Global warming and ocean heat content

The connection between global warming and the changes in ocean heat content has long been a subject of discussion in climate science. This was explicitly discussed in Hansen et al, 1997 where they predicted that over the last few decades of the 20th Century, there should have been a significant increase in ocean heat content (OHC). Note that at the time, there had not been any observational estimate of that change (the first was in 2000 (Levitus et al, 2000)), giving yet another example of a successful climate model prediction. At RC, we have tracked the issue multiple times e.g. 2005, 2008 and 2010. Over the last few months, though, there have been a number of new papers on this connection that provide some interesting perspective on the issue which will certainly continue as the CMIP5 models start to get analysed.

The most recent paper was a new study from NCAR out last week that looked into what happens to OHC in models when there are occasional 10 year periods with no trends in global surface temperatures (Meehl et al, 2011).

It is well-known (or at least it should be) that simulations for late 20th C and early 21st Century do not produce monotonically increasing temperatures at the annual or decadal time-scale. For the models used in AR4, the decadal trends expected under estimates of present day forcings are roughly N(0.2,0.14) (i.e. a Gaussian distribution centered on 0.2 ºC/decade with a standard deviation of ~0.14ºC/decade. This implies that one would expect an 8% probability of getting surface temperature trends less than zero in any one decade.

The Meehl et al study looked at the changes in ocean heat content during these occasional decades and compared that to the changes seen in other decades with positive surface trends. What they found was that decades with cooling surface temperatures consistently had higher-than-average increases in ocean heat content. This makes perfect sense if there is internal decadal variability in the fluxes that connect the deeper ocean to the surface ocean (which of course there is). An anomalous downward heat flux reduces the ocean surface temperature (and hence global surface temperature), which generates an anomalous heat flux into the ocean from the atmosphere (because the flux into the ocean is related to the difference between atmospheric and ocean temperature). And this of course increases total OHC.

A related study from the UK Met. Office looked at the relationship between the ocean heat content changes in the top 700m and the total ocean heat content change in models (Palmer et al, 2011). They found that (unsurprisingly) there is more variability in the top 700m than in the whole ocean. This is important to quantify because we have better estimates of the upper ocean OHC change than we do of changes in the whole ocean. Observational studies indicate that the below-700m increases are not negligible – but they are poorly characterised (von Schuckmann et al, 2009). The Palmer study indicates that the uncertainty on the decadal total OHC change is about 0.15 W/m2 if one only knows the OHC change for the top 700m.

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References

  1. S. Levitus, "Warming of the World Ocean", Science, vol. 287, pp. 2225-2229, 2000. http://dx.doi.org/10.1126/science.287.5461.2225
  2. G.A. Meehl, J.M. Arblaster, J.T. Fasullo, A. Hu, and K.E. Trenberth, "Model-based evidence of deep-ocean heat uptake during surface-temperature hiatus periods", Nature Climate change, vol. 1, pp. 360-364, 2011. http://dx.doi.org/10.1038/nclimate1229
  3. M.D. Palmer, D.J. McNeall, and N.J. Dunstone, "Importance of the deep ocean for estimating decadal changes in Earth's radiation balance", Geophysical Research Letters, vol. 38, pp. n/a-n/a, 2011. http://dx.doi.org/10.1029/2011GL047835
  4. K. von Schuckmann, F. Gaillard, and P. Le Traon, "Global hydrographic variability patterns during 2003–2008", J. Geophys. Res., vol. 114, 2009. http://dx.doi.org/10.1029/2008JC005237