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What causes short term changes in ocean heat?

Posted on 2 June 2009 by John Cook

Over the past 40 years, global ocean heat content has shown a long term warming trend. However, the warming hasn't been monotonic. There are periods where ocean heat drops for several years before the warming trend resumes. On several occasions, this is due to large volcanic eruptions which cause a drop in global temperatures. On other occasions, upper ocean heat drops with no volcanic activity. What causes these breaks in warming?

Figure 1: global upper ocean heat content from 1955 to 2008. Blue line is yearly ocean heat content for the 0–700 m layer (Levitus 2008). Red line is the global mean stratospheric optical depth, indicating the timing of major volcanic eruptions (NASA GISS, data ends in 1999).

This question is examined in Interannual variability in upper ocean heat content, temperature, and thermosteric expansion on global scales (Willis 2004). Willis finds that short term, interannual variability in global upper ocean heat is strongly influenced by variability in the tropics, particularly in the Pacific and Indian Oceans. And much of the variation is related to the El Niño Southern Oscillation (ENSO).

ENSO refers to a cycle between El Niño and La  Niña conditions across the tropical eastern and central Pacific. In normal conditions, trade winds blow towards the west across the tropical Pacific. These winds pile up warm surface water in the west Pacific. This thickened layer of warm water depresses a layer of subsurface water called the thermocline, which lies between the warm surface waters and the much colder deep ocean. On the east Pacific, the warm surface waters are blown off shore and must be replaced. The replacement waters are the cooler waters from the lower parts of the ocean.

Figure 2: conditions in equatorial Pacific during normal conditions (no El Niño or La Niña). Image courtesy of NOAA.

During the La Niña phase, these conditions intensify. This leads to more cold water upwelling in the East Pacific. The thermocline rises as more cold water penetrates into the upper ocean. Sea-surface temperatures are colder than usual in the central and western Pacific.

Figure 3: conditions in equatorial Pacific during La Niña conditions. Image courtesy of NOAA.

How much does this cycle contribute to global upper ocean heat? Figure 4 shows heat content both globally (dashed line) and over the tropics (solid line, 20°N to 20°S). As La Niña intensified, heat content in the tropics decreased rapidly through the end of 1998 and the first half of 1999. Global heat also fell during this time, but somewhat less so, suggesting that some of the tropical heat may have been exported to higher latitudes. The cooling, both globally and in the tropics, gave way to rapid warming in mid-1999 with the subsiding of La Niña.

Figure 4: Interannual variability in heat content integrated over the tropics 20°N to 20°S (solid line) and over the entire globe (dashed line). Graph from Willis 2004.

This puts recent trends in upper ocean heat content in context. Leuliette 2009 discusses how weak El Niño conditions in 2006-2007 were followed by a moderate La Niña in mid-2007. Similarly, Cazenave 2009 suggests that the drop in upper ocean heat in mid-2007 could be related to the particularly strong recent La Niña cold phase.

Figure 5: Southern Oscillation Index (monthly values and 12 month average), courtesy of Australian Bureau of Meteorology.

Globally, upper ocean heat has dropped since mid-2007. However, if one focuses on one piece of the puzzle without understanding the broader picture of the physical mechanisms involved, it can lead to the erroneous conclusion that the long term warming has ended. By recognising that La Niña causes short term cooling in upper ocean waters and that we've been in La Niña conditions since mid-2007, we see that current ocean cooling is a case of internal variability imposed upon the long term trend.

Acknowledgements: thanks to John Cross for his feedback and research for this post.

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Comments 1 to 7:

