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 is a strong mode of variability only after 1800, a persistent negative phase occurring during medieval times (993–1300) which is consistent with La Niña 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. The SST declined along the United States west coast and substantial changes in the populations of salmon, anchovy and sardine were observed as the PDO changed back to a cool "anchovy" phase. However the spatial pattern of the SST change was different with a meridional SST seesaw in the central and western Pacific that resembled 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. The 2014 flip from the cool PDO phase to the warm phase, which vaguely resembles a long and drawn-out El Niño event, contributed to record-breaking surface temperatures across the planet in 2014. Predictability The NOAA Earth System Research Laboratory produces official ENSO forecasts, and Experimental statistical forecasts using a linear inverse modeling (LIM) method to predict the PDO, LIM assumes that the PDO can be separated into a linear deterministic component and a non-linear component represented by random fluctuations. Much of the LIM PDO