INTERNATIONAL JOURNAL OF CLIMATOLOGY Int. J. Climatol. (2012) Published online in Wiley Online Library (wileyonlinelibrary.com) DOI: 10.1002/joc.3431 The impacts of the PNA and NAO on annual maximum snowpack over southern Canada during 1979–2009 Hongxu Zhao, a * Kaz Higuchi, a James Waller, a Heather Auld a and Thomas Mote b a Climate Research Division, Environment Canada, 4905 Dufferin Street, Toronto, ON M3H 5T4, USA b Department of Geography, University of Georgia, Athens, GA 30602-2502, USA ABSTRACT: The Pacific/North American pattern (PNA) and North Atlantic Oscillation (NAO) are two of the most important atmospheric low-frequency variability modes modulating interannual anomalies of snowpacks in North America. However, the quantitative degree to which the PNA and NAO affect the annual maximum snow water equivalent (SWEmax), a quantity directly relevant to the ground snow load calculation in National Building Code of Canada, remains unknown. This study explores the relationship of the winter PNA and NAO to the Canadian SWEmax anomalies and potential changes associated with atmospheric circulation patterns. Significant correlations are identified in eastern and western Canada, where climatological mean values are largest due to the high frequency of snow storm occurrence. The regions in Canada where SWEmax anomalies are influenced by the PNA and NAO experienced a significant change from pre-1998 to post-1998. This change is consistent with the observed changes in annual maximum snow depth and the winter atmospheric circulations during 1979–2009. Copyright  2012 Royal Meteorological Society KEY WORDS annual maximum snow water equivalent; the PNA; the NAO Received 26 April 2011; Revised 3 January 2012; Accepted 7 January 2012 1. Introduction Snow cover, with the largest areal coverage of any components in the cryosphere and high spatial and temporal variability, is believed to be an important indicator of climate change and has been shown to have strong impacts on regional and global energy and water cycles (Groisman et al., 1994; Qu and Hall, 2007; Hall et al., 2008). The heavy snow storms that have occurred in recent years led to significant economic loss and ecological impacts in southeastern Canada, where several observing stations broke long-standing snowfall records, resulting in significant impacts including roof collapses (Descurieux, 2010). The annual maximum snow water equivalent (SWE- max) of snowpack is important for hydrological mod- elling and runoff prediction (Dery et al., 2005) and is also relevant to ground snow load calculations in the National Building Code of Canada (Newark et al., 1989). SWE- max represents accumulated snowfall events during snow season (defined as October to May in this paper) and is influenced by various metamorphosis processes asso- ciated with atmospheric variables (such as temperature, precipitation, and wind), land cover type and topogra- phy. Over North America, the main winter storm track is situated in an elongated area along the border between the United States and Canada, with a primary maximum ∗ Correspondence to: H. Zhao, Climate Research Division, Environ- ment Canada, 4905 Dufferin Street, Toronto, ON M3H 5T4, USA. E-mail: hongxu.zhao@ec.gc.ca centre situated over the Great Lakes and a secondary maximum centre over the lee side of the Rocky Moun- tains (Reitan, 1974), both areas being associated with preferable conditions of cyclogenesis. Atmospheric teleconnection patterns characterize low- frequency climate variability on regional and hemi- spheric scales. The North Atlantic Oscillation (NAO) and the Pacific/North American pattern (PNA) are two of the most important patterns characterising the North- ern Hemisphere extratropical climate variability, espe- cially during the winter (Wallace and Gutzler, 1981; Jones et al., 1997), exerting significant influences on North American regions. Recent studies have revealed that the PNA and the NAO are two of the most important atmospheric low-frequency variability modes modulat- ing snowpack interannual anomalies in North America via both snowfall and temperature pathways (Ghatak et al., 2010). Previous studies have shown that inter- annual variability in snowpack is highly correlated to local temperature and precipitation through controls of large-scale atmospheric circulations (Clark et al., 1999; Saito et al., 2004; Zhao and Fernandes, 2010). Under- standing the role the PNA and the NAO play in the atmosphere-annual maximum snowpack interaction is important for improving the predictability of snow- pack variability and its subsequent impact on infras- tructure design under recent and future climate changes. However, only few studies (Jin et al., 2006; Brown, 2010) have been conducted on this linkage at regional scales. Copyright  2012 Royal Meteorological Society