyclonic circulation.
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Analysis of NCEP/NCAR fields is based on standardized anomalies of mean summer values,
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using the full period 1948-2013 for the mean and the standard deviation of each field (N = 66).
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For the frequency of anticyclonic circulation, for example,
̅ ] / 𝜎𝑓 ,
𝑓̂𝑎𝑐 (𝑡) = [𝑓𝑎𝑐 (𝑡) − 𝑓𝑎𝑐
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where the overbar denotes the mean from 1948-2013,  is the standard deviation, and t refers to
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the year (summer) of interest. We also consider cumulative anomalies for the period 1948-2013
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as a method to assess trends and potential regime shifts.
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We do not detrend the time series for our analysis, as we are interested in both longer-term trends in
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circulation and melt indices over Greenland and interannual variability in these indices. Hence, any trend
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is part of the signal that we wish to detect and understand [e.g., Gardner and Sharp, 2009].
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Contour plots of our fields are shown for a broad region of the western Arctic surrounding
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Greenland, including Iceland and parts of the Canadian Arctic Archipelago. Where we examine
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circulation characteristics and melt indices over Greenland, we restrict the analysis to grid cells
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over Greenland, as determined by the NCEP/NCAR land mask for the relevant latitudes and
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longitudes. The resulting area, which we refer to as ‘Greenland’, is indicated in Figure S1. This
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gives 105 grid cells covering an area of 2.15  106 km2, larger than the Greenland Ice Sheet but
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similar to the actual area of Greenland (2.18  106 km2). Our analysis includes some mixed
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ice/ocean/land cells on the Greenland coast and the ice sheet margin, and should be taken as
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representative of Greenland as a whole. Figure S1b indicated area vs. latitude for the study
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region, along with the latitude zones that we define for discussion of southern, central, and
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northern Greenland.
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Results are discussed in the context of well-known patterns of synoptic weather variability in the
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region: the North Atlantic Oscillation (NAO) [Hurrell, 1995; Hurrell and Deser, 2009], the
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Atlantic Multidecadal Oscillation (AMO) [Enfield et al., 2001], and the raw North Atlantic sea
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surface temperature (SST) time series [updated following Kaplan et al., 1998].
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We supplement our derived melt indices (PDD, Nm) with satellite observations of daily
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Greenland melt extent from 1979-2013, based on SSM/I microwave remote sensing retrieval
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algorithms from Mote [2012] and Tedesco [2012, updated 2013]. Mote melt extent data was
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provided for the period 1979-2012, given as daily melt area AM (km2) for a region that includes
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the Greenland Ice Sheet as well as some grid cells peripheral to the ice sheet. For days with less
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than 95% data coverage over Greenland, we gap-fill with (i) interpolated data from prior and
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subsequent days, or, where this is unavailable, (ii) longterm (1979-2012) mean values for this
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day. Tedesco melt extent data for the period 1981-2013 is processed in a similar way to derive
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daily melt area AT. Both AM and AT are used to calculate mean summer (JJA) Greenland melt
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extent for all available years, giving time series of overall summer melt intensity that can be
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compared with our Greenland-wide summer melt indices, PDD and Nm.
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S2. Results
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Figure S2 plots mean JJA 700-hPa temperature, 500-hPa relative vorticity, anticyclonic
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circulation frequency, and modelled melt days over Greenland. Comparison with Figure 1b
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indicates the high degree of correlation between 500-hPa heights and 700-hPa temperatures.
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Anticyclonic circulation is prevalent in central Greenland in most years, in association with the
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high-elevation central dome of the Greenland Ice Sheet discussed in the main text. Vorticity
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anomalies were exceptional in 2012, with anticyclonic circulation over the entire ice sheet (Fig.
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S2b). This was associated with warm anomalies and melt-index anomalies from 2.8 to 4 standard
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deviations above the 1948-2013 means.
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Figure S2d shows the mean modelled melt days in the 2-m temperature analysis, based on the
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number of days per year with temperatures above 0C for each grid cell over Greenland. Melt
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days increase in northern Greenland due to the lower average topography of NCEP cells in the
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analysis (i.e. coastal and ice-marginal grid cells). Summer 2013 was indistinguishable from the
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1948-2013 mean, while melt day totals in 2012 were 62% above the longterm mean.
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Time series of mean summer fields for the period 1948-2013 are plotted in Figure S3. Increases
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beginning in the mid to late 1990s are evident in all fields. For comparison, fields from NCEP2
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and ERA-Interim are also plotted for all available summers, 1979-2013. NCEP/NCAR and
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NCEP2 fields are highly correlated, with linear correlation coefficient exceeding 0.99 for Z500,
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T700, and circulation indices (, fac). Mean summer ERA-Interim and NCEP/NCAR 500-hPa
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geopotential height and 700-hPa temperature fields over Greenland are also correlated at 0.99
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over the period of overlap (1979-2013), although ERA-Interim 700-hPa temperatures are about
