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Wednesday, October 15, 2008

Atlantic, Indian, and Pacific Ocean SSTs Segmented By Longitude


In a prior post I divided global SST anomaly data into quadrants.

In this post, I first lopped off the high latitudes to eliminate the Arctic and Southern Oceans. Then I divided the globe using longitudes to isolate major portions of individual oceans. Refer to Figure 1. The shapes of the continents and oceans prevent this from being a perfect division, but it is much simpler than downloading multiple data sets and adjusting for area. It also leaves two areas with mixes of two oceanic data sets and requires that I include a portion of the Indian Ocean south of Australia with the Pacific data sets.

Figure 1

In this post, all long-term graphs are made up of monthly data from January 1854 to September 2008 that have been smoothed with 37-month running-average filters. The short-term graphs are of raw monthly data from January 1978 to September 2008.

There are a few eye-opening effects that aren’t revealed when the oceans are divided by hemispheres. Other known behaviors are reinforced.


The graph of the long-term Pacific Ocean SST anomaly data (Figure 2) is very similar in appearance to the global SST anomalies. They’re compared in Figure 3. This, of course, is logical since the Pacific Ocean represents the largest portion of the global oceans. A noteworthy difference in my eyes is the exaggeration of the drop in Pacific Ocean SST in the late 19th century. There are other minor divergences. When, in the future, I isolate the Pacific more thoroughly, I will create a residual data set of the Pacific so we can look at the differences between it and global SST anomalies.
Figure 2

Figure 3

Figure 4 is a graph of the short-term SST data set for the Pacific Ocean. SST anomalies there have been dropping rapidly since 2005. They’re nearing pre-97/98 El Nino ranges.
Figure 4

In Figure 5, I’ve separated the Pacific Ocean at the date line, isolating East and West Pacific data. The 37-month smoothing highlights the opposing cycles in SST. To me, this indicates the transfer of heat from east to west and back again during and between ENSO events. What also stands out for me is the drastic step in the West Pacific data in recent times.
Figure 5

The short-term data for the East and West Pacific (Figure 6) continues to illustrate the opposing cycles. It also shows the step change in the West Pacific SST anomalies following the 1997/98 El Nino.

Figure 6

I’ve isolated the West Pacific data in Figure 7 and added pre-1997 and post-1997 linear trend lines. There appears to have been a significant upward step in West Pacific SST as a result of the 1997/98 El Nino. Unless there was also a coincidental change in coastal upwelling at the same time, the graph further illustrates the long-term impacts of that ENSO event. Are the long-term step changes of smaller ENSO events simply hidden by the noise of Rossby waves and subsequent ENSO variations?
Figure 7


Like the Pacific Ocean, the long-term graph of Atlantic Ocean SST anomalies bears a strong similarity to global SST anomalies. Refer to Figures 8 and 9. The greatest divergence in the Atlantic Ocean data set occurs before 1918, when Atlantic SSTs dropped much lower than Global SSTs.
Figure 8

Figure 9

Figure 10 illustrates the short-term SST anomalies for the Atlantic Ocean. Like many other data sets, it displays an upward step change as a result of the 1997/98 El Nino.
Figure 10

This is emphasized when pre-1997 and post-1997 linear trend lines are added to the plot of the short-term Atlantic Ocean SST anomalies. Refer to Figure 11.

Also note the anomalous spike in Atlantic SST occurring late in 2003. As will be shown later in this post, there was no El Nino at that time.
Figure 11


The long-term SST anomaly graph of the Indian Ocean (Figure 12) is dominated by rise in SST from 1929 to 1942. Part of the drop from 1942 to 1948 can be attributed to the errors in the transition between SST sampling methods, but the rise should remain unaffected as it occurred prior to the time of the known error, 1945. Figure 13 provides a comparison of Indian Ocean and Global SST anomalies.
Figure 12

Figure 13

Figure 14 illustrates the short-term SST anomaly data for the Indian Ocean. Note how there appears to be two major shifts in SSTs, the first occurring as an aftereffect of the 1986/87/88 El Nino and the second as result of the 1997/98 El Nino. SSTs rise sharply as a result of the El Ninos, then gradually decrease until the next major El Nino. Though I’ve seen the effect before, for some reason, I haven’t highlighted it.
Figure 14

The shifts are further emphasized by adding the linear trend lines for the periods from January 1978 to December 1985, and from of January 1988 to December 1996, and from January 1998 to September 2008. Refer to Figure 15. This appears to clearly indicate two things: As discussed, there are regular shifts in the SST anomalies of the Indian Ocean that can be attributed to major El Nino events. Second, there is a significant time period required by the Indian Ocean to dissipate the heat added by major El Nino events. It took almost a decade in the first example, between the 1986/87/88 and the 1997/98 El Nino events. The Indian Ocean is still losing heat from the 1997/98 El Nino.

Figure 15
(Note that I’ve done a quick correction to the Title of Figure 15. The update was posted on 11/3/08, which is after blogspot revised the sizing of uploaded pictures.)

Note how the 1982/83 El Nino, which was nearly the same magnitude as the 1997/98 El Nino, did not cause the same phenomenon. As is well documented, the effects of the 1982/83 El Nino were suppressed by the 1982 El Chichon eruption (and two other explosive volcanic eruptions that same year).


The SST anomalies for the mixed Atlantic-Pacific and the Indian-Pacific data sets are compared to the SST anomalies for the adjoining ocean data sets in Figures 16 and 17. The Southeastern Pacific is the likely cause of the wide variations in mixed Atlantic-Pacific data. There are minor differences in the Indian Ocean and the mixed Indian-Pacific data.

Figure 16

Figure 17


I pointed out the 2003 spike in the short-term Atlantic Ocean SST anomaly data earlier in this post. Figure 18 compares Atlantic SST anomalies with scaled Nino3.4 SST anomalies. The scaling factor used for the NINO3.4 data is 0.1. The 2003 spike appears anomalous.
Figure 18

In this post I’ve shown the long-term effects of major El Nino events on the Indian Ocean. Could the 2003 spike in the Atlantic Ocean data illustrate a long-lasting Rossby wave? There is a study (“Decade-scale trans-Pacific propagation and warming effects of an El NiƱo anomaly” by Jacobs et al, 1994) that discusses the effects of a Rossby wave that existed for a dozen years. http://www.nature.com/nature/journal/v370/n6488/abs/370360a0.html

Is this spike in 2003 a confirmation of long-term Rossby waves? In Figure 19, I’ve shifted the scaled NINO3.4 SST anomaly data 6 years and compared it to the Atlantic data. The eye tends to look for correlations, but there could be a relationship there and over the entire term of the data.
Figure 19


In a future post, I will further investigate the response of the Indian Ocean to El Nino events, especially the long-term decay in temperature. How far back in time is that cause and effect visible? Does it start as part of the Great Pacific Climate Shift of 1976, or does it go back farther?


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

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