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A New Perspective on the Warming of the North Pacific Ocean Over the Last Half of the 20th Century
Volume 8, Number 27: 6 July 2005

Over the past few years, we have reviewed a number of papers that suggest a climatic regime shift may have occurred in the North Pacific Ocean following the last great El Niņo of the 20th century, which suggestion is based on a range of observations made along the west coast of North America (Gedalof and Smith, 2001; Freeland et al., 2002; Barth, 2003; Chavez et al., 2003; Mullin et al., 2003; Peterson and Schwing, 2003).  In fact, the evidence for the regime shift is so strong that it has been speculated by many who are familiar with the phenomenon that this thermal "course correction" may have already returned much of this vast region of the planet to a state of cooler temperatures and associated ecosystem characteristics reminiscent of the pre-1976 climatic regime.

We here review the results of another study that addresses this subject (Di Lorenzo et al., 2005) but that approaches it from the opposite perspective, i.e., that of the warming that preceded the 1999 climatic course correction, and that focuses on the nature of the 1.3°C increase in eastern North Pacific sea surface temperature (SST) that occurred between 1950 and 1999 (Bograd and Lynn, 2003).  In describing this rise in temperature, Di Lorenzo et al. employ the phrase warming trend, but they say that they use it merely "to indicate the overall transition toward warmer temperature from 1950 to 1999," stating that this transition - the bulk of which occurred over 1976 to 1977 - has also (and perhaps more appropriately, to our way of thinking) "been called a regime shift.""

So what did they do?

Utilizing the 52-year-long California Cooperative Oceanic Fisheries Investigations (CalCOFI) temperature time series, Di Lorenzo et al. studied processes controlling long-term changes in temperature in the Southern California Current System (SCCS) "by computing a four-dimensional space-time analysis of the 1949-2000 CalCOFI hydrography and executing a sensitivity analysis of a primitive equation ocean model of the California coast driven by various forcings."  Also employed in this endeavor were 51 years of monthly averaged winds (1950-2000), which they obtained from the National Centers for Environmental Prediction, and net surface heat fluxes at monthly temporal resolution (1950-2000) and 1° spatial resolution, which they obtained from an updated analysis of the Comprehensive Ocean-Atmosphere Data Set.

Di Lorenzo et al.'s analyses revealed that the spatial and temporal variability of CalCOFI upper-ocean temperature is correlated with the Pacific Decadal Oscillation, which they say "is characterized by coherent and in-phase changes of SST along the entire U.S. and Canadian west coasts," indicating further that "temperature changes in the SCCS are part of a large-scale response" that "extends over the top 200 m of the water column and is consistent with large-scale studies of heat content change over the last 50 years."  In fact, they find that "changes in ocean temperature along the coast of California occur as a result of spatially coherent changes in surface heat flux [Q] forcing acting over the entire eastern North Pacific Ocean," and that "the observed warming trend between 1950 and 1999 can be largely explained by decadal variations in Q alone" and is "only weakly controlled locally at the coast by changes in upwelling.""

Commenting further on these findings, Di Lorenzo et al. state that the warming trend (which they use to represent regime shift) between 1950 and 1999 appears to be driven by decadal variations in Q, rather than a trend in Q, and they thus suggest that "the observed trend in ocean temperature is part of natural fluctuations of the climate system rather than associated with global warming."  Or as they describe the situation in another place in their paper, "over the 52-year period of CalCOFI data we cannot distinguish a significant trend in SST that might be associated with global warming."

To not be totally politically incorrect, however, Di Lorenzo et al. add that their work "does not exclude the possibility that there is also a component of warming trend associated with greenhouse gas increases."  They simply find that "the temperature variance associated with decadal changes in heat flux is strong enough to inhibit a clear detection of this global warming signal over the length of the CalCOFI record."  On the other hand, they note that "the observed transition toward cooler temperatures after 1998 is captured by a simple 1D heat flux forced model."

It is refreshing to find a group of scientists who are willing to acknowledge that the 1.3°C increase in eastern North Pacific SST that occurred between 1950 and 1999 was not the result of CO2-induced global warming but merely a natural fluctuation of earth's climate system, and who additionally acknowledge that this non-CO2-induced warming may well have subsequently reversed itself.

Sherwood, Keith and Craig Idso

References
Barth, J.A.  2003.  Anomalous southward advection during 2002 in the northern California current: Evidence from Lagrangian surface drifters.  Geophysical Research Letters 30: 10.1029/2003GL017511.

Bograd, S.J. and Lynn, R.J.  2003.  Long-term variability in the Southern California Current System.  Deep-Sea Research 50B: 2355-2370.

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.

Di Lorenzo, E., Miller, A.J., Schneider, N. and McWilliams, J.C.  2005.  The warming of the California current system: Dynamics and ecosystem implications.  Journal of Physical Oceanography 35: 336-362.

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.

Mullin, M.M., Checkley Jr., D.M. and Thimgan, M.P.  2003.  Temporal and spatial variation in the sizes of California current macrozooplankton: analysis by optical plankton counter.  Progress in Oceanography 57: 299-316.

Peterson, W.T. and Schwing, F.B.  2003.  A new climate regime in northeast Pacific ecosystems.  Geophysical Research Letters 30: 10.1029/2003GL017528.