220.127.116.11 Tropical-Extratropical Teleconnections: PNA and PSA
Circulation variability over the extratropical Pacific features wave-like patterns emanating from the subtropical western Pacific, characteristic of Rossby wave propagation associated with anomalous tropical heating (Horel and Wallace, 1981; Hoskins and Karoly, 1981). These are known as the PNA and Pacific-South American (PSA) patterns and can arise naturally through atmospheric dynamics as well as in response to heating. Over the NH in winter, the PNA pattern lies across North America from the subtropical Pacific, with four centres of action (Figure 3.26). While the PNA pattern can be illustrated by taking a single point correlation, this is not so easy for the PSA pattern (not shown), as its spatial centres of action are not fixed. However, the PSA pattern can be present at all times of year, lying from Australasia over the southern Pacific and Atlantic (Mo and Higgins, 1998; Kidson, 1999; Mo, 2000).
The PNA, or a variant of it (Straus and Shukla, 2002), is associated with modulation of the Aleutian Low, the Asian jet, and the Pacific storm track, affecting precipitation in western North America and the frequency of Alaskan blocking events and associated cold air outbreaks over the western USA in winter (Compo and Sardeshmukh, 2004). The PSA is associated with modulation of the westerlies over the South Pacific, effects of which include significant rainfall variations over New Zealand, changes in the nature and frequency of blocking events across the high-latitude South Pacific, and interannual variations in antarctic sea ice across the Pacific and Atlantic sectors (Renwick and Revell, 1999; Kwok and Comiso, 2002a; Renwick, 2002). While both PNA and PSA activity have varied with decadal modulation of ENSO, no systematic changes in their behaviour have been reported.