Ingo Kirchner

Radiative forcing from the 1991 Mount Pinatubo volcanic eruption

Volcanic sulfate aerosols in the stratosphere produce significant long-term solar and infrared radiative perturbations in the Earths atmosphere and at the surface, which cause a response of the climate system. Here we study the fundamental process of the development of this volcanic radiative forcing, focusing on the eruption of Mount Pinatubo in the Philippines on June 15, 1991. We develop a spectral-, space-, and time-dependent set of aerosol parameters for 2 years after the Pinatubo eruption using a combination of SAGE II aerosol extinctions and UARS-retrieved effective radii, supported by SAM II, AVHRR, lidar and balloon observations. Using these data, we calculate the aerosol radiative forcing with the ECHAM4 general circulation model (GCM) for cases with climatological and observed sea surface temperature (SST), as well as with and without climate response. We find that the aerosol radiative forcing is not sensitive to the climate variations caused by SST or the atmospheric response to the aerosols, except in regions with varying dense cloudiness. The solar forcing in the near infrared contributes substantially to the total stratospheric heating. A complete formulation of radiative forcing should include not only changes of net fluxes at the tropopause but also the vertical distribution of atmospheric heating rates and the change of downward thermal and net solar radiative fluxes at the surface. These forcing and aerosol data are available for GCM experiments with any spatial and spectral resolution.

Climate model simulation of winter warming and summer cooling following the 1991 Mount Pinatubo volcanic eruption

We simulate climate change for the 2-year period following the eruption of Mount Pinatubo in the Philippines on June 15, 1991, with the ECHAM4 general circulation model (GCM). The model was forced by realistic aerosol spatial-time distributions and spectral radiative characteristics calculated using Stratospheric Aerosol and Gas Experiment II extinctions and Upper Atmosphere Research Satellite-retrieved effective radii. We calculate statistical ensembles of GCM simulations with and without volcanic aerosols for 2 years after the eruption for three different sea surface temperatures (SSTs): climatological SST, El Nino-type SST of 1991–1993, and La Nina-type SST of 1984–1986. We performed detailed comparisons of calculated fields with observations. We analyzed the atmospheric response to Pinatubo radiative forcing and the ability of the GCM to reproduce it with different SSTs. The temperature of the tropical lower stratosphere increased by 4 K because of aerosol absorption of terrestrial longwave and solar near-infrared radiation. The heating is larger than observed, but that is because in this simulation we did not account for quasi-biennial oscillation (QBO) cooling and the cooling effects of volcanically induced ozone depletion. We estimated that both QBO and ozone depletion decrease the stratospheric temperature by about 2 K. The remaining 2 K stratospheric warming is in good agreement with observations. By comparing the runs with the Pinatubo aerosol forcing with those with no aerosols, we find that the model calculates a general cooling of the global troposphere, but with a clear winter warming pattern of surface air temperature over Northern Hemisphere continents. This pattern is consistent with the observed temperature patterns. The stratospheric heating and tropospheric summer cooling are directly caused by aerosol radiative effects, but the winter warming is indirect, produced by dynamical responses to the enhanced stratospheric latitudinal temperature gradient. The aerosol radiative forcing, stratospheric thermal response, and summer tropospheric cooling do not depend significantly on SST. The stratosphere-troposphere dynamic interactions and tropospheric climate response in winter are sensitive to SST.

Pinatubo eruption winter climate effects: model versus observations

Large volcanic eruptions, in addition to the well-known effect of producing global cooling for a year or two, have been observed to produce shorterterm responses in the climate system involving non-linear dynamical processes. In this study, we use the ECHAM2 general circulation model forced with stratospheric aerosols to test some of these ideas. Run in a perpetual-January mode, with tropical stratospheric heating from the volcanic aerosols typical of the 1982 El Chichón eruption or the 1991 Pinatubo eruption, we find a dynamical response with an increased polar night jet in the Northern Hemisphere (NH) and stronger zonal winds which extend down into the troposphere. The Azores High shifts northward with increased tropospheric westerlies at 60°N and increased easterlies at 30°N. Surface temperatures are higher both in northern Eurasia and North America, in agreement with observations for the NH winters of 1982–83 and 1991–92 as well as the winters following the other 10 largest volcanic eruptions since 1883.

