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Research Suggests Pacific Trade Winds Cause Pause

09.02.2014
09.02.2014 18:00 Age: 3 yrs

An article published online in Nature Climate Change today investigates how strengthened Pacific trade winds can account for 0.1C–0.2C of cooling through increased subsurface ocean heat uptake— this is enough to account for much of the temperature slowdown observed in the so called global warming pause.

Click to enlarge. Observations are shown as annual anomalies relative to the 1980-2012 mean (grey bars) and a five-year running mean (black solid line). Model projections are shown relative to the year 2000 and combine the CMIP3 and CMIP5 multi-model mean (red dashed line) and range (red shaded envelope). The projections branch o the five-year running mean of observed anomalies and include all simulations as evaluated by the IPCC AR4 and AR5. The cyan, blue and purple dashed lines and the blue shading indicate projections adjusted by the trade-wind-induced SAT cooling estimated by the ocean model (OGCM), under three scenarios: the recent trend extends until 2020 before stabilizing (purple dashed line); the trend stabilizes in year 2012 (blue dashed line); and the wind trend reverses in 2012 and returns to climatological mean values by 2030 (cyan dashed line). The black, dark green and light green dashed lines are as per the above three scenarios, respectively, only using the trade-wind-induced SAT cooling derived from the full coupled model (CGCM). Shading denotes the multi-model range throughout. Courtesy: Nature Climate Change and the authors.

Click to enlarge. This image shows normalized histograms of Pacific trade wind trends (computed over 6 N S and 180 W) for all 20-year periods using monthly data in observations (1980-2011) versus available CMIP5 models (1980-2013). The observed trend strength during 1992-2011 is indicated. Courtesy: Nature Climate Change and the authors.

 

Heat stored in the western Pacific Ocean, due to an unprecedented strengthening of the equatorial trade winds, appears to be largely responsible for the so called pause in global warming.

 

New research published today in the journal Nature Climate Change indicates that the dramatic acceleration in winds has invigorated the circulation of the Pacific Ocean, causing more heat to be taken out of the atmosphere and transferred into the subsurface ocean, while bringing cooler waters to the surface.

 

Global average surface temperatures rose sharply during the second half of the 20th century before leveling off since the late 1990s despite a continuing increase in atmospheric carbon dioxide concentrations. Scientists have yet to fully explain the recent slowdown in the rise of air temperatures attributed to the so called global warming pause that, on at least one measure, has lasted for over 17 years.

 

“Scientists have long suspected that extra ocean heat uptake has slowed the rise of global average temperatures, but the mechanism behind the hiatus remained unclear," said Professor Matthew England, lead author of the study and a Chief Investigator at the ARC Centre of Excellence for Climate System Science at UNSW Australia.


Although cool surface waters in the eastern Pacific Ocean have been identified as a key component of this feature, it has so far been unclear how this occurs. The new paper published online in Nature Climate Change this week investigates how strengthened Pacific trade winds can account for 0.1–0.2°C cooling — much of the temperature slowdown — through increased subsurface ocean heat uptake.


Matthew England from the University of New South Wales and co-workers used observations and climate models to investigate the impact of increased trade winds on climate.


They showed that when the model is forced by the irregularly strong winds, the sea surface response matches observed trends. The shallow ocean circulation loops are sped up by the intensified winds, causing increased equatorial upwelling of cool waters and the subduction of warm water, or heat drawdown, to the subsurface layer.


They find that around 80% of the surface temperature cooling occurred after 2000, indicating that wind acceleration is a key contributor to the slowdown in warming.


These findings suggest that if the stronger trade winds continue, the slowdown in warming will persist, but if they lessen then there will be a return to rapid warming.

 

The paper is called "2014: Recent intensification of wind-driven circulation in the Pacific and the ongoing warming hiatus". Abstract, citation and links below.

 

Seminar

The authors are scheduled to give a present their results at the 2014 Ocean Sciences Meeting in Hawaii later this month that is entitled recently Intensified Pacific Ocean Wind-Driven Circulation And The Ongoing Warming Hiatus. The abstract for their talk follows: “Despite ongoing increases in greenhouse gases, Earth’s global average surface air temperature has remained in a hiatus since 2001, raising questions about its cause, its likely duration, and the implications for global climate. Here we show that a pronounced strengthening in Pacific trade winds observed over the past two decades – not captured by CMIP5 climate models – is sufficient to account for a marked cooling of the tropical Pacific and a substantial slowdown in surface warming via increased subsurface ocean heat uptake. The extra uptake comes about via increased heat subduction in the Pacific subtropical overturning cells, increasing the convergence of heat in the equatorial thermocline, while cooling the surface ocean in the central and eastern Pacific. This process is estimated to have cooled present-day global average surface air temperatures by > 0.1 degrees Celsius; about half the slowdown in surface warming observed since 2001. Model projections assessed by the IPCC AR5 fail to capture the magnitude of this process. The wind-induced anomalous heat uptake could last for around two decades, even if the recent rapid trends in Pacific wind strength reverse in the coming years.”

