The physical environment of the northeastern Pacific fluctuates substantially on a number of seasonal and longer scales, some periodic and predictable, and some not obviously so. El Niño Southern Oscillation (ENSO) events and other perturbations produce profound anomalies in the atmosphere and ocean on interannual to decadal and century time scales. This area also can be separated into discrete spatial regions, dominated by different physical processes that are presumably collocated with unique biological structures. The timing and intensity of large-scale climate events may not be coherent between regions. The boundaries defining these regions may change over time as well. Our objective is to describe decadal-scale differences in the spatial texture of the northeastern Pacific, to provide a base from which to evaluate the effect of climate change on the area's ecosystems, and particularly on its salmonid populations and fisheries.
The data analyzed in this paper are summarized from the Comprehensive Ocean-Atmosphere Data Set (COADS). Monthly time series for 1946-90 were constructed from COADS sea surface temperature (SST) and wind stress data for 2° latitude boxes along the U.S. West Coast. A nonlinear trend was estimated for the monthly series using a state-space model (Schwing and Mendelssohn in press). The 2° averaged data from the entire North Pacific Ocean also are meaned seasonally for two decade-long periods (1966-75 and 1977-86) to compare seasonal differences before and after a significant "climate shift" in 1976 (Trenberth 1990). This analysis places the coastal time series in a basin-scale context.
Based on the monthly time series, the U.S. West Coast can be divided into three distinct geographical regions. The northern region (north of 40°N) features a rapid transition from strongly equatorward to poleward wind stress with distance north (Fig. 1). The mean stress north of 44°N is poleward and has become increasingly poleward over time. The SSTs north of 40°N vary temporally in unison and exhibit little spatial difference in magnitude (Fig. 2).
Winds south of 40°N are equatorward and can be described in terms of central and southern regions (Fig. 1). The central region (32-40°N) exhibits the greatest coastal wind stress magnitudes. This region features the greatest scales of interannual-to-decadal variation in stress (Fig. 1) and SST (Fig. 2) as well. Stress in the southern region (22-32°N) has become increasingly equatorward over time, like the central region, but with relatively little interannual variation (Fig. 1). Mean SST (Fig. 2) decreases consistently with increasing latitude in the central and southern regions. The SST over about 30-38°N appears to warm rapidly in response to the 1957 and 1983 ENSO events as well as the 1976 regime shift (Trenberth 1990).
The SSTs off Washington and Oregon, on the other hand, take several months to years following the 1976 shift to warm by similar amounts, and they are less sensitive to ENSO events. Although SST series exhibit a warming tendency south of 36°N, the COADS SSTs show a cooling tendency north of 36°N (Fig. 2). However, shore-based SST trends along the entire U.S. West Coast display a significant warming tendency over the past several decades. A lack of correspondence between the COADS and shore SST time series north of 36° N suggests there is considerable cross-shelf as well as latitudinal variability in the northeastern Pacific. This is confirmed by analysis of the decadal differences before and after 1976 for the North Pacific, as well as examination of COADS data in the northeastern Pacific on 1° space scales (Schwing et al. in press).
The SST shows decadal-scale periods of warming and cooling that extend along the entire coast. Wind stress anomalies are less coherent latitudinally and are uncorrelated with local SST, suggesting that decadal-scale SST variability in the coastal northeastern Pacific is controlled by the basin- to global-scale meteorological fields, rather than local wind forcing. The decadal comparison of data over the entire basin substantiates this conclusion.
Figure 3 shows the linear trends of the poleward stress and SST time series. The correlation between poleward stress and SST is positive in much of the central region; increasing equatorward stress coincides with a cooling trend. Outside of the central region, however, the trends in stress and SST are negatively correlated. Increasing equatorward stress coincides with warming south of 34°N, while greater poleward stress accompanies a cooling trend north of 44°N.
It is apparent that over the last several decades, surface coastal waters in the northern portion of the California Current have either experienced a different set of forcing conditions from those farther south, or have responded differently to large-scale atmospheric forcing. Not only are the tendencies of wind and SST different in these regions, but the different linear relationships between stress and SST imply that the primary mechanisms driving variability in SST, and probably the general ocean circulation, on decadal time scales is fundamentally different in these regions.
Preliminary analysis of decadal changes beginning in 1976 shows a strong correlation between anomalously high wind divergence, hence greater upwelling, and cool SST in winter over the central North Pacific. Anomalously weak divergence and warm SSTs are seen in a broad stretch along the North American west coast. Cool anomalies propagate eastward until they extend along the West Wind Drift to within a few degrees of the coast off Oregon and Washington. Thus, it appears that wintertime anomalies in wind forcing over the central Pacific, rather than changes in coastal wind, lead to the observed regional SST pattern in the coastal northeastern Pacific. A complex interaction of spatially as well as temporally varying Ekman transport, wind mixing, and direct heating appears to be responsible for the long-term fluctuations in SST.
The temporal and spatial variability of the physical environment of the northeastern Pacific must be considered when analyzing changes in the biological structure of its ecosystems. Roemmich and McGowan (1995) have found that zooplankton biomass off southern California has decreased by 80% since the 1950s. Over a similar period, Brodeur and Ware (1992) detected a doubling in zooplankton biomass in the Gulf of Alaska. The results reported here show that regional differences in environmental variability have existed in the northeastern Pacific over the last five decades; therefore, it is conceivable that analogous differences in biological productivity, coupled with environmental variability, have occurred as well.
The results presented here clearly demonstrate the highly variable nature of the northeastern Pacific environment in time and space, and they argue against assuming climate change is constant, or that it can be represented by a record from a single location. The distinct latitudinal regionalization and cross-shelf variability of wind and SST fields has key implications for ecosystem studies as well as fisheries management. For example, which time series or regions are more important to study in terms of defining a salmon stock's ocean environment? Regional differences also mean that, over their life history, salmon face a spatially heterogeneous, changing climate. Widespread stocks also may display a very different long-term variability from species whose domain is limited to one of the homogeneous regions described here. Fisheries scientists must evaluate the relative environmental differences in each region as they pertain to the climate signal and its variability, and compare them to a species' distribution and behavior as a function of its life stage in order to fully understand the consequences of climate change on populations.
The long-term trends described here cannot be described without considering changes in seasonal patterns. Schwing and Mendelssohn (in press) demonstrated, with state-space models, that long-term changes in the seasonal cycle of environmental time series can be independent from interannual-to-interdecadal fluctuations. The results presented here demonstrate the importance of evaluating the entire spectrum of temporal and spatial variability, rather than simply at global climate scales, when examining long-term environmental fluctuations. This will improve our understanding of the linkages between long-term variations in atmospheric forcing and the coastal ocean's response to this variability on regional scales, and ultimately improve our assessment of how climate variability impacts marine ecosystems and living marine resources.
Brodeur, R. D., and D. M. Ware. 1992. Long-term variability in zooplankton biomass in the subarctic Pacific Ocean. Fish. Oceanogr. 1:32-38.
Roemmich, D., and J. McGowan. 1995. Climatic warming and the decline of zooplankton in the California Current. Science 267:1324-1326.
Schwing, F. B., and R. Mendelssohn. In press. Increased coastal upwelling in the California Current System. J. Geophys. Res.
Schwing, F. B., R. Mendelssohn, and R. H. Parrish. In press. Recent trends in the spatial variability of the SST and wind fields of the California Current System. Committee on Earth Observations Satellites (CEOS) Workshop Proceedings.
Trenberth, K. E. 1990. Recent observed interdecadal climate changes in the Northern Hemisphere. Bull. Am. Meteorol. Soc. 71:988-993.