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