Monday, August 8, 2011

C. A. Katsman and G. J. van Oldenborgh (2011), Tracing the upper ocean's “missing heat” , Geophys. Res. Lett. , 38 , L14610, doi:10.1029/2011GL048417

I find this an interesting study, as it deals with the probability of having stagnant periods in the warming - at least in the artificial world of climate models, where we know that the increase in GHGs has a significant effect (even if skeptics may want to argue: falsely so):

The Abstract reads:
"Over the period 2003–2010, the upper ocean has not gained any heat, despite the general expectation that the ocean will absorb most of the Earth's current radiative imbalance. Answering to what extent this heat was transferred to other components of the climate system and by what process(‐es) gets to the essence of understanding climate change. Direct heat flux observations are too inaccurate to assess such exchanges. In this study we therefore trace these heat budget variations by analyzing an ensemble of climate model simulations. The analysis reveals that an 8‐yr period without upper ocean warming is not exceptional. It is explained by increased radiation to space (45%), largely as a result of El Niño variability on decadal timescales, and by increased ocean warming at larger depths (35%), partly due to a decrease in the strength of the Atlantic meridional overturning circulation. Recently‐observed changes in these two large‐scale modes of climate variability point to an upcoming resumption of the upward trend in upper ocean heat content."


Anonymous said...

The probability they find for an eight-year stagnation is only 3%.

eduardo said...

This is not quiet correct. Quoting Katsmann and Oldenborgh '
Figure 2a, for the period 1990–2020, representing the conditions around the time of the observed flattening over 2003–2010. In Figure 2a, 11% of the distribution consists of zero or negative trend values; in Figure 2b this is reduced to 3%. This corresponds to a 97% chance of at least one period with an 8‐yr negative trend occurring for 1969–1999 and a 57% chance for 1990–2020.

This is admittedly somewhat obscure. I interpret that ' 3% of the overlapping trends are zero or negative' corresponds to a '57% chance of at least a negative trend being zero or negative' occurs because the trends of 8-year-trends are not independent. For instance they are calculated over all overlapping 8-year periods in 1969-1999 or 1990-2020.

The study by Katsmann and Oldeborgh is interesting, but there are some points that are not clear. For instance, in the analysis of observations Trenberth and Fasullo attribute the gain of ocean heat during La Nina events to a reduced cloud cover, but it seems that this is not what happens in the model runs. Katsmann and Oldeborg say that '[Trenberth and Fasullo, 2010], but the record has large uncertainties and is too short to separate trends from decadal variability. Observations of spatially resolved net outgoing TOA radiation are not of sufficient length and accuracy to assess the verisimilitude of the modeled radiation pattern. This seems to me a bit of wriggling out.

The other factor that Katsmann and Oldeborgh invoke to explain the missing heat gain in the upper ocean is that part of heat has been transported down to deeper layers. If the thermal expansion of water did not depend on temperature and pressure, this heat redistribution should have no effect on the sea-level trend, and then in the model the thermal expansion trend would remain unchanged by this mechanism. However, thermal expansion does depend on temperature and pressure, and so in the model sea-level rise should be changed by a bit (compared to all the heat remaining in the top layers) depending on the details of the vertical redistribution of heat. It would have been interesting if Katsmann and Oldenborgh had also reported on the simulated sea-level, and on whether it is compatible with the observed thermal expansion over the last 8 years.