Monday, July 18, 2016

Super El Nino Over? Perhaps Not. Base Periods Matter!

In May-June, 2016, it was announced by various official climate agencies, organizations and media outlets that one of the strongest El Nino's recorded was "dead." 

For example, on April 11, 2016, the Washington Post's Angela Fritz, called the "Godzilla El Nino" dead. 
Washington Post: https://www.washingtonpost.com/news/capital-weather-gang/wp/2016/04/11/godzilla-el-nino-is-dead/  In reality, the Nino 3.4 SST's had only dropped below 1C, and it was not till the end of May that Australia's BOM called it over.

El Nino certainly looks over in the June, 2016 SST anomaly map from Climate.gov:
Source: https://www.climate.gov/news-features/featured-images/tropical-pacific-continues-cool-june-2016

This last week (July 11), we were well into ENSO neutral, based upon the CPC Weekly ENSO update which depicted this: 
Source: http://www.cpc.ncep.noaa.gov/products/analysis_monitoring/enso_advisory/index.shtml

Each of these SST anomalies are based upon the SST averages for the years 1981-2010, the current climatological base for determining ENSO SST anomaly values. The science community consensus is to use a 30 year scale which slides every 5 years, to map an El Nino against the most recent observed climate. I understand the rationale and support it.
Source: http://www.cpc.noaa.gov/products/analysis_monitoring/ensostuff/ONI_change.shtml

However, the question that kept bugging me was....so how do these anomalies compare to actual Nino 3.4 sea surface temperatures over time? 

NCDC ERSSTv4 Nino Indices SST's

Fortunately the National Climate Data Center (NCDC), makes available the ERSSTv4 reconstructed and observed sea surface temperatures for the Nino regions from January, 1854 through the most recent month. 

The "Read Me" is here: http://www1.ncdc.noaa.gov/pub/data/cmb/ersst/v4/index/Readme

The actual SST's by month are here: http://www1.ncdc.noaa.gov/pub/data/cmb/ersst/v4/index/ersst.v4.el_nino.sst.dat

The NOAA Climate.gov explanation for ERSSTv4 is well worth the read at:
https://www.climate.gov/news-features/blogs/enso/exactly-same-completely-different-why-we-have-so-many-different-ways

The basis for measuring El Nino/La Nina is warmth or coolness in what is called Nino 3.4, in the central Pacific.
Source: https://www.ncdc.noaa.gov/teleconnections/enso/indicators/sst.php

So what is the actual monthly sea surface temperature trend in this region from 1854-2016? Well, this:


Source NCDC ERSSTv4 SSTs (link above), A4R graphics

What is noticeable, is that beginning in the 1970's the trend curves upward as SST's begin to warm globally, a topic for another post

Also, with the ERSSTv4 actual temperatures. prior climatological bases can be calculated. For the sake of this post, I calculated 1854-1883, 1881-1910, 1951-1980, 1971-2000 and the current one, 1981-1910. Here is what the actual average SST's look like by month for those bases:


Source: NCDC ERSSTv4 SSTs (link above), A4R graphics

So how much difference is there between climatological base periods? There is an "El Nino" difference between 1854-1883 vs 1981-2010 and for many months in the 1881-1910 vs 1981-2010.
Source: NCDC data (link above), A4R graphics

Also El Ninos and La Ninas are rated by strength or severity based upon how strong they are. They are classified as Weak (with a 0.5 to 0.9 SST anomaly), Moderate (1.0 to 1.4), Strong (1.5 to 1.9) and Very Strong (≥ 2.0). No category has been officially created for SST anomalies above 2.5 C.

With these differences in mind, here is the impact of using the earlier base periods to create SST anomalies.

We'll work backwards through time to observe what happens to our current ENSO neutral, and the "Godzilla El Nino."

Here is the current monthly Nino 3.4 graph for January, 1854 through June, 2016, using the 1981-2010 base period.
Source: NCDC data, A4R graphics

There are four very strong El Nino's, with anomalies above 2C: in 1887, 1982, 1997, and 2015.

When we step back in time to a 1971-2000 base, the SST anomalies are higher:
Source: NCDC Data (link above), Graphics, A4R

Note the SSTAs move higher, and the 1889 El Nino joins the "Very Strong" El Nino category. However, our "current" El Nino moves above 2.5 C, making it a new category - perhaps to be called a "Very Very Strong" (VVS) event, which barely nudges out the 1997 event as the all time high anomaly. The 1971-2000 base is used by the NCDC for its El Nino reporting. See the "Read Me" http://www1.ncdc.noaa.gov/pub/data/cmb/ersst/v4/index/Readme

GISS TEMP

Goddard issues its standard monthly reports on the 1951-1980 base. To compare to the global Goddard ERSSTv4 reporting, here is the NCDC Nino 3.4 using that base:
Source: NCDC data (link above), A4R graphics

Each El Nino SSTA is stronger, with both 1997 and 2015 in the "Very Very Strong" (VVS) with a 2.66 C anomaly in November, 2015.

At times, the earliest Goddard SST base available is used for "pre-industrial" comparison of SST and air temperature change. The base is 1881-1910 data. What happens when we move the SST anomaly base back to this "pre-industrial" period is sobering:
Source: NCDC data (link above), A4R graphics

We now have almost four "Very Very Strong" (VVS) El Nino events, 1877-1878 is almost there, along with 1983, 1997 and 2015. With 1997 at 2.85 C anomaly and 2015 at 2.86 C anomaly, we are close to needing new superlatives.

While there are larger uncertainties in the 1854-1883 sea surface temperatures, the following demonstrates how powerful the 1997 and 2015 El Nino high point were compared to past conditions:
Source: NCDC data, A4R Graphics

With this oldest available SST base, the power of these El Nino's becomes even more pronounced. The 1878, 1889, 1972 and 1983 El Ninos are all "Very Very strong" (VVS) anomalies. However, the 1997 and 2015 El Nino peaks deserve that new superlative - Godzilla El Ninos. Perhaps a "GZ" designation.

Sadly, if we keep increasing global heat content, we may have to find a scarier monster for naming SST anomalies above 3.5 C.

Finally, here is the comparison between the 1981-2010 and 1854-1883 bases:
Source: NCDC data (link above). Graphics A4R.

The SSTA numbers above the red peaks are the 1854-1883 base readings, those beside the peaks are the 1981-2010 SST anomalies. The difference is a "weak" El Nino in itself.

So what about the current El Nino not being over?

The June NCDC SST anomalies against these different bases are:

1981-2010:   .41 C - ENSO neutral, as generally reported.
1971-2000:   .54 C - Weak El Nino
1951-1980:   .66 C - Weak El Nino
1881-1910:   .88 C - Weak El Nino
1854-1883: 1.03 C - Moderate El Nino

Given the above bases, El Nino is not over - even worse, we are in historical hot water - a "Global El Nino."

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