Ensemble Oceanic Nino Index

Ensemble Oceanic NINO Index (ENS ONI)

**UPDATED: MAR 4, 2024**

Figure 1. ENS ONI time series (1850 - Nov 2023)
Created using Matlab 2022b

Figure 2. Pearson correlation coefficient (R-value) between the Southern Oscillation Index (SOI) and SST anomalies (1891-2010), from a mean of COBE SST2, HADISST2, ERSSTv5, ERSSTv4, Kaplan Extended SSTv2, HADISST, & COBE SST (all datasets bi-linearly interpolated to 5x5° grid spacing using nctoolkit python package), adapted from figure 1 in Webb & Magi (2021) & is intended to be analogous to figure 2 in Barnston (1997). The NINO 3.4 region (5°N - 5°S, 170°W - 120°W) is demarcated by a solid white box. An R-value of -0.8 is denoted by the dotted white line.
Note: The land-sea mask in Kaplan Extended SSTv2 causes discontinuities (white space) near the edges of the figure.
Created in Python v3.9

Figure 3. Leading mode (1st EOF (unitless)) of Tropical Pacific (25°N - 25°S, 140°E - 90°W) SST anomalies (1891-2010), from a mean of COBE SST2, HADISST2, ERSSTv5, ERSSTv4, Kaplan Extended SSTv2, HADISST, & COBE SST (all datasets bi-linearly interpolated to 5x5° grid spacing using nctoolkit python package), adapted from figure 1 in Webb & Magi (2021) & is intended to be analogous to figure 3 in Barnston (1997).
The NINO 3.4 region (5°N - 5°S, 170°W - 120°W) is demarcated by a solid white box.
Note: The land-sea mask in Kaplan Extended SSTv2 causes discontinuities (white space) near the edges of the figure.
Created in Python v3.9

MAJOR UPDATES TO THE ENSEMBLE OCEANIC NINO INDEX (ENS ONI)

November 2023: Due to the dropout of a few long-term SST datasets (e.g. Kaplan Extended SSTv2), several high-resolution SST datasets were added and converted a 1x1° degree grid to improve the near real-time estimates. These datasets include: The Operational Sea Surface Temperature & Sea Ice Analysis (OSTIA), Coral Reef Watch (CRW) SST, ODYSSEA SST, European Space Agency SST Climate Change Initiative (ESA SST CCI)

July 2022: MERRA-2 reanalysis was extended from 2016 to the present

April 2022: Raw monthly ENS ONI input from all 32 SST + reanalysis datasets have been released

February 2022: ENS ONI published in the International Journal of Climatology
(Accepted PDF): The Ensemble Oceanic Nino Index (Webb & Magi, 2022)

May 2021: ENS ONI median definitions are now presented in percent instead of raw numeric values to allow for more intuitive interpretations of the data.

April 2021: ENS ONI median definitions (described below) were added to the suite of available output.

March 2021: ENS ONI data for 1850-1864 has now been released, several datasets were added, and the juxtaposition of ENS ONI's 30-year sliding base periods were updated to be more symmetric with the 5-year periods (Dr Brian Magi, person comm). The pre-1865 extension of the ENS ONI reveals a potential El Nino event in 1855-56, which is strongly supported by recent work from Lin et al (2020) depicting this particular El Nino to be associated with the 2nd worst drought during the Qing dynasty in China. Applying a 30-year sliding median to Madras SLP (1796-2003) reconstructed by Allan et al (2002) also reveals substantial evidence of an El Nino in 1855-56 w/ 30-year median SLP anomalies exceeding +1.2σ in November 1855, which is near the top 10% of all Novembers in 1796-2003.

Experiments with pre-1850 instrumentally-based data like the aforementioned Madras SLP and available reanalysis over this period (like NOAA 20CRv3 (Slivinski et al (2019)) and corroboration with documentary sources such as
Garcia-Herrera et al (2008), Ortileb (2000), & Quinn et al (1987) reveal fairly substantial evidence of an El Nino in the winter of 1845-46.

Simple Ocean Data Assimilation version 2.2.4 (SODAv2.2.4) (1871-2010) (Giese and Ray (2011)), ECMWF's Ocean Reanalysis System 5 (ORAS5) (Zuo et al (2019)) (which is used for the operational ECMWF model), & NASA's Modern-Era Retrospective analysis for Research and Applications Version 2 (NASA MERRA2) (Gelaro et al (2017)), have been added to the ENS ONI, further improving the overall quality of the index and accompanying estimates of inter-dataset spread, especially for the 1871-1900 period when few datasets are available and most SST reconstructions (e.g. HADISST1) that rely on modern ENSO behavior to reconstruct ENSO when data coverage is sparse, may underestimate the amplitude of major ENSO events like 1877-78 (Giese and Ray (2011)).

For comparison purposes and to further validate the legitimacy of ENS ONI's first-order estimates of structural uncertainty that emerge between datasets, ENS ONI's inter-dataset spread is now also directly accompanied by other published estimates of NINO 3.4 region spread from NOAA's 20CRv3 (Slivinski et al (2019)) and HADISST2 (Titchner and Rayner (2014)) (See figure and the excel sheet below).

The previous quality control procedure used to filter out potentially spurious data was discontinued as the median used to derive the index here is fairly robust to outliers.

An additional excel sheet with peak ENSO event intensities (in °C) has been added to the list of deliverables provided below on this web page.

October 2020: HADISST2.1 was added to the ENS ONI, significantly increasing the quality of the ENS ONI prior to 1900, and the new quality control procedure used to filter out potentially spurious data in the ENS ONI is significantly less stringent. OISSTv2 was upgraded to OISSTv2.1, which may slightly impact recent ENS ONI data.
ENS ONI now uses median SSTs instead of mean SSTs from available datasets at each time step to better protect the ENS ONI against outliers and preserve the amplitude of pre-1950 ENSO events. Utilizing the median SST also more accurately depicts well known asymmetries in ENSO amplitude stemming from more efficient Bjerknes feedbacks and stronger non-linearities between SSTs, surface winds, and locally forced convection in El Ninos. To better reflect this asymmetry, ENSO event intensity definitions were altered from, with "Super La Nina" no longer being utilized. However, keeping in spirit of the original ONI, ENS ONI still utilizes 3-month running means of monthly ONI values to determine tri-monthly ENS ONI values. The ENS ONI is also now accompanied by uncertainty estimates, determined by inter-dataset spread at each monthly time step, approximated as 2σ, generally corresponding to 95% level statistical significance.

July 2019: ERA-5 was added to the analysis & interpolated to a 1x1 degree grid to make it more comparable to other datasets used in the ENS ONI. Its addition resulted in little notable change in the ENS ONI after 1964. The ENS ONI quality control procedure was reevaluated for the 1877-78 El Nino, resulting in more realistic amplitude evolution during the growth phase of this "Super" El Nino in 1877. Changes in the 1877-78 El Nino also implicated nearby portions of the record, leading to an extremely modest modification of other ONI values from 1865 to 1890.

June 2019: HADSST4 was added to the analysis, resulting in a slight increase in amplitude of most pre-1950 ENSO events due to better constraint of quality control procedures. The standard deviation of available SST datasets used in the ENS ONI is now also provided.