Changing lower stratospheric circulation: The role of ozone and greenhouse gases

Stratospheric climate has changed significantly during the last decades. The causes of these changes are discussed on the basis of two different general circulation model experiments forced by observed greenhouse gas and ozone concentration. There is a clear and significant response of the lower stratosphere temperature and geopotential in the model simulations forced by observed ozone changes that is in accord with observed trends in summer in middle and high latitudes of the northern hemisphere. Little effect is seen in the tropics. In spring there occur the strongest anomalies/trends in both hemispheres at polar latitudes; however, the model response is late by 1 to 2 months and is much weaker than the observed effects. The ozone-forced model in winter of both hemispheres produces slight warming or no change instead of the slight cooling observed. The effects of enhanced greenhouse gases as taken from a transient IPCC scenario AGCM run do enhance the cooling in high latitudes in spring, but the effect is much smaller than observed. Hence neither of the two forcings (reduced ozone and increased greenhouse gases) in the cold seasons is able to produce the recent stratospheric and tropospheric trend patterns alone. These trends clearly resemble a natural mode of variability both in the model and in the real world. This mode associates a strengthened polar night vortex with an enhanced North Atlantic oscillation. The excitation of this mode cannot yet be attributed to anthropogenic forcing.

A one and half year interactive MA/ECHAM4 simulation of Mount Pinatubo Aerosol

The Mount Pinatubo volcanic eruption in June 1991 had significant impact on stratospheric and tropospheric climate and circulation. Enhanced radiative heating caused by the aerosol absorption of solar and terrestrial radiation changed stratospheric temperature and circulation. Using the stratospheric mesospheric version of the Hamburg climate model MA/ECHAM4, we performed an interactive Pinatubo simulation with prognostic stratospheric aerosol. Interactive and noninteractive model results for the years 1991 and 1992 are compared with satellite data and in situ measurements. The on-line calculated heating rates are in good agreement with radiation transfer models indicating maximum heating rates of about 0.3 K/d in October 1991. The dynamic feedback in the MA/ECHAM4 simulation is similar to observations. The model is able to reproduce the strengthening of the polar vortex in winter 1991/1992 and a minor warming in January. The importance of an interactive treatment of the volcanic cloud for the aerosol transport is evidenced by the analysis of effects such as aerosol lifting and meridional transport. In general, the model results agree well with observations from the northern midlatitudes, especially in the first months after the eruption. The MA/ECHAM4 model is successful in reproducing the formation of two distinct maxima in the optical depth but is unable to simulate the persistence of the tropical aerosol reservoir from the end of 1991. Better agreement may be achieved if the influence of the quasi-biennial oscillation and ozone changes is also taken into account.

Volcanos and El Niño: signal separation in Northern Hemisphere winter

The frequent coincidence of volcanic forcing with El Niño events disables the clear assignment of climate anomalies to either volcanic or El Niño forcing. In order to select the signals, a set of four different perpetual January GCM experiments was performed (control, volcano case, El Niño case and combined volcano/El Niño case) and studied with advanced statistical methods for the Northern Hemisphere winter. The results were compared with observations. The signals for the different forcings are discussed for three variables (temperature, zonal wind and geopotential height) and five levels (surface, 850 hPa, 500 hPa, 200 hPa and 50 hPa). The global El Niño signal can be selected more clearly in the troposphere than in the stratosphere. In contrast, the global volcano signal is strongest in the stratospheric temperature field. The amplitude of the perturbation for the volcano case is largest in the Atlantic region. The observed effect of local cooling due to the volcanic reduction of shortwave radiation over large land areas (like Asia) in subtropical regions, the observed advective warming over Eurasia and the advective cooling over Greenland are well simulated in the model. The radiative cooling near the surface is important for the volcano signal in the subtropics, but it is weak in high latitudes during winter. A statistically significant tropospheric signal of El Niño forcing occurs in the subtropics and in the midlatitudes of the North Pacific. The local anomalies in the El Niño forcing region in the tropics, and the warming over North America in middle and high latitudes are simulated as observed. The combined signal is different from a simple linear combination of the separate signals. It leads to a climate perturbation stronger than for forcing with El Niño or stratospheric aerosol alone and to a somewhat modified pattern.

