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1. Introduction
Extratropical cyclones control weather variability. They preferentially occur in regions that are commonly referred to as storm tracks. Near the storm-track entrances, equator-pole temperature gradients are strongest and baroclinic instability drives the release of available potential energy. Many properties of storm tracks are controlled by processes affecting the equator-pole temperature gradients and the static stability of the atmosphere (e.g., Fyfe 2003; Yin 2005; Bengtsson et al. 2006; Schneider and Walker 2008; O’Gorman and Schneider 2008; O’Gorman 2010; Harvey et al. 2014). Consequently, the future development of midlatitude equator-pole temperature gradients and static stability is a key to understanding future storm-track behavior, with the relative importance of various processes still under debate [see Chang et al. (2002), Schneider et al. (2010), and Shaw et al. (2016) for reviews].
Baroclinic instability is widely accepted as the formation mechanism of extratropical cyclones, and baroclinicity, which is proportional to the meridional temperature gradient and inversely proportional to static stability, quantifies their growth potential (Charney 1947; Eady 1949; Lindzen and Farrell 1980). As noted by classical theory and supported by observations, higher baroclinicity usually leads to deeper and more rapidly intensifying extratropical cyclones. Because extratropical cyclone activity is intimately linked to poleward heat transport, increased baroclinicity generally also implies intensified poleward eddy heat flux (e.g., Schneider and Walker 2008; Thompson and Birner 2012). Observational studies suggest that variations in the equator-to-pole temperature contrast often dominate baroclinicity variations (Ambaum and Novak 2014; Thompson and Barnes 2014). In fact, Stone and Miller (1980) found that seasonal variations of the meridional eddy energy flux show an excellent correlation with variations in the meridional surface temperature gradient.
However, the midwinter evolution of the North Pacific storm track appears to challenge these classical theories and observations (Nakamura 1992). Although the surface temperature gradient peaks during midwinter, several measures of eddy activity, such as the meridional eddy energy flux and the transient eddy kinetic energy (EKE), have a minimum in midwinter over the North Pacific (Nakamura 1992). Several mechanisms explaining this phenomenon have been proposed:
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The increased group velocity of eddies in winter can result in wave packets passing too quickly through the main baroclinic zone, causing a suppression of their baroclinic amplification (Chang 2001).
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The narrow jet stream in midwinter (Harnik...