The 1976 Pacific Climate Shift

INTRODUCTION

There are a good number of studies of the 1976 Pacific Climate Shift. Many of them discuss the processes that initiated the shift as modeled in GCMs; others illustrate the resulting effects on climate in the North Pacific and adjoining land surfaces; while still others show its impact on the PDO and ENSO. In “The Evolution of ENSO and Global Atmospheric Temperatures”, Trenberth et al illustrate that even the development of El Nino events changed near that time.
http://www.cgd.ucar.edu/cas/papers/jgr2001b/jgr2.html

Recently, in a post on Tropical SSTs, I noted the upward step change in the Tropical East Pacific SST data that happened in 1976. Refer to Figure 6 here:
http://bobtisdale.blogspot.com/2008/09/tropical-sst-anomalies-revisited-east.html

In this post, I’ll illustrate the step changes, or lack thereof, in various Pacific SST data sets that occurred at 1976. I’ll employ linear trend lines for 30 years before and 30 after 1976 and for 10 years before and after 1976 to help highlight the shifts in SST and the changes in the trends. The trend lines are only being used for emphasis; they are not being used to forecast or hindcast changes outside the periods plotted in the graphs.

As always, there are a few revelations that some might find unusual or unexpected.

BASIN-WIDE PACIFIC OCEAN SHIFT (60S-65N, 120E-100W)

Figures 1 and 2 show the shifts in the Pacific basin SST that occurred at 1976, using the long-term and short-term trend lines as reference. The changes in trends are obvious in the long-term data. The step change is apparent in the short-term data.

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Figure 1

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Figure 2

SHIFT IN THE NORTH PACIFIC (0-65N, 100E-90W)

In Figure 3, the change in trend in the North Pacific is also tough to miss. The step change in the North Pacific, Figure 4, is exaggerated compared to the basin-wide Pacific data.
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Figure 3

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Figure 4

SOUTH PACIFIC (0-60S, 145E-70W) APPEARS TO SHIFT EARLIER

If the South Pacific data is divided at 1976, the change in the two long-term trends is not as severe. Refer to Figure 5. The pre-1976 trend in the South Pacific was positive, reducing the visual impact at the change point. A look at the short-term South Pacific data, Figure 6, reveals a negative trend up to 1976.
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Figure 5

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Figure 6

Take another look at the long-term South Pacific data. A shift appears 8 years before 1976, in 1968. This is emphasized if the trend lines are segmented at 1968. Refer to Figure 7.
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Figure 7

THE INFLUENCE OF THE SOUTHERN OCEAN ON THE SOUTH PACIFIC

The Southern Ocean SST anomalies are shown in Figure 8. I’ve marked 1968 on the graph. It, the Southern Ocean, has a major influence on the South Pacific. Through the Humboldt Current (Figure 9), the Antarctic Circumpolar Current (ACC) feeds the South Pacific. Could it take 8 years for the shift in the Southern Ocean and South Pacific to be reflected in the basin wide and North Pacific data sets?

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Figure 8

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Correction: The end date on Figure 8 originally read 2008. It’s been changed to its correct year, 2006.

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Figure 9

Figure 9 courtesy of Wikipedia. http://en.wikipedia.org/wiki/Image:Humboldt_current.jpg

Note: In Figure 8, I’ve also marked the year that another shift took place in the Southern Ocean SST anomalies, 2000. Seven to eight years later, there was a significant La Nina, and the PDO is said to have shifted to its cold phase. http://www.jpl.nasa.gov/news/news.cfm?release=2008-066 I enjoy coincidences like that. Will the recent shift in the Southern Ocean SSTs drive global temperatures down for a few decades now? To answer yes, one would have to believe the Southern Ocean was in fact one of the ultimate causes of the increase in temperature from the mid-1970s to the early 2000s.

SHIFT IN THE TROPICAL EAST PACIFIC (20S-20N, 90-180W)

Segmenting the Pacific more, Figures 10 and 11 illustrate the significant step changes in the long-term and short-term trends in the Eastern Tropical Pacific.
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Figure 10

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Figure 11

SHIFT IN NINO3.4 (5S-5N, 120-170W)

The shifts in the NINO3.4 SST anomalies, Figures 12 and 13, are similar to the Eastern Tropical Pacific. Variations in NINO3.4 SST anomalies are known to cause changes in global climate.
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Figure 12

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Figure 13

SHIFT IN THE EAST PACIFIC (60S-65N, 100-180W)

Consistent with the North Pacific and basin-wide Pacific data, the East Pacific also experienced a substantial upward shift in 1976. Refer to the long-term and short-term East Pacific SST anomaly graphs, Figures 14 and 15.
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Figure 14

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Figure 15

To counter that there’s a…

DOWNWARD STEP IN THE WEST PACIFIC (60S-65N, 120-180E)

As could be expected, for the short-term data set, Figure 17, the change in the West Pacific SST anomalies opposes the change in the East Pacific. The long-term trends react somewhat differently, Figure 16. I have done a quick search for a shift in the West Pacific data set, looking for one that occurs during the 1960s and 1970s, but none is readily visible. However, note the rise and fall in the West Pacific SSTs during the late 1980s to early 1990s (Figure 16). The surge and retreat appears to be a response to the multiyear El Nino preceding it. Starting in 1998, it is followed by another rise to a plateau, a result of the 97/98 El Nino, which has taken longer to subside.
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Figure 16

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Figure 17

The decrease in the West Pacific anomalies at 1976 (while the temperature rises in the East Pacific) is consistent with the relationship between the Tropical East and Tropical West Pacific, Figure 18. In the Tropical Pacific SST anomaly data, the variations in the West SST anomalies oppose the major variations in the East SST anomalies.
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Figure 18

SOURCE

Smith and Reynolds Extended Reconstructed SST (ERSST.v2) is available through the NOAA National Operational Model Archive & Distribution System (NOMADS).
http://nomads.ncdc.noaa.gov/#climatencdc