Kevin Trenberth on El Nino: Part 2

May 12, 2014

To create the most recent Yale Forum video, I interviewed Dr. Kevin Trenberth, of the National Center for Atmospheric Research, at length.
Part one is below on this page. Here is the rest, above.

Dr. Kevin Trenberth illuminates the mechanics of ocean heat, and the coming El Nino event, and makes a bold prediction.

Reporting Climate Science:

Leading climate scientist Kevin Trenberth has told reportingclimatescience.com that he believes the pause in global warming may be caused by long term changes in the Pacific Ocean.

Trenberth and colleague John Fasullo argue in a new scientific paper that the massive El Nino Pacific Ocean warming event that occurred in 1997 and 1998 triggered the pause. They say that the El Nino caused a large loss of heat from the deep ocean to the sea surface that resulted in a cooling of the oceans. Since then the deep ocean has been absorbing heat back from the upper ocean and so cooling the atmosphere.

The implication is that the heat being absorbed from the atmosphere by the oceans has offset the underlying and ongoing warming of the atmosphere due to green house gases. As the deep ocean waters have slowly warmed they have taken heat from the upper ocean which has then cooled the atmosphere. This is the cause of the apparent hiatus in global warming that has manifested itself as a halt in the rise in global mean atmospheric temperatures seen in the second half of the 20th century.

Trenberth and Fasullo, from the US National Center for Atmospheric Research in Boulder Colorado, suggest 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.

“It is not so much that the atmosphere warms up rather that the upper levels of sea get warmer and these interact more directly with the atmosphere,” Trenberth said. So a warmer sea surface leads to a warmer atmosphere. “More heat penetrates to the deep ocean in the negative phase and that is not the case in the positive phase,” he explained.

trenfasull

“We can speculate that the huge 1997–1998 El Niño event was a trigger for the change in the PDO; certainly, it led to a large loss of heat in the Pacific… that has taken years to recover from, if the recovery is even complete. Past behavior of the PDO… suggests that regimes can last for 25 years,” Trenberth and Fasullo write in their paper.

“The picture emerging is one where the positive phase of the PDO from 1976 to 1998 enhanced the surface warming somewhat by reducing the amount of heat sequestered by the deep ocean, while the negative phase of the PDO is one where more heat gets deposited at greater depths, contributing to the overall warming of the oceans but cooling the surface somewhat. The Pacific Ocean appears to account for the majority of the decadal variability… Nevertheless, the events in the Pacific undoubtedly also affect the Atlantic, Indian, and Southern Oceans as the system acts collectively to equilibrate to these changes in the flow of energy,” they write.

The paper, entitled “An apparent hiatus in global warming?”, appears the new scientific journal Earth Futures.

“There are really deep teleconnections between the Pacific and the Atlantic and Southern Oceans,” Trenberth explained. “The centre of action is the Pacific Ocean but the main places where heat goes deep into the ocean are the Atlantic and Southern Oceans rather than the Pacific.”

There is also a very strong relationship with winds and sea level, according to Trenberth. 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.”

 

 

 

 

21 Responses to “Kevin Trenberth on El Nino: Part 2”

  1. dumboldguy Says:

    Great questions from Peter, and excellent responses from Trenbreth. WOW indeed, on the “bold prediction”. We have been teetering on the brink of quite a few tipping points—-will going up one more “step” to a “new normal” take us over some brinks into truly dangerous territory? And I love the gallows humor at the end—-it seems that some climate scientists are starting to talk a bit like policemen and combat vets—-laugh because you don’t want to cry.


  2. For those who may not have heard, a paper was recently published which suggests the frequency of super El Ninos will double with global warming…

    Cai, Wenju, et al. Increasing frequency of extreme El Nino events due to greenhouse warming. Nature Climate Change (2014).

    • dumboldguy Says:

      Don’t have the time to read and parse every word, but a quick look says they are really on to something—-great graphics.

