The Tropical Response to Extratropical Thermal Forcing in an Idealized GCM: The Importance of Radiative Feedbacks and Convective Parameterization

Abstract

The response of tropical precipitation to extratropical thermal forcing is reexamined using an idealized moist atmospheric GCM that has no water vapor or cloud feedbacks, simplifying the analysis while retaining the aquaplanet configuration coupled to a slab ocean from the authors' previous study. As in earlier studies, tropical precipitation in response to high-latitude forcing is skewed toward the warmed hemisphere. Comparisons with a comprehensive GCM in an identical aquaplanet, mixed-layer framework reveal that the tropical responses tend to be much larger in the comprehensive GCM as a result of positive cloud and water vapor feedbacks that amplify the imposed extratropical thermal forcing.The magnitude of the tropical precipitation response in the idealized model is sensitive to convection scheme parameters. This sensitivity as well as the tropical precipitation response can be understood from a simple theory with two ingredients: the changes in poleward energy fluxes are predicted using a onedimensional energy balance model and a measure of the ''total gross moist stability'' [Dm, which is defined as the total (mean plus eddy) atmospheric energy transport per unit mass transport] of the model tropics converts the energy flux change into a mass flux and a moisture flux change. The idealized model produces a low level of compensation of about 25% between the imposed oceanic flux and the resulting response in the atmospheric energy transport in the tropics regardless of the convection scheme parameter. Because Geophysical Fluid Dynamics Laboratory Atmospheric Model 2 (AM2) with prescribed clouds and water vapor exhibits a similarly low level of compensation, it is argued that roughly 25% of the compensation is dynamically controlled through eddy energy fluxes. The sensitivity of the tropical response to the convection scheme in the idealized model results from different values of Dm: smaller Dm leads to larger tropical precipitation changes for the same response in the energy transport.

Green Mountains and White Plains: The Effect of Northern Hemisphere Ice Sheets on the Global Energy Budget

“…We can thus assess whether the change in the radiation budget over the ice sheet is balanced by a compensating TOA flux to the south of, but in the same hemisphere as, the ice sheet (NTrop) or into the SH (SHem). If the compensating flux is in the SH this implies that there must be a cross-equatorial energy fluxsuch a flux may imply movements of the ITCZ (Kang et al 2008). We summarize these results in Fig.…”

“…Since it has been argued that movements in the ITCZ are in some way a requirement for transporting energy across the equator (Chiang and Bitz 2005;Kang et al 2008Kang et al , 2009Donohoe et al 2013;Cvijanovic and Chiang 2013), a first step to understanding the ITCZ is to understand the actual energy budget that movements in the ITCZ may be responding to. The first comment is that we do not find that imposing an ice sheet, with its concomitant impact on the TOA energy budget, necessarily implies that there is an atmospheric heat transport across the equator.…”

“…In the Green Mountain case we found that a radiative flux anomaly over the ice sheet can be balanced by fluxes out of the atmosphere in the same hemisphere as the ice sheet, not therefore requiring any cross-equatorial heat fluxes. The idea that an imposed forcing in the extratropics can be offset by heat fluxes in different places is not new: Kang et al (2008) discuss this ''compensation'' in depth. Kang et al (2008), however, consider how different physical parameters affect this compensation.…”

“…While the local effects of the ice sheet on the midlatitude circulation have long been considered (e.g., Cook and Held 1988;Kutzbach and Wright 1985), their global impact has been somewhat ignored and never examined with a fully coupled oceanatmosphere climate model (Manabe and Broccoli 1985). There has been much written recently about the effect that changing the contrast between the temperature and energy balance in the Northern and Southern Hemispheres can have on the position of tropical rain belts and transports of energy across the equator (e.g., Yoshimori and Broccoli 2008;Kang et al 2008;Chiang and Friedman 2012;Donohoe et al 2013). This framework has proven itself quite useful for understanding changes in the large-scale circulation.…”

“…The importance of moving heat from one hemisphere to another in response to a hemispheric cooling has been emphasized in a number of studies (Kang et al 2008;Cvijanovic and Chiang 2013). Although these studies do show that when the high latitudes are cooled there is a change in the cross-equatorial heat transport, there are a number of problems if we wish to relate these studies to how the climate responds to an ice sheet.…”