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Tambora and the strange loss of Arctic Ice


knocker
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  • Location: Camborne
  • Location: Camborne

    Being in introspective mode today I got to musing awhile about an interesting puzzle that baffled many for years. That was why, in the summers of 1816 and 1817-while the inhabitants of temperate zones from China to New England shivered and starved-, the Arctic Circle basked in relative warmth and shed its ice at an amazing, unprecedented rates?

     

    Answers to why this should be so may be found in studies of the 1991 eruption of Mount Pinatubo in the Philippines. The Pinatubo eruption has served scientists well as a model from which environmental impacts of the nearby Tambora event might be extrapolated.

     

    A notable consequence of Pinatubo's eruption, and the global cooling it produced, was the "substantial decrease" in rainfall overland for a year following the eruption and a subsequent "record decrease" in runoff to the oceans. The cause was the chilled, volcanic atmosphere, which repressed evaporation and reduced the amount of water vapor in the air. Put in its broadest terms, reduced solar radiation in Pinatubo's aftermath altered the flow of energy through the coupled ocean-atmosphere system, with significant implications for the global hydrological cycle. Accordingly, the first post-Pinatubo year, 1992, witnessed the largest recorded percentage of the global landmass suffering drought conditions. A recent computer simulation of the influence of volcanic activity on global climate since 1600 produced the same "general precipitation decrease" in the high latitudes of the northern hemisphere, especially pronounced over land.

     

    In the case of Tambora, a volcanic event six times the magnitude of Pinatubo, hydrological disruption at the hemispheric scale must have been nothing short of catastrophic. In 1816 and 1817, with extreme drought conditions prevailing across the high North American landmass, the Atlantic Ocean received only a fraction of its standard allotment of warm freshwater discharge from rivers and streams. As a result, surface waters in the North Atlantic became colder and saltier, sinking with greater force. The subsequent destabilization of the water column in turn enhanced the motive energy of the Atlantic thermohaline circulation.

    Convective currents released increased quantities of heat into the Arctic Circle, melting the ice cap, while a bulked-up southward current delivered great volumes of Greenland glacial ice into the Atlantic. The increased surface temperatures likewise inhibited the formation of new ice in the subpolar region, hence the magical-seeming open seas visible from the mastheads of British whalers off the coast of Greenland in 1816 and 1817.

     

    Volcanic enhancement of the Atlantic Meridional Overturning Circulation (AMOC) is produced not only by reduced air temperature, which suppresses evaporation. Wind also plays its part in draining water vapour and energizing ocean circulation. Gales swept across the North Atlantic with unusual force and frequency in the Tambora period. A major tropical eruption enhances the normal gradations in temperature between the equator and the poles. Differentiated temperatures in turn influence the density and pressure gradients that power winds, strengthening wafts into breezes and stiff breezes into gales. For the North Pole region, this meant an amplified, positive phase of the Artie Oscillation, its major circulatory weather system. These stronger-than-usual winds further cooled the sea-surface temperatures of the North Atlantic, adding to the positive inputs at work on the AMOC. In short, environmental change in the Arctic in the Tambora period was driven mostly by changes in oceanic currents and winds, which overrode the general atmospheric cooling of the planet. And because wind and ocean currents behave nonlinearly in response to atmospheric change, the Arctic ice pack was vulnerable in turn to extreme transformations.

     

    References

     

    Gillen D’Arcy Wood, Tambora:the eruption that changed the world, Princetown University Press

     

    Effects of Mount Pinatubo volcanic eruption on the hydrological cycle

    as an analog of geoengineering

     

    Kevin E. Trenberth and Aiguo Dai

     

    http://www.cgd.ucar.edu/cas/adai/papers/TrenberthDai_GRL07.pdf

     

    Volcanic signals in oceans

     

    Georgiy Stenchikov, Thomas L. Delworth, V. Ramaswamy, Ronald J. Stouffer, Andrew Wittenberg, and Fanrong Zeng

     

    http://www.gfdl.noaa.gov/~atw/yr/2009/stenchikov_jgra_2009.pdf

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    Edited by knocker
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    Posted
  • Location: Yorkshire Puddin' aka Kirkham, Lancashire, England, United Kingdom
  • Weather Preferences: cold winters, cold springs, cold summers and cold autumns
  • Location: Yorkshire Puddin' aka Kirkham, Lancashire, England, United Kingdom

    So the "cold mid latitudes-warm Arctic" pattern usually associated with the new Arctic Dipole patterns isn't a new thing after all, though this would have been an extreme and rare example in the Little Ice Age.

    Edited by Craig Evans
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