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Recent seasonal asymmetric changes in the NAO (a marked summer decline and increased winter variability) and associated changes in the AO and Greenland Blocking Index

Journal Article · · International Journal of Climatology
DOI:https://doi.org/10.1002/joc.4157· OSTI ID:1565268
 [1];  [1];  [2];  [3];  [3]
  1. Univ. of Sheffield (United Kingdom). Dept. of Geography
  2. Univ. of East Anglia. Norwich (United Kingdom). Climatic Research Unit; King Abdulaziz Univ., Jeddah (Saudi Arabia). Center of Excellence for Climate Change Research/Dept. of Meteorology
  3. Met Office Hadley Centre, Exeter (United Kingdom)
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Recent changes are found in the means and variability of the North Atlantic Oscillation (NAO) index. There has been a sustained significant recent decrease in the summer NAO since the 1990s and, at the same time, a striking increase in variability of the winter – especially December – NAO that resulted in three of five (two of five) record high (record low) NAO Decembers occurring during 2004–2013 in the 115-year record. These NAO changes are related to an increasing trend in the Greenland Blocking Index (GBI, high pressure over Greenland) in summer and a more variable GBI in December. The enhanced early winter NAO variability originates mainly at the southern node of the NAO but is also related to the more variable GBI in December. Transition seasons (spring and autumn) have remained relatively unchanged over the last 30 years. These results are corroborated using several NAO indices. The Arctic Oscillation (AO) index, although strongly correlated with the NAO, does not show the recent sustained significant summer decrease, but it does show enhanced early winter variability. These recent observed changes are not present in the current generation of global climate models, although the latest process studies do offer insight into their causes. We invoke several plausible climate forcings and feedbacks to explain the recent NAO changes.
Research Organization:
Oak Ridge National Laboratory (ORNL), Oak Ridge, TN (United States). Oak Ridge Leadership Computing Facility (OLCF); Univ. of East Anglia. Norwich (United Kingdom)
Sponsoring Organization:
USDOE Office of Science (SC)
DOE Contract Number:
SC0005689
OSTI ID:
1565268
Journal Information:
International Journal of Climatology, Journal Name: International Journal of Climatology Journal Issue: 9 Vol. 35; ISSN 0899-8418
Publisher:
Royal Meteorological Society
Country of Publication:
United States
Language:
English

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