      Looks like SHTF time may be approaching—-get out your umbrellas


      • From page 3 of Cai (2014):The models reproduce the contrasts between moderate and extreme El Niño events (Fig. 2a,b) as seen in observations (Fig. 1a,b and Supplementary Fig. 2), associated with large reductions in meridional and zonal SST gradients (Supplementary Fig. 14). In aggregation, the total number of El Niño events decreases slightly but the total number of extreme El Niño events increases (Fig. 2c,d). The frequency of extreme El Niños doubles from about one event every 20 years (101 events in 2,000 years) in the control, to one every 10 years (212 events in 2,000 years) in the climate change period (Fig. 2c,d). This is statistically significant according to a bootstrap test[27], underscored by a strong inter-model consensus, with 17 out of 20 models simulating an increase (Supplementary Tables 1–2). These robust statistics are particularly compelling given the large inter-model differences in convective parameterizations[28].… and page 6:

        In summary, our result of greenhouse-induced increased occurrence of extreme El Niño events is in stark contrast with previous findings of no consensus in El Niño change; our robust results arise from the use of process-based metrics, such as SST gradients and the impacts of reorganization of atmospheric convection, that isolate the mechanism of extreme El Niño events.

        The bit with slightly reduced frequency of El Niños but increased frequency of super El Niños reminds me of the slight decrease in the frequency of hurricanes but increase in the frequency of strong hurricanes, and similarly, the slight decrease in precipitation (made worse by increased evaporation) but increase in extreme precipitation events.


      • From page 3 of Cai (2014):

        The models reproduce the contrasts between moderate and extreme El Niño events (Fig. 2a,b) as seen in observations (Fig. 1a,b and Supplementary Fig. 2), associated with large reductions in meridional and zonal SST gradients (Supplementary Fig. 14). In aggregation, the total number of El Niño events decreases slightly but the total number of extreme El Niño events increases (Fig. 2c,d). The frequency of extreme El Niños doubles from about one event every 20 years (101 events in 2,000 years) in the control, to one every 10 years (212 events in 2,000 years) in the climate change period (Fig. 2c,d). This is statistically significant according to a bootstrap test[27], underscored by a strong inter-model consensus, with 17 out of 20 models simulating an increase (Supplementary Tables 1–2). These robust statistics are particularly compelling given the large inter-model differences in convective parameterizations[28].

        … and page 6:

        In summary, our result of greenhouse-induced increased occurrence of extreme El Niño events is in stark contrast with previous findings of no consensus in El Niño change; our robust results arise from the use of process-based metrics, such as SST gradients and the impacts of reorganization of atmospheric convection, that isolate the mechanism of extreme El Niño events.

        The bit with slightly reduced frequency of El Niños but increased frequency of super El Niños reminds me of the slight decrease in the frequency of hurricanes but increase in the frequency of strong hurricanes, and similarly, the slight decrease in precipitation (made worse by increased evaporation) but increase in extreme precipitation events.


  3. I checked, and Trenberth’s article is open access:

    Trenberth, Kevin E., and John T. Fasullo. An apparent hiatus in global warming?, Earth’s Future (2013).


  4. One thing that also comes to mind…. Trenberth suggests that there may be a step-like behavior to global warming. Now as a cautionary note, it is worth pointing out that trend plus noise may be mistaken for step-like behavior. This is a point made over at Skeptical Science and by Tamino a while back. However, Tamino also seems to think that there is regime-change behavior in the Atlantic with respect to tropical cyclones. If Cai (2014) is right, it would seem to make sense that we can expect step-like behavior with respect to the trend in global surface temperature along the same lines as what Trenberth suggests. Super El Niños would be the step up and the periods of reduced frequency El Niños (and one would presume increased frequency La Niñas) would be the relatively flat steps in between.


