Changing Winds Explain Most Pacific Coast Warming

Changes in winds over the eastern Pacific Ocean explain most of the warming trend along the West Coast of North America in the last century, according to a new analysis published in the Proceedings of the National Academy of Sciences.  (The previous link is a link to Non-Federal government web site. Click to review NOAA Fisheries disclaimer.)

Similar atmospheric shifts are known to drive fluctuations in Pacific climate over decades in the form of the Pacific Decadal Oscillation  (The previous link is a link to Non-Federal government web site. Click to review NOAA Fisheries disclaimer.) (PDO), a long-recognized pattern of seesawing ocean temperatures. The new research indicates that similar changes in regional pressure and winds can also drive trends in sea surface and coastal air temperatures that extend over a century or more.

This study used independently measured ocean and land-surface air temperature records from 1900 to 2012 to confirm an already well-documented increase of approximately 0.5 to 1 degree Celsius in the northeast Pacific Ocean and nearby land areas. What’s especially interesting and new about this work is that independently measured atmospheric sea level pressures over the past century show that circulation changes account for nearly all of the year to year, decade to decade, and century long surface temperature changes in the northeast Pacific Ocean and West Coast states since 1900.

An evaluation of climate model simulations used in the latest IPCC  (The previous link is a link to Non-Federal government web site. Click to review NOAA Fisheries disclaimer.) reports finds that the atmospheric changes that account for most of the 113 year warming trend are not predicted to result from historical radiative changes, including both natural and anthropogenic forcings. It is also notable that most of the West Coast warming since 1900 took place between 1910 and 1940. These facts suggest that the temperature trend most likely occurred through natural changes in regional climate dynamics, the new study concludes.

“Surface winds and wind-driven ocean currents have large effects on temperatures in and around the northeast Pacific Ocean; they dominate the overall temperature variability and also account for a large fraction of the warming trend,” said Jim Johnstone, formerly of the NOAA Joint Institute for the Study of the Atmosphere and Ocean at the University of Washington and lead author of the research. “West Coast sea surface and coastal air temperatures evolved in lockstep with changing patterns of atmospheric pressure and winds.”

The new research is a regional climate study of historical sea level pressures, winds and temperatures over the eastern Pacific Ocean and draws no conclusions about climate change on a global scale. The study demonstrates the importance of understanding how climate variability on a regional scale may at least temporarily obscure larger forces acting on the global climate system.

While greenhouse gases trap heat in the upper atmosphere, processes in the lower atmosphere, particularly heat transfers linked with surface winds, dominate ocean temperature changes in the coastal NE Pacific, Johnstone said. Winds over the ocean vary in large-scale patterns that regulate evaporative cooling and surface currents. When northeast Pacific winds have a more cyclonic (counterclockwise) tendency, coastal evaporation is reduced, warmer currents prevail, and sea-surface temperatures rise. The new analysis identifies trends that suggest a century-long cyclonic tendency in northeast Pacific winds drove most of the coastal sea surface warming.

Land surface temperatures over the West Coast states also closely track the nearby ocean climate.

“People want to know what’s going to happen to the climate in their backyard, but for the West Coast that’s a real challenge to say because whatever happens is still likely to be dominated by natural variability over the next few decades,” said Nate Mantua, leader of the Landscape Ecology Team at NOAA Fisheries’ Southwest Fisheries Science Center and coauthor of the research.

“Even with global climate change going on in the background, regional changes like what we’re describing continue to dominate the climate we live in,” said Mantua, who was among the group of scientists that first described the Pacific Decadal Oscillation  (The previous link is a link to Non-Federal government web site. Click to review NOAA Fisheries disclaimer.) in 1996. “At local and regional scales, we have a lot of work to do in distinguishing natural variability from the forced climate changes expected from human influences.”

The research grew out of a 2010 study  (The previous link is a link to Non-Federal government web site. Click to review NOAA Fisheries disclaimer.) that found a 30 percent reduction in the frequency of summer fog in the coast redwood region of Northern California over the course of the 20th century. In the new research, Johnstone and Mantua found that the same coastal temperatures that affect fog frequency are regulated by offshore changes in the atmosphere.

Sea level pressures, simply measured by barometers that indicate the weight of the atmosphere over a given area at sea level, capture regional atmospheric variations on time scales of days, months or years, as with El Niño and the PDO, and, as the new research shows, longer scales of a century or more. A weakening of the North Pacific High, consistent with weaker winds, accounts for more than 80 percent of the warming trend from 1900 to 2012 in northeast Pacific sea surface temperatures and air temperatures in the coastal Pacific Northwest, including Washington, Oregon and Northern California, the study found.

The same factors account for about 60 percent of the warming trend over Southern California, indicating other factors are also affecting temperatures there.

Tree-ring studies also demonstrate that century-long temperature trends have occurred along the West Coast before the influence of human-generated greenhouse gases. That provides evidence that changes in atmospheric circulation and the effect of winds are longstanding factors in Pacific climate.

