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Storm Severity and Frequency in North-Eastern New Zealand
Reference
Page, M.J., Trustrum, N.A., Orpin, A.R., Carter, L., Gomez, B., Cochran, U.A., Mildenhall, D.C., Rogers, K.M., Brackley, H.L., Palmer, A.S. and Northcote, L. 2010. Storm frequency and magnitude in response to Holocene climate variability, Lake Tutira, North-Eastern New Zealand. Marine Geology 270: 30-44.

Background
The authors say "there is growing evidence that climate during the Holocene has been highly variable, with broad global or hemispheric change, upon which are superimposed marked regional variability," noting that "this is certainly the case for mid-latitude Southern Ocean areas such as New Zealand, where climate responds to atmospheric and oceanic forcing from polar and sub-tropical regions." However, they report there are few such real-world records of sub-annual events, such as storms.

What was done
Working with sediment cores extracted from Lake Tutira on the eastern North Island of New Zealand, Page et al. developed a 7200-year history of the frequency and magnitude of storm activity, based on analyses of (1) sediment grain size, (2) diatom, pollen and spore types and concentrations, plus (3) carbon and nitrogen concentrations, together with (4) tephra and radiocarbon dating.

What was learned
The ten New Zealanders plus one U.S. researcher report that "the average frequency of all storm layers is one in five years," but that "for storm layers >= 1.0 cm thick, the average frequency is every 53 years." And in this regard, they say that over the course of their record, "there are 25 periods with an increased frequency of large storms," the onset and cessation of which stormy periods "was usually abrupt, occurring on an inter-annual to decadal scale." They also note that the duration of these stormy periods "ranged mainly from several decades to a century," but that "a few were up to several centuries long," while "intervals between stormy periods range from about thirty years to a century." In addition, they find that millennial-scale cooling periods tend to "coincide with periods of increased storminess in the Tutira record, while warmer events match less stormy periods."

What it means
Page et al. write that in today's world there is growing concern -- driven by climate models -- that there may be abrupt changes in various short-term meteorological phenomena caused by global warming, "when either rapid or gradual forces on components of the earth system exceed a threshold or tipping point." However, as is demonstrated by the results of their work in the real world, the sudden occurrence of a string of years -- or even decades -- of unusually large storms is something that can happen at almost any time on its own, or at least without the necessity of being driven by human activities such as the burning of fossil fuels.

Reviewed 14 July 2010