Feedbacks and eddy diffusivity in an energy balance model of tropical rainfall shifts

Henry G Peterson,William R Boos

doi:10.1038/s41612-020-0114-4

Abstract

Radiative feedbacks are known to strongly modify horizontal shifts of the intertropical convergence zone (ITCZ) produced by remote atmospheric energy sources. This study uses a one-dimensional moist energy balance model to understand how radiative feedbacks and the structure of an imposed eddy diffusivity can influence such ITCZ shifts. The Planck feedback is shown to damp ITCZ shifts more strongly for extratropical forcings than for tropical ones, because lower moisture content in cold regions makes the temperature response larger there. The water vapor feedback on ITCZ shifts is shown to be dominated by changes in the cross-equatorial asymmetry of the relative humidity of subtropical dry zones, with additional contributions by the changes in mixing ratio that occur at fixed relative humidity and by the meridional shift of the humid ITCZ. Finally, the ITCZ response is found to be highly sensitive to the meridional structure of the diffusivity; the ITCZ shift increases with the tropical diffusivity, even when the global mean diffusivity is fixed.

Highlights

Atmospheric circulations transport both water and energy, coupling the spatial structure of precipitation to the spatial structure of atmospheric energy transport
This is consistent with the PL feedback on extratropical forcings having a contribution to NEI0 that is more localized (Fig. 4c) and a flux form with greater magnitude on the equator (Fig. 4d). This enhanced localization of T0 anomalies and the associated larger effect of the PL feedback on energy flux equator (EFE) shifts occurs in the absence of any rotational confinement of temperature anomalies in our diffusive moist energy balance model (MEBM). This contrasts with an argument presented by Seo et al.[23], who stated that extratropical forcings produced a larger clear-sky OLR response in their aquaplanet global climate models (GCMs) because of a larger Coriolis parameter at higher latitudes: “Since large temperature gradients cannot be sustained in the tropics (e.g. Sobel et al 2001; Yano and Bonazzola 2009), the tropical thermal forcing ... is mostly compensated by atmospheric energy transport divergence with only a small change in TOA radiative fluxes”
We used an MEBM to investigate the influences of climate feedbacks and the eddy diffusivity of moist energy on the sensitivity of the EFE to remote inputs of energy through the top and bottom of the atmosphere. This MEBM has no representation of cloud-radiative effects or the seasonal cycle, which we hope to explore in future work

Summary

INTRODUCTION

Atmospheric circulations transport both water and energy, coupling the spatial structure of precipitation to the spatial structure of atmospheric energy transport. When the measure of energy is chosen to include the latent heat stored in water vapor, the net input of this moist energy through the top and bottom of the atmosphere can be related to the position and intensity of time-mean tropical precipitation maxima, without requiring specification of the latent heat release that accompanies precipitation[1] Such moist energy frameworks have improved understanding of a variety of tropical climate phenomena, including tropical cyclones, monsoons, and the Madden−Julian Oscillation (see reviews by refs 2–4). Most frameworks that relate horizontal shifts in precipitation to anomalous sources of atmospheric moist energy are diagnostic rather than prognostic: they require knowledge of the net energy source anomaly in order to provide information on the precipitation shift This is because feedbacks act on any imposed energy source, producing changes in radiative fluxes and in surface turbulent fluxes of latent and sensible heat, altering the horizontal atmospheric energy transport needed to balance the imposed forcing (e.g. refs 8,16,17). We view this as a small first step toward a theoretical framework that may eventually be used to quantitatively predict feedbacks on ITCZ shifts

RESULTS

DISCUSSION

METHODS

CODE AVAILABILITY