Europeans who couldn't care less about El Nino may sit up and take notice when some-body mentions the North Atlantic Oscillation. Although it's less important on a global scale than ENSO, the NAO is a much bigger player in Europe's climate. Unlike El Nino, it's an atmospheric cycle that operates more or less independently of the ocean beneath it, although possible connections to faraway tropical oceans are emerging. The NAO is basically a see-saw in air pressure between the Azores and Greenland. When the NAO is positive – higher pressure over the Azores, lower pressure over Greenland – the west-to-east jet stream cuts more directly and strongly across the Atlantic. This tends to bring wet, mild winter weather to northern Europe and makes the Mediterranean winter even more invitingly sunny and dry. By contrast the NAO's negative phase blocks the jet stream, forcing it to arc around the North Atlantic. This can bring periods of cold, dry air across northern Europe and leave southern Europe open to renegade systems that can bring lingering dampness. Since the NAO is detached from the ocean beneath it, there can be no early-warning system like the one provided by Pacific buoys for El Nino. Moreover, the NAO may swing from positive to negative or vice versa after only a few weeks in a given mode. The NAO favoured its positive phase for much of the 1980s and 1990s, giving northern Europe a string of wet and mild winters. Several computer models project a continued preference for the positive NAO in the century to come.
More widespread displacements of the polar jet stream throughout the Northern and Southern Hemisphere are dubbed the Northern Annular Mode and Southern Annular Mode. For both the NAM and SAM a positive phase means the jet is contracted poleward in a tight ring that tends to limit incursions of polar air into midlatitudes. According to basic physics as well as computer modelling, the positive modes of the NAM and SAM should become more prevalent over time as global warming pushes the jet streams poleward. This should help feed the positive NAO trend noted above; it may also tend to push critical winter rains away from parts of southern Australia.
Then there's the Pacific Decadal Oscillation, a circulation pattern centreed south of Alaska that was named in 1996. The PDO is a measure of regions of warm and cool water across the North Pacific that typically takes 20 to 30 years to shift When the PDO is positive, as was the case from the late 1970s into the 1990s, it tends to enhance El Nino's effects and diminish La Nina's; the converse is true for a negative PDO. The most evident PDO correlations to date involve fisheries: salmon and other fish in the North Pacific tend to cluster further north, toward Alaska, during the PDO's positive or warm phase. As with the PDO, the North Atlantic may also have warm and cool cycles that switch every 20 or 30 years, alternately stoking or suppressing hurricane activity. Scientists are honing the statistical tools needed to separate cycles like these from day-to-day weather as well as from long-term warming.