Introduction

The Ensemble Oceanic NINO Index (ENS-ONI) is one of the longest running, real-time, instrumentally-based ENSO indices that has been developed to date, spanning from the mid 19th century to the present. The ENS ONI also arguably provides the most complete and up-to-date description of structural uncertainty amongst a multitude of SST reconstructions and reanalyses (for more on structural uncertainties see: Kennedy (2014)). The ENS ONI closely follows the methodology of the Climate Prediction Center (CPC), and is computed by using seasonally averaged sea surface temperature (SST) anomalies in the NINO 3.4 region (5S-5N, 120-170W) over the Equatorial Pacific as defined by Barnston et al (1997) using 32 SST and reanalysis datasets (unlike CPC's ONI which only uses Extended Reconstructed Sea Surface Temperature Version 5 (ERSSTv5) (see Huang et al (2017) ). Following the Barnston et al (1997) publication, and the realization that the NINO 3.4 index was highly correlated with other measures of ENSO (figures 2 and 3 above), the NINO 3.4 index became one of the most popular ENSO indices at the turn of the 21st century. The rather exceptional combined intensity and longevity of the 2014-16 multi-year "Super" El Nino event in the context of modern single-year modern "Super" El Nino events in 1972-73, 1982-83, & 1997-98 spurred the creation of the ENS ONI. The ENS ONI attempts to provide as long and reliable of an instrumentally-based ENSO record as possible maintaining simplicity in its construction and being available in real-time, while also providing uncertainty estimates. While the quality and quantity of data platforms and spatiotemporal coverage is relatively less reliable, less numerous, and more sparse respectively, the number of ENSO event samples under the ENS ONI is more than double the modern "reliable" record (1950-present).

Aside from the large number number of datasets and uncertainty estimates, one of the primary methodological differences between the ENS ONI and the CPC ONI is the choice to use climatological base periods based on median SSTs instead of mean SST. This allows the ENS ONUI to better characterize ENSO asymmetry, as SST climatologies may be slightly skewed towards El Ninos (e.g. An & Jin (2004)) and makes the ENS ONI more robust to outliers, which is important early in the record when SST uncertainty is large and there are few(er) datasets available to constrain the ENS ONI. A more detailed description of methods is described in Webb and Magi (2021)

The ENS ONI is intended to be a predecessor product to the Extended MEI version 2 (MEI.extv2), which is currently in production. For more on MEI.extv2 see my 100th Annual American Meteorological Society (AMS) Conference poster presentation: "Reanalysis of the Extended Multivariate ENSO Index"

2020 Master's Thesis on the "Reanalysis of the Extended Multivariate ENSO Index"

Figure 4. December 1877 Tropical Pacific sea surface temperature anomalies via mean of HADISST2.1's 10 ensemble members

Created in Python v3.9

Figure 5. NINO SST regions
NINO 1+2: (0°-10°S, 80-90°W), NINO 3: (5°N-5°S, 90°W-150°W), NINO 3.4 (5°N-5°S, 120°W-170°W), NINO 4 (5°N-5°S, 160°E-150°W)

Obtained at: https://tel.archives-ouvertes.fr/tel-02301101/document

Data

The ENS ONI utilizes World Meteorological Organization (WMO) 30 year climatological base periods that are updated every 5 years (Table 1) to account for significant changes and inhomogeneities in the background climatic and observational mean state in the last few centuries. Raw data rounded to the nearest 0.01C was attained and datasets such as Kaplan's Extended SST Version 2, were converted from anomalies to raw data using their respective climatologies.

Although this index exhibits very robust correlations with the Bivariate ENSO Timeseries (BEST), Multivariate ENSO Index (MEI), Extended Multivariate ENSO Index (MEI.ext), this product should be used with caution, especially for prior to 1877 and in/around the first and second World War. The following 32 sea surface temperature, satellite, and reanalysis datasets were utilized in the creation of the ENS ONI at a grid spacing that varied between 1°x1° and 5°x5° latitude and longitude to ensure all were resolving the same types of phenomena in their respective fields. If these datasets weren't explicitly provided with this grid spacing, they were averaged over a number of grid boxes until this criteria was met.

HADISST2.1 (1850-2010)
COBE SST 2 (1850-2019)
HADSST4 (1850-2021)
NOAAs 20th Century Reanalysis Version 3 (1850-2015)
NOAAs 20th Century Reanalysis Version 2c (1854-2014)
ERSST Version 5 (1854-Present)
ERSST Version 4 (1854-2020)
Kaplan's Extended SST Version 2 (1856-2023)
HADISST (1870-Present)
NOAAs 20th Century Reanalysis Version 2 (1871-2012)
SODAv2.2.4 (1871-2010)
IOCADSv3 (1878-2020)
ERSST Version 3b (1880-2020)
COBE SST (1891-Present)
ERA-20CM (1900-2010)
ERA-20C (1900-2010)
CERA-20C (1901-2010)
NCEP/NCAR Reanalysis Version 1 (1948-Present)
HADSST2 (1950-2014)
HADSST3 (1950-2020)
ERA-5 (1950-Present)
WHOI OA Flux Version 3 (1958-2019)
ECMWF ORAS5 (1958-2019)
Climate Analysis Center (CAC) (now known as the CPC) SST (1970-March 2003)
NCEP CFSR (1979-2015)
NCEP DOE R2 (1979-Present)
ERA-Interim (1979-2019)
NASA MERRA (1979-2016)
NASA MERRA2 (1980-2023)
European Space Agency SST Climate Change Initiative (Sep 1981 - 1988) (ESA SST CCI)
Operational Sea Surface Temperature & Sea Ice Analysis (OSTIA) (Oct 1981-Present)
OISSTv1 (1 degree) (Nov 1981-March 2003)
OISSTv2.1 (1 degree) (Nov 1981-Present)
Aqua/MODIS SST (1 degree) (July 2002-Present)
Coral Reef Watch (CRW) SST (Apr 1985-Present)
ODYSSEA SST (Jan 2021 - Present)

30-Year Climatological Base Period	ENS ONI Values
1850-1879	1850-1867
1855-1884	1868-1872
1860-1889	1873-1877
1865-1894	1878-1882
1870-1899	1883-1887
1875-1904	1888-1892
1880-1909	1893-1897
1885-1914	1898-1902
1890-1919	1903-1907
1895-1924	1908-1912
1900-1929	1913-1917
1905-1934	1918-1922
1910-1939	1923-1927
1915-1944	1928-1932
1920-1949	1933-1937
1925-1954	1938-1942
1930-1959	1943-1947
1935-1964	1948-1952
1940-1969	1953-1957
1945-1974	1958-1962
1950-1979	1963-1967
1955-1984	1968-1972
1960-1989	1973-1977
1965-1994	1978-1982
1970-1999	1983-1987
1975-2004	1988-1992
1980-2009	1993-1997
1985-2014	1998-2002
1990-2019	2003-2021

Table 1. ENS ONI 30-year base periods (updated every 5 years) (left) with corresponding ENS ONI values (right).

