Sep 8, 2014

Recent seasonal asymmetric changes in the NAO and associated changes in the AO and Greenland Blocking Index

Hanna, Edward, Cropper, Thomas E., Jones, Philip D., Scaife, Adam A., Allan, Rob
International Journal of Climatology
  • Invokes several plausible climate forcings and feedbacks to explain recent changes in the means and variability of the North Atlantic Oscillation (NAO) index
  • States 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
  • States 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
  • States 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
  • States transition seasons (spring and autumn) have remained relatively unchanged over the last 30 years
  • Corroborates findings using several NAO indices
  • States 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