The Pacific storm track and the Atlantic storm track are two major features of the general circulation of the northern hemisphere in winter. They stem from repeated passage of synoptic disturbances over those two oceanic sectors. It has been documented with 40 years of NCEP reanalysis data that the Atlantic storm track is distinctly more intense than the Pacific storm track in 30 out of 40 winters. On the average, the former is about 10% more intense, although the Atlantic jet is about 30% weaker than the Pacific jet (see Figs. 1 and 3 in Mak and Deng, 2007).

This counterintuitive characteristic may be accounted for if the seeding disturbances entering the Atlantic jet are statistically stronger than those entering the Pacific jet. A case can be made that the difference in the seeding disturbances is a natural consequence of the geographical distribution of continents and oceans in the northern hemisphere. Euro-Asia is a huge landmass, with the Pacific being on the downstream side, while the Atlantic is on the downstream of the much smaller North America. Disturbances typically suffer greater dissipation over land than water because of the differences in their surface roughness. It follows that eddies (seeding disturbances) reaching the Pacific jet after having traversed through Euro-Asia would typically be weaker than those reaching the Atlantic jet after having traversed through North America, so much so that eddies over the North Atlantic may intensify to a greater equilibrated intensity, even though the Pacific jet is substantially stronger. This is the hypothesis for the phenomenon under consideration. The difference in the horizontal structure of the two jets may be an additional factor.

The literature on storm tracks has been extensively reviewed by Chang et al. (2002). The constituent disturbances of the storm tracks are largely baroclinic. One would naturally use a baroclinic model to investigate storm track dynamics (e.g. Lee and Mak, 1996; Whi-taker and Sardeshmukh, 1998; Deng and Mak, 2005). Orlanski (2005) and Zurita-Gotor and Chang (2005) have examined the role of seeding disturbances in the context of a single storm track. The latter authors conjecture that greater damping over Asia could result in a weaker Pacific storm track. The hypothesis for the relative intensity of the two winter storm tracks has been specifically shown to be valid by Mak and Deng (2007) in the context of a two-layer baroclinic model.

The purpose of this study is to further demonstrate that the hypothesis is valid even in a barotropic model setting, which is a maximally simplified model. Large-scale barotropic and baroclinic dynamics have much in common at the most fundamental level. Barotropic instability and baroclinic instability can both be succinctly interpreted in terms of mutual reinforcement of the constituent elements in an unstable disturbance (the so-called wave resonance mechanism, e.g. Mak, 2002). Indeed, some intrinsic aspects of storm track dynamics can be delineated with a barotropic model (Swanson et al., 1997; Lee, 2000).

The design of a particular forced dissipative barotropic model with two jets is presented in Sec. 2. Subsection 3.1 reports the inviscid modal instability properties of such a flow, and Subsec. 3.2 reports the instability properties under the influence of differential friction. The properties of eddy feedback and nonlinear model storm tracks are presented in Sec. 4 with uniform friction (Subsecs. 4.1 and 4.2) and with differential friction (Subsec. 4.3). The article ends with concluding remarks in Sec. 5.

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