Data for the ENS ONI was obtained via:
Climate Prediction Center (CPC): Monthly Atmospheric and SST Indices
Copernicus Marine Service
ECMWF Climate Reanalysis
ECMWF Copernicus Climate Change Service
Global Climate Observing System (GCOS) Working Group on Surface Pressure (WG-SP)
International Research Institute for Climate and Society
Koninklijk Nederlands Meteorologisch Instituut (KNMI) Climate Explorer
Met Office Hadley Centre observation datasets
NASA Earth Observations
NCAR's Research Data Archive
NOAA's Earth System Research Laboratory
NOAA's Science Data Integration Group Live Access Server
Pacific Islands Ocean Observing System (PacIOOS) ERDDAP

Discussion and analysis

Uncertainty in SST observations

Although the ENS ONI is comprised of a multitude of datasets with disparate spatial and temporal resolutions, and only utilizes SST anomalies in a seemingly inconsequential region of the equatorial Pacific, the standard deviation amongst various available datasets appears to be somewhat reflective of global trends during the instrumental record. This very crude estimate of parametric uncertainty reflects structural uncertainties amongst available datasets and their sensitivity to data input, which occurs because most datasets in the ENS ONI are effectively not constructed the same way and many datasets include different versions of ICOADS and/or other observational sources (especially reanalysis datasets) respectively. Both are important points regarding uncertainty in the instrumental record that tend to be less frequently discussed in literature. Nonetheless, from the standard deviation of SSTs in the NINO 3.4 region, three primary periods of contention and notable disagreement emerge from the observed record, namely before the 1877-78 "Super" El Nino, and during the first and second World Wars. The period before 1877-78 and the first World War is largely explained by a lack of data with the latter data outage being associated with ships being sunk during the war. The apparent discrepancy amongst available datasets in and around World War 2 however is multi-faceted, featuring both a similar loss of potential data from ships sunk in the war in addition to how SST observations were taken, with many ships switching from insulated bucket to engine room intake (ERI) measurements of SST and sampling more frequently during the daytime. This paradigm shift in SST collection strategies in/around World War 2 is potentially attributable to safer wartime sampling conditions during daylight as utilization of lights on-board at night to carry out more traditional bucket measurements of SST could attract enemy ships (Freeman et al (2017), Kent et al (2017) (also see figure 6 below). A comparison of the ENS ONI's uncertainty estimate against the HADISST2.1 ensemble and NOAA 20CRv3 is also shown in figure 7 and available for download further down the page.

Figure 6. ICOADS R3.0 Tropical Pacific [30S-20N, 100E-80W] percent of SST (black) and SLP (blue) observations made in daylight hours.
Created using Matlab 2019a

Figure 7. NOAA's 20th Century Reanalysis Version 3 (NOAA 20CRv3) (red), Hadley Centre Sea Ice and Sea Surface Temperature version 2.1 (HADISST2.1) (blue), & Ensemble Oceanic Nino Index spread (black) (annually smoothed) (1851-2010)
Created using Matlab 2019a

COMPARING ENS ONI TO Independent proxy & documentary records

The potential for under-dispersiveness in how instrumental-based SST datasets and reanalyses may represent pre-modern era (or prior to 1950) ENSO variability due to similarity in methods from different dataset curators (e.g. (Giese and Ray (2011)), it's important to also look at independently-derived records of ENSO (namely, proxy (tree rings, ice cores, corals, etc.) and documentary/written records) that don't have any dependence whatsoever on instrumentally-based derivations of SST. Doing so allows for an independent evaluation of instrumentally-based records of ENSO prior to the modern-era (~ 1950), when in-situ observations are sparse, and uncertainty in what limited data exists is much larger.

Here, 13 literature sources (Quinn et al (1987), Stahle et al (1998), Cook et al (2008), Braganza et al (2009), Gergis & Fowler et al (2009), McGregor et al (2010), Wilson et al (2010), Li et al (2011), Hakim et al (2016), Anderson et al (2019), Freund et al (2019), Tardif et al (2019), Dätwyler et al (2020), Sanchez et al (2020)) are utilized during their shared, overlapping period (1850-1977), Most of these proxy records have vastly different units and variables, and the written records are qualitative in their classification of ENSO. To facilitate a comparison amongst them, an approach modified from Wolter and Timlin (2011), whereby the annual percentile ranks are analyzed from all proxy + documentary records, and compared to the fraction of institution and literature based definitions (below) based on the Ensemble ONI. The upper, middle, and lower terciles represent El Niño, Neutral ENSO, and La Niña, respectively, except for Stahle et al (1998), which is an SOI-based proxy index where the signs are inverted. The annual fraction of proxy + documentary records that designate El Nino or La Nina conditions is shown by the black line, whereas the ENS ONI % of institution + literature definitions met shaded (red = El Nino, blue = La Nina). For reference, the ENS ONI time series is also plotted (bottom).

Overall, the agreement between instrumental and proxy + documentary records of ENSO is pretty strong even prior to 1950. Unsurprisingly, the largest discrepancies between these 2 independent sources of data exist during the 1850s. For example, proxy + documentary records suggest an El Nino in 1852-53 and a La Nina in 1863 are significantly muted/underestimated by instrumental data, whereas a moderately-intense La Nina depicted during the winter of 1856-57 in the ENS ONI may be an artifact.

Figure 8 (from Webb and Magi (2021)): 1850–2021 time series of (a) the percent agreement with 33 literature-based definitions (see Table 2) of El Niño (positive values in light red) and La Niña (negative values in light blue), and the percent agreement with multiple proxy + documentary records of ENSO events from using percentile ranks for the overlapping 1850 to 1977 period (Wolter and Timlin (2011) methodology), except for documentary records, where ENSO phase/intensity is explicitly stated (e.g. Quinn et al (1987)), (black lines), and (b) the Ensemble ONI values shaded to only show El Niños (red) and La Niñas (blue) when greater than 60% of the literature-based definitions are met. Values in grey are when neither El Niño or La Niña conditions are met by greater than 60% of the definitions

ENSO phase and intensity

Attempts at reconstructing ENSO using multiple datasets and the Oceanic Nino Index has been attempted before as recently as Huang et al (2016), however their analysis was strictly limited to only several SST datasets and over the period 1950-present.

Following the methodology of Wolter and Timlin (2011), El Nino and La Nina tri-monthly periods are further binned into Weak, Moderate, Strong, and "Super" based on their tri-monthly ENS ONI percentiles shown below in table 2. El Ninos, Neutral, and La Nina are generally assumed to each occur roughly one-third of the time, however unlike Wolter and Timlin (2011), the ranks are flipped such that El Ninos occupy the lowest ranks and La Ninas the highest, we use all bi-monthly periods to rank ENSO instead of individual bi-monthly periods, and the El Nino and La Nina thresholds correspond to the lower tercile (33%) and uppermost tercile (67%) respectively as opposed to the end of the 3rd decile (30%) and beginning of the 7th decile (70%), which does not change the conclusions we draw here, although it increases the detectability of ENSO events ever-so-slightly in ENS ONI. The distribution of El Nino and La Ninas using the ENS ONI with NOAA CPC’s definition of 5 successive tri-monthly periods greater than or equal to +/-0.5C, found that La Nina and El Nino occur about 30% of the time. Therefore, the choice of ENSO threshold here is well within reason and similar to other popular ENSO indices and previous published studies.

Thus, going by the definition of ENSO noted here, El Ninos occur when the ENS ONI values are in the top 33% of all ENS ONI values for each tri-monthly period, while Neutral ENSO and La Nina correspond to the middle third and lower third of bi-monthlies respectively. Further distinctions to denote Weak, Moderate, Strong, and Super (very strong) ENSO events were also made here as was done with MEI.ext and MEI. Similar to Wolter and Timlin (2011), moderate El Ninos and moderate La Ninas were assumed to be ongoing when the MEI.extv2 bi-monthly percentiles were in the 2nd (10-20%) and 9th deciles (80-90%) respectively. Also similar to Wolter and Timlin (2011), the minimum threshold for Strong El Nino and Strong La Nina is defined as the 10th and 90th percentiles respectively.
A further distinction is made to identify extraordinarily powerful El Nino and La Nina events. The motivation for this is attributable to the fact that 4 very intense El Nino events (1877-78, 1982-83, 1997-98, 2015-16) are all more intense than not only any other El Nino, but all other ENSO events in the observed record. Furthermore, all of these El Ninos events have triggered exceptionally large and widespread climate anomalies across the globe, and the term “Super El Nino” has become increasingly popular and acceptable nomenclature in the atmospheric science community (Hameed et al (2018); Zhu et al (2018); Bing and Xie (2017); Chen et al (2016); Hong (2016); Latif et al (2015).

