Hemispherically asymmetric volcanic forcing of tropical hydroclimate during the last millennium

Christopher M Colose,Mathias Vuille,Allegra N Legrande

doi:10.5194/esd-7-681-2016

Abstract

Abstract. Volcanic aerosols exert the most important natural radiative forcing of the last millennium. State-of-the-art paleoclimate simulations of this interval are typically forced with diverse spatial patterns of volcanic forcing, leading to different responses in tropical hydroclimate. Recently, theoretical considerations relating the intertropical convergence zone (ITCZ) position to the demands of global energy balance have emerged in the literature, allowing for a connection to be made between the paleoclimate simulations and recent developments in the understanding of ITCZ dynamics. These energetic considerations aid in explaining the well-known historical, paleoclimatic, and modeling evidence that the ITCZ migrates away from the hemisphere that is energetically deficient in response to asymmetric forcing.Here we use two separate general circulation model (GCM) suites of experiments for the last millennium to relate the ITCZ position to asymmetries in prescribed volcanic sulfate aerosols in the stratosphere and related asymmetric radiative forcing. We discuss the ITCZ shift in the context of atmospheric energetics and discuss the ramifications of transient ITCZ migrations for other sensitive indicators of changes in the tropical hydrologic cycle, including global streamflow. For the first time, we also offer insight into the large-scale fingerprint of water isotopologues in precipitation (δ18Op) in response to asymmetries in radiative forcing. The ITCZ shifts away from the hemisphere with greater volcanic forcing. Since the isotopic composition of precipitation in the ITCZ is relatively depleted compared to areas outside this zone, this meridional precipitation migration results in a large-scale enrichment (depletion) in the isotopic composition of tropical precipitation in regions the ITCZ moves away from (toward). Our results highlight the need for careful consideration of the spatial structure of volcanic forcing for interpreting volcanic signals in proxy records and therefore in evaluating the skill of Common Era climate model output.

Highlights

The intertropical convergence zone (ITCZ) is the narrow belt of deep convective clouds and strong precipitation that develops in the rising branch of the Hadley circulation
The two general circulation models (GCMs) that we use as our laboratory are NASA GISS ModelE2-R and the National Center for Atmospheric Research (NCAR) Community Earth System Model Last Millennium Ensemble
Ents (Fig. S1 in the Supplement) and a positive mode of the Arctic–North Atlantic Oscillation (Robock and Mao, 1992, 1995; Stenchikov et al, 2002; Shindell et al, 2004; Ortega et al, 2015). This effect is weak in the ASYMMNH composite, likely because the maximal radiative forcing is located in the Northern Hemisphere (NH), offsetting any dynamical response, but is present in the SYMM and ASYMMSH composites in both models

Summary

Introduction

The intertropical convergence zone (ITCZ) is the narrow belt of deep convective clouds and strong precipitation that develops in the rising branch of the Hadley circulation. Meridional displacements of the ITCZ are constrained by requirements of reaching a consistent energy balance on both sides of the ascending branch of the Hadley circulation (e.g., Kang et al, 2008, 2009; Schneider et al, 2014). The ITCZ is a convergence zone in near-surface meridional mass flux, it is a divergence zone energetically. The stratification of the tropical atmosphere is such that moist static energy (MSE) is greater aloft than near the surface, compelling Hadley cells to transport energy in the direction of their upper tropospheric flow (Neelin and Held, 1987).

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