## 1. Introduction

[2] Several dynamical responses to increased greenhouse gasses (GHGs) have been identified. These include an increase in the dry static stability [*Frierson*, 2006]; an increase in the tropopause height [*Lorenz and DeWeaver*, 2007]; and a poleward shift of the mid-latitude jet streams and storm tracks [e.g., *Meehl et al.*, 2007b, and references therein], which is also associated with the expansion of the tropics [*Lu et al.*, 2008]. Here we show that another robust dynamical response to increasing GHGs is an increase in the eddy length scale. The cause remains uncertain, but it appears to be linked to the increase in dry static stability.

[3] In the mid-latitudes the tropospheric circulation is dominated by eddies, which are born primarily of baroclinic instability. A fundamental characteristic of these eddies is the horizontal length scale. For a given background flow, the length scale affects both the height and the latitude at which the eddies dissipate. The location of eddy genesis and dissipation is of first order importance in determining the time-mean atmospheric circulation [*Holton*, 1992; *Vallis*, 2006].

[4] The dynamics that determine the dominant eddy length scales is a matter of ongoing research. In so far as linear theory applies, the fastest-growing normal mode will dominate. In the Eady model the relevant measure is the Rossby radius, *L*_{R} = *NH*/*f* where *N* is the buoyancy frequency, *H* a scale height, and *f* the Coriolis parameter. Inclusion of a planetary vorticity gradient (*β*) as in the Charney model makes the linear problem less tractable, but if the deep Charney mode is relevant then again a length scale that arises is *L*_{R}. However, if the flow is nonlinear then there may be a transfer of energy to larger scales, which can be arrested by *β*, friction, or a combination of the two [*Rhines*, 1975; *Vallis and Maltrud*, 1993]. If the *β* effect were of primary importance then the Rhines scale, *L*_{β} = where *U*′ is the RMS eddy velocity, might be expected to determine the eddy scale. However, in the Earth's atmosphere this scale is not significantly larger than the deformation radius *L*_{R}, and certainly no well-developed inverse cascade exists. Whether the dominant eddy length-scale is primarily determined by the Rhines scale, the deformation scale, or some combination or other factors entirely remains an open question [*Vallis*, 2006]. For example, using a dry primitive equation model [*Schneider and Walker*, 2006] found that *L*_{β} and *L*_{R} essentially scale in the same way as parameters vary, whereas *Thuburn and Craig* [1997] and *Frierson et al.* [2006] found that *L*_{β} was a better fit for moist primitive equation models.