Climate response to volcanic eruptions

Major volcanic eruptions are an important natural cause of climate variations. Understanding how much of the observed climate variability at continental and seasonal scale is a response to natural variations, as opposed to anthropogenic contributions or internal variability is a fundamental challenge and a crucial test for climate models attempting to predict future climate. Analysis of observational data indicate that over the Northern Hemispheric (NH) land regions radiative cooling is dominant only in the summer half-year, whereas during boreal winters atmosphere dynamical effects prevail producing anomalously warm conditions. Volcanic aerosols injected into the lower stratosphere by explosive eruptions produce a substantial perturbation of the Earth's radiative balance, causing stratospheric warming and tropospheric cooling at the same time. Strongly enhanced reflection of incoming solar radiation causes global annual cooling at the surface for typically 2 to 3 years (Fig. 1).

Figure 1: Schematic diagram of the impact of quiescent and explosive volcanism on the Earth’s radiative balance (Fischer et al. 2006). Redrawn after Robock (2000).

We present new evidence on the seasonal European temperature and precipitation response as well as the circulation anomalies related to 15 major tropical volcanic eruptions. All eruptions are selected on the basis of the historical records and ice-core data (Table 1). Independent seasonally resolved reconstructed land surface temperature (period 1500--2000), precipitation (1769--2000) and geopotential height (1766--2000) fields are analysed to determine the mean response to eruptions. The length of the reconstruction allows us to include a larger set of eruptions than previous studies, thereby minimising the risk of other forcings and internal variability obscuring the volcanic signal. Furthermore, to our knowledge, this is the first detailed analysis of the volcanic influence on high-resolved mid-latitudinal precipitation patterns. The fact that the geopotential height fields were reconstructed independently from temperature allows us to rule out any circular statement in the comparison of the winter temperature and circulation anomalies.

During the first and second post-eruption years we find significant continental scale summer cooling (Fig. 2) and somewhat drier conditions over Central Europe. In winter, the volcanic signal induces a positive North Atlantic Oscillation (NAO) associated with a significant overall warming (Fig. 2b) and wetter conditions over Northern Europe. Our results compare well with GCM studies and observational studies covering the instrumental period (Fischer et al. 2007).

Figure 2: Anomaly composite of European summer temperature
(°C) in year 1 following 15 tropical volcanic eruptions over the past
half millennium (Fischer et al. 2007).

Figure 3: Anomaly composite of European winter temperature
(°C) in year 1 following 15 tropical volcanic eruptions over the past
half millennium (Fischer et al. 2007).

References:
Fischer, E.M., J. Luterbacher, E. Zorita, S.F.B. Tett, C. Casty, and H. Wanner, 2007: European climate response to tropical volcanic eruptions over the last half millennium, Geophys. Res. Lett., VOL. 34, L05707, doi:10.1029/2006GL027992, 2007. PDF

Fischer E.M., 2006: Climate response to tropical eruptions, PAGES Newsletter, Vol. 13, Nr. 3, Ed.: Valerie Masson-Delmotte, Jürg Beer, Howard Cattle and Christoph Kull. PDF