Second largest volcanic eruption of the 20th century
The volcanic eruption on Mount Pinatubo on the 15th of June 1991 was the second largest eruption of the 20th century. It had a large, but relatively short-lived, impact on the climate system. Such large and explosive eruptions typically lead to a global cooling of the surface due to the increased reflection of solar radiation from stratospheric aerosols. In addition, a warming is observed in the lower stratosphere near the Equator mostly due to increased absorption of solar near-infrared and outgoing longwave radiation. Furthermore, observations and model simulations have shown that large tropical volcanic eruptions, like Mount Pinatubo, typically is followed by a positive phase of the Arctic Oscillation in the subsequent winters in the northern hemisphere.
Important test for climate models
Numerical models are currently being used to give scenarios on future climate change. The observed changes after volcanic eruptions like Mount Pinatubo give us an unique opportunity to test how realistic state-of-the-art climate models respond to global-scale radiative forcings. One example of such a test is presented in the article, where the effect of the changed radiative forcing after the Mount Pinatubo volcanic eruption have been examined in ARPEGE, the atmospheric component in Bergen climate model.
The results show that the model is able to reproduce many of the observed features after the eruption. A global-averaged cooling in excess of 0.3 degrees the following summer agrees reasonably well with observations. Furthermore, the model simulates a warming over the landmasses in the northern hemisphere the following winter that is in qualitative agreement with the observations. In the model this feature is due to the fact that the model simulates a positive phase of the Arctic Oscillation in the first few winters after the eruption. This leads to increased flow of warm air to Europe and Eurasia where anomalously high winter surface air temperatures are simulated.
The response of the Arctic Oscillation is most plausibly explained by a dynamical coupling between the large-scale tropospheric circulation and the state of the stratospheric vortex