GISS Spackle and Caulk

11 08 2008

Guest post by John Goetz
Cross posted from Climate Audit

Earlier this year I did a post on the amount of estimation done to the GHCN temperature record by GISS before generating zonal and global averages. A graphic I posted compared the amount of real temperature data with the amount of estimation over time. To read the graphic, consider 2000 as an example. As of February 7, 2008 there were 3159 station records in the GHCN data with an entry for the year 2000. Of those station records, 62% were complete and an annual average could be fully calculated. Another 29% were incomplete, but contained enough monthly data that the GISS estimation method kicked in. The final 9% were so incomplete that no estimation could be done.

What I did not explore at the time and would like to look more closely here is the accuracy of the estimation. One would hope with so much infilling going on that the accuracy would be rather high (I will leave the determination of “high accuracy” for a later time). Because I did not have real data to compare with the GISS estimations, I took another approach. I used the GISS method to estimate real temperature data as if that data were missing.

Recall that GISS never explicitly estimates missing monthly temperatures. What they do is estimate seasonal averages when one monthly temperature is missing but the other two are present. Similarly, an annual temperature can be estimated when one seasonal value is missing but the other three are present. Using this methodology GISS can estimate an annual temperature when as many as six monthly values are missing.

While no explicit monthly estimate is recorded by GISS, it certainly can be derived from the seasonal estimate. I have shown several times a one-line equation that exactly reproduces the GISS seasonal estimate. Leaving a subsequent derivation as an exercise for the reader, the implied monthly estimate can be found from that equation and is expressed as follows:

where the average values for A, B, and C are calculated from all valid entries for the given month in a particular station record.

Now to test the estimation accuracy. In Connecticut, December 2006 was warmer than normal, but February 2007 was colder than normal. Looking at the records for Hartford, CT, we see the following monthly and seasonal temperatures:

Dec 2006: 3.3
Jan 2007: -0.3
Feb 2007: -4.6
DJF: -0.5

If the December 2006 record were missing from Hartford, GISS would estimate a value of -0.7 C, which would yield a seasonal average of -1.9 C. Similarly, if February 2007 were missing, GISS would estimate it at 1.7 C and produce a seasonal average of 1.6 C. That’s a 4.0 degree miss for Dec, a 6.3 degree miss for February, and a 3.5 degree swing at the seasonal level.

The winter of 06-07 in Connecticut was a bit of an oddball. I really wanted to know what the typical error looked like. To do that, I performed the same calculation on all GHCN v2.mean records.

Read the rest of this entry »


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Categories : Uncategorized

NASA Sees Arctic Ocean Circulation Do an About-Face

11 08 2008

NOTE: You may recall a story I posted some months ago titled: “NASA: It’s the wind” regarding Arctic wind circulation patterns and the way it drove sea ice further south into melt zones. Commenter Paul Marek brought this story to attention recently, and given the sea ice trend this summer, I thought it was worth bringing to light again. Then and now, “The results suggest not all the large changes seen in Arctic climate in recent years are a result of long-term trends associated with global warming. ” Given our less than predicted catastrophic sea ice loss this year, coupled with this study, it looks like Arctic ice could be on the mend. – Anthony

Scientists used measurements from Arctic Bottom Pressure Recorders
Click for Larger image
This shows contours of the trend in ocean bottom pressure from 2002 to 2006 as measured by GRACE along with hypothetical trends that would apply at the circles if ocean salinity reverted from 1990s values to climatological conditions over the same period.

NASA Sees Arctic Ocean Circulation Do an About-Face
November 13, 2007

PASADENA, Calif. – A team of NASA and university scientists has detected an ongoing reversal in Arctic Ocean circulation triggered by atmospheric circulation changes that vary on decade-long time scales. The results suggest not all the large changes seen in Arctic climate in recent years are a result of long-term trends associated with global warming.

The team, led by James Morison of the University of Washington’s Polar Science Center Applied Physics Laboratory, Seattle, used data from an Earth-observing satellite and from deep-sea pressure gauges to monitor Arctic Ocean circulation from 2002 to 2006. They measured changes in the weight of columns of Arctic Ocean water, from the surface to the ocean bottom. That weight is influenced by factors such as the height of the ocean’s surface, and its salinity. A saltier ocean is heavier and circulates differently than one with less salt.

The very precise deep-sea gauges were developed with help from the National Oceanic and Atmospheric Administration; the satellite is NASA’s Gravity Recovery and Climate Experiment (Grace). The team of scientists found a 10-millibar decrease in water pressure at the bottom of the ocean at the North Pole between 2002 and 2006, equal to removing the weight of 10 centimeters (four inches) of water from the ocean. The distribution and size of the decrease suggest that Arctic Ocean circulation changed from the counterclockwise pattern it exhibited in the 1990s to the clockwise pattern that was dominant prior to 1990.

Reporting in Geophysical Research Letters, the authors attribute the reversal to a weakened Arctic Oscillation, a major atmospheric circulation pattern in the northern hemisphere. The weakening reduced the salinity of the upper ocean near the North Pole, decreasing its weight and changing its circulation.

“Our study confirms many changes seen in upper Arctic Ocean circulation in the 1990s were mostly decadal in nature, rather than trends caused by global warming,” said Morison. Read the rest of this entry »


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Categories : Science, sea ice