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Thursday, November 6, 2008

Another Look at the Saw-Tooth Trends in the Indian Ocean


In many prior posts, I’ve illustrated the step changes in numerous data sets that resulted from the 1997/98 El Nino. In this post I’ll examine the response of the Indian Ocean SST to El Nino and La Nina events for the period of 1978 to present. The result: two step changes. This post also illustrates that Indian Ocean can react differently to El Nino events based on their magnitude and timing. Nothing surprising there.


Preliminary note: The 1982/83 El Nino will be excluded from the discussion, because its effects were severely dampened by the timing of the El Chichon eruption. Keep in mind that there was also a major volcanic eruption in 1991, Mount Pinatubo. It should also have suppressed Indian Ocean SST.


In a prior post titled Atlantic, Indian, and Pacific Ocean SSTs Segmented By Longitude, I identified what appeared to be saw-tooth trends in the Indian Ocean SST anomalies. Refer to Figure 1, which is the corrected-on-the-fly version of Figure 15 in that post.
Figure 1

If NINO3.4 SST anomaly data is scaled by a factor of 0.2 and compared to the Indian Ocean SST anomaly data, Figure 2, a few things stand out. First, following an El Nino event that causes a major reaction in the Indian Ocean (the 1986/87/88 and 1997/98 El Ninos), the subsequent lesser El Ninos appear to have little to no impact on the Indian Ocean. This is apparent in the NINO3.4 and Indian Ocean SST anomalies for the period between the 1986/87/88 and 1997/98 El Ninos and the period after the 1997/98 El Nino. The 1991 Mount Pinatubo eruption may be responsible for part of this effect during the early 1990s, but there were no cataclysmic volcanic eruptions following the 1997/98 El Nino.
Figure 2

Second, the Indian Ocean appears to respond proportionately to the entire rise from trough to peak in NINO3.4 SST anomaly during the 1986/87/88 and 1997/98 El Ninos. Accounting for the scaling factor, the Indian Ocean SST reacts by approximately 0.2 deg C for each 1 deg C rise in Nino 3.4 SST during those events. Refer to Figure 3.
Figure 3

Third, the Indian Ocean does NOT respond proportionately to the entire drop in NINO3.4 SST anomaly from peak to trough after the 1986/87/88 and 1997/98 El Ninos. See Figure 4. Written another way, it appears as though the decreases in the Indian Ocean SST do not occur until after the Nino3.4 SST drops below a threshold slightly above zero.
Figure 4

Fourth, as a result of the disparity in the response of the Indian Ocean to significant El Nino events, Indian Ocean SST anomalies rose in two steps from the mid-1980s to present. Refer again to Figures 1 and 2.


I’ve comment before about the base years used for NINO3.4 SST anomaly data. Which thread, I can’t recall. But the base year of the anomaly data dictates the relative magnitude of El Nino and La Nina events in graphs. Example, if the anomaly data were calculated against a period with lower average NINO3.4 SST, El Ninos would grow in size and La Ninas would shrink. What’s the correct base period to properly illustrate NINO3.4 SST anomalies? It’s probably debatable, but it has to be kept in mind.


If, like the Indian Ocean, Global temperatures also respond to the full rise from trough to peak in NINO3.4 SST during El Nino events but only a portion of the decrease from peak to trough during the subsequent decline, then Global temperatures would rise in steps even during periods when the frequency and magnitude of El Ninos equaled the frequency and magnitude of La Ninas.


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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