GEOPHYSICAL RESEARCH LETTERS, VOL. 30, NO. 3, 1132, doi:10.1029/2002GL015797, 2003

Recent cooling in coastal southern Greenland and relation with theNorth Atlantic Oscillation
Edward Hanna1 and John Cappelen2
Received 3 July 2002; revised 11 September 2002; accepted 11 September 2002; published 12 February 2003.

[1] Analysis of new data for eight stations in coastal
southern Greenland, 1958 – 2001, shows a significant
cooling (trend-line change 1.29C for the 44 years), as
do sea-surface temperatures in the adjacent part of theLabrador Sea, in contrast to global warming (+0.53C overthe same period). The land and sea temperature series
follow similar patterns and are strongly correlated but withno obvious lead/lag either way. This cooling is significantly
inversely correlated with an increased phase of the North
Atlantic Oscillation (NAO) over the past few decades (r =0.76), and will probably have significantly affected theINDEX TERMS:
mass balance of the Greenland Ice Sheet.
3309 Meteorology and Atmospheric Dynamics: Climatology(1620); 3319 Meteorology and Atmospheric Dynamics: Generalcirculation; 3349 Meteorology and Atmospheric Dynamics: Polar
meteorology; 1610 Global Change: Atmosphere (0315, 0325);
1827 Hydrology: Glaciology (1863). Citation: Hanna, E., andJ. Cappelen, Recent cooling in coastal southern Greenland andrelation with the North Atlantic Oscillation, Geophys. Res. Lett.,
30(3), 1132, doi:10.1029/2002GL015797, 2003.

[2] Greenland, the world’s largest island, is important
climatically and glaciologically. The Greenland Ice Sheet, if
it were to melt in its entirety, could contribute 6 – 7 m to
global sea-level rise [Hvidberg, 2000]. The Ice Sheet iseither approximately currently in balance or losing mass
[Abdalati, 2001] but with a large uncertainty. It experiences
large interannual variations in snow accumulation andablation [Braithwaite, 1994; McConnell et al., 2001; Hanna
et al., 2002]. Mean annual temperatures around the southernedge of the Ice Sheet are only a few degrees below freezing,5C being typical [Ohmura, 1987], and summer temperatures rise a few degrees above freezing. The marginal
zones of the Ice Sheet, from which most of the surface
meltwater runoff occurs, are therefore critically sensitive toclimatic change (especially to a summer warming). Here we
explore recent climatic change in southern Greenland, its
relation to the North Atlantic Oscillation (NAO) and possible effects on the Ice Sheet mass balance.
[3] Annual temperature series from eight Danish Meteorological Institute stations in coastal and near-coastal
southern Greenland (Figure 1), were averaged to producea ‘‘Composite Greenland Temperature’’ (CGT) (Figure 2).
Most of the stations are in south-west Greenland. CGT isbased on recently published data [Cappelen et al., 2001],with an update to 2001, and the presence of data from at
1
2

Institute of Marine Studies, University of Plymouth, UK.
Danish Meteorological Institute, Copenhagen, Denmark.

Copyright 2003 by the American Geophysical Union.
0094-8276/03/2002GL015797$05.00

