Volcanic - PowerPoint Presentation

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Volcanic Impacts on the Atlantic Multidecadal Oscillation and Initiation of the Little Ice Age Alan Robock and Joanna Slawinska (robock@envsci.rutgers.edu), Dept. Environmental Sciences, Rutgers University, New Brunswick, New Jersey USARobert A. Tomas, National Center for Atmospheric Research, Boulder, CO; Dimitrios Giannakis, Courant Institute of Mathematical Sciences, New York University

Introduction

Ice cover in coupled climate system

High correlation of north hemispheric ice area with poleward oceanic heat

transfer

indicates on strong dependence of Arctic ice on Atlantic circulation of the ocean.

Centennial perturbation of Arctic ice

Multidecadal perturbation of climate and their response to volcanic eruptions

Lagged correlation of ice area and poleward heat transfer at 66 N (right) captures their coupling that is associated with internal oscillation of 20 year and slower timescale.

Arctic ice response to fluctuations of poleward heat transfer happens predominantly through its coupling with North Atlantic SST (AMO) that response with 5-10 years lag (left)

to AMOC.

AMOC response to volcanic eruptions does not exhibit pronounced centennial signal.

The strongest response is found among others (right) for volcanic eruptions usually associated with the beginning of the Little Ice Age (1274, 1456, and 1600).On average (left) (ensemble - mean)eruptions preferentially lead to / prolong negative phase of AMOC.

Pronounced centennial fluctuations of Arctic ice occur in simulations with all the forcings, e.g. over 1600-1750 period.The magnitude cannot be explained solely by volcanic forcing.

It is well known that large volcanic eruptions produce stratospheric sulfate aerosol clouds with a lifetime of 1-2 years, and that these clouds scatter some of the incoming sunlight back to space, cooling Earth (Robock, 2000). The aerosol layer also warms the stratosphere through absorption of infrared radiation. This results in a change to Earth’s energy balance. In addition, there are atmospheric and oceanic dynamical responses to large eruptions, producing characteristic regional and seasonal patterns of climate response. It seems that a series of very large eruptions at the end of the 13th Century, starting with the 1257 Samalas eruption (

Lavigne et al. 2013), reduced North Atlantic oceanic heat flux into the Arctic so much that a feedback perpetuated this cool climate for centuries, starting the Little Ice Age (Zhong et al. 2011, Miller et al. 2012). The “Little Ice Age” refers to the 1350-1850 CE worldwide cooling of Earth’s climate. While in the Zhong et al. (2011)and Miller et al. (2012) climate model simulations, only some cases resulted in these large volcanic eruptions producing a Little Ice Age, Lehner et al. (2013) have identified and clarified the mechanism by which this response should happen. Here we take advantage of a new set of climate model simulations, the Last Millennium Ensemble (LME, Otto-Bliesner et al. 2016), to investigate that response, and its dependence on the forcing. The LME conducted a series of 1000-year simulations with the National Center for Atmospheric Research Community Earth System Model version 1.1 (Hurrell et al. 2013), forced by changing insolation, volcanic eruptions, land surface, greenhouse gases, and aerosols, in combination and separately. Were volcanic eruptions alone enough to produce a Little Ice Age? If not, why? Or was a combination of volcanic and solar forcing necessary? Large eruptions can also produce decadal scale shifts in the North Atlantic circulation, with impacts during the next decade (Otterå et al. 2010, Booth et al. 2012, Zanchettin et al. 2012, 2013a). These oceanic changes are commonly studied as perturbations to the Atlantic Meridional Overturning Circulation (AMOC, Buckley and Marshall 2016), whose associated time series is sometimes measured statistically as the Atlantic Multidecadal Oscillation (AMO). Swingedouw et al. (2015) showed that the response to volcanic eruptions depends on the phase of the AMO at the time of the eruption. Here we also examine the AMOC response to large volcanic eruptions in the LME and how this might be linked to the long-term impacts on Arctic sea ice.

Acknowledgments. This work is supported by National Science Foundation grant AGS-1430051.References

NH sea ice

area

(x 106 km2)

FIG. 12. Northern hemisphere ice area anomaly (1-year average) as a function of time, t=0 corresponds

to 1600 eruption. Color dotted line - individual members, solid black line - ensemble mean for simulations with all the forcings (top), volcanic forcing (middle), and solar forcing (bottom).

Conclusions

The NCAR CESM model produces a Little Ice Age with all

forcings

, but produces a much smaller Little Ice Age with only volcanic forcing.

The coincidence of reduced solar insolation at the same time as large volcanic eruptions was crucial to producing the Little Ice Age.

While volcanic eruptions produce a negative phase of the AMOC on decadal time scales, this does not produce the centennial scale Little Ice Age response.