The climatic effects of the direct injection of water vapour into the stratosphere by large volcanic eruptions

M M Joshi,G S Jones

doi:10.5194/acp-9-6109-2009

Abstract

Abstract. We describe a novel mechanism that can significantly lower the amplitude of the climatic response to certain large volcanic eruptions and examine its impact with a coupled ocean-atmosphere climate model. If sufficiently large amounts of water vapour enter the stratosphere, a climatically significant amount of water vapour can be left over in the lower stratosphere after the eruption, even after sulphate aerosol formation. This excess stratospheric humidity warms the tropospheric climate, and acts to balance the climatic cooling induced by the volcanic aerosol, especially because the humidity anomaly lasts for a period that is longer than the residence time of aerosol in the stratosphere. In particular, northern hemisphere high latitude cooling is reduced in magnitude. We discuss this mechanism in the context of the discrepancy between the observed and modelled cooling following the Krakatau eruption in 1883. We hypothesize that moist coignimbrite plumes caused by pyroclastic flows travelling over ocean rather than land, resulting from an eruption close enough to the ocean, might provide the additional source of stratospheric water vapour.

Highlights

Volcanic eruptions have the capacity to dramatically alter global climate on long climatic timescales
We describe a novel mechanism that might significantly alter the response of the climate to a large volcanic eruption: so much water vapour is directly injected into the stratosphere during certain eruptions that a climatically significant amount is left over after the formation of stratospheric sulphate aerosol
This stratospheric water vapour anomaly exerts a positive radiative forcing on the surface, offsetting the negative climate forcing from the volcanic stratospheric aerosol, leading to a surface temperature response that is smaller in amplitude than if forced by the volcanic aerosol alone

Summary

Introduction

Volcanic eruptions have the capacity to dramatically alter global climate on long climatic timescales. We describe a novel mechanism that might significantly alter the response of the climate to a large volcanic eruption: so much water vapour is directly injected into the stratosphere during certain eruptions that a climatically significant amount is left over after the formation of stratospheric sulphate aerosol. Joshi and Shine (2003) proposed that heating of the tropical tropopause layer by volcanic aerosol in the 2 years following the eruption allowed more water vapour into the stratosphere Another mechanism is that water vapour enters the plumes of a major volcanic eruption by a number of different ways: from the magma itself (Scaillet et al, 2003); from interaction of the magma with water (phreatomagmatic eruptions) (Carey et al, 1996); from entrainment from the local environment (Glaze et al, 1997; Dartevelle et al, 2002). Whilst much of the water condenses out of the plume, enough may reach the stratosphere to significantly affect climate

Results

Discussion

Conclusion