Table 2. ENS ONI ENSO phase and intensity (left), rank (middle), and percentiles (right). Generally analogous to Wolter & Timlin (2011)

Instead of using percentiles for each tri-monthly period as previously described, another of popular way of defining ENSO intensity, as is done by Jan Null, is to use the peak intensity of the totality of a boreal winter and/or ENSO event (Table 3). This leads to slightly different numbers of events in each classification compared to Table 2 above, but both methods are arguably equally valid ways of subjectively defining ENSO, depending on the preference and intentions of the user.

Table 3. ENS ONI ENSO phase (left) peak tri-monthly intensity (in °C) (right)

ENSO Event Phase and Peak Intensity List (1850-2024)

"Super" El Nino: 1877-78, 1888-89, 1972-73, 1982-83, 1997-98, & 2015-16

Strong El Nino: 1896-97, 1899-00, 1902-03, 1918-19, 1930-31, 1940-41, 1957-58, 1965-66, 1987-88, 1991-92, & 2009-10, 2023-24

Moderate El Nino: 1855-56, 1868-69, 1885-86, 1904-05, 1905-06, 1911-12, 1913-14, 1925-26, 1939-40, 1941-42, 1951-52, 1963-64, 1968-69, 1976-77, 1977-78, 1986-87, 1994-95, & 2002-03

Weak El Nino: 1852-53(?), 1864-65, 1865-66, 1867-68(?), 1868-69, 1876-77, 1880-81, 1883-84, 1884-85, 1887-88(?), 1891(?), 1895-96(?), 1900-01, 1914-15, 1919-20, 1929-30, 1923-24, 1952-53, 1953-54(?), 1958-59, 1969-70, 1979-80, 2004-05, 2006-07, 2014-15(?), 2018-19, & 2019-20(?)

Weak La Nina: 1869-70, 1870-71, 1872-73, 1873-74(?), 1874-75(?), 1879-80(?), 1898-99(?), 1903-04, 1906-07(?), 1908-09, 1910-11, 1915-16, 1917-18, 1921, 1922-23(?), 1938-39, 1944-45, 1950-51, 1954-55, 1956-57, 1964-65, 1971-72(?), 1974-75, 1978(?), 1983-84(?), 1989-90, 1995-96, 2000-01, 2005-06, 2008-09, 2016-17, 2017-18

Moderate La Nina: 1849-50, 1856-57, 1886-87, 1889-90, 1892-93, 1893-94, 1909-10, 1924-25, 1933-34, 1942-43, 1949-50, 1970-71, 1975-76, 1984-85, 1998-99, 2011-12, 2020-21, 2021-22, 2022-23

Strong La Nina: 1916-17, 1955-56, 1973-74, 1988-89, 1999-00, 2007-08, 2010-11

Table 4. ENSO Event Intensity List (1850-2024)

Figure 7. Standardized ENS ONI time series (1850 - Feb 2024) MEI.ext style
Created using Matlab 2022b

ENSO Definitions

Shown below is a table with 50 ENSO definitions based on either SST anomalies, SST standardized anomalies, or percentiles. The corresponding reference from where this definition originates is hyperlinked in the far left column. While the NOAA/CPC ONI definition (5 successive tri-monthly periods where SST anomalies are > +/- 0.5°C) is arguably the most commonly used in the scientific community, it's very far from the only one that's been frequently cited, utilized, and/or mentioned for real-time and post-mortem research based applications. Users of the ENS ONI should be aware that no matter how good any particular definition sounds or might seem, there is always some level of nuance involved, and that even slight modifications to a definition can lead to very different statistical and/or dynamical interpretations depending on the intention(s) of the individual and uncertainty in the underlying data. However, like combining a multitude of datasets, synthesizing a large ensemble of definitions from many prominent institutions and researchers increase the robustness, stability, and confidence that the output and conclusions derived from them are truly legitimate and significant.

This unique analysis of many ENSO definitions also allows us to obtain an estimate of opinions from experts in the field on what is and is not ENSO at some time "t" based on output from the ENS ONI; a crude measure of consensus.

Considering all of the above, it seems reasonable to believe that when at least a majority of the 50 definitions (25 of 50 (or more)) reach a consensus that an El Nino or La Nina is ongoing for a given monthly, bi-monthly, tri-monthly, and/or pentad period based on the ENS ONI, that an ENSO event is indeed probably, if not, likely ongoing. We will tentatively refer to this measure of ENSO as the ENS ONI Definitions Index. In order to ensure an apples-apples comparison between all the definitions, each one is given equivalent weight of up to 1 w/ a binary output (1 or 0/pass or fail) signaling the specified criteria in each definition was or was not met at a particular time step. To determine the time that this actually corresponds to for instances where tri-monthly or pentad averaged values of this ENS ONI are used, the middle month of that period is utilized. For ex, if we're looking at an ENSO definition that uses a pentad-averaged period to categorize ENSO and we're analyzing Oct-Feb (ONDJF), the binary output would be placed in the month of December, w/ January being used in place of NDJFM, etc. For bi-monthlies, these ENS ONI Definitions index values are binned into the first month of the bi-monthly period. To designate both El Ninos and La Ninas in this paradigm, La Nina (El Nino) values are negative (positive) and shaded blue (red).

Depicted in Table 6 is the ENS ONI Median Definitions Index for 2010-present. Raw values (0 to (+ or -) 50) are converted to percent, with values <-49(%) signifying that a majority of ENSO definitions recognize La Nina conditions, whereas values >+49 (%) correspond to where most definitions contend that El Nino conditions are occurring. Note that this definitions index will lag in real-time by a few months, due to the inclusion of literature-based ENSO definitions that are dependent on pentad (5-month) averaged data.

Hyperlinked Excel file for the ENS ONI definitions index (evaluated on the median SST anomaly value presented here)
ENS ONI Median Definition Index (1850 - Feb 2024)
**Assessments and further applications of this ENS ONI Definitions Index (esp as it pertains to uncertainty) are currently being experimented with and may be released in the future**

