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Variability of Climate (Decadal Variability - Oceans: Pacific) -- Summary
Many are the types of environmental phenomena that vary on decadal timescales.  For some examples from the terrestrial world, see our Summary of Variability of Climate (Decadal Oscillations - Europe).  We here review what has been learned about a few such phenomena that occur in and over the Pacific Ocean.

Andrus et al. (2002) analyzed the δ18O histories of several large otoliths - aragonite (CaCO3) structures in fish that are used for acoustic perception and balance - obtained from catfish living in near-shore habitats at two sites off the coast of Peru (650'S and 810'S) that were caught during and after the strong 1997-98 ENSO event.  They then analyzed the δ18O histories of several otoliths from two archaeological sites -- Ostra (6010 90 yr BP) at 855'S, and Siches (6450 110 yr BP) at 440'S -- after which they compared the results from the different sites and time periods.  The first of these exercises yielded a 10-year-long δ18O history, which in addition to providing a link to recent sea surface temperatures, clearly revealed the occurrence of both the 1991-92 and 1997-98 El Nios.  The older δ18O histories suggested that (1) sea surface temperatures some 6000 years ago were 3 to 4C warmer than what they were over the decade of the 1990s, and that (2) sea surface temperatures of that earlier time period exhibited better-defined seasonal signals but provided little evidence of any El Nio activity.  These results suggest that higher temperatures are typically associated with less decadal variability of the type that is generated by ENSO events.

McPhaden and Zhang (2002) analyzed hydrographic data between 20S and 50N latitude for the period 1950-99 in the depth range of 50-400 meters in the tropical and subtropical Pacific Ocean together with empirical wind data in addressing the question of "whether the meridional overturning circulation [MOC] in the upper Pacific [which was only recognized in the 1990s] may be changing on decadal time-scales."  They found that the MOC has indeed "been slowing down since the 1970s, causing a decrease in upwelling of about 25% in an equatorial strip between 9N and 9S."  They further note that "this reduction in equatorial upwelling of relatively cool water ... is associated with a rise in equatorial sea surface temperatures of about 0.8C."  In addition, they indicate that the onset of the change occurred at about the same time as the pronounced shift in the Pacific Decadal Oscillation, which occurred in 1976-77.

The ultimate significance of these observations remains to be determined.  As things stand currently, McPhaden and Zhang note that the oceanic circulatory slowdown "can account for the recent anomalous surface warming in the tropical Pacific," which means it can also account for a significant portion of the slight warming of the globe over the period studied.  On the other hand, they say that the slowdown may possibly "have been influenced by global warming," while on yet another hand they acknowledge that natural variability may have played a major role, and that "the observed decadal changes may simply be the low-frequency residual of random or chaotic fluctuations in tropical ocean-atmosphere interactions that give rise to the ENSO cycle."

Much speculation thus abounds with respect to this subject; and more research will clearly be needed to sort things out.  In addition, there are questions related to the impact of the new findings on earth's carbon cycle, as mediated by changes in the outgassing of CO2 from the equatorial Pacific Ocean and as influenced by changes in nutrient supply that affect phytoplanktonic productivity there.  All in all, therefore, things of a global change nature just got a little more complex for everyone, including climate modelers, who now have an important new phenomenon to replicate, for as McPhaden and Zhang describe it, their results provide "an important dynamical constraint for model studies that attempt to simulate recent observed decadal changes in the Pacific basin."

Chu and Clark (1999) analyzed the frequency and intensity of tropical cyclones that either originated in or entered the central North Pacific (0-70N, 140-180W) over the 32-year period 1966-1997.  They found that tropical cyclone activity (tropical depressions, tropical storms, and hurricanes combined) experienced an increase of about 3.2 cyclones over the period of their study, and that this increase appears to be due to a step-change in the record such that there are "fewer cyclones during the first half of the record (1966-81) and more during the second half of the record (1982-1997)."  They conclude, however, that the observed increase in tropical cyclone activity cannot be due to global warming, because "global warming is a gradual process" and "it cannot explain why there is a steplike change in the tropical cyclone incidences in the early 1980s," which brings us to the subject of regime shifts.

Taking a long view of the topic, Gedalof and Smith (2001) compiled a transect of six tree ring-width chronologies from stands of mountain hemlock growing near the treeline that extends from southern Oregon to the Kenai Peninsula of Alaska for the period 1599 to 1983, analyzing the data in such a way as to "directly relate changes in radial growth to annual variations in the North Pacific ocean-atmosphere system."  They determined that "much of the pre-instrumental record in the Pacific Northwest region of North America is characterized by alternating regimes of relatively warmer and cooler sea surface temperature in the North Pacific, punctuated by abrupt shifts in the mean background state," which were found to be "relatively common occurrences."  The two scientists concluded, for example, that "regime shifts in the North Pacific have occurred 11 times since 1650" and that "another regime-scale shift in the North Pacific is almost certainly imminent."