 

Consistent

 

This research appears to be consistent with other recent work. Late last year global warming may be caused by long term changes in the Pacific Ocean. Trenberth and colleague John Fasullo, both from the US National Center for Atmospheric Research in Boulder Colorado, suggested that long term oscillations in the Pacific Ocean, known as Pacific Decadal Oscillations (or PDOs) drive alternate 20-plus year cycles of upper ocean warming and cooling which also involve heat being exchanged with the atmosphere. The implication of this is that when the Pacific is in a negative phase the upper ocean loses heat and so cools the atmosphere, and that when it is in a positive phase the upper ocean warms and so heats the atmosphere.

 

There is also a very strong relationship with winds and sea level, Trenberth told reportingclimatescience.com. Water is piling up in the western Pacific Ocean at a rate of around 10mm per year which is three times the global average. This has led to a difference in sea level, measured by satellite radars, between the western and eastern Pacific. “The sea level is 20cm higher in the western Pacific and the only way to keep it there is for strong winds to pile up the water. It is these changes in the winds that change the ocean currents and affect where the heat is going,” he explained. “But this can't keep going for ever. The ocean wants to slop back to the east.” Trenberth pointed to three lines of evidence that support this idea: measurable and recorded changes in the wind strength, satellite altimeter radar measurements of sea level and an analysis of ocean heat data. 

 

See our story "Global Warming Pause Due To Pacific Says Trenberth" here.

 

Here is the news release issued by the University of New South Wales


Pacific trade winds stall global surface warming - for now

10 February 2014

Heat stored in the western Pacific Ocean, due to an unprecedented strengthening of the equatorial trade winds, appears to be largely responsible for the hiatus in surface warming observed over the past 13 years.

 

New research published today in the journal Nature Climate Change indicates that the dramatic acceleration in winds has invigorated the circulation of the Pacific Ocean, causing more heat to be taken out of the atmosphere and transferred into the subsurface ocean, while bringing cooler waters to the surface.

 

“Scientists have long suspected that extra ocean heat uptake has slowed the rise of global average temperatures, but the mechanism behind the hiatus remained unclear," said Professor Matthew England, lead author of the study and a Chief Investigator at the ARC Centre of Excellence for Climate System Science at UNSW Australia.

 

“But the heat uptake is by no means permanent: when the trade wind strength returns to normal – as it inevitably will – our research suggests heat will quickly accumulate in the atmosphere.  So global temperatures look set to rise rapidly out of the hiatus, returning to the levels projected within as little as a decade."

 

The strengthening of the Pacific trade winds began during the 1990s and continues today. Previously, no climate models have incorporated a trade wind strengthening of the magnitude observed, and these models failed to capture the hiatus in warming.  Once the trade winds were added by the researchers, the global average temperatures very closely resembled the observations during the hiatus.

 

“The winds lead to extra ocean heat uptake, which stalled warming of the atmosphere.  Accounting for this wind intensification in model projections produces a hiatus in global warming that is in striking agreement with observations,” Professor England said.

 

“Unfortunately, however, when the hiatus ends, global warming looks set to be rapid."

 

The impact of the trade winds on global average temperatures is caused by the winds forcing heat to accumulate below the surface of the Western Pacific Ocean.

 

“This pumping of heat into the ocean is not very deep, however, and once the winds abate, heat is returned rapidly to the atmosphere,” Professor England said.

 

“Climate scientists have long understood that global average temperatures don't rise in a continual upward trajectory, instead warming in a series of abrupt steps in between periods with more-or-less steady temperatures. Our work helps explain how this occurs," Professor England said.

 

“We should be very clear: the current hiatus offers no comfort  – we are just seeing another pause in warming before the next inevitable rise in global temperatures.”

News release ends.

 

 

 

 

Abstract

Despite ongoing increases in atmospheric greenhouse gases, the Earth’s global average surface air temperature has remained more or less steady since 2001. A variety of mechanisms have been proposed to account for this slowdown in surface warming. A key component of the global hiatus that has been identified is cool eastern Pacific sea surface temperature, but it is unclear how the ocean has remained relatively cool there in spite of ongoing increases in radiative forcing. Here we show that a pronounced strengthening in Pacific trade winds over the past two decades—unprecedented in observations/reanalysis data and not captured by climate models—is sufficient to account for the cooling of the tropical Pacific and a substantial slowdown in surface warming through increased subsurface ocean heat uptake. The extra uptake has come about through increased subduction in the Pacific shallow overturning cells, enhancing heat convergence in the equatorial thermocline. At the same time, the accelerated trade winds have increased equatorial upwelling in the central and eastern Pacific, lowering sea surface temperature there, which drives further cooling in other regions. The net effect of these anomalous winds is a cooling in the 2012 global average surface air temperature of 0.1–0.2 °C, which can account for much of the hiatus in surface warming observed since 2001. This hiatus could persist for much of the present decade if the trade wind trends continue, however rapid warming is expected to resume once the anomalous wind trends abate.  

 

Citation

England, M. H., S. McGregor, P. Spence, G. A. Meehl, A. Timmermann, W. Cai, A. Sen Gupta, M. J. McPhaden, A. Purich and A. Santoso, 2014: Recent intensification of wind-driven circulation in the Pacific and the ongoing warming hiatus, Nature Climate Change, doi: 10.1038/nclimate2106.

Read the abstract and get the paper here


Source

Nature press release.

University of New South Wales news release here.

 

Note

This story updated 10 February 2014 to include news release from the University of New South Wales