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Zonal-mean circulation response to reduced air-sea momentum roughness

The impact of uncertainties in surface layer physics on the atmospheric general circulation is comparatively unexplored. Here the sensitivity of the zonal-mean circulation to reduced air-sea momentum roughness ($Z_{0m}$) at low flow speed is investigated with the Community Atmosphere Model (CAM3). In an aquaplanet framework with prescribed sea surface temperatures, the response to reduced $Z_{0m}$ resembles the La Ni$\tilde{\text{n}}$a minus El Ni$\tilde{\text{n}}$o response to El Ni$\tilde{\text{n}}$o Southern Oscillation variability with: i) a poleward shift of the mid-latitude westerlies extending all the way to the surface; ii) a weak poleward shift of the subtropical descent region; and iii) a weakening of the Hadley circulation, which is generally also accompanied by a poleward shift of the inter-tropical convergence zone (ITCZ) and the tropical surface easterlies. Mechanism-denial experiments show this response to be initiated by the reduction of tropical latent and sensible heat fluxes, effected by reducing $Z_{0m}$. The circulation response is elucidated by considering the effect of the tropical energy fluxes on the Hadley circulation strength, the upper tropospheric critical layer latitudes, and the lower-tropospheric baroclinic eddy forcing. The ITCZ shift is understood via moist static energy budget analysis in the tropics. The circulation response to reduced $Z_{0m}$ carries over to more complex setups with seasonal cycle, full complexity of atmosphere-ice-land-ocean interaction, and a slab ocean lower boundary condition. Hence, relatively small changes in the surface parameterization parameters can lead to a significant circulation response.