About Pacific Northwest Climate

Comparing ENSO and PDO

Distinctions Between PDO and ENSO

The PDO is distinct from ENSO in three ways:

• Location. The strongest signature of the PDO is in the North Pacific, instead of the tropical Pacific.
  
  
• Duration. PDO phases last much longer – typically 20 to 30 years for a single warm or cool phase – than ENSO events – 6 to 18 months for a single warm (El Niño) or cold (La Niña) phase. This conclusion is based on 20th century observations and has been confirmed to a significant degree by historic analysis of Pacific Northwest (PNW) tree rings (Gedalof et al. 2002) and geoduck shells (Strom 2003).
  
  
• Cause and Predictability. The scientific community is reasonably agreed on the factors contributing to ENSO events, making it possible to provide skilled forecasts of ENSO events several seasons in advance of the event’s onset (for a sample of forecast centers, see Seasonal to Interannual Forecasts). The causes of the PDO, on the other hand, are not understood.

  Part of the difficulty in understanding what triggers PDO phase shifts is the persistence of PDO events. Accurate instrumental records for the north Pacific begin around 1900; because of the persistence of the PDO phases, we have seen only two complete PDO cycles in that time, making it difficult to determine the cause for – and therefore the predictability of – the PDO.

  Even in the absence of a theoretical understanding, PDO climate information improves season-to-season and year-to-year climate forecasts for North America because of its strong tendency for multi-season and multi-year persistence. Simply assuming persistence of observed PDO-related North Pacific SST anomalies in the fall in any given year provides some skill in predicting PDO-related winter climate anomalies in the PNW region. However, this persistence based forecast will always fail to predict the relatively infrequent switches from one PDO phase to another.

Table of PDO-ENSO Years

Table 1 categorizes past winters according to ENSO and PDO phases since 1900. The table assumes the PDO pattern is truly regime-like, with continuous epochs switching phase in 1925 (from cool to warm phase), 1947 (from warm to cool phase), and 1977 (from cool to warm). It is believed that the PDO shifted back to cool phase in mid-1998, but a recent (mid-2002) shift back to warm phase PDO makes it difficult to determine at this time if the 1998 shift was a true shift to a cold phase.

Years are categorized as El Niño or La Niña when temperatures in the Niño-3.4 region of the tropical Pacific average one-half standard deviation, or approximately 0.5°C, above (El Niño) or below (La Niña) mean temperature for three consecutive months. Years that do not exceed the one-half standard deviation threshold are considered ENSO Neutral. The base period for calculating departures from the mean is 1971-2000.

The ENSO and PDO states assigned to any given year span the fall of the prior year and the winter of the listed year (e.g., "2004" refers to fall [Oct-Dec] 2003 and winter [Jan-March] 2004). The potential for precipitation and temperature extremes is higher when ENSO and PDO are in the same phase.

Table 1 ENSO/PDO phases since 1900.

		Negative PDO: 1900-1924, 1947-1976, 1999-2002	Warm phase PDO: 1925-1946, 1977-1998, 2003-2005*
ENSO Phase	La Niña (cool)	1904, 1907, 1909, 1910, 1911, 1917, 1918, 1921, 1923, 1950, 1951, 1955, 1956, 1963, 1965, 1968, 1971, 1972, 1974, 1975, 1976, 1999, 2000, 2001	1925, 1932, 1934, 1938, 1939, 1943, 1944, 1945, 1984, 1985, 1986, 1989, 1996
	ENSO Neutral	1901, 1902, 1908, 1913, 1916, 1922, 1947, 1948, 1949, 1953, 1954, 1957, 1960, 1961, 1962, 1967, 2002	1927, 1928, 1929, 1933, 1935, 1936, 1937, 1946, 1979, 1981, 1982, 1990, 1991, 1993, 1994, 1997, 2004
	El Niño (warm)	1900, 1903, 1905, 1906, 1912, 1914, 1915, 1919, 1920, 1924, 1952, 1958, 1959, 1964, 1966, 1969, 1970, 1973	1926, 1930, 1931, 1940, 1941, 1942, 1977, 1978, 1980, 1983, 1987, 1988, 1992, 1995, 1998, 2003, 2005

* It is believed that the PDO shifted to cool phase in mid-1998, but a recent (mid-2002) shift back to warm phase PDO makes it difficult to determine at this time if the 1998 shift was a true shift to a cold phase.