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Sunday, December 21, 2008

The Indo-Pacific Warm Pool Index

I’ve moved to WordPress.  This post can now be found at The Indo-Pacific Warm Pool Index

I will be using the Indo-Pacific Warm Pool Index (IPWPI) in an upcoming post and I wanted to introduce it beforehand.

The IPWPI is defined as the average monthly SST for the Indo-Pacific Warm Pool greater than or equal to 28 Deg C. The coordinates listed for the Indo-Pacific Warm Pool are 20S-20N, 90E-180E. I ran across it in the PowerPoint presentation “Decadal Variability of the Indo-Pacific Warm Pool and Its Association with Atmospheric and Oceanic Variability in Winter and Summer” by Mehta and Wang, dated May 7, 2007.

In the presentation, Mehta and Wang also appear to have converted the difference between the IPWP monthly mean temperature and 28 Deg C into an anomaly. I have not taken this extra step in the following. Their illustration on Slide 5 also uses annual data from approximately 1945 to present, where I will use monthly data in the following from 1854 to present for the long-term data and from 1978 and 1980 to present for the short term. They also appear to use a different SST data set, possibly HADSST.


Figure 1 shows the IPWPI for the period of January 1854 to November 2008. The monthly data has been smoothed with a 12-month running-average filter. The major shift (~0.3 Deg C) in 1945 looks to be the result of the “bucket adjustment.” Hopefully, that correction will be made soon. Also note the dip and rebound that occurred in early 20th Century. Then there’s rise from the 1940s to present, which is obviously skewed by the bucket adjustment.

Note: The 0.3 Deg C shift is discussed in “A large discontinuity in the mid-twentieth century in observed global-mean surface temperature”, by Thompson et al (2008).

Figure 1

In Figure 2, the period has been shortened to the last 30 years. Note the significant step change in the IPWPI after the 1997/98 El Nino. Working back in time, there’s also a smaller upward step from 1994 to 1997, one from 1986 to 1988, and one from 1979 to 1982.
Figure 2

Adding scaled NINO3.4 data to the graph for timing purposes illustrates the cause of those step changes. Refer to Figure 3.
Figure 3

If the upward step changes in the IPWPI result from El Nino events, why then weren’t there step changes during the 1982/83 and the 1991/92 El Nino events? By overlaying scaled Sato Index data, Figure 4, the impact of the El Chichon and Mount Pinatubo eruptions on the IPWPI becomes apparent. They suppressed the 1982/83 and the 1991/92 El Nino events, and might have impacted the short upsurge in NINO3.4 SST anomaly that occurred during the Spring of 1993. In May of 1993, NINO3.4 SST anomalies reached 1.4 deg C, which is well above the threshold of an El Nino.
Figure 4


It is becoming more evident that the relatively high number of El Nino events since the late 1970s may be caused by the suppression of the El Nino events that occurred at the same time as the El Chichon and Mount Pinatubo volcanic eruptions. During those periods, the volcanic eruptions appear to reduce El Nino heat transfer within the tropics. It would therefore seem likely that the normal export of heat from the tropics poleward would be diminished as well.


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

Sato Mean Optical Thickness data is available from GISS:


Adolfo Giurfa said...

So you are saying that the energy budget was increased due to aerosols, say SO2 acting as a positive feedback.

Bob Tisdale said...

Adolfo, no, I'm not saying that the energy budget was increased due to aerosols. I am also not saying that SO2 was acting as a positive feedback.

I was saying the the volcanic aerosols reduced (to nill) the normal response of the IPWPI to El Nino events. If the normal response of the IPWPI is represented by the upward step changes that occurred after the 1986/87/88 and 1997/98 El Nino events, then the El Chichon prevented that upward step after the 1982/83 El Nino.

JimSteele said...

Confounding the volcanic explanation is the regime shift caused by the PDO in the late 70's, which I would favor. I suspect the interplay of the harmonics of the major oscillations, like the PDO and ENSO, cause significant shifts in fluid transfer, similar to the way tides can be shifted. The different spatial pattern of the warming, not seen in graphs that just show SST anomalies, makes me suspect the PDO, because it would more likely effect location of upwellings and downwelling. Whereas a shift by Volcanics more likely would uniformly effect the speed of radiative transfer between the atmosphere and sea surface.

Trenberth's 2002 paper describes the late seventies as a shift in El Nino's pattern of SST's and distribution of heat as follows.

"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. "

And more interesting:

"The eastern Pacific south of the equator warms 4–8 months later for 1979–1998 but cools from 1950 to 1978."


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