least seven out of the eight stations in any year. The
Greenland station temperature records were closely examined and compared with records for related climatic elementsfrom the same stations. Station data were also inter-compared to search for systematic offsets which (where they
could not be accounted for by local conditions) were alsocorrected. A new series was calculated using a Gaussian
filter with a standard deviation of three years [G3, equivalent
to a 10-year running mean (RM)]. This series shows a leastsquares linear regression trend-line decrease of 1.29C (s =0.63C) from 1961 –2001, compared with a simultaneous
trend-line increase of 0.53C (s = 0.16C) for similarly
filtered global mean temperature (GMT). GMT is from theHadley Centre and Climatic Research Unit [Jones et al.,
1999; Parker et al., 1995]. In other words while conditions
warmed significantly globally, they cooled significantly incoastal southern Greenland.
[4] Sea-surface temperatures off SW Greenland for an
area centred on 62.5N, 52.5W (Labrador Sea), also seemto have cooled over a similar time period (Figure 3). G3
series of HadSST1 [Parker et al., 1995; Rayner et al., 1996]
decreased 0.44C (s = 0.20C) and NOAA (NationalOceanographic and Atmospheric Administation) SST
blended to the National Centers for Environmental Prediction (NCEP) Reanalysis skin temperature [Kalnay et al.,1996] decreased 0.80C (s = 0.29C) during 1961 –99 and1961– 2001 respectively (data from HadSST1 were available only to 1999). There are common patterns to theobserved land and sea coolings and no obvious lag: perhaps
changes in air temperature/atmospheric circulation in southern Greenland predominantly influence SST in the Labrador
Sea rather than vice versa, although establishing the exact
cause and effect is challenging. Correlation coefficients
between 5-year RMs of the series are: 0.69 between thetwo SST series, 0.71 between CGT and HadSST1, and 0.92
between CGT and NCEP SST. These ’r’ values are all
statistically significant at the 5% level, and the latter at the1% level (see Dr. David Stephenson’s Web site at http://www.met.rdg.ac.uk/cag/ ), even allowing for autocorrelationin the time series.
[5] The stronger/increasing phase of the NAO during thelast 35 years (see Dr. James Hurrell’s Web site "NorthAtlantic Oscillation (NAO) Indices Information" atwww.cgd.ucar.edu/jhurrell/nao.html links in with theobserved cooling (Figure 4). The 5-yr. RM of NAO wassignificantly negatively correlated with 5-yr. RM CGT (r =0.76) during 1961 – 2000. This is to be expected assouthern Greenland is near Iceland, one of the ‘ends ofthe seasaw’ of the NAO [van Loon and Rodgers, 1978].
Recent milder conditions over north-west Europe wereaccompanied by stronger cold northerly winds over Greenland. It is unknown whether this is a long-term trend,
possibly forced by anthropogenic greenhouse warming, or

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HANNA AND CAPPELEN: NORTH ATLANTIC OSCILLATION AND RECENT COOLINGFigure 3. Composite Greenland temperature (CGT) andsea-surface temperatures [Hadley Centre (HadSST1) & USNational Centers for Environmental Prediction (NCEP)] off
SW Greenland, all 5-yr. running means, with trend lines.
Figure 1. Map showing eight Danish Meteorological
Institute stations in coastal southern Greenland, data fromwhich were used in this study.
simply natural/random climatic variation. The NAO-temperature link doesn’t explain what caused the observed
cooling in coastal southern Greenland (what ‘drives’ theNAO?) but it does lend it credibility.

Figure 2. (a): Temperatures at the southern Greenland
weather stations and factored (x10) global temperature, all
5-year running means. Greenland stations are shown by
their World Meteorological Organization codes. Trend lines
are shown for the 4250 (Nuuk, SW Greenland) and global
temperature series. (b): Composite Greenland temperatureand factored global temperature, 5-yr. running means withtrend lines.

[6] The observed cooling in coastal southern Greenlandmeans that the southern part of the Ice Sheet most likely
also cooled overall over the past few decades. Althoughinterior (ice sheet) and coastal climates are different, themost relevant part of the Ice Sheet is that around the edges(i.e. nearest the coast) where summer melt rates andvariability are highest. Ablation-temperature modellingindicates that an annual or summer temperature rise of1C increases ablation by 20– 50% [Oerlemans, 1991;
Braithwaite and Olesen, 1993; Ohmura et al., 1996;
Janssens and Huybrechts, 2000]. So recent cooling may
have significantly added to the mass balance of at least thesouthern half of the Ice Sheet. However, a small temperature rise during the mid-late 1990s is in agreement withobserved thinning of the marginal ice/glaciers during that
period [Krabill et al., 2000]. So in an extension of thisstudy, we will be very interested to compare temperature
with laser/radar altimetry data of Ice Sheet surface-elevation
change.
[7] Continued systematic observations from Greenland
met. stations are of greatest importance for studies of polarand global climate change. We leave it to future investigators to explore the reasons behind this important regional
exception to recent ‘global warming’.

Figure 4. Comparison of Composite Greenland Temperature (CGT) and North Atlantic Oscillation (NAO) index(5-yr. running means with trend lines).

HANNA AND CAPPELEN: NORTH ATLANTIC OSCILLATION AND RECENT COOLING[8] Acknowledgments. We thank Sirpa Hakkinen at NASA/Goddardfor the NOAA/NCEP SSTs, and Roger Braithwaite, Philippe Huybrechts
and Atsumu Ohmura for useful advice.

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J. Cappelen and E. Hanna, Institute of Marine Studies, University ofPlymouth, Drake Circus, Plymouth, Devon, PL4 8AA, UK. (e.hanna@plymouth.ac.uk)