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FIG. 1. Winter land surface air temperature (LSAT) anomalies averaged over the Arctic (60°–90°N) (thick solid line), midlatitude (45°–60°N) (dashed), and globe (dotted) based on CRUTS2.0 in units of °C. Also shown is the Arctic averages based on CRUTEM2v dataset. The anomalies are relative to a 1901–80 base period. All curves are smoothed with 5-yr running mean.
FIG. 2. Time series of LSAT anomalies over the Arctic (60°–90°N) based on 63 realizations from 20 models investigated in their 20C3M simulations. The observed series based on CRUTS2.0 is shown by a thick orange line. The anomalies are relative to the mean of 1901–80. All curves are smoothed with 5-yr running mean, in units of °C.
FIG. 3. Mean winter arctic LSAT anomalies for the 1979–99 period from observation (first bar in each panel, light shaded) and model simulations in the 20C3M scenario. (top) Individual realizations of each model (bars 2–61), and (bottom) the ensemble mean for models when more than one realization is provided, or the only realization available. The confidence limits are two standard deviations derived from the detrended control run time series. Due to an abrupt change in the GISS-EH control run, the confidence limit is not shown. The last bar in the bottom panel shows the ensemble mean from all runs of all models.
FIG. 4. Decadal mean winter LSAT anomalies for the 1939–49 period based on individual realizations from each model over the region of 60°–90°N. The first bar on the left is the observed mean (CRUTS2.0). Units are in °C.
FIG. 5. Winter LSAT anomalies over the Arctic for individual realizations of each model. Thick solid black line is the observed time series based on CRUTS2.0. (left), (middle) the models with natural forcing included, (right) the models without natural forcing in their simulations. All the time series are smoothed with 5-yr running mean. Units are in °C.
FIG. 6. LSAT anomalies averaged over a decade that is centered in the peak value detected during the 1910–60 period in the 20C3M simulation. The thin black line indicates a value that is two-thirds of the observed amplitude. The first gray bar is based on CRUTS2.0. Units are in °C.
FIG. 7. The ratio of standard deviation of model control runs to the observed (CRUTS2.0) on (a) interannual, (b) decadal, and (c) interdecadal time scales. GISS-EH is excluded from the figure due to a large abrupt change found in its control run. All standard deviations are calculated after the time series is detrended and a (b) 5-yr and (c) 15-yr running mean applied, respectively. The dashed line indicates the lower range of the 90% confidence limit on the standard deviation normalized by CRUTS2.0.
FIG. 8. (top) Winter LSAT anomalies averaged over the Arctic based on model ensemble runs that passed the proposed 2/3CRU criterion in their 20C3M simulations. (bottom) The truncated 100-yr-long time series from control runs of the nine models that pass the variance test. All time series are smoothed with a 5-yr running mean. Models with natural forcing are shown in solid lines, while those without are shown in dashed lines. All models show upward warming trend in the Arctic for the last two decades in 20C3M scenario, while none is shown in the control runs. The range of variation during 1911–60 is about the same in the 20C3M simulations as well as in the control runs.
FIG. 9. Spatial distribution of LSAT anomalies based on (left) CRUTS2.0 and (right) one realization each from the eight models that pass the 2/3CRU criterion. All patterns have a 5-yr running mean applied. The years selected are around maxima of the warm anomalies during 1911–60 for both the observation and models. Contour interval is 0.5°C.
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