Institution/Reference(s)	Criteria Amplitude	Criteria Length
Abhik et al (2019)	> +/- 0.75°C SST	1 tri-monthly
Ashok et al (2007)	> +/- 0.7σ SST	3 tri-monthlies
Australian Bureau of Meteorology (BOM)	> +/- 0.8°C SST	Any month
Bove et al (1998)	> +/- 0.5°C SST	6 consecutive pentads
Cai et al (2020)	> +/- 0.75σ SST	1 tri-monthly
Chen et al (2004) & Yu et al (2011)	> +/- 1.0°C SST	Any month
Coelho et al (2002)	> +/- 2.0°C SST	Any month
Gergis & Fowler (2005)	> +/- 0.5°C SST	6 consecutive months
Glantz & Ramirez (2020)	> +/- 1.0°C SST	1 tri-monthly
Goddard & Dilley (2005)	El Nino: 25th %ile, La Nina: 75th %ile	1 tri-monthly
Guilyardi (2006)	> +/- 1.0σ SST	Any month
Hanley et al (2003), Goddard & Dilley (2005), Callahan et al (2021)	El Nino: 25th %ile, La Nina: 75th %ile	Any month
Hanley et al (2003)	> +/- 1.0σ SST	4 consecutive months
Hendon et al (2009)	> +/- 0.7σ SST	1 tri-monthly
Japan Meteorological Agency (JMA)	> +/- 0.5σ SST	6 consecutive pentads
Kao & Yu (2009)	> +/- 1.0σ SST	3 consecutive months
Kug et al (2009) and Ham & Kug (2011)	> +/- 1.0σ SST	6 consecutive months
Luan et al (2012)	> +/- 1.2σ SST	Any month
Lucena et al (2011)	> +/- 0.5°C SST	2 consecutive tri-monthlies
McGregor et al (2010) and Khordi et al (2017)	> +/- 0.5σ SST	Any month
Cai et al (2020), Hurwitz et al (2011), Meehl et al (2006), Tian et al (2017), and Yuan & Yang (2012)	> +/- 1.0σ SST	1 tri-monthly
Nguyen et al (2020)	> +/- 0.6°C SST	1 tri-monthly
NOAA/CPC	> +/- 0.5°C SST	5 consecutive tri-monthlies
Phillips et al (1998)	> +/- 0.7σ SST	4 month period
Predybaylo et al (2017)	> +/- 1.4°C SST	1 tri-monthly
Predybaylo et al (2017)	> +/- 1.0°C SST	1 tri-monthly
Qian et al (2011) and Voskresenskaya & Marchukova (2017)	> +/- 0.5°C SST	5 consecutive months
Rimbu et al (2004) and Roundy et al (2010)	> +/- 0.75σ SST	Any month
Santoso et al (2017) and Amaya & Foltz (2014)	> +/- 0.5σ SST	1 tri-monthly
Sardeshmukh et al (2000)	El Nino: 33rd %ile, La Nina: 67th %ile	1 pentad
Shabbar et al (2006)	> +/- 0.5σ SST	1 pentad
Shrestha & Kostaschuk (2005)	> +/- 0.5σ SST	5 consecutive pentads
Shukla et al (2011)	> +/- 0.4°C SST	Any month
Shukla et al (2011)	> +/- 0.5°C SST	Any month
Song et al (2015)	> +/- 0.4σ SST	4 consecutive months
Takahashi et al (2014)	> +/- 1.0°C SST	3 consecutive months
Takahashi et al (2014) and Lemmon & Karnauskas (2019)	> +/- 0.4°C SST	3 consecutive months
Trenberth & Hoar (1997)	> +/- 0.4°C SST	6 consecutive tri-monthlies
Tsonis & Swanson (2008)	> +/- 1.0σ SST	1 pentad
Turkington et al (2019)	> +/- 0.25°C SST	5 consecutive tri-monthlies
Wang et al (2000)	> +/- 0.5°C SST	8 months
Wang et al (2001)	> +/- 1.5°C SST	1 tri-monthly
Wang et al (2019)	> +/- 0.6°C SST	1 pentad
Wolter & Timlin (2011)	El Nino: 30th %ile, La Nina: 70th %ile	1 bi-monthly
Xie & Jin (2018)	> +/- 0.5σ SST	5 consecutive months
Yeh et al (2009) and Ren & Jin (2011)	> +/- 0.5°C SST	1 tri-monthly
Zanchettin et al (2007)	> +/- 0.4°C SST	1 pentad
Zanchettin et al (2007), Butler and Polvani (2011), and Sullivan et al (2016)	> +/- 0.5°C SST	1 pentad
Zhai et al (2016)	> +/- 1.0°C SST	5 consecutive months
Zhang et al (2015)	> +/- 0.6σ SST	1 tri-monthly

Table 4. Institution and literature-based ENSO Definitions. From left-to-right Institution/reference(s) (left), criteria amplitude (middle), and criteria length (right)

*ENS ONI Literature-Based Definitions Index (% ENSO Definitions Met)*
YR	JAN	FEB	MAR	APR	MAY	JUN	JUL	AUG	SEP	OCT	NOV	DEC
2010	94%	92%	88%	28%	-4%	-82%	-94%	-94%	-98%	-98%	-98%	-98%
2011	-96%	-94%	-88%	-86%	-76%	-40%	-34%	-62%	-70%	-82%	-84%	-84%
2012	-70%	-66%	-60%	-36%	-12%	0%	2%	22%	16%	2%	0%	0%
2013	-4%	-6%	-2%	-4%	-16%	-28%	-18%	-10%	-2%	0%	0%	0%
2014	-2%	-8%	-2%	0%	0%	4%	0%	0%	2%	20%	44%	42%
2015	42%	38%	50%	80%	86%	88%	94%	98%	100%	100%	100%	100%
2016	100%	100%	98%	86%	18%	0%	-20%	-42%	-40%	-46%	-42%	-32%
2017	-6%	0%	0%	0%	2%	4%	2%	0%	-14%	-48%	-68%	-68%
2018	-68%	-68%	-68%	-54%	-16%	0%	0%	0%	12%	54%	66%	68%
2019	58%	60%	72%	66%	60%	40%	20%	0%	0%	6%	10%	10%
YR	JAN	FEB	MAR	APR	MAY	JUN	JUL	AUG	SEP	OCT	NOV	DEC
2020	12%	4%	10%	8%	-4%	-22%	-32%	-62%	-82%	-92%	-92%	-88%
2021	-88%	-82%	-70%	-62%	-40%	-18%	-18%	-32%	-50%	-80%	-78%	-80%
2022	-74%	-82%	-92%	-92%	-92%	-86%	-84%	-88%	-88%	-90%	-86%	-70%
2023	-68%	-34%	-2%	0%	22%	80%	86%	94%	96%	98%	98%	98%
2024	-	-	-	-	-	-	-	-	-	-	-	-

Table 5. ENS ONI Definitions Index evaluated on the median ENS ONI value (in %) (2010-present).

ENS ONI Data

Given all the above, Data for the ENS ONI are available here as hyperlinks in Excel format:

Raw Monthly NINO 3.4 SST Input (1850 - Feb 2024)
Monthly NINO 3.4 SSTs (1850 - Feb 2024)
ENS ONI Median Definitions Index (1850 - Feb 2024)
ENS ONI Monthly Data (1850 - Feb 2024)
ENS ONI tri-monthly (CPC-like) data (1850 - Feb 2024)
ENS ONI standardized tri-monthly data (1850 - Feb 2024)
ENS ONI tri-monthly ranks (1850 - Feb 2024)
ENS ONI, NOAA 20CRv3, & HADISST2.1 uncertainty (1850 - Feb 2024)

The following ENS ONI table (also linked above) uses a modified version of Webb and Magi (2021)'s definition of ENSO. Here, 50 literature-based definitions (table 5) are used instead of 33, and if at least 50% of these ENSO definitions are met surrounding a particular month, it is recognized as a legitimate El Nino (red) or La Nina (blue), irrespective of the event's duration and/or potential discontinuities wrt adjacent months.

