Circus tents, convective thresholds and the non-linear climate response to tropical SSTs

Williams, Andrew; Jeevanjee, Nadir; Bloch‐Johnson, Jonah

doi:10.1002/essoar.10512543.1

Cited by 9 publications

(10 citation statements)

References 47 publications

(59 reference statements)

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“…In the low cloud regions, at 700 hPa level, temperature warms up by 4.8 K (Figure S10c in Supporting Information ), which increases the EIS, and subsidence weakens by 4.2 hPa/day (Figure S11c in Supporting Information ). These are consistent with the understanding that deep convection affects low clouds by changing the tropical overturning circulation and temperature in the free troposphere (e.g., Andrews & Webb, 2018; Erfani & Burls, 2019; Schiro et al., 2022; Silvers & Robinson, 2021; Williams et al., 2023), and both the warming and the weakening of subsidence will tend to increase the low clouds (e.g., Myers & Norris, 2013; Qu, Hall, Klein, & Caldwell, 2015). Regarding the AMIP‐p4Krad experiment, the SST warming leads to reduction of precipitation over the western tropical Pacific and the tropical Indian oceans (Figure S9b in Supporting Information ), which is related to suppressed latent heating by the deep convection.…”

Section: Resultssupporting

confidence: 87%

Positive Low Cloud Feedback Primarily Caused by Increasing Longwave Radiation From the Sea Surface in Two Versions of a Climate Model

Ogura,

Webb,

Lock

2023

Geophysical Research Letters

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Low cloud feedback in global warming projections by climate models is characterized by its positive sign, the mechanism of which is not well understood. Here we propose that the positive sign is primarily caused by the increase in upward longwave radiation from the sea surface. We devise numerical experiments that enable separation of the feedback into components coming from physically distinct causes. Results of these experiments with a climate model indicate that increases in upward longwave radiation from the sea surface cause warming and absolute drying in the boundary layer, leading to the positive low cloud feedback. The absolute drying results from decrease in surface evaporation, and also from decrease in inversion strength which enhances vertical mixing of drier free tropospheric air into the boundary layer. This mechanism is different from previously proposed understanding that positive low cloud feedback is caused by increases in surface evaporation or vertical moisture contrast.

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Section: Resultssupporting

confidence: 87%

Positive Low Cloud Feedback Primarily Caused by Increasing Longwave Radiation From the Sea Surface in Two Versions of a Climate Model

Ogura,

Webb,

Lock

2023

Geophysical Research Letters

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“…All of these nonlinearities may have a common explanation, proposed by Williams et al (2022b), which documented similar nonlinearities with the ICON model. In order to set off deep convection, the surface conditions of tropical regions must reach a threshold in which their subcloud moist static energy is larger than the saturated moist static energy aloft (Williams and Pierrehumbert, 2017).…”

Section: Download and Upload Informationmentioning

confidence: 55%

The Green’s Function Model Intercomparison Project (GFMIP) Protocol

Bloch‐Johnson

Rugenstein

Alessi

et al. 2023

Preprint

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The atmospheric Green’s function method is a technique for modeling the response of the atmosphere to changes in the spatial field of surface temperature. While early studies applied this method to changes in atmospheric circulation, it has also become an important tool to understand changes in radiative feedbacks due to evolving patterns of warming, a phenomenon called the “pattern effect.’ To better study this method, this paper presents a protocol for creating atmospheric Green’s functions to serve as the basis for a model intercomparison project, GFMIP. The protocol has been developed using a series of sensitivity tests performed with the HadAM3 atmosphere-only general circulation model, along with existing and new simulations from other models. Our preliminary results have uncovered nonlinearities in the response of the atmosphere to surface temperature changes, including an asymmetrical response to warming vs. cooling patch perturbations, and a change in the dependence of the response on the magnitude and size of the patches. These nonlinearities suggest that the pattern effect may depend on the heterogeneity of warming as well as its location. These experiments have also revealed tradeoffs in experimental design between patch size, perturbation strength, and the length of control and patch simulations. The protocol chosen on the basis of these experiments balances scientific utility with the simulation time and setup required by the Green’s function approach. Running these simulations will further our understanding of many aspects of atmospheric response, from the pattern effect and radiative feedbacks to changes in circulation, cloudiness, and precipitation.

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“…For example, estimates of EffCS from the observed historical energy budget constraints are lower than those from long‐term warming under CO 2 quadrupling, primarily owing to changes in the tropical Pacific SST patterns (Andrews et al., 2018, 2022; Dong et al., 2019; Gregory et al., 2020; Zhou et al., 2016). This “pattern effect” has been studied with a Green's function approach (Dong et al., 2019; Zhang et al., 2023; Zhou et al., 2017), which shows that the global feedback has a predominant dependence on tropical convective regions (Williams et al., 2023), and is less sensitive to the North Atlantic SSTs. This tropical Pacific SST pattern effect has been found to be a leading mechanism for the time evolution of EffCS estimates.…”

Section: Discussionmentioning

confidence: 99%

Non‐Monotonic Feedback Dependence Under Abrupt CO₂ Forcing Due To a North Atlantic Pattern Effect

Mitevski

Dong

Polvani

et al. 2023

Geophysical Research Letters

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Effective climate sensitivity (EffCS), commonly estimated from model simulations with abrupt 4×CO2 for 150 years, has been shown to depend on the CO2 forcing level. To understand this dependency systematically, we performed a series of simulations with a range of abrupt CO2 forcing in two climate models. Our results indicate that normalized EffCS values in these simulations are a non‐monotonic function of the CO2 forcing, decreasing between 3× and 4×CO2 in CESM1‐LE (2× and 3×CO2 in GISS‐E2.1‐G) and increasing at higher CO2 levels. The minimum EffCS value, caused by anomalously negative radiative feedbacks, arises mainly from sea‐surface temperature (SST) relative cooling in the tropical and subtropical North Atlantic. This cooling is associated with the formation of the North Atlantic Warming Hole and Atlantic Meridional Overturning Circulation collapse under CO2 forcing. Our findings imply that understanding changes in North Atlantic SST patterns is important for constraining near‐future and equilibrium global warming.

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Circus tents, convective thresholds and the non-linear climate response to tropical SSTs

Cited by 9 publications

References 47 publications

Positive Low Cloud Feedback Primarily Caused by Increasing Longwave Radiation From the Sea Surface in Two Versions of a Climate Model

Positive Low Cloud Feedback Primarily Caused by Increasing Longwave Radiation From the Sea Surface in Two Versions of a Climate Model

The Green’s Function Model Intercomparison Project (GFMIP) Protocol

Non‐Monotonic Feedback Dependence Under Abrupt CO₂ Forcing Due To a North Atlantic Pattern Effect

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Circus tents, convective thresholds and the non-linear climate response to tropical SSTs

Cited by 9 publications

References 47 publications

Positive Low Cloud Feedback Primarily Caused by Increasing Longwave Radiation From the Sea Surface in Two Versions of a Climate Model

Positive Low Cloud Feedback Primarily Caused by Increasing Longwave Radiation From the Sea Surface in Two Versions of a Climate Model

The Green’s Function Model Intercomparison Project (GFMIP) Protocol

Non‐Monotonic Feedback Dependence Under Abrupt CO2 Forcing Due To a North Atlantic Pattern Effect

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Product

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Non‐Monotonic Feedback Dependence Under Abrupt CO₂ Forcing Due To a North Atlantic Pattern Effect