Arctic oscillation or North Atlantic oscillation?

The definition and interpretation of the Arctic oscillation (AO) are examin ed and compared with those of the North Atlantic oscillation (NAO). It is s hown that the NAO reflects the correlations between the surface pressure va riability at its centers of action, whereas this is not the case for the AO . The NAO pattern can be identified in a physically consistent way in princ ipal component analysis applied to various fields in the Euro-Atlantic regi on. A similar identification is found in the Pacific region for the Pacific -North American (PNA) pattern, but no such identification is found here for the AO. The AO does reflect the tendency for the zonal winds at 35 degrees and 55 degreesN to anticorrelate in both the Atlantic and Pacific regions associated with the NAO and PNA. Because climatological features in the two ocean basins are at different latitudes, the zonally symmetric nature of t he AO does not mean that it represents a simple modulation of the circumpol ar flow. An increase in the AO or NAO implies strong, separated tropospheri c jets in the Atlantic but a weakened Pacific jet. The PNA has strong relat ed variability in the Pacific jet exit, but elsewhere the zonal wind is sim ilar to that related to the NAO. The NAO-related zonal winds link strongly through to the stratosphere in the Atlantic sector. The PNA-related winds d o so in the Pacific, but to a lesser extent. The results suggest that the N AO paradigm may be more physically relevant and robust for Northern Hemisph ere variability than is the AO paradigm. However, this does not disqualify many of the physical mechanisms associated with annular modes for explainin g the existence of the NAO.