  1. Personally, I don't think the SOI is anything like the whole story on ocean heat content. If you take the SOI numbers from your Australia BOM link, you find three other instances where the SOI was as strongly cooling as it was now(that lie in the period covered by LEvitus 2008). 1955-56 the ocean heat content is falling in line with the SOI 1970-71 the ocean heat is flat or rising slighly when the SOI is strongly cooling 1973-75 the ocean heat is rising strongly while the SOI should be cooling 2008-9(?) the ocean heat is falling strongly while the SOI should be cooling. It appears that there must be something else going on here, too. Cheers, :)
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    Response: SOI is not the whole story by any means. There are other ocean cycles that also have an influence. But Willis 2004 found that it is has the strongest influence on global upper ocean heat - of all the ocean cycles, it is the most dominant signal.
  2. Hi John, could you confirm or refute this please?: You are using the words "ocean heat content", but I think you should be saying "estimates of upper ocean heat content". You indicate that the heat content of the ocean goes up and down due to ENSO, but it seems to me that ENSO is just redistributing that heat content.
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    Response: I use 'ocean heat' and 'upper ocean heat' interchangeably in this post - technically I should use upper ocean heat everytime to be perfectly precise but the wording gets a little clunky. The ocean heat estimates are measured by ARGO and XBT floats which measure the upper 0 to 3000 metres, but the upper 700 metres more thoroughly. Figure 1 shows ocean heat for the upper 700 metres. What is happening with ENSO is colder water from deeper waters are upwelling into the upper ocean waters.
  3. I helped John a little with this post and while I can't speak for him, I will comment on the questions above. Shawnhet: I agree that the SOI is not the whole story and there are a number of other factors going on. I think the main point of the post was that there is at least one phenomena that explains why the ocean heat is not rising monotonically. Steve L: Again, I do not speak for John and I am sure he will provide better insight, but - yes, I think you are correct that it is for the upper part of the ocean (IIRC 700 m and up). However I would disagree that the heat content is just redistributed. I just read a paper but I can't recall the name (it is at work) that shows that based on a metric they use there is a difference in the heat gained and lost in an El-nino and a La nina. If it was just redistribution you would expect the highs and lows to cancel out. This does not appear to be the case. Regards, John
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  4. Thanks John Cross and John Cook for your responses to my question (#2). It seemed to me that a very warm upper ocean means greater loss of heat to outer space, so the heat energy of the planet should increase more during La Nina instead. On the other hand, I thought that high ocean surface temperatures would more likely lead to greater fractions of water vapour in the atmosphere which should warm things faster. So having confused myself on those issues, I framed my question under "redistribution" although I suspect it is much more complicated. I'd like to know more about the paper at work if you get a chance. Thanks.
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  5. Steve L: I think that your comment about water vapour is correct and the latent heat of the vapour spreads the heat enough so that the radiation balance is not that much affected. The paper I was talking about is "Observations of Warm Water Volume Changes in the Equatorial Pacific and Their Relationship to El Nin˜o and La Nin˜a" by Meinen My point is supparized by this quote from it.
    We found that the amplitudes of WWV anomalies are linearly related to the amplitudes of the Nin˜o-3 SST anomalies, with larger anomalies in WWV preceding larger anomalies in SST. However, for a given magnitude of WWV anomaly, positive values are associated with larger-magnitude SST anomalies than negative values. This asymmetry implies differences in the relative importance of physical processes controlling SST during El Nin˜o and La Nin˜a events.
    Of course this is a 10 year old paper which in oceanography terms is a lifetime! Regards, John
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  6. I wonder if Willis 2004 is only addressing the correlation of the ENSO with ocean heat over the most recent period. Personally, I think that if the correlation goes in opposite directions at different times(as it seems to in the 70s as opposed to recent events, it is premature to ascribe the more recent cooling to ENSO shift. Further, I am not convinced that claiming strong La Ninas lead to less ocean heat actually makes physical sense. Assuming a constant input of energy into the oceans, warmer sea surface temperatures should, it seems to me, lead to more emission of heat from the ocean(and hence, less ocean heat gain). However, claiming that La Ninas cause less ocean heat to be stored is a situation where there is both less emission of OH and less storage of it. Cheers, :)
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  7. "Global heat also fell during this time, but somewhat less so, suggesting that some of the tropical heat may have been exported to higher latitudes. You are mangling the English language, and destroying logic. You aren't writing what you mean. The higher latitudes are part of the globe. If heat left one part of the globe and trotted up to the higher latitudes, the globe got neither warmer nor cooler. This sort of basic confusion mangles all sorts of reports on all sides of the debate. Be more careful. The Primary effect of any ENSO event is to simply move heat around. Secondary effects may change the energy balance by altering the IR radiation from the earth. For example, by altering clouds. I think we can agree ENSO does not alter solar brightness. Moving heat around can, by itself, change air temps, but that is not global warming or cooling.
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