A while back, there was quite a “kerfluffle” (as Lucia called it) regarding the comparisons between the GISS model projections for Ocean Heat Content (OHC) and those calculated by NODC. However, the thing that struck me at the time was that I could not easily find any available averages for the 700 m OHC projections for any of the model runs. If I recall correctly, RealClimate, Tamino, and Bob Tisdale were simply using linear extrapolations.
Since recently I had been doing my own OHC calculations from the NOAA/NODC data, I decided I would register for an account at PCMDI, download the gridded ocean potential temperature data, and perform the averaging myself to derive estimates of OHC for some model projections. Today, I will be showing the five runs from GISS-ER under the SRESA1B scenario (720 ppm stabilization) from CMIP3. I’d also like to do GISS-EH, but those files are a lot bigger, and taking longer to download and process. I’m assuming they have a higher resolution.
Anyhow, the package that includes the data and scripts mentioned in this post is available here. There are a few points I want to note:
- I convert from average temperature to Ocean Heat Content simply by multiplying by a factor (specific heat * volume * density) which assumes a constant value for all of those as we increase depth, which is not strictly accurate. However, as we saw in that previous post on OHC, the results are pretty darned close (r^2 = .998 between my NODC calculations and that available from Climate Explorer), and I will be using my calculated values from observations to compare against my calculations from the projections, so it will be more of an apples-to-apples comparison, even if the factor is slightly off.
- I start everything in 2004, because that is when the GISS-ER projections start for the SRESA1B scenario. I have baseline everything to the overlapping period…that is, 1st quarter of 2004 through the 2nd quarter of 2011.
- If you want to personally do a more in-depth conversion from layer temperatures to OHC, I have also included the intermediate calculations of globally-averaged ocean potential temperatures for the top 16 layers (down to 700m).
Anyhow, here is the comparison between the 5 GISS-ER model runs and the observations:
For kicks, I would note that the simple OLS trend for the overlapping period is 0.089 +/- 0.163 J*10^22/year for the observations, compared to the 0.715 +/- .054 J * 10^22/year for the model mean. On the one hand, I’ll note that this is a short period, and there are some other things – such as a prolonged solar minimum – taking place here. On the other hand, I’d like to point out that calculating the smallest trend over ANY 7.5 year period in the runs of the chart above yields the following results:
Run1: 0.673 J * 10^22/year
Run2: 0.714 J * 10^22/year
Run3: 0.583 J * 10^22/year
Run4: 0.555 J * 10^22/year
Run5: 0.633 J * 10^22/year
So, the smallest trend for any 7.5 year period in the 36 years for any of the five runs is 0.555 J*10^22/year, which is clearly much larger than our currently observed value. I’ll need to take a look at Meehl et. al 2011, and there are certainly other models, but it seems to me – at least at initial glance – that there is nothing in the GISS-ER runs to suggest that we could potentially see a slow-down in the OHC of the upper 700 meters that we are currently seeing. I’d welcome other opinions.