The response of stratospheric water vapor to climate change driven by different forcing agents

Xun Wang,Andrew E Dessler

doi:10.5194/acp-20-13267-2020

Abstract

Abstract. We investigate the response of stratospheric water vapor (SWV) to different forcing agents within the Precipitation Driver and Response Model Intercomparison Project (PDRMIP) framework. For each model and forcing agent, we break down the SWV response into a slow response, which is coupled to surface temperature changes, and a fast response, which is the response to external forcing but before the sea surface temperatures have responded. Our results show that, for most climate perturbations, the slow SWV response dominates the fast response. The slow SWV response exhibits a similar sensitivity to surface temperature across all climate perturbations. Specifically, the sensitivity is 0.35 ppmv K−1 in the tropical lower stratosphere (TLS), 2.1 ppmv K−1 in the northern hemispheric lowermost stratosphere (LMS), and 0.97 ppmv K−1 in the southern hemispheric LMS. In the TLS, the fast SWV response only dominates the slow SWV response when the forcing agent radiatively heats the cold-point region – for example, black carbon, which directly heats the atmosphere by absorbing solar radiation. The fast SWV response in the TLS is primarily controlled by the fast adjustment of cold-point temperature across all climate perturbations. This control becomes weaker at higher altitudes in the tropics and altitudes below 150 hPa in the LMS.

Highlights

Stratospheric water vapor (SWV) plays an important role in global climate change
In most climate perturbations analyzed in this study, the equilibrium response of water vapor in both the tropical lower stratosphere (TLS) and the lowermost stratosphere (LMS) is dominated by SWVfast) and slow response ( (SWVslow), which is the component mediated by sea surface temperature change
We investigate the response of stratospheric water vapor (SWV) to a range of different climate forcing mechanisms using a multi-model and multiple-forcing-agent framework

Summary

Introduction

Stratospheric water vapor (SWV) plays an important role in global climate change. It is an important greenhouse gas (GHG), which affects the Earth’s radiative budget (Forster and Shine, 2002; Solomon et al, 2010), and it plays an important role in stratospheric ozone chemistry (Solomon et al, 1986; Dvortsov and Solomon, 2001).SWV in the overworld (above the 380 K isentropic surface) (e.g., Hoskins, 1991) and SWV in the extratropical lowermost stratosphere (LMS, between the extratropical tropopause and the 380 K isentropic surface) (e.g., Holton et al, 1995) are distinguished according to different mechanisms that control them. The addition of a radiatively active constituent to the atmosphere can influence the atmosphere even before the surface temperature changes, leading to changes in SWV This is often referred to as an “adjustment” to the forcing and is generally considered part of the external forcing (e.g., Sherwood et al, 2015). The slow response is the component in the SWV change that is coupled to changes in the surface temperature, which occur on longer timescales This slow response means that SWV could be an important positive feedback to global warming (Forster and Shine, 2002; Dessler et al, 2013; Huang et al, 2016; Banerjee et al, 2019). This slow response means that SWV could be an important positive feedback to global warming (Forster and Shine, 2002; Dessler et al, 2013; Huang et al, 2016; Banerjee et al, 2019). Banerjee et al (2019) have shown that, when CO2 is abruptly quadru-

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