    • Shorter version of unpublished comment:

      If Cai (2014) is right, it would seem to make sense that we can expect step-like behavior with respect to the trend in global surface temperature along the same lines as what Trenberth suggests. Super El Niños would be the step up and the periods of reduced frequency El Niños (and one would presume increased frequency La Niñas) would be the relatively flat steps in between.

      • dumboldguy Says:

        It definitely does make sense, and as Andy says, the deniers will be talking about a new pause once we reach the higher step and level out for a bit. Of course, the ocean will then be sucking up heat again and getting ready for the next Super El Nino. Let’s hope that this one gets everyone’s attention, because by the next one it may be too late.

  5. Andy Lee Robinson Says:

    Next year we’ll have a new pause – “There’s been no global warming for a year now!”


  6. […] To create the most recent Yale Forum video, I interviewed Dr. Kevin Trenberth, of the National Center for Atmospheric Research, at length. Part one is below on this page. Here is the rest, above. D…  […]

  7. redskylite Says:

    Great interview with bold predictions from Dr Trenberth, some more predictions in the New Scientists:

    http://www.newscientist.com/article/mg22229682.400-world-is-unprepared-for-major-el-nino-later-this-year.html

  8. redskylite Says:

    What interested me greatly was his views on the PDO and the teleconnection with El Nino, which appears to fly in the face of Ms Curry’s view (that the PDO would diminish the effects of the El Nino, seems the other way around), looking at the data the PDO is certainly on a short upward trend, interesting to see if the trend maintains through the El Nino development period.

    2013** -0.13 -0.43 -0.63 -0.16 0.08 -0.78 -1.25 -1.04 -0.48 -0.87 -0.11 -0.41
    2014** 0.30 0.38 0.97 1.13


    • redskylite, I know of at least a couple of teleconnections between PDO and ENSO. For example, ENSO tends to lead PDO, such that an El Nino should precede a change in the phase of PDO similar to what we saw in 1998.

      Please see:

      ENSO also leads the PDO index by a few months throughout the year (Fig. 1d), most notably in winter and summer. Simultaneous correlation is lowest in November–March, consistent with Mantua et al. (1997). The lag of maximum correlation ranges from two months in summer (r ~ 0.7) to as much as five months by late winter (r ~ 0.6).

      Matthew Newman et al (1 Dec 2003) ENSO-Forced Variability of the Pacific Decadal Oscillation, Journal of Climate, Vol 16, No 23


    • Alternatively, when PDO is in its positive phase El Ninos are more common whereas when PDO is in its negative phase La Ninas are more common.

      Please see:

      Comparing ENSO and PDO: Table of PDO-ENSO Years (Climate Impacts Group at the University of Washington)


    • If you look at the temperature fields associated with both oscillations it seems quite natural that these sorts of relationships would exist. Their temperature fields look very much the same:

      Figure 1 shows the sea surface temperature (SST) anomalies that are associated with the warm phase of PDO. The spatial patterns are very similar: both favor anomalously warm sea surface temperatures near the equator and along the coast of North America, and anomalously cool sea surface temperatures in the central North Pacific. The cool phases for PDO and ENSO, which are not shown, have the opposite patterns of SST anomalies: cool along the equator and the coast of North America and warm in the central north Pacific.

      About PDO: The Pacific Decadal Oscillation.
      Climate Impacts Group, University of Washington

      … but with the PDO being more intense in the North Pacific while ENSO is more intense in the equatorial region. If their phases are sustained in part by positive feedback mechanisms, then one should expect constructive interference when they are in sync and deconstructive interference when they are out of sync.

      As for one oscillation diminishing the effects of the other, I noticed that on the page with “Table of PDO-ENSO Years” that I just refered to, it states, “Since 1999, the PDO has shifted every few years between cool and warm phase, making it difficult to determine at this time if the 1998 shift was a true shift to a cold phase.” But I am not exactly sure what passage by Trenberth you are referring to.


  9. […] Video-Interview mit US-Klimaforscher Kevin Trenberth zum El Niño […]


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