Large volcanic eruptions are a natural, impulse-like perturbation to the climate system. The sulfur particles ejected into the stratosphere during these eruptions are rapidly converted to sulfate aerosols that diminish the net incoming solar flux at the top of the atmosphere resulting in a cooling of the near-surface climate. The sulfate aerosols have a long residence time (about 1-2 years in the stratosphere), but have been observed to cause surface cooling for many more years after the eruption.
In their Journal of Climate paper "The climate response to multiple volcanic eruptions mediated by ocean heat uptake: damping processes and accumulation potential," Mukund Gupta, a graduate student in MIT's Department of Earth, Atmospheric and Planetary Sciences (EAPS), working with EAPS advisor John Marshall, the Cecil and Ida Green Professor of Oceanography, investigate this further. Using a hierarchy of models -- among them MIT's general circulation model (MITgcm) as a simple slab model and as a fully coupled, spatially resolving GCM) -- they explore the role of the ocean in mediating the response of the climate to single and serial volcanic eruptions that lead to cold temperature anomalies in the ocean interior, as measured by the ocean heat exchange parameter q.
In this study, the researchers leverage the classic “double-drake” aquaplanet configuration of MITgcm to simulate the physics of an ocean-covered planet coupled to an atmosphere, absent land, sea-ice or clouds. In the “double-drake” two narrow barriers, the depth of the model ocean, set 90° apart extend from the North Pole to 35°S separate the ocean into a large and a small basin connected in a circumpolar region to the south providing geometrical constraints on the circulation. Box models are then used to help in the interpretation of the globally-averaged sea surface temperature (SST) responses of the MITgcm aquaplanet to the idealized volcanic forcings imposed.
The pair finds that the response to a single (Pinatubo-like) eruption comprised two primary timescales, one fast (year) and one slow (decadal).
Over the fast timescale, their experiments suggested that the ocean sequesters eruption-induced cooling anomalies at depth, resulting in a damping of the rate of change of SST relative to that expected if the atmosphere acted alone. They also found that while this effect compromises the ability to constrain atmospheric feedback rates (as measured by the relaxation rate λ), it yields information about the transient climate sensitivity proportional to λ + q with their results suggesting that q can significantly exceed λ in the immediate aftermath of an eruption.
Once shielded from damping to the atmosphere, the effect of the volcanic eruption can persist on longer decadal timescales.
Indeed, the researchers assess the "accumulation potential" of a succession of volcanic eruptions over time. They find that this process could, in part, explain the observed prolongation of cold surface temperatures experienced during, for example, the Little Ice Age.
Story Image: An ash plume billows from the crater atop Mount St. Helens hours after its eruption began on May 18th, 1980, in Washington state. The column of ash and gas reached 15 miles into the atmosphere, depositing ash across a dozen states. (Photo: USGS / Robert Krimmel)
EAPS graduate student Mukund Gupta is interested in understanding climate sensitivity to perturbations and in developing simple climate models to investigate this.
John Marshall is an oceanographer interested in climate and the general circulation of the atmosphere and oceans, which he studies through the development of mathematical and numerical models of key processes. His research has focused on problems of ocean circulation and coupled climate dynamics involving interactions between motions on different scales, using theory, laboratory experiments, observations and new innovative approaches to global ocean modeling pioneered by his group at MIT.