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.
INDO-PACIFIC WARM POOL INDEX
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).
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.
Adding scaled NINO3.4 data to the graph for timing purposes illustrates the cause of those step changes. Refer to 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.
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: