The first fisheries researcher to suggest that environmental changes were a controlling factor in multidecade fluctuations of fish stocks was Ljungman (1880). In his paper "Contribution Toward Solving the Question of the Secular Periodicity of the Great Herring Fisheries," Ljungman presented the known fluctuations of the Bohuslan herring fisheries from the 19th century back to the 9th century and suggested that there was about a 55-year periodicity in the abundance of herring. He further suggested that the fishery fluctuations were caused by "periodical and secular changes in the direction and intensity of the currents of the sea, by which a change in the occurrence of herring food and the consequent migrations of the large schools of herring could be explained."
There was little advancement in multidecadal-scale fisheries analyses for nearly a century; then, a number of researchers on sardines and anchovies from several major current systems came to a consensus that not only were major stocks of sardines and anchovies fluctuating on a multidecadal or "regime" time scale, but there were obvious similarities in the timing of regimes in widely separated current systems (Kawasaki and Omori 1988, Lluch-Belda et al. 1989, Lluch-Belda et al. 1992). More recent work has shown that other stocks were affected by the now well-known 1976 regime shift in the northeastern Pacific (Beamish 1993, Beamish and Bouillon 1993).
In an attempt to determine oceanic features associated with the observed biological regimes, I have used the Climate and Eastern Ocean Systems (CEOS) version of the Comprehensive Ocean-Atmosphere Data Set (COADS) (Mendelssohn and Roy in press) to develop time series and decadal difference maps for a variety of marine surface observations in an attempt to describe recent regime shifts. The widespread changes associated with the 1976 regime shift are clearly seen in the differences in decadal sea-level atmospheric pressures, surface winds, and sea-surface temperatures in the mid-latitude North Pacific.
Sea level pressure data show that the principal difference between the pre- and post-1976 decades is an intensification and eastward expansion of the winter (December-February) Aleutian low pressure system (Fig. 1). The pressure fields give the visual impression that the major change was centered around lat. 48°-55°N. From an oceanographic point of view this is misleading, since the principal oceanographic effect of the pressure field is not associated with the position or intensity of the low but with the winds which are a function of the gradient in atmospheric pressure.
In the decade prior to 1976, the region of maximum winter winds, as seen in fields of eastward pseudostress (i.e., the means of the squares of the east component of the wind observations), was centered at about lat. 40°N and long. 160°W (Fig. 2). In contrast, in the period of 1977-86, there was a major increase in eastward pseudostress and the maxima were displaced well to the southeast. Thus, the major change was not in the vicinity of the Aleutian low pressure system but on the subtropical side of the West Wind Drift region.
The third major aspect of the regime shift is the change in sea surface temperature (SST). In winter, the eastern North Pacific was between 0.5° and 1.5°C warmer in the post-1976 decade than in the pre-1976 decade (Fig. 3). The Aleutian Islands and southern Bering Sea also had warmer winter SST in the recent decade. In contrast, the majority of the central North Pacific (i.e., lat. 20°-45°N and long. 150°E-150°W) was colder in the post-1976 decade. The maximum difference occurred in the Oyashio-Kuroshio mixing area, just west of Northern Honshu where winter SST was between 1.5° and 2°C lower during 1977-86 than during 1966-75. The winter pattern of colder SST suggests that much of the colder, subtropical side of the North Pacific Current is moved southwards in the region to the east of the dateline. In summer, the SST decadal difference pattern is somewhat different than in winter, with the colder water region displaced to the east with an extension of colder water almost to the coast off of Oregon and Washington. There is also an intensification and expansion of the warmer SST region in the Alaska Stream region. The summer temperature difference in the Oyashio-Kuroshio mixing area is considerably less than in winter.
While the sea level pressure and SST changes are relatively easy to visualize, the physical effects of the wind changes are much more complex. Wind has a number of physical effects on the sea, and these effects are different functions of the wind. For example, turbulent mixing is a function of the cube of the wind speed and is independent of wind direction, whereas Ekman transport is a function of the square of the wind vector. Divergence and convergence patterns are a function of the gradient of the wind, and geostrophic transport is a function of the wind curl or the divergence of Ekman transport. Thus, changes in the wind field alter a wide range of factors, including sea surface temperature, the depth of the upper mixed layer, upwelling, frontal formation, wind-driven currents and wind stress curl-driven currents. Polovina et al. (1995) have shown that there was a major decadal shift in winter and spring mixed layer depths and mixed layer temperatures in association with the 1976 regime shift. In particular, there was a sharp increase in mixed layer depth in a region centered at about lat. 35°N and extending from about long. 155°E to about 155°W. The geographical pattern of decadal differences in wind speed cubed, an index of turbulent mixing, is very similar to this reported increase in mixed layer depth. The region of increased mixed layer temperatures described by Polovina et al. (1995) is very similar to the area of increased SST from the COADS data (Fig. 3).
Although there is a distinct difference in the environmental data fields from the two decades, time series show that the regime shift occurred earlier in the northwestern Pacific than in the northeastern Pacific. Winter sea surface temperatures from the Oyashio-Kuroshio mixing region show that the shift occurred in this region in 1971, whereas the shift is generally considered to have occurred in 1976-77 in the northeastern Pacific.
The principal force altering the oceanographic conditions associated with the 1976 regime shift in the northeastern Pacific appears to be the great increase in winter eastward wind stress which was centered in the central and western North Pacific at about lat. 32-38°N (Fig. 2). This alteration in the winter winds resulted in a significant change in the surface wind-stress curl, which is the forcing function for the vertically integrated mass transport of the North Pacific Current. Decadal averages of mid-North Pacific winter, eastward pseudostress (averaged by 2° of latitude and 20° of longitude) suggest that the wind curl-driven flow of the North Pacific Current was intensified and centered at a lower latitude in the post-1976 decade (Fig. 4). In the pre-1976 decade, winter eastward pseudostress increased between lat. 57°N and 47°N, with the region between 43°N and 35°N having values near 40 (m/sec)2, and then decreased between 33°N and 21°N. In the post-1976 decade, eastward stress increased to a maximum of 85 (m/sec)2 at 35°N and decreased rapidly between 33°N and 21°N. In contrast, there was little difference in the divergence-convergence pattern during spring in the two decades (Fig. 5). The increase and equatorward expansion of the winter divergence pattern in the post-1976 decade would result in a lower latitude, more subtropical source for water entering the California and Alaska Currents; this, in turn, represents the probable major cause of the anomalously warm SST which occurred in both regions.
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Beamish, R. J., and D. R. Bouillon. 1993. Pacific salmon production trends in relation to climate. Can. J. Fish. Aquat. Sci. 50:1002-16.
Kawasaki,T., and M. Omori. 1988. Fluctuations in the three major sardine stocks in the Pacific and the global trend in temperature. In T. Wyatt and M. G. Larreñeta (editors), Long-term changes marine fish populations, p. 37-53. Instituto de Investigaciones Marinas de Vigo, Vigo, Spain.
Ljungman, A. 1880. Contribution toward solving the question of the secular periodicity of the great herring fisheries. Rep. U.S. Fish. Comm. 1879:497-503.
Lluch-Belda, D., R. J. M. Crawford, T. Kawasaki, A. D. MacCall, R. H. Parrish, R. A. Schwartzlose, and P. E. Smith. 1989. World-wide fluctuations of sardine and anchovy stocks: The regime problem. S. Afr. J. Mar. Sci. 8:195-205.
Lluch-Belda, D., R. A. Schwartzlose, R. Serra, R. Parrish, T. Kawasaki, D. Hedgecock, and R. J. M. Crawford. 1992. Sardine and anchovy regime fluctuations of abundance in four regions of the world oceans: A workshop report. Fish. Oceanogr. 1(4):339-347.
Mendelssohn, R., and C. Roy. In press. CODE (Comprehensive Ocean Data Extraction) Users Guide. U.S. Dep. Commerc., NOAA-TM-NMFS-SWFSC-228.
Polovina, J. J., G. T. Mitchum, and G. T. Evans. 1995. Decadal and basin-scale variation in mixed layer depth and the impact on biological production in the Central and North Pacific, 1960-88. Deep-Sea Res. 41(10):1701-1716.