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1. Introduction
Enhanced warming of the polar surface temperature with respect to the global-mean temperature, referred to as polar warming amplification (PWA), has been a robust feature of externally forced climate model simulations for more than 30 years (e.g., Manabe and Wetherald 1975; Manabe and Stouffer 1980; Hansen et al. 1984; Rind 1987; Lu and Cai 2010; Cai and Tung 2012) and is considered an inherent characteristic of the climate system. Modern general circulation models (GCMs) from phase 3 of the Coupled Model Intercomparison Project (CMIP3) have indicated that polar regions are likely to warm at least twice as much as the global average (Meehl et al. 2007). PWA is further supported by observations, which indicate that the Arctic likely (66%–90% confidence) warmed over the last 50 years (ACIA 2005) and has outpaced global-mean warming by 2–3 times from 1970 to 2008 (Chylek et al. 2009). Despite the robustness of PWA predictions amongst CMIP3 models, the largest intermodel spread in regional warming magnitudes is found in polar regions (Bony et al. 2006; Meehl et al. 2007). In addition, the magnitude of polar warming is an important constraint on sea and land ice melt projections, sea level rise, and global climate sensitivity.
The polar regions, specifically the Arctic, are the most climatically sensitive areas of the globe (e.g., Solomon et al. 2007). The enhanced sensitivity of high-latitude regions to an external forcing is linked to several radiative and nonradiative feedback processes (Rind et al. 1995; Holland and Bitz 2003; Hall 2004; Cai 2005, 2006; Cai and Lu 2007; Winton 2006; Lu and Cai 2010; Hwang et al. 2011; Alexeev and Jackson 2013). The surface albedo feedback is cited as the most significant contributor to PWA (e.g., Manabe and Wetherald 1975; Hall 2004; Winton 2006; Serreze and Barry 2011). Studies suggest that the surface albedo feedback accounts for nearly half of the polar surface temperature increase under a CO2 forcing (Hall 2004). However, Winton (2006) concludes, using a top-of-atmosphere (TOA) perspective, that the surface albedo feedback may not be the most important contributor to PWA but rather combined longwave (LW) feedbacks. Idealized climate simulations demonstrate that PWA can occur in the absence of the surface albedo feedback (Cai 2005, 2006; Cai and Lu 2007; Graversen...