Sea Surface Temperatures of the Oceans Surrounding Australia – A Note from Bob Tisdale
The extended title of the post is “Sea Surface Temperatures of the Oceans Surrounding Australia and the Magnitude of ENSO Events, ” because the following illustrations of Sea Surface Temperatures (SSTs) of Australian waters present an interesting effect, the impact of the magnitude of El Nino-Southern Oscillation (ENSO) events on SST. While I’ve seen it before in other data sets, I haven’t yet singled it out for discussion.
(Note: To economize words, throughout the rest of the post, I’ll use “Australian waters” in place of “oceans surrounding Australia” or the “combined Southwest South Pacific and Southeast South Indian Ocean data set.”)
First, to determine the area to evaluate, I downloaded data for two geographical areas from the NOAA NOMADS system. They’re identified by the red and blue boxes in Figure 1. The coordinates used are 10-45S, 105-165E for the red area and 0-55S, 95-175E for the blue.
Figure 2 illustrates the SST anomalies from January 1854 to June 2008 for the two ocean areas surrounding Australia. Both data sets have been smoothed with a 37-month filter. Picking the start date in the trough at 1905 and the end date at a peak in 2000 (There must be an alarmist side to me.), both data sets show rises in SST that are on the order of 1.0 to 1.2 deg C. The two signals are similar, with the smaller area having the greater variations. For the remainder of this post, I’ll use the data from the smaller area, the red curve.
Figure 3 shows the raw long-term data for the SST anomalies of Australian waters. Also illustrated is data that’s been smoothed with a 12-month running-average filter. Typical of many other oceanic data sets, there is an overall decline in SSTs from the late 1800s to 1910 and a rebound in SSTs until 1940. Then, though there are underlying oscillations, SSTs rise almost continuously from 1940 to present. El Nino events appear to stand out.
Note: The step change (temperature drop) at 1945 has been identified as an error in a recent Thompson et al letter to “Nature” with the title “A Large Discontinuity in the Mid-Twentieth Century in Observed Global-Mean Surface Temperature”.
But there’s something that seems to be missing from the data in later years. Refer to Figure 4, which illustrates short-term (January 1978 to June 2008) SST anomalies for Australian waters. The delayed responses to the 82/83, the 86/87/88, and the 97/98 El Nino events are again easy to find with the sudden rises in SST, but…
Where are the effects of the La Ninas?
The 97/98 El Nino is so much larger than the subsequent multiyear La Nina that it gives the impression that it supplied enough heat to create a step change in the SSTs of Australian waters and that the heat then dissipated over a ten year period. But that impression is only partly correct. The effects of the La Ninas are there, but they are overwhelmed by the magnitudes of those major El Ninos.
In Figure 5, I’ve added NINO3.4 SST anomaly data to the graph of short-term SST anomalies for Australian waters. A scaling factor of 0.3 has been applied to the NINO3.4 data. I’ve also noted the timing of the two major volcanic eruptions in case someone feels they’re relevant.
The time lags between an ENSO event and the response of SST for Australian waters appear to be on the order of a few months to a year. And with the scaling factor used (0.3), the magnitude of the NINO3.4 SSTs during the 82/83, the 86/87/88, and the 97/98 events appear to generate similarly sized reactions in the SSTs of Australian waters.
Then, starting at 1998, it would appear that the entire drop in NINO3.4 SST should result in a similarly sized response in SST, but the response of the Australian waters falls far short. The logic behind the “Would-Should” statement is wrong.
In simple terms, ENSO events supply heat to sea surfaces when the NINO3.4 SST anomalies are greater than zero and remove heat when they’re less than zero. So the reference point for the NINO3.4 data is zero when comparing it to other anomaly data. Looking at the data again and using zero as the reference for the multiyear La Nina episode after the 97/98 El Nino, the reaction by the SSTs of Australian waters is in the proper scale. There are minor differences in the cause and effect and in the time lags in prior ENSO events, but all in all, there are no surprises.
The effects of ENSO events on SST trends are obvious once we’re reminded of them. Keep in mind that it is not only the frequency of El Nino and La Nina events but also the magnitude of those events that must be considered during discussions of their impacts on global or local climate.
In “The Evolution of ENSO and Global Atmospheric Temperatures”, Trenberth et al identify the linear trend in global temperatures that result from ENSO events: “For 1950-98, ENSO linearly accounts for 0.06 deg C of global warming.” http://www.cgd.ucar.edu/cas/papers/jgr2001b/jgr2.html
The effects of ENSO events appear much greater on regional levels.
Closing Note: The additional problems with measuring and calculating global mean sea surface temperature are discussed at length in numerous posts at ClimateAudit and in the papers that are the subjects of or the references used for those posts. For further information, refer to the following ClimateAudit posts:
Sea Surface Temperature Data is Smith and Reynolds Extended Reconstructed SST (ERSST.v2) available through the NOAA National Operational Model Archive & Distribution System (NOMADS).