Robust Wind and Precipitation Responses to the Mount Pinatubo Eruption, as Simulated in the CMIP5 Models

Abstract

The volcanic eruption of Mount Pinatubo in June 1991 is the largest terrestrial eruption since the beginning of the satellite era. Here, the monthly evolution of atmospheric temperature, zonal winds, and precipitation following the eruption in 14 CMIP5 models is analyzed and strong and robust stratospheric and tropospheric circulation responses are demonstrated in both hemispheres, with tropospheric anomalies maximizing in November 1991. The simulated Southern Hemisphere circulation response projects strongly onto the positive phase of the southern annular mode (SAM), while the Northern Hemisphere exhibits robust North Atlantic and North Pacific responses that differ significantly from that of the typical northern annular mode (NAM) pattern. In contrast, observations show a negative SAM following the eruption, and internal variability must be considered along with forced responses. Indeed, evidence is presented that the observed El Niño climate state during and after this eruption may oppose the eruption-forced positive SAM response, based on the El Niño-Southern Oscillation (ENSO) state and SAM response across the models. The results demonstrate that Pinatubo-like eruptions should be expected to force circulation anomalies across the globe and highlight that great care must be taken in diagnosing the forced response as it may not fall into typical seasonal averages or be guaranteed to project onto typical climate modes.

Modeling the 1783–1784 Laki Eruption in Iceland: 1. Aerosol Evolution and Global Stratospheric Circulation Impacts

“…This response is qualitatively similar to the expected response to a large tropical volcanic eruption (e.g. Barnes et al, , Karpechko et al, ). Specifically, we find, like Karpechko et al () found for simulations of the low‐latitude El Chichón and Pinatubo eruptions, that the SH polar vortex is strengthened, with zonal wind anomalies above 8 m/s in austral spring, when planetary waves have the most impact on the stratospheric circulation there.…”

“…Perhaps somewhat surprisingly, the most robust circulation changes occur in the SH, with a response that is similar to the simulated stratospheric response to the 1991 Pinatubo eruption (e.g., Barnes et al, ). As the aerosols make their way to the equator and farther south, significant heating anomalies occur in the tropical stratosphere of both hemispheres.…”

“…In Figures a and b, the spatial patterns of the SAM and NAM, respectively, are shown. A positive tropospheric annular mode index is traditionally defined such that the jet stream is strengthened and shifted poleward relative to climatology; a positive value in the stratosphere denotes strengthened zonal winds relative to climatology (e.g.Baldwin & Dunkerton, , Barnes et al, ). Figures c and d show the Laki ensemble mean SAM and NAM time series, respectively.…”

“…The NAO is an index of the wintertime variability of north‐south NH sea level pressure gradients between 110°W and 70°E and north of 20°N (Hurrell, ), and a positive phase of the NAO has been associated with an anomalously strong polar vortex (Thompson & Wallace, ) or a positive phase of the Northern Annular Mode (NAM). While observations and—to varying degrees—models have shown a tendency toward a strengthened polar vortex (Barnes et al, ; Bittner, Schmidt, et al, ; Bittner, Timmreck, et al, ; Zambri et al, ; Zambri & Robock, ) and a positive NAO (Christiansen, ) in the first winter after large tropical volcanic eruptions, the circulation response to high‐latitude eruptions is less well understood.…”

The evolution and volcanic forcing of the southern annular mode during the past 300 years

“…The heating difference between low and high latitudes produces an enhanced pole‐to‐equator temperature gradient (Robock, ). The strengthening of the temperature gradient produces a stratospheric zonal wind anomaly and strengthens the polar vortex in both the NH and SH (Barnes et al, ). The positive zonal winds on the polarwards flanks of the jet streams and the negative zonal winds on the equatorwards flanks are projected onto the hemispheric mode through their propagation from the stratosphere into the troposphere.…”