One of the significant consequences of these findings is that the abrupt 1976-77 shift in this Pacific Decadal Oscillation, as it is generally called, is what is responsible for the vast majority of the past half-century's warming in Alaska, which climate alarmists wrongly hype as evidence of gradual CO2-induced global warming.  Take away what occurred in that single year, and Alaska is no different from the rest of the world, with most of its temperature stations showing either no subsequent warming or an actual cooling trend.

Chavez et al. (2003) reviewed "physical and biological fluctuations with periods of about 50 years that are particularly prominent in the Pacific Ocean," including air and ocean temperatures, atmospheric CO2 concentration, landings of anchovies and sardines, and the productivity of coastal and open ocean ecosystems.  They found that "sardine and anchovy fluctuations are associated with large-scale changes in ocean temperatures: for 25 years, the Pacific is warmer than average (the warm, sardine regime) and then switches to cooler than average for the next 25 years (the cool, anchovy regime)."  They also report that "instrumental data provide evidence for two full cycles: cool phases from about 1900 to 1925 and 1950 to 1975 and warm phases from about 1925 to 1950 and 1975 to the mid-1990s."  These warm and cool regimes, which they respectively call El Viejo (the old man) and La Vieja (the old woman), are manifest in myriad similar-scale biological fluctuations that may, according to Chavez et al., be even better indicators of climate change than actual climate data.

The findings of this important study have many ramifications.  The one we highlight here is the challenge they present for the detection of the CO2-induced global warming that is predicted by state-of-the-art climate models.  In the words of Chavez et al., data used in climate change projections are "strongly influenced by multidecadal variability of the sort described here, creating an interpretive problem."  Hence, they conclude that "these large-scale, naturally occurring variations must be taken into account when considering human-induced climate change."

In this regard, we note that the warming of the late 1970s to late 1990s, which returned much of the world to the level of warmth experienced during the 1930s and 1940s, may well be about to end.  In fact, Chavez et al. cite considerable evidence that indicates that a change from El Viejo to La Vieja conditions may already be in progress.  If this is indeed true, we could well see global temperatures begin to drop in the very near future.

Investigating this latter possibility, Freeland et al. (2003) analyzed water temperature and salinity measurements that were made at a number of depths over a period of several years along two lines emanating from central Oregon and Vancouver Island westward into the Pacific Ocean.  Their work revealed that subsurface waters in an approximate 100-meter-thick layer located between 30 and 150 meters depth off central Oregon were, in their words, "unexpectedly cool in July 2002."  Specifically, mid-depth temperatures over the outer continental shelf and upper slope were more than 0.5C colder than the historical summer average calculated by Smith et al. (2001) for the period 1961-2000, which they say "might be cooler than a longer-term mean because the 1961-71 decade coincided with a cool phase of the Pacific Decadal Oscillation (Mantua et al., 1997)."  At the most offshore station, in fact, the four researchers report that "the upper halocline is >1C colder than normal and about 0.5C colder than any prior observation."  In addition to being substantially cooler, the anomalous water was also considerably fresher; and the combined effects of these two phenomena made the water less spicy, as Freeland et al. describe it, so much so, in fact, that they refer to the intensity of the "spiciness anomaly" as "remarkable."

Along the line that runs from the mouth of Juan de Fuca Strait to Station Papa at 50N, 145W in the Gulf of Alaska, similar low spiciness was observed, and they say there is little doubt it was the same feature as that detected off the coast of central Oregon.  In this case, the four scientists report that "conditions in June 2002 [were] well outside the bounds of all previous experience [our italics]," and that "in summer 2001 the spiciness of this layer was already at the lower bound of previous experience."

Freeland et al. say their data imply that "the waters off Vancouver Island and Oregon in July 2002 were displaced about 500 km south of their normal summer position."  Is this observation an indication the Pacific Ocean is beginning to experience a shift from what Chavez et al. call a warm, sardine regime to a cool, anchovy regime?  It is tempting to suggest that it is.  However, Freeland et al. caution against jumping to that conclusion too quickly, saying there are no obvious signals of such a regime shift in several standard climate indices and that without evidence of a large-scale climate perturbation, the spiciness anomaly may simply be, well, anomalous.  Hence, although the pattern of Pacific Ocean regime shifts documented by Chavez et al. suggests that a change from warmer to cooler conditions is imminent, there is not yet sufficient climatic evidence to claim that it is indeed in process of occurring.

On the other hand, in reference to the 1976-77 regime shift in the Pacific, Chavez et al. note that "it took well over a decade to determine that a regime shift had occurred in the mid-1970s" and, hence, that "a regime or climate shift may even be best determined by monitoring marine organisms rather than climate," as suggested by Hare and Mantua (2000).  Enlarging on this concept, they cite several recent studies that appear to provide such evidence, including "a dramatic increase in ocean chlorophyll off California," which would appear to be a logical response to what Freeland et al. describe as "an invasion of nutrient-rich Subarctic waters."

Other pertinent evidence cited by Chavez et al. includes "dramatic increases in baitfish (including northern anchovy) and salmon abundance off Oregon and Washington," as well as "increases in zooplankton abundance and changes in community structure from California to Oregon and British Columbia, with dramatic increases in northern or cooler species [our italics]."

One final paper that comes to bear upon this issue is that of Bratcher and Giese (2002), who explored "the recent record of Southern Hemisphere subsurface temperature anomalies and whether they may be an indicator of future global surface air temperature trends."  With respect to this question, they found that "low frequency changes of tropical Pacific temperature lead global surface air temperature changes by about 4 years," and that "anomalies of tropical Pacific surface temperature are in turn preceded by subsurface temperature anomalies in the southern tropical Pacific by approximately 7 years."  In addition, they document a distinct cooling of the southern tropical Pacific over the last 8 years, leading them to conclude that "the warming trend in global surface air temperature observed since the late 1970s may soon weaken."  Indeed, they report that "conditions present in the southern tropical Pacific resemble those prior to the 1976 climate shift [after which the temperature of the region rose by a full 1C], except with the opposite sign [our italics]," stating that "a climate shift to pre-1976 conditions could lessen the warming trend that has existed since 1976."  Although they say their results "do not preclude the possibility that anthropogenic sources of greenhouse gases have contributed to global warming," they feel they "indicate that the human forced portion of global warming may be less than previously described."

Clearly, something dramatic appears to be in the works; and it could well be a return to cooler conditions in the Pacific Ocean.  Biological and climatic studies over the next few years should enlighten us considerably on this point, as well as indicate what such a decadal-scale regime shift would portend for the global warming debate.

Andrus, C.F.T., Crowe, D.E., Sandweiss, D.H., Reitz, E.J. and Romanek, C.S.  2002.  Otolith δ18O record of mid-Holocene sea surface temperatures in Peru.  Science 295: 1508-1511.

Bratcher, A.J. and Giese, B.S.  2002.  Tropical Pacific decadal variability and global warming.  Geophysical Research Letters 29: 10.1029/2002GL015191.

Chavez, F.P., Ryan, J., Lluch-Cota, S.E. and Niquen C., M.  2003.  From anchovies to sardines and back: multidecadal change in the Pacific Ocean.  Science 299: 217-221.

Chu, P.-S. and Clark, J.D.  1999.  Decadal variations of tropical cyclone activity over the central North Pacific.  Bulletin of the American Meteorological Society 80: 1875-1881.

Freeland, H.J., Gatien, G., Huyer, A. and Smith, R.L.  2003.  Cold halocline in the northern California Current: An invasion of subarctic water.  Geophysical Research Letters 30: 10.1029/2002GL016663.

Gedalof, Z. and Smith, D.J.  2001.  Interdecadal climate variability and regime-scale shifts in Pacific North America.  Geophysical Research Letters 28: 1515-1518.

Hare, S.R. and Mantua, N.J.  2000.  Empirical evidence for North Pacific regime shifts in 1977 and 1989.  Progress in Oceanography 47: 103-145.

Mantua, N.J., Hare, S.R., Zhang, Y., Wallace, J.M. and Francis, R.C.  1997.  A Pacific interdecadal climate oscillation with impacts on salmon production.  Bulletin of the American Meteorological Society 78: 1069-1079.

McPhaden, M.J. and Zhang, D.  2002.  Slowdown of the meridional overturning circulation in the upper Pacific Ocean.  Nature 415: 603-608.

Smith, R.L., Huyer, A. and Fleischbein, J.  2001.  The coastal ocean off Oregon from 1961 to 2000: Is there evidence of climate change or only of Los Nios?  Progress in Oceanography 49: 63-93.

Last updated 4 May 2005