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1C colder. Derived relative vorticity fields are slightly less correlated, at 0.91. This is probably
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related to the finer grid in ERA-Interim, which gives different values of the constructed Z500
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curvature in the relative vorticity calculations. The main results of our analysis are not sensitive
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to the choice of reanalysis product, however; strong anomalies in circulation, 500-hPa height,
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and lower tropospheric temperature over Greenland begin in the late 1990s or early 2000s in all
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of the reanalyses.
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This result also extends to the satellite-derived melt-extent records. Figure S4 plots the relation
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between our derived melt indices (Nm and PDD) and the average summer (JJA) melt extent in
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Greenland for the period 1979-2013. Both the Mote and Tedesco datasets are shown, and give
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similar results. Nm and PDD correlate well with the observed melt extent for the period of
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overlap: r = 0.83 and 0.87, respectively (Table S1). None of these melt indices is a measure of
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actual melting/runoff on the ice sheet (no such data exists), but average summer melt extent is a
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function of the duration and spatial extent of the melt season in Greenland, and is probably the
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best available measure of interannual changes and trends in Greenland melt over the last three
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decades. Figure S4 demonstrates that the NCAR/NCEP melt proxies do a reasonable job of
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representing this, allowing us to extend the reconstruction to earlier decades.
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In Figure S5 we plot time series of modeled melt-day anomalies and relative vorticity as a
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function of latitude bands over the ice sheet (Fig. S1b). There is relatively little spatial structure
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for the melt-day anomalies, with similar responses in each latitude band. The 2000s appear as the
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only period of persistent positive anomalies in the 66-year record, evident across the whole ice
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sheet. Anticyclonic circulation trends are less coherent. Southern Greenland has a different
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temporal structure to central and northern Greenland, switching from predominantly cyclonic
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circulation in the first half of the record to frequent dominance of anticyclonic circulation in
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summers after 1983. At higher latitudes, anticyclonic circulation prevails in most summers, but it
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has become stronger in the last decade, particularly in central Greenland.
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Cumulative melt-day anomalies discussed in the main text (Fig. 3) offer a clear view of these
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temporal transitions. Other temperature and melt indices for Greenland are similar to the melt-
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day anomalies. In Figure S6a we plot the cumulative anomalies for melt days, PDD, T700, Z500
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and the satellite-derived melt extent. All time series show the shift to positive anomalies in the
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late 1990s or early 2000s. Upper-air temperatures and geopotential heights have a less sharp
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reversal, levelling off in the late 1990s before the period of strong positive anomalies from 2001-
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2012. This is inextricably linked with the anomalous anticyclonic circulation (Fig. S6b). Both the
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frequency and strength of ridging anomalies undergo a persistent increase post-2001. The
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satellite-derived melt extent in Fig. S6a also exhibits a marked reversal to sustained positive
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anomalies in the 1990s, but the inflection begins earlier than the NCEP/NCAR melt proxies.
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Given the evidence for a potentially different circulation and melt regime around the year 2000,
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we plot the mean circulation and melt anomalies for the period 2001-2013, relative to the mean
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conditions for the full period, 1948-2013. Figure S7 shows the anomalies in the 2000s. This is
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essentially a moderate version of the extreme temperature and melt conditions that Greenland
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experienced in 2012.
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Interannual temperature and melt variability in the 66-year NCEP record are highly correlated
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with our two main synoptic proxies, the relative vorticity and residual 500-hPa heights (Fig. 3c).
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Figure S8 plots the relations between annual melt days and mean summer values of these two
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fields. This relation holds up for the SSM/I-derived melt extent, although with more scatter.
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Collectively, relative vorticity and residual 500-hPa heights explain from 51-76% of the variance
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in the different melt proxies over Greenland. The unexplained variance must arise from surface
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effects (e.g., albedo and sea ice changes, low stratiform clouds) and other synoptic-scale weather
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systems that are not captured by our relatively simple ridging index.
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References
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Supplementary Table.
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Table S1. Linear correlation coefficients for mean summer (JJA) climate fields in Greenland,
1948-2013 (N = 66). Variables are average Greenland 500-mb geopotential height (Z500) and
relative vorticity (), 700-mb temperature (T700), 2-m temperature (T2m), positive degree days
(PDD), melt days (Nm), anticyclonic circulation frequency (fac), NAO and AMO indices, North
Atlantic SST, and the Tedesco mean annual melt extent, AT (1981-2013). Values above the
diagonal are for the raw time series, and values below the diagonal (grey, italic) are for detrended
time series. Detrended SST is equivalent to the AMO.
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Variables
PDD
Nm
T700
T2m
Z500

