Changes in Local and Global Climate Feedbacks in the Absence of Interactive Clouds: Southern Ocean–Climate Interactions in Two Intermediate-Complexity Models

Patrik L Pfister,Thomas F Stocker

doi:10.1175/jcli-d-20-0113.1

Patrik L Pfister, Thomas F Stocker

Open Access

https://doi.org/10.1175/jcli-d-20-0113.1

Copy DOI

Abstract

AbstractThe global-mean climate feedback quantifies how much the climate system will warm in response to a forcing such as increased CO2 concentration. Under a constant forcing, this feedback becomes less negative (increasing) over time in comprehensive climate models, which has been attributed to increases in cloud and lapse-rate feedbacks. However, out of eight Earth system models of intermediate complexity (EMICs) not featuring interactive clouds, two also simulate such a feedback increase: Bern3D-LPX and LOVECLIM. Using these two models, we investigate the causes of the global-mean feedback increase in the absence of cloud feedbacks. In both models, the increase is predominantly driven by processes in the Southern Ocean region. In LOVECLIM, the global-mean increase is mainly due to a local longwave feedback increase in that region, which can be attributed to lapse-rate changes. It is enhanced by the slow atmospheric warming above the Southern Ocean, which is delayed due to regional ocean heat uptake. In Bern3D-LPX, this delayed regional warming is the main driver of the global-mean feedback increase. It acts on a near-constant local feedback pattern mainly determined by the sea ice–albedo feedback. The global-mean feedback increase is limited by the availability of sea ice: faster Southern Ocean sea ice melting due to either stronger forcing or higher equilibrium climate sensitivity (ECS) reduces the increase of the global mean feedback in Bern3D-LPX. In the highest-ECS simulation with 4 × CO2 forcing, the feedback even becomes more negative (decreasing) over time. This reduced ice–albedo feedback due to sea ice depletion is a plausible mechanism for a decreasing feedback also in high-forcing simulations of other models.

Highlights

Energy balance models (EBMs) are invaluable diagnostic tools for assessing the behavior of more comprehensive climate models and their differences (e.g., Gregory et al 2004; Winton et al 2010; Geoffroy et al 2013; Ceppi and Gregory 2019)
We have investigated the causes of the time dependence of the transient climate sensitivity in two EMICs: the Bern3D-LPX and the LOVECLIM model
This time dependence can be understood from two different perspectives: First, the global mean feedback becomes less negative due to a combination of increasingly polar-amplified warming and changing local feedback patterns; second, ocean heat uptake (OHU) efficacy increases due to strong OHU in the Southern Ocean, where an anomalously strong local feedback dominates

Summary

15 JANUARY 2021

Changes in Local and Global Climate Feedbacks in the Absence of Interactive Clouds: Southern Ocean–Climate Interactions in Two Intermediate-Complexity Models. STOCKERa,b a Climate and Environmental Physics, Physics Institute, University of Bern, Bern, Switzerland b Oeschger Center for Climate Change Research, University of Bern, Bern, Switzerland (Manuscript received 18 February 2020, in final form 6 October 2020)

Introduction

Time-dependent global feedback

Testing the approximation of constant local feedbacks

The ocean heat uptake efficacy perspective

Findings

Conclusions