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California summer: where did it go?

13 Sep 2023 | Science Notes

From the CO2Science archive: Concentrating on California, authors Schwing and Moore review a number of meteorological and oceanographic phenomena that have occurred off the west coast of the United States over the past couple of years. Schwing and Moore report that October 1999 marked a 13-month run of below-normal temperatures at Monterey, California, with a March-July mean that was 1.4C below the long-term average. This remarkable run of unusually cool weather was linked to extreme oceanic conditions that prevailed over the same time period. As they describe it, “in less than 2 years, sea-surface temperatures (SSTs) in this region went from being the warmest on record, during the height of El Nino in late 1997, to the lowest in decades,” with a rapid ocean cooling of nearly 10C in some areas that was “unprecedented.” This cooling was “especially impressive,” they said, “given that the annual range of SST in coastal waters off central California is about 3C, and its interannual variance is only 1C.”

Paper reviewed: Schwing, F. and Moore, C. 2000. A year without summer for California, or a harbinger of a climate shift. EOS, Transactions, American Geophysical Union 81: 301,304-305.

These dramatic deviations in climate were largely driven by unusually high coastal upwelling rates linked to stronger-than-normal southward winds that prevailed throughout the latter half of 1998 and much of 1999; and the climate changes, in turn, produced a number of changes in the local ecology of the ocean, as zooplankton communities off Oregon and British Columbia shifted from predominantly warm-water to exclusively sub-arctic species. Marine bird populations along the west coast of the continent also shifted from a prevalence of subtropical to subarctic species.

What it means
Although it is clearly too early to tell for certain, the authors state that “the unusual physical and biological conditions of 1999 may be the first signs of a climate shift,” bringing conditions that are “reminiscent of the climatic and ecological patterns common in the years prior to 1997.” Whether or not such ultimately turns out to be the case, they note that “the northeast Pacific Ocean can change swiftly and dramatically and that its marine populations can respond nearly as rapidly.”

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