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Tuesday, November 11, 2008

The Ishii and Kimoto Proposed Update to Ocean Heat Content


The November 5, 2008, NASA Earth Observatory article “Correcting Ocean Cooling” renewed interest in Ocean Heat Content at Jennifer Marohasy’s blog with her post Correcting Ocean Cooling: NASA Changes Data to Fit the Models.

NASA Earth Observatory Article:

If you search the comments on the Jennifer Marohasy thread, you can find my second attempt to post a comment with links. Unfortunately, Jennifer’s spam blocker discarded my “working version”. This post is an expansion of my post there, with copies of the Ocean Heat Content graph from the paper I linked.

About the same time the NASA article was published, I discovered a paper “Reevaluation of Historical Ocean Heat Content Variations With An XBT Depth Bias Correction” (2008), in which Ishii and Kimoto proposed a bias correction methodology that resulted in the following Ocean Heat Content graph, Figure 6 from the paper.

Ishii and Kimoto Figure 6

Link to the Ishii and Kimoto paper:

I highlighted the proposed update in red in the following.
Ishii and Kimoto Figure 6 with Proposed OHC Corrections Highlighted

The Abstract of the paper includes: “In comparison with the previous temperature analysis, large differences are found in the present analysis as follows; the duration of large ocean heat content in the 1970s shortens dramatically, the global warming trend decreases, and recent ocean cooling becomes insignificant.”

Ishii and Kimoto end the differences with “and recent ocean cooling becomes insignificant”, but the clause before that “the global warming trend decreases” is significant. They explain on page 10: “The OHC trend of the present analysis for 1955-2005 becomes smaller by 20% than in ver. 6.2, while that for recent 13 years (1993-2005) becomes larger by 70%. These changes in global warming trends are substantial.” The reason for the substantial increase in trend from 1993 to 2005 is the major downward correction in Ocean Heat Content from 1991 to 1997. It wasn’t due to an increase, because most of the upward correction from 2003 through 2006 was not included in the trend.

Therefore, the decrease in the long-term trend of 20% is, in my opinion, the significant difference.


UPDATE (November 14, 2008): Please note that I am not implying any wrongdoing on the parts of those who calculate Ocean Heat Content. I simply found the following to be an odd coincidence, one that should be discussed.

The following is a graph of the number of global ocean data profiles accumulated monthly from 1979 to 2006 by the NCEP Global Ocean Data Assimilation System for a depth of 0-250 Meters from 90S to 90N. It’s odd that in recent years the Ocean Heat Content rises above its previous high in the 1970s, while at the same time the number of subsurface ocean temperature readings increases drastically. Then the Ocean Heat Content peak and decline about the same time as the number of readings.
Number of Global Ocean Data Profiles

The Climate Prediction Center NCEP Global Ocean Data Assimilation System: Monthly Products webpage is here:

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