fac
NAO
AMO
SST
AT
PDD
Nm
T700
T2m
Z500
1.0
0.97
0.87 0.88 0.82
0.98 1.0
0.92 0.94 0.84
0.87 0.92
1.0
0.97 0.82
0.90 0.94
0.97 1.0
0.85
0.82 0.84
0.81 0.84 1.0
0.71 0.68 0.63 0.70 0.61
0.69 0.67
0.61 0.67 0.57
0.54 0.54 0.46 0.52 0.87
0.57 0.58
0.59 0.55 0.46





0.66
0.63
0.68
0.58
0.47

0.73
0.70
0.65
0.71
0.63
1.0
0.97
0.35
0.28

fac
0.71
0.69
0.63
0.68
0.58
0.97
1.0
0.30
0.32

0.43 0.36
NAO AMO SST
AT
0.52 0.62 0.65 0.87
0.53 0.61 0.62 0.82
0.46 0.62 0.62 0.75
0.52 0.57 0.56 0.71
0.86 0.49 0.49 0.86
0.35 0.33 0.36 0.62
0.30 0.36 0.38 0.64
1.0 0.26 0.26 0.44
0.26 1.0
0.98 0.72

1.0
1.0
0.76
0.20 0.40 0.40 1.0
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Supplementary Figures
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Figure S1. (a) NCEP/NCAR grid cells over Greenland (N = 105) and (b) area of Greenland at
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different latitudes in the NCEP/NCAR grid (106 km2). Dashed lines indicate the division of
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southern, central, and northern Greenland in our analysis.
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Figure S2. Latitudinal variation of NCEP/NCAR fields over Greenland. (a) 700-hPa JJA
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temperatures (C), (b) 500-hPa JJA relative vorticity (105 s1), (c) frequency of summer
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anticyclonic circulation (%), and (d) summer (May-Sept) melt days. Solid black lines show the
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1948-2013 mean and dashed lines indicate +/ one standard deviation; red lines are for 2012;
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blue lines are for 2013.
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Figure S3. Mean summer (JJA) fields over Greenland, 1948-2013: (a) 500-hPa geopotential
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heights (m), (b) frequency of days with anticyclonic circulation (%), (c) 700-hPa temperatures
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(C), and (d) relative vorticity (s1  105). Black lines are from the NCEP/NCAR Reanalysis, red
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lines are from NCEP2 (1979-2012), and green lines are from ERA-Interim (1979-2013).
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Figure S4. Satellite-derived melt extent vs. (a) NCEP/NCAR melt days and (b) modeled PDD,
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for the Mote data (diamonds) and Tedesco data (stars), expressed as mean summer melt extent in
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Greenland. Summer 2012 is the point in the upper right corner of each plot.
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Figure S5. Summer (a) melt day anomalies and (b) 500-hPa relative vorticity for different
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latitudes over Greenland, 1948-2013. Red, green and blue are for southern, central, and northern
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Greenland, and the heavy black line is the Greenland-averaged value.
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Figure S6. Cumulative anomalies of (a) melt indices and (b) anticyclonic circulation indices
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over Greenland, 1948-2013. All lines are Greenland-averaged values. AT refers to the Tedesco
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mean-summer melt extent (standardized anomalies), from 1981-2013.
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Figure S7. Anomalies over Greenland for the period 2001-2013, relative to the 1948-2013 mean:
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(a) 700-hPa temperature, C; (b) modeled melt days; (c) 500-hPa relative vorticity, s1  105; (d)
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anticyclonic circulation frequency, days.
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Figure S8. (a,b) NCEP/NCAR summer melt days over Greenland, 1948-2013 vs. (a) mean JJA
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500-hPa relative vorticity and (b) residual 500-hPa geopotential heights, Z500res. (c,d). As per
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(a,b) but for SSM/I-derived mean annual Greenland summer melt extent, 1981-2013.
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