“We’ve known that these fluctuations are in play over decadal scales and what we’re now seeing is that they can extend to century scales as well,” Johnstone said. Mantua added that “previous studies have found similarly strong circulation controls on snowmelt timing in California’s Sierras, and springtime snowpack trends in the Cascade Range of Washington and Oregon. So in addition to temperatures and ocean currents, changes in wind patterns play important roles in the kind of climate we experience in many different ways. We think this research improves the understanding for this region’s recent climate change origins. We also hope it will inform efforts to improve models used to predict future climate, and to aid the interpretation of paleoclimate data used to inform people about pre-historical climate.”

A comparison of observed and predicted regional sea surface temperatures in the NE Pacific study region. Our model predictions for sea surface temperature changes are based on monthly sea level pressure variations.

Sea surface temperature graph

The red curve shows monthly observations of regionally averaged NE Pacific sea surface temperature deviations from the long-term mean. The blue curve shows our sea level pressure based model predictions for sea surface temperature deviations in the same region, again using monthly data. The “r” value is the correlation coefficient between the predicted and observed monthly time series.

Our study focused on observations from 1900 to 2012. Light gray shading near the very end of this graphic highlights the model predictions for 2013 through August 2014 using observed sea level pressure (also in blue), while the monthly sea surface temperature observations are shown in red. Note that temperatures in the northeast Pacific Ocean in recent months have warmed considerably, even moreso than predicted by our simple model.

In essence, what does this study say?

This study shows that from 1900 to 2012, winds were the primary cause of monthly to century-long sea surface temperature (SST) variations and changes in the coastal northeast Pacific. Wind forcing (represented by an index of regional sea-level pressure) can reproduce the observed history of SST variability and change with a high degree of accuracy (correlation of ~0.8).

What is significant about these findings?

These results present a significant reinterpretation of the region’s recent climate change origins, showing that wind and pressure patterns have changed substantially over the last century. This research also shows that these changes in the region’s wind, pressure and temperature are not likely related to anthropogenic causes (from changes in land cover, greenhouse gas and aerosol concentrations) or natural radiative forcing (from changes in the sun or explosive volcanic eruptions).

How do winds affect ocean temperatures?

In the NE Pacific, winds vary in large-scale patterns that regulate evaporation rates and upper-ocean currents. When regional sea level pressure is unusually low, a cyclonic (counterclockwise) wind tendency results. Coastal wind speeds, evaporation rates, and southward currents weaken, leading to broad warming along the North American coast. This research highlights a century-long sea-level pressure reduction that suggests coastal warming from 1900-2012 was generated by increasingly cyclonic wind tendencies.

Does this mean anthropogenic warming is not affecting the West Coast/eastern Pacific Ocean?

This study shows that atmospheric circulation changes, essentially changes in winds, were the proximate cause for West Coast/NE Pacific temperature changes from 1900 to 2012. If anthropogenic effects were important for NE Pacific / west coast warming from 1900 to 2012, they likely occurred through an indirect circulation pathway that that is not well simulated by current global climate models. The more we understand the regional climate dynamics discussed in this research the better we can understand how they may interact with human-caused climate change, and what the combined effect on the region’s climate may be.

How do you know that anthropogenic climate change is not creating the trend in declining winds you find responsible for the warming trend?

Global climate models used to simulate changes from 1850 to 2005 do not generate anything like the observed decline in winds that we identify as the key driver of the observed coastal warming trends.

Furthermore, the timing of warming and cooling periods in the region do not match the timing of relatively strong and weak trends in greenhouse gas concentrations. Regional climate changes from 1980 to 2012 suggest, for example, that winds strengthened and the coastal ocean cooled while greenhouse gas concentrations rose rapidly. The strongest implied wind reductions and strongest regional warming occurred from ~1910 to 1940, when greenhouse gas increases were relatively modest.

Does this call into question the veracity of the global climate models if they cannot reproduce such basic influences?

It raises questions about the ability of global climate models to simulate regional climate changes, which is a known shortcoming of these models. A better understanding of model strengths and weaknesses may guide future research efforts.

Does this mean Southern California is most vulnerable to anthropogenic greenhouse forcing?

Not necessarily. There may be other patterns of circulation change (including, for example, those over land) that play a role in Southern California warming. This is a topic that warrants further study.

Should we be concerned about human-caused global climate change even if it can be outpaced by regional climate factors?

Of course. Climate at regional and global scales is subject to a combination of natural and human-caused factors. The present study suggests that care should be taken in trying to identify and separate these factors. It may not be safe to assume that regional warming trends over the span of a century are dominated by human causes, nor that global climate models faithfully reproduce the relevant processes. We hope that this study will motivate further research on circulation changes and their role in overall climate change.

So how does this research help us?

This study shows how changing weather patterns, known to be important for West Coast temperature variations, can also contribute to century-long trends in the region’s climate. We think it provides valuable insights into past climate changes, with a relatively simple story about the proximate causes for West Coast/NE Pacific Ocean warming since 1900. The study opens new questions about mechanisms of century-long climate changes.


James Johnstone (Independent Climate Researcher)

Nate Mantua (SWFSC Fisheries Ecology Division, Landscape Ecology Team)

For press inquiries: Michael Milstein (NOAA Fisheries West Coast Region, Communications and External Affairs Office)

(September 22, 2014)