Ensemble Oceanic Nino Index (ENS ONI)
YR	DJF	JFM	FMA	MAM	AMJ	MJJ	JJA	JAS	ASO	SON	OND	NDJ
1850	-1.0	-1.1	-1.2	-0.6	-0.1	0.2	0.2	0.1	0.0	-0.1	-0.2	-0.1
1851	-0.2	-0.1	-0.1	0.1	0.2	0.1	0.2	0.2	0.3	0.4	0.3	0.2
1852	0.0	-0.1	-0.1	0.0	0.1	0.2	0.3	0.5	0.5	0.7	0.6	0.6
1853	0.3	0.1	0.1	0.1	0.1	0.1	-0.1	-0.1	-0.1	0.2	0.4	0.4
1854	0.3	0.2	0.2	0.2	0.4	0.3	0.1	-0.1	-0.1	-0.1	-0.3	-0.2
1855	0.1	0.4	0.4	0.3	0.2	0.0	-0.1	0.1	0.5	0.9	1.2	1.2
1856	1.0	0.9	0.6	0.3	-0.1	-0.3	-0.6	-0.8	-1.0	-0.9	-1.1	-1.1
1857	-0.9	-0.7	-0.4	-0.4	-0.1	0.0	0.0	0.0	0.1	0.2	0.1	0.1
1858	0.2	0.2	0.1	0.0	-0.1	-0.2	-0.3	-0.3	-0.1	0.1	0.1	0.1
1859	0.0	0.0	-0.2	-0.3	-0.3	-0.1	0.1	0.2	0.1	0.0	0.1	0.1
YR	DJF	JFM	FMA	MAM	AMJ	MJJ	JJA	JAS	ASO	SON	OND	NDJ
1860	-0.1	-0.3	-0.4	-0.3	-0.2	0.0	0.0	0.1	0.0	-0.1	-0.3	-0.2
1861	0.0	0.0	0.0	-0.2	-0.2	-0.1	0.1	0.1	0.1	0.0	0.0	0.0
1862	0.1	0.2	0.2	0.1	0.2	0.3	0.3	0.2	0.0	-0.3	-0.4	-0.5
1863	-0.3	-0.2	-0.1	-0.4	-0.5	-0.6	-0.4	-0.3	-0.2	-0.1	0.0	0.0
1864	0.0	-0.1	-0.1	-0.1	-0.1	-0.1	0.1	0.3	0.6	0.9	0.9	0.8
1865	0.7	0.8	0.6	0.4	0.2	0.4	0.5	0.4	0.3	0.4	0.6	0.9
1866	0.9	0.7	0.5	0.4	0.4	0.4	0.4	0.3	0.1	0.1	0.1	0.2
1867	0.1	0.1	0.0	0.3	0.5	0.6	0.5	0.4	0.4	0.5	0.5	0.6
1868	0.6	0.6	0.4	0.4	0.4	0.4	0.3	0.2	0.4	0.8	1.1	1.1
1869	0.9	0.7	0.5	0.3	0.3	0.3	0.1	-0.3	-0.6	-0.6	-0.4	-0.5
YR	DJF	JFM	FMA	MAM	AMJ	MJJ	JJA	JAS	ASO	SON	OND	NDJ
1870	-0.6	-0.6	-0.5	-0.6	-0.7	-0.8	-0.6	-0.4	-0.3	-0.4	-0.5	-0.4
1871	-0.3	-0.1	0.0	0.0	0.0	-0.2	-0.2	-0.2	0.0	0.0	0.1	0.1
1872	0.0	-0.2	-0.2	-0.3	-0.3	-0.3	-0.3	-0.4	-0.5	-0.5	-0.6	-0.6
1873	-0.7	-0.7	-0.8	-0.7	-0.5	-0.3	-0.2	-0.1	-0.1	-0.1	-0.2	-0.3
1874	-0.5	-0.7	-0.8	-0.7	-0.7	-0.7	-0.7	-0.8	-0.8	-0.7	-0.7	-0.5
1875	-0.3	-0.2	-0.3	-0.3	-0.4	-0.5	-0.5	-0.6	-0.5	-0.4	-0.2	-0.1
1876	-0.2	-0.3	-0.5	-0.5	-0.6	-0.4	-0.3	-0.1	0.1	0.4	0.7	0.9
1877	0.9	1.0	1.0	1.0	0.9	1.0	1.3	1.8	2.2	2.6	2.8	2.9
1878	2.9	2.5	2.0	1.6	1.3	1.0	0.6	0.3	0.1	0.0	-0.1	-0.2
1879	-0.2	-0.1	0.0	-0.1	-0.3	-0.4	-0.5	-0.6	-0.6	-0.5	-0.5	-0.7
YR	DJF	JFM	FMA	MAM	AMJ	MJJ	JJA	JAS	ASO	SON	OND	NDJ
1880	-0.6	-0.5	-0.3	-0.3	-0.2	-0.1	0.0	0.1	0.4	0.5	0.7	0.8
1881	0.7	0.7	0.6	0.6	0.5	0.4	0.3	0.2	0.1	0.1	0.0	0.0
1882	-0.1	0.0	0.1	0.2	0.2	0.1	-0.1	-0.2	-0.2	-0.3	-0.3	-0.2
1883	-0.1	0.2	0.3	0.4	0.3	0.3	0.2	0.2	0.1	0.2	0.2	0.3
1884	0.3	0.4	0.5	0.5	0.4	0.4	0.4	0.5	0.6	0.8	0.9	1.0
1885	0.8	0.5	0.3	0.2	0.3	0.4	0.4	0.4	0.8	1.2	1.3	1.2
1886	0.8	0.3	-0.1	-0.3	-0.5	-0.7	-0.8	-0.8	-0.8	-0.8	-1.0	-1.1
1887	-1.2	-1.0	-0.9	-0.7	-0.6	-0.3	-0.2	0.0	0.1	0.3	0.5	0.5
1888	0.7	0.7	0.7	0.6	0.6	0.7	0.8	1.0	1.3	1.7	2.0	2.2
1889	2.2	1.9	1.4	0.9	0.6	0.1	-0.4	-0.8	-1.0	-1.1	-1.1	-1.4
YR	DJF	JFM	FMA	MAM	AMJ	MJJ	JJA	JAS	ASO	SON	OND	NDJ
1890	-1.4	-1.3	-1.1	-0.9	-0.8	-0.7	-0.7	-0.7	-0.8	-0.8	-0.6	-0.3
1891	0.0	0.1	0.3	0.3	0.4	0.4	0.4	0.3	0.3	0.3	0.3	0.1
1892	0.0	-0.1	-0.3	-0.4	-0.5	-0.5	-0.7	-0.9	-1.2	-1.4	-1.4	-1.2
1893	-1.1	-1.1	-1.1	-1.1	-1.1	-1.0	-1.0	-1.0	-1.1	-1.1	-0.9	-0.9
1894	-0.9	-0.9	-0.8	-0.8	-0.7	-0.6	-0.4	-0.4	-0.4	-0.5	-0.4	-0.4
1895	-0.3	-0.2	-0.2	-0.1	0.0	0.0	0.2	0.3	0.5	0.6	0.7	0.6
1896	0.4	0.3	0.3	0.2	0.2	0.4	0.7	1.0	1.2	1.5	1.8	1.8
1897	1.7	1.3	0.8	0.3	0.0	0.1	0.1	0.1	0.0	-0.2	-0.3	-0.3
1898	-0.3	-0.4	-0.4	-0.4	-0.3	-0.3	-0.5	-0.5	-0.5	-0.4	-0.4	-0.5
1899	-0.5	-0.4	-0.2	0.0	0.2	0.3	0.4	0.5	0.8	1.2	1.5	1.7

