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1. Introduction
The North Atlantic storm track moderates climate over the Atlantic–European sector, and an understanding of the physical mechanisms behind its variability is needed to better constrain the regional impacts. Factors that control the mean strength, position, and orientation of the storm track are well documented (Shaw et al. 2016): these include underlying SST patterns (Brayshaw et al. 2008, 2011), the equator-to-pole temperature gradient (Chang et al. 2002; Schneider 2006), and the upper-tropospheric jet stream (Hoskins and Valdes 1990; Chang et al. 2002; Schneider 2006). However, factors that control variations in its anchor point (i.e., the main region of cyclogenesis over the warm Gulf Stream) have received less attention. The current study examines the influence of tropically induced flow perturbations on cyclogenesis and the storm track.
The Gulf Stream acts as an “anchor” for the North Atlantic storm track. It maintains a low-level zone of enhanced baroclinicity (Hoskins and Valdes 1990; Kwon et al. 2010), which, together with the overlying jet stream, makes the region between Cape Hatteras and Nova Scotia amenable for explosive cyclogenesis and deepening of existing cyclones during all seasons (Sanders 1986; Hoskins and Valdes 1990; Neiman and Shapiro 1993; Bosart 1999; Jacobs et al. 2005; Booth et al. 2012). However, the interaction between the Gulf Stream SSTs and the storm track is complex. While surface turbulent fluxes have been shown to be one driving agent behind the tropospheric circulation in this region (Nakamura et al. 2008; Kwon et al. 2010), much of their variability on and below synoptic time scales is controlled by the atmosphere itself (Shaman et al. 2010). For example, Booth et al. (2012) showed that the spatial variability of the surface storm track over the Gulf Stream, in particular the 10-m wind, is strongly affected by the stability of the marine planetary boundary layer and preconditioning by surface fluxes, but much of the temporal variability of the surface storm track is controlled by variability in the free troposphere.
Tropical Pacific SST variability is known to be an important driver of extratropical climate variability in both hemispheres, with consequences for regional weather patterns (Trenberth et al. 1998; Hoerling et al. 2001; Alexander et al. 2002; Seager et al. 2003; Ding et al. 2014). The remote influence of El...