This post was first posted at WattsUpWithThat here a few days ago.
This post presents a number of problems the 2005 Hartmann and Wendler paper “The Significance of the 1976 Pacific Climate Shift in the Climatology of Alaska.” These include the misuse of the Pacific Decadal Oscillation (PDO) index and the erroneous conclusions derived from that misuse.
On page 14 of the 16-page paper, Hartmann and Wendler (2005) note:
This current debate presents the fact that more work is needed to further define North Pacific climate indices and implications.
Much of the obvious confusion could be eliminated if researchers stopped using abstract forms of sea surface temperature data like the PDO and began to use actual sea surface temperature anomalies. In other words, they should use the right tool for the job.
MISUSE OF THE PACIFIC DECADAL OSCILLATION INDEX
The abstract of Hartmann and Wendler (2005) begins:
The 1976 Pacific climate shift is examined, and its manifestations and significance in Alaskan climatology during the last half-century are demonstrated. The Pacific Decadal Oscillation index shifted in 1976 from dominantly negative values for the 25-yr time period 1951–75 to dominantly positive values for the period 1977–2001.
And the abstract concludes with following two sentences:
When analyzing the total time period from 1951 to 2001, warming is observed; however, the 25-yr period trend analyses before 1976 (1951–75) and thereafter (1977–2001) both display cooling, with a few exceptions. In this paper, emphasis is placed on the importance of taking into account the sudden changes that result from abrupt climatic shifts, persistent regimes, and the possibility of cyclic oscillations, such as the PDO, in the analysis of long-term climate change in Alaska.
The authors should be commended for considering the impacts of natural variability on Alaskan climate. However, they fail to recognize that the Pacific Decadal Oscillation (PDO) does not represent the sea surface temperature anomalies of the North Pacific north of 20N (the area on which the PDO data is based), or the Pacific Ocean as a whole, or the coastal waters of Alaska. This is clearly visible in Figure 1, which compares the Pacific Decadal Oscillation Index data (red) to the sea surface temperature anomalies of the North Pacific north of 20N (brown), the Pacific Ocean as a whole (light blue) and of the coastal waters of Alaska (dark blue).
The decadal variability of the PDO does not agree with the variations in the other three datasets. This can be shown by detrending and standardizing the sea surface temperature anomalies of the North Pacific north of 20N, the Pacific Ocean and of the coastal waters of Alaska and smoothing them (and the PDO index) with 121-month running-average filters. See Figure 2.
The Pacific Decadal Oscillation index is a statistically created dataset that does not represent the sea surface temperatures of the region of the North Pacific from which it is derived, nor does it represent the sea surface temperature anomalies of the Alaskan coastal waters, which should have a direct impact on the land surface air temperatures of Alaska. It therefore appears as though the authors used the wrong dataset as their reference for the sea surface temperatures.
There’s another way to illustrate the confusion caused by the use of the PDO index as a reference for North Pacific sea surface temperature anomalies, and that is in the context of climate shifts.
The primary focus of the paper was the impact of the 1976 Pacific Climate Shift on Alaskan climate—hence the title of the paper. The authors used temperature data starting in 1951 so we’ll use the same start year, and we’ll end the sea surface temperature data at present times.
Contrary to assumptions made using the PDO index, the sea surface temperature anomalies of the North Pacific north of 20N (which is the area from which the PDO index is derived) do not include a climate shift in 1976. The climate shift for the extratropical North Pacific occurred in 1988-89. See Figure 3. That’s about the same time as the climate shift in North Pacific ocean heat content.
On the other hand, the sea surface temperature anomalies of the Alaskan coastal waters did experience a climate shift in 1976, Figure 4. Sea surface temperature anomalies cooled from 1951 to 1975, and cooled again from 1977 to present. The two cooling trends before and after the 1976 climate shift suggest that the 1976 shift is responsible for the long-term trend of 0.055 deg C per decade for the Alaskan coastal waters.
Hartmann and Wendler (2005) could have used the actual sea surface temperature anomalies for the Alaskan Coastal Waters in their paper. It would have kept to their overall objective of citing the 1976 Pacific Climate Shift as a cause for the long-term warming and would have avoided the confusion associated with the use of the PDO index, which is an abstract form of sea surface temperature data.
MORE ON THE 1976 PACIFIC CLIMATE SHIFT
To help clarify a few things, let’s borrow the next two illustrations from an earlier post, Blog Memo to John Hockenberry Regarding PBS Report “Climate of Doubt”. As noted in Figure 3, the 1976 climate shift is not a North Pacific phenomenon. It’s a shift in East Pacific sea surface temperature anomalies, Figure 5. It effectively shifted the sea surface temperatures of the East Pacific (90S-90N, 180-80W) up 0.17 deg C. In other words, that’s a natural rise in sea surface temperatures of almost 0.2 deg C for 33% of the surface area of the global oceans.
The Great Pacific Climate Shift also refers to the change in the basic state of the ocean processes taking place in the tropical Pacific. See Figure 6. After 1976, El Niño events dominated, but for the period from the early-1940s to 1976, El Niños and La Niñas were more evenly matched, with La Niñas just a little bit stronger.