The influence of direct radiative forcing versus indirect sea surface temperature warming on southern hemisphere subtropical anticyclones under global warming

Abdullah A Fahad,Natalie J Burls

doi:10.1007/s00382-021-06006-1

Abdullah A Fahad, Natalie J Burls

Open Access

https://doi.org/10.1007/s00382-021-06006-1

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Abstract

Southern hemisphere subtropical anticyclones are projected to change in a warmer climate during both austral summer and winter. A recent study of CMIP 5 & 6 projections found a combination of local diabatic heating changes and static-stability-induced changes in baroclinic eddy growth as the dominant drivers. Yet the underlying mechanisms forcing these changes still remain uninvestigated. This study aims to enhance our mechanistic understanding of what drives these Southern Hemisphere anticyclones changes during both seasons. Using an AGCM, we decompose the response to CO2-induced warming into two components: (1) the fast atmospheric response to direct CO2 radiative forcing, and (2) the slow atmospheric response due to indirect sea surface temperature warming. Additionally, we isolate the influence of tropical diabatic heating with AGCM added heating experiments. As a complement to our numerical AGCM experiments, we analyze the Atmospheric and Cloud Feedback Model Intercomparison Project experiments. Results from sensitivity experiments show that slow subtropical sea surface temperature warming primarily forces the projected changes in subtropical anticyclones through baroclinicity change. Fast CO2 atmospheric radiative forcing on the other hand plays a secondary role, with the most notable exception being the South Atlantic subtropical anticyclone in austral winter, where it opposes the forcing by sea surface temperature changes resulting in a muted net response. Lastly, we find that tropical diabatic heating changes only significantly influence Southern Hemisphere subtropical anticyclone changes through tropospheric wind shear changes during austral winter.

Highlights

Subtropical Anticyclones (SA)s are semi-permanent highpressure systems that are crucial components of the largescale atmospheric circulation in both hemispheres
The total diabatic heating increase in the tropical Pacific Ocean is ~ 0.6 k/day in the CMIP6 multi-model mean (MMM), and ~ 1.0 k/day in the Community Earth System Model version 2 (CESM2) during austral summer in the SSP585 scenario compared to the Historical experiment
Sea Surface Temperature (SST) acts as the primary driver of these Southern Hemisphere (SH) DJF SA Sea Level Pressure (SLP) and tropical diabatic heating changes (Fig. 6c, g), whereas the C O2 increase acts as a secondary forcing during austral summer (Fig. 6d, h)

Summary

Introduction

Subtropical Anticyclones (SA)s are semi-permanent highpressure systems that are crucial components of the largescale atmospheric circulation in both hemispheres. To understand how the weather and climate of SH subtropical and midlatitude regions will change in response to global warming, understanding how the SH SAs will respond is crucial. In simulations conducted by the models participating in the Coupled Model Intercomparison Project (CMIP), it is well documented that the subtropical to midlatitude atmospheric circulation is significantly affected in both hemispheres under future warming (Li et al 2012, 2013; Shaw and Voigt 2015; He et al 2017; Song et al 2018a, Fahad et al 2020) (Fig. 1a, b). The SH SAs have received less attention, with open questions surrounding our understanding of what drives the

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