1Supplementary Materials for2“Evolution of Arctic Sea Ice Concentration Trends and the Role of Atmospheric

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Circulation Forcing, 1979-2007” by Clara Deser and Haiyan Teng

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Submitted to Geophysical Research Letters, Sep 13 2007; Revised Nov 16 2007

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Empirical orthogonal function analysis of sea ice concentration anomalies

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Are the Sea Ice Concentration (SIC) trend patterns shown in Fig. 1 of the8manuscript preferred structures of variability, or are they simply a result of dividing the9record in half? To address this question, we have applied Empirical Orthogonal Function

10(EOF) analysis to the covariance matrix of SIC anomalies over the full period of record,

11using a separate EOF analysis for winter and for summer. The two leading EOFs in each

12season and their associated principal component (PC) time series are shown in Fig. S1. In

13each season, the first and second EOFs account for approximately 30% and 17% of the14variance, respectively, and are well separated according to the criterion of North et al.

15(1982).

16In winter, the leading EOF exhibits out-of-phase variations between the eastern

17and western Atlantic and between the eastern and western Pacific, strongly reminiscent of18

the trend pattern during the first half of the record (see Fig. 1 in the manuscript). This19

EOF is nearly identical to that in Ukita et al. (2007) based on February-March averages

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over the period 1979-2003, and consistent with results obtained using data sets beginning21

in the early 1950s (Walsh and Johnson, 1979; Fang and Wallace, 1994; Deser et al.,

222000). The associated PC time series exhibits an upward trend from 1979 to 1995, near

23zero values from 1996 through 2004, and positive values from 2005 through 2007. EOF2

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24of winter SIC is characterized by uniform polarity throughout the Arctic marginal ice

25zones, with largest amplitudes in the Labrador Sea. This EOF resembles the trend pattern

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during the second half of the record (see Fig. 1 in the manuscript). Its PC time series

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exhibits generally negative values before 1995 and positive values thereafter, indicative

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of a decreasing trend of winter SIC in the peripheral seas. To our knowledge, such an29

EOF pattern has not been reported previously for winter SIC. It is notable that the first30

and second EOFs of winter SIC anomalies during 1979-2007 correspond to the winter31

SIC trend patterns in the first and second halves of the record, respectively, indicating

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that these two trend patterns dominate the variability over the period of study.

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The leading EOF of summer SIC anomalies during 1979-2006 exhibits uniform

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polarity throughout most of the Arctic marginal ice zone, with largest amplitudes from35

the Laptev Sea eastward to the Beaufort Sea. This EOF resembles closely the patterns of36

summer SIC trends during 1979-2006 and 1993-2006, and projects substantially onto the37

trend pattern for 1979-1993 (see Fig. 1 in the manuscript). This similarity is consistent38

with the fact that the leading PC time series exhibits an upward trend over the period of

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record. The second EOF of summer SIC anomalies consists of out-of-phase variations

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between the Barents/Kara Seas and the East Siberian/Beaufort Seas, with no discernible

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trend in its PC time series. This EOF does not correspond closely to any of the summer

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SIC trend patterns shown in Fig. 1 in the manuscript, although it captures some of the43

out-of-phase behavior evident in the early period.

4445References

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Deser, C., J.E. Walsh, and M.S. Timlin, 2000: Arctic sea ice variability in the context of47

recent atmospheric circulation trends. J. Clim., 13, 617-633.

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Fang, Z. and J. M. Wallace, 1994: Arctic sea ice variability on a timescale of weeks: Its49

relation to atmospheric forcing. J. Climate, 7, 1897-1913.

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North, G. R., T. L. Bell, R. F. Calahan, and F. J. Moeng, 1982: Sampling errors in the51estimation of empirical orthogonal func tions. Mon. Wea. Rev., 110, 699-706.

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Ukita, J., M. Honda, H. Nakamura, Y. Tachibana, D.J. Cavalieri, C.L. Parkinson, H.

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Koide, and K. Yamamoto, 2007: Northern hemisphere sea ice variability: lag structure54

and its implications. Tellus, 59, doi: 10.111/j.1600-0870.2006.00223.x.

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Walsh, J. E. and C. M. Johnson, 1979: An analysis of arctic sea ice fluctuations, 1953-56

1977. J. Phys. Oceanogr., 9, 580-591.

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Figure Caption

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Figure S1. Sea ice concentration EOFs 1 and 2 and their associated PC time series based

60on the period 1979-2007 for winter (panels a – c) and 1979-2006 for summer (panels d –61f). EOF 1 (2) accounts for 30% (17%) of the variance in each season

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Figures

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Figure S1. Sea ice concentration EOFs 1 and 2 and their associated PC time series basedon the period 1979-2007 for winter (panels a – c) and 1979-2006 for summer (panels d –f). EOF 1 (2) accounts for 30% (17%) of the variance in each season

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