The impact of doubled CO₂ on the energetics and hydrologic processes of mid-latitude transient eddies

Abstract

Atmospheric transient eddies contribute significantly to mid-latitude energy and water vapor transports. Changes in the global climate, as induced by greenhouse enhancement, will likely alter transient eddy behavior. Unraveling all the feedbacks that occur in general circulation models (GCMs) can be difficult. Here, we isolate the transient eddies from the feedbacks and focus on the response of the eddies to simulated zonal-mean climate change that results from CO₂-doubling. Using a primitive-equation spectral model, we examine the impact of such climate change on the life cycles of transient eddies. We compare transient eddy behavior in experiments with initial conditions that are given by the zonal-mean climates of GCMs with current and doubled amounts of CO₂. The smaller meridional temperature gradient in a doubled CO₂ climate leads to a reduction in eddy kinetic energy, especially in the subtropics. The decrease in subtropical eddy energy is related to a substantial reduction in equatorward flux of eddy activity during the latter part of the life cycle. The reduction in equatorward energy flux alters the moisture cycle. Eddy meridional transport of water vapor is shifted slightly poleward and subtropical precipitation is reduced. The water vapor transport exhibits a relatively small change in magnitude, compared to changes in eddy energy, due to the compensating effect of higher specific humidity in the doubled-CO₂ climate. An increase in high-latitude precipitation is related to the poleward shift in eddy water vapor flux. Our experiments indicate that the simulation of climate change in eddy water vapor transport and eddy-generated precipitation is sensitive to model resolution and requires a minimum truncation of rhomboidal 30 in a spectral model. Surface evaporation amplifies climatic changes in water vapor transport and precipitation in our experiments.