Modelling Mt. Pinatubo Climate Effects

The climatic impact of the Mt. Pinatubo volcanic eruption of June 1991 was studied by observations and model simulation for the first winter and summer after the eruption. A low resolution (T21) atmospheric GCM was driven by additional stratospheric aerosol derived from observations. The main results as observed and simulated are: Warming of the aerosol containing stratospheric layers resulting in an enhanced polar night jet. Reduction of the incoming shortwave radiation by 2 to 4 W/m-2. Modification of the tropospheric planetary wave structure in winter. During winter the induced advection leads to positive temperature anomalies in the lower troposphere over the midlatitude continents, while in summer the local radiation balance determines the volcanic impact and leads to slight cooling.

The Influence of Zonally Asymmetric Stratospheric Ozone on the Coupling of Atmospheric Layers

The radiation perturbation due to zonal asymmetries in stratospheric ozone (\(\mathrm{O}_{3}^{*}\)) is an important factor in the atmosphere-ocean circulation system, affecting the coupling processes between the middle atmosphere, the troposphere and the ocean. We investigate the effects of observed \(\mathrm{O}_{3}^{*}\) in general circulation model simulations without and with an interactively coupled ocean (GCM, AOGCM). Observations show a pronounced wave one pattern in \(\mathrm{O}_{3}^{*}\) during northern winter, with amplitudes of 10–20 % of zonal mean values. Based on sensitivity studies with the GCM MAECHAM5, and accompanying linear solutions of forced Rossby waves, we demonstrate that the radiative forcing due to \(\mathrm{O}_{3}^{*}\) affects the stratospheric circulation by imposing a wave perturbation in geopotential height (in the order of 10–20 % of the zonal mean values) and the associated large-scale flow. Subsequent changes due to non-linear stratosphere-troposphere coupling indicate an intensification of the tropospheric Aleutian low but a weakening of the Iceland low, i.e. a shift of tropospheric circulation towards negative phase of North Atlantic Oscillation (NAO). Multi-decadal simulations with the AOGCM COSMOS show that this signal is more pronounced and spatially southward shifted due to additional interactions between the troposphere and the ocean. In particular, we identify significant changes in surface temperatures, precipitation, wind-driven ocean currents and sea ice thickness induced by the wave one pattern in \(\mathrm{O}_{3}^{*}\). The results suggest that including the effects of zonal asymmetries in stratospheric ozone and other important absorbers may help to improve current general circulation models and climate change studies.

On the Interrelationship Between Recent Climate Trends, Ozone Changes and Increased Greenhouse Gas Forcing

Recently considerable trends in ozone concentration with positive values mainly in the upper troposphere, and decreasing concentrations in the lower stratosphere of the Northern Hemisphere middle and high latitudes were shown (WMO, 1992). The data basis consists approximately of 15 to 20 years of observations at various locations.

Evaluation of the tropospheric chemistry general circulation model ECHAM5-MOZ and its application to the analysis of the chemical composition of the troposphere with an emphasis on the late RETRO period 1990-2000

The Tropospheric Chemistry General Circulation model ECHAM5-MOZ was developed between 2001 and 2005 and was used to investigate the variability and trends of ozone, CO and NOx in the second half of the 20th century in the framework of the RETRO project. The multi–decadal simulation of the period of 1960 to 2000 was one of the first of that kind. The model captures many features of the seasonal cycle and vertical gradients of trace gas concentrations measured on the ground or from balloons, aircraft or satellite. We diagnose a significant high bias in the simulated ozone concentrations in the 1990s, which can in part be attributed to an overestimated stratosphere troposphere exchange and possibly underestimated dry deposition of ozone. Wintertime CO concentrations in the northern hemisphere are underestimated by up to 30%. The observed interannual variability of the tropospheric NO2 column, surface CO concentrations and ozone is generally captured by the simulation, but the model fails to capture the surface ozone increase observed at several stations around the world during the 1980s and 1990s. The increase in the tropospheric ozone column between the 1960s and 1990s is consistent with model simulations of preindustrial conditions. The global ozone burden and chemical formation and loss are continuously rising during the entire 41-year simulation period. The dry deposition flux increases until the early 1980s and shows a more irregular behavior afterwards. Until around 1980 regionally averaged precursor emissions correlate well with surface ozone changes. Thereafter, the emission trend in Europe and North America is reversed, while ozone levels remain high. Asian emissions and ozone concentrations continue to rise, but the slope of the correlation changes.