Pacific Decadal Oscillation - Reconstructions and Regime Shifts

Reconstructions and Regime Shifts

The PDO index has been reconstructed using tree rings and other hydrologically sensitive proxies from west North America and Asia.

MacDonald and Case reconstructed the PDO back to 993 using tree rings from California and Alberta. The index shows a 50-70 year periodicity but this is a strong mode of variability only after 1800, a persistent negative phase occurred during medieval times (993-1300) which is consistent with la nina conditions reconstructed in the tropical Pacific and multi-century droughts in the South-West United States.

Several regime shifts are apparent both in the reconstructions and instrumental data, during the 20th century regime shifts associated with concurrent changes in SST, SLP, land precipitation and ocean cloud cover occurred in 1924/1925,1945/1946 and 1976/1977:

• 1750: PDO displays an unusually strong oscillation.
• 1924/1925: PDO changed to a "warm" phase.
• 1945/1946: The PDO changed to a "cool" phase, the pattern of this regime shift is similar to the 1970s episode with maximum amplitude in the subarctic and subtropical front but with a greater signature near the Japan while the 1970s shift was stronger near the American west coast.
• 1976/1977: PDO changed to a "warm" phase.
• 1988/1989:A weakening of the Aleutian low with associated SST changes was observed, in contrast to others regime shifts this change appears to be related to concurrent extratropical oscillation in the North Pacific and North Atlantic rather than tropical processes.
• 1997/1998: Several changes in Sea surface temperature and marine ecosystem occurred in the North Pacific after 1997/1998, in contrast to prevailing anomalies observed after the 1970s shift SST declined along the United States west coast and substantial changes in the populations of salmon, anchovy and sardine were observed, however the spatial pattern of the SST change was different with a meridional SST seesaw in the central and western Pacific that resemble a strong shift in the North Pacific Gyre Oscillation rather than the PDO structure, this pattern dominated much of the North Pacific SST variability after 1989.