Correction
LETTER (ONLINE ONLY)
Correction for “Reply to Li and Wu: Arctic sea ice and winter snowfall,” by Jiping Liu, Judith A. Curry, Huijun Wang, Radley M. Horton, and Mirong Song, which appeared in issue 28, July 10, 2012, of Proc Natl Acad Sci USA (109:E1899–E1900; first published June 14, 2012; 10.1073/pnas.1206848109).
The authors wish to note, “We erroneously uploaded a test plot for the sea ice extent in the reply to Li and Wu, rather than the plot for the sea ice area that we intended.” The corrected Figure 1 and its legend appear below.
Fig. 1. Time series of actual and detrended autumn Arctic sea-ice area anomaly (×106 km2) and winter AO index for the period 1979–2011. SON, SeptemberOctober-November; DJF, December-January-February. www.pnas.org/cgi/doi/10.1073/pnas.1400315111
E530 | PNAS | February 4, 2014 | vol. 111 | no. 5
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LETTER
Reply to Li and Wu: Arctic sea ice and winter snowfall We respond here to three comments by Li and Wu (1) that we find to be either incorrect or lacking in relevance. As stated in Liu et al. (2), “extended from previous studies,” we demonstrated that autumn Arctic sea-ice anomalies, instead of winter North Atlantic Oscillation/Arctic Oscillation (NAO/AO) and El Niño, show a consistent relationship with winter snow anomalies for recent winters. We then demonstrated that reduction of sea ice is linked to winter atmospheric circulation change that is different from the classic winter AO and increased local moisture sources to Europe and the United States, which contributed to recent snowy winters. The statement by Li and Wu, “Wu et al. [3] discovered the importance of autumn Arctic sea ice in triggering regional cold extremes. . .” is incorrect. Earlier than Wu et al. (3), studies have suggested that cold conditions over Eurasia are associated with reduced Arctic sea ice and demonstrated that Arctic sea-ice change could have hemispheric consequences on atmospheric circulation [references 13–21 in Liu et al. (2)]. These works, in contrast to Wu et al. (3), are relevant to our study and are cited in Liu et al. (2). Interestingly, none of these well-known works are cited in Wu et al. (3). In fact, Wu et al. (3) did not cite a single article about Arctic sea-ice change and its possible impacts. Wu et al. (3) focused on associations between the third mode of empirical orthogonal function (EOF3) of surface temperature over East Asia (not snow), a mode with questionable statistical significance, and sea surface temperature (SST) in the Arctic Ocean and central/northeastern Pacific (not Arctic sea ice). Furthermore, Li and Wu (1) claim a link between reduced autumn sea ice and the classic winter AO, but neither the AO nor the link is mentioned in Wu et al. (3). The numerical experiments in Liu et al. (2) and Wu et al. (3) differ in their purposes. Additionally, Wu et al. (3) chose arbitrary heating sources, whereas Liu et al. (2) used observed and physically based sea-ice anomalies, including winter anomalies persisting from autumn. Liu et al. (2) used an atmospheric general circulation model that is widely used by the modeling community, and more ensemble runs. Liu et al. (2) simulated and discussed responses of moisture sources and snowfall to
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reduced sea ice. All of these are well beyond the scope of Wu et al. (3). Second, Liu et al. (2) focused on demonstrating possible mechanisms linking anomalously large snowfall in recent winters to diminishing Arctic sea ice, not seasonal forecasts. We looked at the practice forecast in Wu et al. (3) for the principal component of EOF3 (not snow) based on SST (not Arctic sea ice). Excepting the single winter 2007–2008, there is almost no forecast skill for other years, including no skill for the winter of 2008–2009, despite anomalously low Arctic sea ice. Finally, Li and Wu’s central premise is wrong. We did use sea-ice area (SIA) in Liu et al. (2). We cannot discern the source of Li and Wu’s misconception. Li and Wu might not note that the National Snow and Ice Data Center sea-ice index excludes the area near the pole not imaged by sensors—1.19 (0.31) million square kilometers for Scanning Multichannel Microwave Radiometer (Special Sensor Microwave/Imager). We adjusted this discontinuity in Liu et al. (2). We double-checked our results. The correlation between the detrended autumn Arctic SIA and winter AO is indeed 0.28 for 1979–2010. The correlation is 0.27 when sea ice extent (SIE) is used instead of SIA. We now extend sea ice and AO time series to 2011 (Fig. 1). The correlations are reduced to 0.22/0.20 (SIA/SIE), only explaining ∼4% of the shared variance. This reinforces the claim in Liu et al. (2) that atmospheric circulation change linked to reduction of autumn sea ice is different from the classic winter AO. Jiping Liua,b,1, Judith A. Currya, Huijun Wangb, Radley M. Hortonc, and Mirong Songb a School of Earth and Atmospheric Sciences, Georgia Institute of Technology, Atlanta, GA 30332; bState Key Laboratory of Numerical Modeling for Atmospheric Sciences and Geophysical Fluid Dynamics (LASG), Institute of Atmospheric Physics, Chinese Academy of Sciences Beijing 100029, China; and cColumbia University Center for Climate Systems Research, New York, NY 10025 1. Li J, Wu Z (2012) Importance of autumn Arctic sea ice to northern winter snowfall. Proc Natl Acad Sci USA 109:E1898. 2. Liu J, Curry JA, Wang H, Song M, Horton RM (2012) Impact of declining Arctic sea ice on winter snowfall. Proc Natl Acad Sci USA 109:4074–4079. 3. Wu Z, Li J, Jiang Z, He J (2011) Predictable climate dynamics of abnormal East Asian winter monsoon: Once-in-a-century snowstorms in 2007/2008 winter. Clim Dyn 37: 1661–1669.
Author contributions: J.L. designed research; J.L. and M.S. analyzed data; and J.L., J.A.C., H.W., and R.M.H. wrote the paper. The authors declare no conflict of interest. 1
To whom correspondence should be addressed. E-mail:
[email protected].
PNAS | July 10, 2012 | vol. 109 | no. 28 | E1899–E1900
Fig. 1. Time series of actual and detrended autumn Arctic sea-ice area anomaly (×106 km2) and winter AO index for the period 1979–2011. SON, SeptemberOctober-November; DJF, December-January-February.
E1900 | www.pnas.org/cgi/doi/10.1073/pnas.1206848109
Liu et al.