YR	DJF	JFM	FMA	MAM	AMJ	MJJ	JJA	JAS	ASO	SON	OND	NDJ
1900	1.6	1.5	1.3	1.1	1.0	0.9	0.7	0.6	0.5	0.4	0.5	0.5
1901	0.6	0.5	0.4	0.2	0.1	0.0	0.0	0.0	0.0	0.0	0.0	0.0
1902	0.1	0.2	0.4	0.5	0.8	1.1	1.3	1.5	1.6	1.7	1.8	1.8
1903	1.6	1.4	1.1	0.7	0.3	-0.1	-0.3	-0.5	-0.6	-0.7	-0.8	-0.8
1904	-0.7	-0.5	-0.5	-0.3	-0.1	0.1	0.4	0.4	0.6	0.7	0.9	1.1
1905	1.1	1.1	1.0	1.0	1.0	1.2	1.3	1.4	1.4	1.3	1.3	1.3
1906	1.3	1.1	0.9	0.6	0.3	0.0	-0.3	-0.4	-0.5	-0.5	-0.4	-0.3
1907	-0.4	-0.4	-0.4	-0.3	-0.2	-0.1	-0.1	0.0	0.1	0.1	0.0	0.0
1908	0.0	-0.1	-0.3	-0.5	-0.4	-0.4	-0.3	-0.4	-0.5	-0.7	-0.7	-0.8
1909	-0.8	-0.8	-0.7	-0.6	-0.7	-0.8	-0.8	-0.9	-1.0	-1.1	-1.1	-1.1
YR	DJF	JFM	FMA	MAM	AMJ	MJJ	JJA	JAS	ASO	SON	OND	NDJ
1910	-1.1	-1.2	-1.2	-1.1	-1.0	-0.9	-0.8	-0.9	-0.9	-0.9	-0.7	-0.6
1911	-0.6	-0.6	-0.7	-0.6	-0.4	-0.2	0.1	0.2	0.3	0.6	1.1	1.4
1912	1.4	1.3	1.1	0.9	0.6	0.3	0.1	-0.1	-0.1	-0.1	-0.1	-0.1
1913	-0.1	-0.1	0.0	0.0	0.0	0.2	0.3	0.4	0.3	0.4	0.7	1.0
1914	1.1	0.9	0.7	0.6	0.6	0.6	0.7	0.8	0.8	0.8	0.8	1.0
1915	1.0	1.0	0.9	0.9	1.1	0.9	0.6	0.1	-0.1	-0.1	-0.3	-0.5
1916	-0.6	-0.4	-0.3	-0.3	-0.5	-0.8	-1.0	-1.1	-1.4	-1.7	-2.1	-2.1
1917	-1.9	-1.7	-1.3	-1.1	-0.8	-0.7	-0.6	-0.6	-0.7	-0.9	-1.0	-0.9
1918	-0.7	-0.6	-0.4	-0.3	0.1	0.2	0.5	0.6	0.8	1.1	1.4	1.6
1919	1.6	1.4	1.1	0.8	0.6	0.5	0.3	0.2	0.3	0.5	0.6	0.6
YR	DJF	JFM	FMA	MAM	AMJ	MJJ	JJA	JAS	ASO	SON	OND	NDJ
1920	0.8	0.7	0.6	0.3	0.3	0.2	0.3	0.3	0.3	0.2	0.1	-0.1
1921	-0.3	-0.6	-0.8	-0.8	-0.6	-0.5	-0.3	0.0	0.0	-0.1	-0.1	-0.1
1922	0.0	0.0	0.0	0.0	-0.1	-0.1	-0.4	-0.5	-0.5	-0.3	-0.3	-0.4
1923	-0.5	-0.4	-0.3	0.0	0.1	0.2	0.3	0.5	0.7	0.9	0.9	1.0
1924	1.0	0.9	0.5	0.1	-0.3	-0.6	-0.7	-0.9	-0.9	-1.0	-1.0	-1.0
1925	-0.8	-0.6	-0.3	-0.2	-0.1	0.1	0.3	0.5	0.7	1.0	1.2	1.4
1926	1.5	1.5	1.3	1.1	0.9	0.7	0.5	0.3	0.0	-0.2	-0.3	-0.2
1927	-0.1	-0.1	-0.2	-0.3	-0.2	-0.2	-0.1	-0.1	0.1	0.1	0.2	0.3
1928	0.4	0.3	0.1	0.0	0.0	0.0	-0.1	-0.1	-0.1	0.0	0.1	0.1
1929	0.1	-0.1	-0.1	-0.1	0.0	0.0	0.0	0.2	0.4	0.5	0.5	0.6
YR	DJF	JFM	FMA	MAM	AMJ	MJJ	JJA	JAS	ASO	SON	OND	NDJ
1930	0.7	0.7	0.5	0.5	0.5	0.5	0.6	0.8	1.1	1.4	1.6	1.8
1931	1.7	1.6	1.3	1.0	0.8	0.5	0.2	0.0	-0.1	-0.2	-0.2	-0.2
1932	-0.1	0.0	0.2	0.3	0.4	0.2	0.1	0.0	0.0	0.0	0.0	0.0
1933	-0.1	-0.2	-0.4	-0.4	-0.5	-0.7	-0.9	-1.1	-1.1	-1.1	-1.1	-1.1
1934	-1.1	-1.0	-0.8	-0.6	-0.4	-0.2	-0.1	-0.1	-0.1	-0.1	-0.1	-0.1
1935	-0.2	-0.2	-0.3	-0.3	-0.3	-0.3	-0.2	-0.1	0.2	0.4	0.4	0.4
1936	0.4	0.3	0.2	0.1	-0.1	-0.3	-0.4	-0.4	-0.3	-0.2	-0.1	-0.1
1937	0.1	0.1	0.2	0.1	0.0	-0.1	-0.1	0.0	0.1	0.2	0.1	0.0
1938	-0.2	-0.3	-0.5	-0.5	-0.6	-0.8	-1.0	-1.0	-0.9	-0.8	-0.8	-0.8
1939	-0.8	-0.7	-0.5	-0.2	0.0	0.1	0.2	0.2	0.2	0.1	0.1	0.5
YR	DJF	JFM	FMA	MAM	AMJ	MJJ	JJA	JAS	ASO	SON	OND	NDJ
1940	0.8	1.0	0.9	0.9	0.7	0.6	0.7	0.7	0.8	0.8	1.1	1.4
1941	1.6	1.6	1.6	1.4	1.3	1.1	0.9	0.8	0.9	1.1	1.3	1.3
1942	1.1	0.8	0.6	0.4	0.2	-0.2	-0.6	-0.9	-1.1	-1.1	-1.2	-1.2
1943	-1.2	-1.1	-1.1	-0.8	-0.4	-0.1	0.1	0.1	-0.1	-0.2	-0.3	-0.3
1944	-0.2	-0.1	0.0	0.1	0.2	0.2	0.2	0.1	0.0	-0.2	-0.3	-0.3
1945	-0.4	-0.5	-0.7	-0.6	-0.3	-0.2	-0.2	-0.3	-0.1	0.0	0.0	0.0
1946	0.0	-0.1	-0.2	-0.2	-0.1	0.1	0.1	0.0	0.0	0.2	0.3	0.5
1947	0.4	0.3	0.1	0.0	0.0	0.1	0.1	0.1	0.0	-0.1	-0.1	0.0
1948	0.2	0.3	0.4	0.4	0.3	0.3	0.2	0.2	0.1	0.0	0.0	0.1
1949	0.1	0.0	0.0	0.0	-0.1	-0.2	-0.3	-0.2	-0.2	-0.4	-0.7	-0.9

