Arctic Oscillation response to volcanic eruptions in the IPCC AR4 climate models

Abstract

[1] Stratospheric sulfate aerosol particles from strong volcanic eruptions produce significant transient cooling of the troposphere and warming of the lower stratosphere. The radiative impact of volcanic aerosols also produces a response that generally includes an anomalously positive phase of the Arctic Oscillation (AO) that is most pronounced in the boreal winter. The main atmospheric thermal and dynamical effects of eruptions typical of the past century persist for about two years after each eruption. In this paper we evaluate the volcanic responses in simulations produced by seven of the climate models included in the model intercomparison conducted as part of the preparation of the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4). We consider global effects as well as the regional circulation effects in the extratropical Northern Hemisphere focusing on the AO responses forced by volcanic eruptions. Specifically we analyze results from the IPCC historical runs that simulate the evolution of the circulation over the last part of the 19th century and the entire 20th century using a realistic time series of atmospheric composition (greenhouse gases and aerosols). In particular, composite anomalies over the two boreal winters following each of the nine largest low-latitude eruptions during the period 1860-1999 are computed for various tropospheric and stratospheric fields. These are compared when possible with observational data. The seven IPCC models we analyzed use similar assumptions about the amount of volcanic aerosols formed in the lower stratosphere following the volcanic eruptions that have occurred since 1860. All models produce tropospheric cooling and stratospheric warming as in observations. However, they display a considerable range of dynamic responses to volcanic aerosols. Nevertheless, some general conclusions can be drawn. The IPCC models tend to simulate a positive phase of the Arctic Oscillation in response to volcanic forcing similar to that typically observed. However, the associated dynamic perturbations and winter surface warming over Northern Europe and Asia in the post-volcano winters is much weaker in the models than in observations. The AR4 models also underestimate the variability and long-term trend of the AO. This deficiency affects high-latitude model predictions and may have a similar origin. This analysis allows us to better evaluate volcanic impacts in up-to-date climate models and to better quantify the model Arctic Oscillation sensitivity to external forcing. This potentially could lead to improving model climate predictions in the extratropical latitudes of the Northern Hemisphere.

The Role of Volcanic Activity in Climate and Global Change

“…This basic zonal-mean pattern is modulated by a very strong regional structure with an intensified high-pressure anomaly over the North Atlantic and Mediterranean sectors called North Atlantic Oscillation (NAO). Consistent with this are poleward shifts in the Atlantic storm track and an increased flow of warm air to Northern Europe and Asia, where anomalously high winter surface temperatures are observed [7,8,40]. It seems that only low-latitude volcanic eruptions could affect the AO/NAO phase and the AO/NAO remain fairly insensitive to the high-latitude NH eruptions [90].…”

“…Amman et al [35], however, used a fixed effective radius of 0.42 mm for calculating aerosol optical properties and, in general, provided higher values of optical depth than in Hansen [38]. Stenchikov et al [40] used UARS observations to modified effective radii from Hansen [38], implementing its variations with altitude, especially at the top of the aerosol layer where particles became very small. They conducted Mie calculations for the entire period since 1850 and implemented these aerosol characteristics in the new Geophysical Fluid Dynamics Laboratory (GFDL) climate model.…”

“…The mechanisms that govern the climate response to volcanic impacts very likely play an important role in global climate change [4,40,91]. The northern polar circulation modes (NAO/AO) experienced significant climate variations during the recent two decades and also are sensitive to volcanic forcing.…”

“…The aerosol optical characteristics were calculated following Stenchikov et al [2] using optical depth from Sato et al [39] and Hansen [38]. The aerosol size distribution was assumed lognormal with fixed width of 1.8 mm [40]. p0110…”

“…CLAES and ISAMS stopped working after 20 months. The SAGE III instrument aboard the Russian Meteor III-3M satellite continued the outstanding SAGE aerosol data record [33,34] Available satellite and ground-based observations were used to construct volcanic aerosol spatiotemporal distribution and optical properties [2,[35][36][37][38][39][40]. Hansen [38] improved a Goddard Institute for Space Studies (GISS) volcanic aerosols data set for 1850-1999, providing zonal mean vertically resolved aerosol optical depth for visible wavelength and column average effective radii.…”