HR: 0830h
AN: GC31B-0197    [PDF]
TI: The Global Climate Anomaly in 1940-1942
AU: * Br\"onnimann, S
EM: stefanb@lpl.arizona.edu
AF: Lunar and Planetary Laboratory, Univ. Arizona, P.O. Box 210092, Tucson, AZ 85721 United States
AU: Luterbacher, J
EM: juerg@giub.unibe.ch
AF: NCCR Climate, Inst. Geography, Univ. Bern, Hallerstr. 12, Bern, CH-3012 Switzerland
AU: Staehelin, J
EM: staehelin@iac.umnw.ethz.ch
AF: IAC ETH Z\"urich, H\"onggerberg HPP, Z\"urich, CH-8093 Switzerland
AU: Svendby, T M
EM: t.m.svendby@fys.uio.no
AF: Dept. Physics, Univ. Oslo, P.O. Box 1048, Blindern, Oslo, N-0316 Norway
AB: An unprecedented climatic anomaly occurred in the tropics and in the Northern Hemisphere in 1940-1942. During a strong and prolonged El Ni\~{n}o [Bigg \& Inou\'{e}, {\it QJRMS} 118 (1992), 125], extremely cold winters were observed in Europe, accompanied by very warm temperatures in Alaska and large parts of the Arctic and a cold North Pacific. The anomalies were strong (comprising the two coldest European winters of the 20th century) and extraordinarily persistent. In addition, exceptionally high values of total ozone are reported [Langlo, {\it Geofys. Publ.} 18/6 (1952)], pointing to an anomalous stratospheric circulation. Events of this magnitude have a strong economical and environmental impact; the 1940s anomaly even affected World War II. Studying this anomaly in detail contributes to (1) document the extent of 20th century climate variability, (2) understand large-scale coupling processes between the tropics and the extratropics and between the troposphere and the stratosphere and (3) develop tools to analyze past upper-level climate variability prior to 1948, i.e., the reanalysis period. For this study we have compiled, digitized, and re-evaluated several tens of thousands of temperature and pressure profiles from aircraft and radiosonde ascents up to 50 hPa [Br\"{o}nnimann, {\it Int. J. Clim.} 23 (2003), 769]. The upper-air data were supplemented with data from the Earth's surface and used to statistically reconstruct monthly upper-level fields for the extratropical Northern Hemisphere up to 100 hPa [Br\"{o}nnimann \& Luterbacher, {\it Clim. Dyn.}, submitted]. Although the quality of the reconstructed stratospheric fields is not comparable to more recent data, it is sufficient to allow a broad characterization of the circulation at 100 hPa during the early 1940s. In addition to upper-air data, several total ozone series from the 1940s were re-evaluated [Br\"{o}nnimann {\it et al., QJRMS} 129 (2003), 2819], providing further information on the stratosphere. In this paper we present an analysis of these new data sets and compare the results to climate model data. It is demonstrated that the climate anomaly at the ground was accompanied in the lower stratosphere by a weak polar vortex and warm temperatures over the polar region, Eurasia, and the North Pacific. The total ozone data show a peak in 1940-1942 in all available records, at sites as far apart as China, North America, central Europe, and the Arctic. The co-occurrence of warm tropical SSTs (due to El Ni\~{n}o), a weak polar vortex and warm lower stratosphere over polar regions, and a total ozone increase is in agreement with findings by van Loon and Labitzke [{\it Mon. Wea. Rev.} 115 (1987), 357]. Using the 290-yr control run of the Community Climate System Model CCSM-2.0 provided by UCAR we show that such large-scale coupling events are related to an exceptionally large difference between tropical and northern-extratropical SSTs such as during strong El Ni\~{n}os. The coupling most likely proceeds through a change in planetary wave activity in the northern extratropics that manifests itself in a strong Aleutian low and a weak Icelandic low and in a disturbance of the polar vortex in the stratosphere. The 1940-1942 climate anomaly is not well known among scientists, but it is unprecedented in strength, yet exemplary in character, providing a unique opportunity to study large-scale climate variability.
DE: 0341 Middle atmosphere--constituent transport and chemistry (3334)
DE: 1694 Instruments and techniques
DE: 3309 Climatology (1620)
SC: Global Climate Change [GC]
MN: 2003 Fall Meeting