PACIFIC OCEAN — Since 1900, sea surface and land-based surface temperatures in the northeastern Pacific Ocean have risen up to 1 degree Centigrade (1.8 degrees Fahrenheit) with most of that temperature rise occurring by 1940.

The main cause of the temperature change in the near North Pacific Arc is natural atmospheric dynamics and not anthropogenic, or man-made, causes usually attributed to greenhouse gases, according to a report released last week. The Pacific Arc includes Washington, Oregon and northern California.

Using a history of sea surface temperature, land-based air temperature and sea-level pressure, a way of understanding local winds, from 1900 to 2012, researchers found that natural circulation changes in the northeast Pacific Ocean accounted for more than 80 percent of the warming over U.S. Pacific states and more than 90 percent of the warming in Washington, Oregon and Northern California.

While the study does not question global climate change and its potential anthropogenic causes, it does offer another option, that local winds are important in generating changes in the northeastern Pacific Ocean.

The study does say that its findings are consistent with what the Northwest is seeing in Cascade Mountain snowpack and earlier snow melt in northern California.

“An ensemble of climate model simulations run under the same historical radiative forcings fails to reproduce the observed regional circulation trends,” the report says of climate change models. “These results suggest that natural internally generated changes in atmospheric circulation were the primary cause of coastal NE Pacific warming from 1900 to 2012 and demonstrate more generally that regional mechanisms of interannual and multidecadal temperature variability can also extend to century time scales.”

The study, “Atmospheric controls on northeast Pacific temperature variability and change, 1900-2012,” was published in the Proceedings of the National Academy of Sciences last week (tinyurl.com/p9q3glm)

Its authors are Northwest climatologists James A. Johnstone, Joint Institute for the Study of the Atmosphere and Ocean at the University of Washington, and Nathan J. Mantua, also with the Joint Institute and with NOAA Fisheries Southwest Fisheries Science Center in Santa Cruz, Calif.

The 1900 to 2012 data shows clearly the circulation-driven temperature variability during the time period. Conditions were near the mean in 1900, but show “historic anomalies of high pressure and low coastal SST from 1910 to 1920. Steep trends toward lower SLP and coastal warming then followed from 1920 to 1940,” the report says.

The periods 1945 to 1957 and 1971 to 1976 saw relatively high sea-level pressure and cooling coastal temperatures. There was a period in between (1958 to 1970) of low pressure and higher temperatures.

“Elevated temperatures from the late 1970s to the early 2000s appear to be consequences of persistently negative SLP anomalies over the NE Pacific that reduced wind speeds, thereby increasing the net downward turbulent heat flux and warm poleward SST advection over the NE Pacific Arc,” the report says.

Over the final decade of the study (2003 to 2012) there was a cooling trend with higher pressure, stronger winds, “greater turbulent heat losses, and cool equatorward advection, leading to a partial reversal of century-long circulation and temperature trends.”

The results are consistent with other global climate model simulations, the report says.

“Recent GCM results from large-ensemble experiments show that anthropogenically forced SLP changes in the NE Pacific are considerably weaker than natural internal variability over the 2010-2060 period,” the report says, looking forward for the next 50 years. “We conclude that internal variability is the most likely source of the observed SLP decline and that current GCMs may not adequately capture the underlying processes.”

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