Data

Monthly values of the Pacific Decadal Oscillation Index (PDOI), January 1900 - present, are available from the Joint Institute for the Study of Atmosphere and Oceans. For the Bering Sea, three intervals are used:

Winter = Dec Jan Feb avg, where Dec is from the year before
Summer = Jun Jul Aug avg
Annual = Jan thru Dec avg

Reference

Mantua, N.J. and S.R. Hare, Y. Zhang, J.M. Wallace, and R.C. Francis,1997: A Pacific interdecadal climate oscillation with impacts on salmon production. Bulletin of the American Meteorological Society, 78, pp. 1069-1079. (available via the internet at url: http://www.atmos.washington.edu/~mantua/abst.PDO.html)

Relevance to Ecosystem

The Pacific Decadal Oscillation (PDO) is a long-lived El Niño-like pattern of Pacific climate variability. The PDO is strongly correlated with the North Pacific Index through air-sea interactions in the North Pacific (see the correlation table for the Aleutian Low-related indices). The average life span for a PDO regime is about 20-30 years. In its warm phase, characterized by the correlation map in Fig.1-R, sea surface temperature (SST) anomalies are negative from the coast of Japan eastward with the strongest signal in the east-central North Pacific, and positive along the North American coast, extending to the south-eastern Bering Sea. For the cold PDO phase the distribution of SST anomalies is the opposite.

Fig. 1-R. Correlation between annual values of the PDO index and SSTs.

The winter correlation map (Fig. 2-R) shows the opposition is SST anomalies between the eastern and western parts of the Bering Sea, while in summer (Fig. 3-R) the PDO effect on the south-eastern Bering Sea appears to be the strongest.

Fig. 2-R. Correlation between mean winter (DJF) values of the PDO index and SSTs.

Fig. 3-R. Correlation between mean summer (DJF) values of the PDO index and SSTs.

The PDO regime shift in 1977 from a cold to warm phase had far reaching consequences for the Bering Sea ecosystem (Hare and Mantua 2000; Benson and Trites 2002). As an example, this shift resulted in the major expansion for the recruitment, standing stock and fishery for pollock and Bristol Bay sockeye salmon. The regime shift of 1989 was not as persuasive as the 1977 shift, and was expressed mainly in the winter PDO index. A notable feature of the 1989 shift is the relative clarity that was found in the biological records (Hare and Mantua 2000).

References

Benson, A.J., and A.W. Trites, 2002: Ecological effects of regime shifts in the Bering Sea and eastern North Pacific Ocean. Fish and Fisheries, 3, 95-113.

Hare S.R., and N.J. Mantua, 2000: Empirical evidence for North Pacific regime shifts in 1977 and 1989. Progr. Oceanog., 47, 103-146.

Additional Links

Nate Mantua's website
NASA/JPL PDO page
Steven Hare's post-1977 regime shift page

Recent Trends

On the multi-decadal time scale, there were two major regime shifts in the winter PDO index that occurred in 1946 and 1977 (Fig. 1-T a). There appears to be no sign of reversal of the current positive PDO regime. Within this multi-decadal regime, however, there is quite prominent variability on the decadal time scale (Fig. 1-T b). Thus, during the period 1989-2002 the mean value of the winter PDO index was slightly below zero. The shift of 1989 is statistically significant at the 0.008 level. Starting with the winter of 2003 (El Niño year), the PDO index increased again.

The pattern of temporal variability of the summer (Fig. 1-T c and d) and annual (Fig. 1-T e and f) PDO indices is different from that of the winter PDO index. The negative index values in the late 1980s and early 1990s were not large enough to qualify for a regime shift. Instead, a regime shift was detected in the late 1990s. This is clearly seen for the summer PDO anomalies calculated as deviations from the mean value of the positive PDO regime of 1979-1997 (Fig. 2-T). Despite the summer PDO index was positive in 2003-2005, both its values were below the 1979-1997 average, and the regime shift in 1998 still remains statistically significant at the 0.02 level.

a)	b)	Winter (DJF)
c)	d)	Summer (JJA)
e)	f)	Annual

Fig. 2-T. Anomalies of the summer (JJA) PDO index, 1979-2005, calculated as deviations from the mean value of the positive PDO regime of 1979-1997.

References