The NAO, the AO, and Global Warming: How Closely Related?
Journal of Climate
The North Atlantic Oscillation (NAO) and closely related Arctic Oscillation (AO) strongly affect Northern Hemisphere (NH) surface temperatures with patterns reported similar to the global warming trend. The NAO and AO have been in a positive trend for much of the 1970s and 1980s with historic highs in the early 1990s, and it has been suggested that they contribute significantly to the global warming signal. The trends in standard indices of the AO, NAO, and NH average surface temperature for
... temperature for Dec-Feb, 1950, and the associated patterns in surface temperature anomalies are examined. Also analyzed are factors previously identified as relating to the NAO, AO, and their positive trend: North Atlantic sea surface temperatures (SSTs), Indo-Pacific warm pool SSTs, stratospheric circulation, and Eurasian snow cover. Recently, the NAO and AO indices have been decreasing; when this data is included, the overall trends for the past thirty years are weak to non-existent, and strongly dependent on the choice of start and end date. In clear distinction, the wintertime hemispheric warming trend has been vigorous and consistent throughout the entire period. When considered for the whole hemisphere, the NAO/AO patterns can also be distinguished from the trend pattern. Thus the Dec-Feb warming trend may be distinguished from the AO and NAO in terms of the strength, consistency, and pattern of the trend. These results are insensitive to choice of index or dataset. While the NAO and AO may contribute to hemispheric and regional warming for multi-year periods, these differences suggest that the large scale features of the global warming trend over the last 30 years is unrelated to the AO and NAO. The related factors may also be clearly distinguished, with warm pool SSTs linked to the warming trend, while the others are linked to the NAO and AO.