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Tuesday, October 7, 2008

Peruvian Coast SST Anomalies


A number of blogs, including IceCap, have picked up the Reuters story about the anomalous SSTs off the Peruvian Coast. The article “Peru studies climate riddle as the world heats up” begins with: “Scientists are using everything from a yellow submarine to weather balloons and special airplanes to solve a climate conundrum: why is Peru getting colder while the rest of the world heats up?” A few paragraphs later Reuters provides the following quote, "’Peru has a very important role in global climate,’ said Alexis Chaigneau, a French scientist leading experiments in Peru. ‘Over the past 50 years, the Peruvian coast has gotten colder, mainly because of stronger winds that have pulled up the deep cold waters of the ocean current.’"

IceCap Link:
Link to Reuters article:

Hmm. What strange phenomena are taking place off the Peruvian Coast?


Figure 1 illustrates the SST anomalies, smoothed with a 12-month running average filter, for the waters of the coast of Peru, for the period of January 1854 to Aug 2008. The coordinates used are 5 to 15S, 75 to 82W. With the two spikes in 1982 and 1997, there is an obvious ENSO influence.

You could pick a point 50 years ago and state that Peruvian Coast SSTs have gotten colder. In fact, you could pick any number of starting points in that curve and state that it’s currently colder.

Figure 1

Note, however, that the 1982 and 1997 spikes in temperature seem disproportionately large when compared to other ENSO events. I’ve provided Figure 2 , NINO3.4 SST anomalies, as reference. Also note that there are no disproportionately large La Nina events reflected in the Peruvian Coast SST data.
Figure 2

In “Evolution of El Nino-Southern Oscillation and global atmospheric surface temperatures”, Trenberth et al identified a difference in the formation of El Nino events before and after the 1976 Great Pacific Climate Shift: “The 1976/1977 climate shift and the effects of two major volcanic eruptions in the past 2 decades are reflected in different evolution of ENSO events. At the surface, for 1979–1998 the warming in the central equatorial Pacific develops from the west and progresses eastward, while for 1950–1978 the anomalous warming begins along the coast of South America and spreads westward. The eastern Pacific south of the equator warms 4–8 months later for 1979–1998 but cools from 1950 to 1978.” Refer to:

It appears the increase in the magnitude of the Peruvian coast reactions to the 1982 and 1997 El Nino events may reflect the Trenberth et al observation about ENSO SST development.


I’ve tried to dampen the effects of the ENSO events on the Peruvian Coast SST data by smoothing it with a 37-month filter, with limited success. Refer to Figure 3. The trends and their switching points begin to take shape.
Figure 3

Smoothing it with an 85-month filter, Figure 4, helps the shape take form, but not as well as I would have liked. The sheer sizes of the two anomalous late 20th century responses to El Nino events make it virtually impossible to smooth them away. But…
Figure 4

Do the SSTs of the waters off the Peruvian Coast reflect the overall trends of the ERSST.v3 version of the Southern Ocean, Figures 5 and 6? They should, but as noted before, they are masked by the high-magnitude spikes of the ENSO events. The correlation may not be exact, but the influence is obvious. Of course, having the Southern Ocean as a major influence on Peruvian Coastal SSTs makes sense, since the Humboldt Current transports waters from the Antarctic Circumpolar Current (the dominant current in the Southern Ocean), up along the west coast of South America, to the coast of Peru.

Southern Ocean SST Anomalies Smoothed with a 12-Month Filter:
Figure 5

Southern Ocean SST Anomalies Smoothed with a 37-Month Filter:

Figure 6


Smith and Reynolds Extended Reconstructed SST (ERSST.v2) is available through the NOAA National Operational Model Archive & Distribution System (NOMADS).

The more recent version of the Smith and Reynolds Extended Reconstructed SST (ERSST.v3), along with land surface temperature and combined (land + ocean) surface temperatures, are available in various latitudinal bands at:
Don’t let the PDO in the address confuse you. There’s much more there. The overview for the update is here:

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