YR	DJF	JFM	FMA	MAM	AMJ	MJJ	JJA	JAS	ASO	SON	OND	NDJ
1950	-1.0	-1.0	-1.1	-1.0	-0.9	-0.8	-0.6	-0.6	-0.5	-0.5	-0.6	-0.7
1951	-0.7	-0.6	-0.3	-0.1	0.1	0.3	0.5	0.8	1.0	1.2	1.1	0.9
1952	0.7	0.5	0.4	0.3	0.1	-0.1	-0.1	0.0	0.2	0.3	0.2	0.3
1953	0.4	0.5	0.5	0.5	0.5	0.4	0.4	0.6	0.6	0.7	0.5	0.5
1954	0.4	0.3	0.0	-0.3	-0.6	-0.7	-0.8	-0.8	-0.7	-0.6	-0.6	-0.6
1955	-0.6	-0.6	-0.8	-0.8	-0.9	-0.9	-0.9	-0.9	-1.1	-1.4	-1.5	-1.4
1956	-1.1	-0.8	-0.7	-0.7	-0.8	-0.8	-0.7	-0.6	-0.5	-0.5	-0.5	-0.4
1957	-0.3	-0.0	-0.3	0.6	0.6	0.7	0.8	1.0	1.1	1.2	1.4	1.7
1958	1.8	1.6	1.2	0.8	0.6	0.5	0.5	0.4	0.3	0.3	0.5	0.6
1959	0.7	0.7	0.6	0.4	0.2	0.0	-0.1	-0.1	0.1	0.1	0.2	0.1
YR	DJF	JFM	FMA	MAM	AMJ	MJJ	JJA	JAS	ASO	SON	OND	NDJ
1960	0.0	0.1	0.0	0.1	0.0	0.0	0.1	0.2	0.2	0.2	0.1	0.1
1961	0.1	0.1	0.0	0.0	0.0	0.1	0.0	-0.1	-0.1	-0.2	-0.1	-0.2
1962	-0.2	-0.2	-0.3	-0.3	-0.3	-0.3	-0.1	-0.1	-0.1	-0.2	-0.3	-0.3
1963	-0.4	-0.2	0.0	0.0	0.1	0.3	0.7	1.0	1.0	1.1	1.1	1.1
1964	1.0	0.6	0.1	-0.4	-0.6	-0.8	-0.7	-0.6	-0.7	-0.8	-0.9	-0.8
1965	-0.5	-0.2	-0.1	0.1	0.3	0.7	1.0	1.3	1.6	1.7	1.7	1.6
1966	1.4	1.2	1.0	0.6	0.4	0.3	0.3	0.3	0.2	0.1	0.0	-0.1
1967	-0.2	-0.3	-0.4	-0.3	-0.2	0.0	0.0	-0.1	-0.2	-0.2	-0.2	-0.3
1968	-0.4	-0.6	-0.6	-0.5	-0.2	0.1	0.4	0.4	0.5	0.7	0.9	1.1
1969	1.2	1.1	0.9	0.7	0.6	0.4	0.4	0.5	0.8	0.9	0.9	0.9
YR	DJF	JFM	FMA	MAM	AMJ	MJJ	JJA	JAS	ASO	SON	OND	NDJ
1970	0.8	0.6	0.4	0.2	0.0	-0.4	-0.6	-0.7	-0.7	-0.7	-0.8	-1.0
1971	-1.1	-1.1	-1.0	-0.9	-0.8	-0.7	-0.6	-0.5	-0.6	-0.6	-0.7	-0.6
1972	-0.4	-0.2	0.1	0.3	0.5	0.8	1.1	1.4	1.7	1.9	2.2	2.2
1973	1.9	1.4	0.8	0.2	-0.3	-0.7	-0.9	-1.0	-1.1	-1.3	-1.5	-1.7
1974	-1.6	-1.4	-1.1	-0.8	-0.7	-0.6	-0.5	-0.3	-0.3	-0.5	-0.6	-0.5
1975	-0.3	-0.3	-0.4	-0.6	-0.7	-0.9	-1.0	-1.0	-1.1	-1.2	-1.3	-1.4
1976	-1.3	-1.0	-0.6	-0.4	-0.2	0.0	0.3	0.6	0.8	1.0	1.0	1.0
1977	0.9	0.7	0.4	0.3	0.3	0.4	0.4	0.5	0.6	0.9	1.0	0.9
1978	0.7	0.4	0.1	-0.3	-0.5	-0.6	-0.6	-0.5	-0.4	-0.3	-0.2	-0.1
1979	-0.1	0.0	0.2	0.2	0.1	0.0	0.0	0.2	0.4	0.5	0.4	0.5
YR	DJF	JFM	FMA	MAM	AMJ	MJJ	JJA	JAS	ASO	SON	OND	NDJ
1980	0.4	0.3	0.2	0.2	0.3	0.2	0.0	-0.1	-0.1	0.0	0.0	-0.1
1981	-0.3	-0.5	-0.4	-0.3	-0.3	-0.4	-0.4	-0.3	-0.1	-0.2	-0.3	-0.3
1982	-0.2	0.0	0.1	0.4	0.7	0.7	0.8	1.1	1.6	1.9	2.2	2.3
1983	2.3	2.1	1.6	1.3	1.0	0.6	0.3	0.0	-0.2	-0.4	-0.7	-0.8
1984	-0.7	-0.5	-0.4	-0.4	-0.5	-0.5	-0.4	-0.2	-0.2	-0.4	-0.8	-1.1
1985	-1.1	-1.0	-0.9	-0.8	-0.7	-0.5	-0.4	-0.3	-0.2	-0.2	-0.2	-0.4
1986	-0.6	-0.7	-0.5	-0.3	-0.1	0.0	0.3	0.6	0.9	1.1	1.2	1.2
1987	1.2	1.2	1.1	1.0	1.1	1.3	1.6	1.7	1.8	1.7	1.4	1.1
1988	0.7	0.3	0.0	-0.5	-1.0	-1.4	-1.5	-1.4	-1.5	-1.7	-2.1	-2.1
1989	-1.9	-1.6	-1.3	-1.1	-0.8	-0.6	-0.5	-0.4	-0.4	-0.3	-0.3	-0.2
YR	DJF	JFM	FMA	MAM	AMJ	MJJ	JJA	JAS	ASO	SON	OND	NDJ
1990	-0.1	0.0	0.1	0.2	0.2	0.1	0.1	0.2	0.3	0.3	0.2	0.3
1991	0.3	0.2	0.1	0.3	0.5	0.6	0.7	0.6	0.8	0.9	1.3	1.5
1992	1.6	1.6	1.4	1.2	1.0	0.7	0.3	0.1	-0.1	-0.1	-0.1	0.1
1993	0.2	0.3	0.4	0.6	0.7	0.5	0.3	0.3	0.3	0.4	0.4	0.2
1994	0.1	0.0	0.0	0.1	0.2	0.2	0.3	0.3	0.7	0.9	1.2	1.2
1995	1.0	0.7	0.4	0.1	0.0	-0.2	-0.3	-0.4	-0.6	-0.7	-0.8	-0.9
1996	-0.9	-0.7	-0.6	-0.5	-0.4	-0.4	-0.3	-0.3	-0.2	-0.2	-0.4	-0.5
1997	-0.5	-0.4	-0.1	0.2	0.7	1.1	1.6	1.9	2.3	2.5	2.6	2.7
1998	2.5	2.2	1.6	1.1	0.3	-0.4	-1.1	-1.2	-1.2	-1.2	-1.4	-1.4
1999	-1.3	-1.1	-0.9	-0.8	-0.8	-0.9	-1.0	-1.0	-1.1	-1.1	-1.3	-1.5

YR	DJF	JFM	FMA	MAM	AMJ	MJJ	JJA	JAS	ASO	SON	OND	NDJ
2000	-1.5	-1.3	-1.0	-0.8	-0.6	-0.6	-0.5	-0.5	-0.4	-0.6	-0.7	-0.7
2001	-0.6	-0.4	-0.3	-0.2	-0.1	0.0	0.0	0.0	-0.1	-0.1	-0.2	-0.1
2002	0.1	0.2	0.2	0.3	0.5	0.6	0.8	0.9	1.1	1.3	1.5	1.4
2003	1.1	0.8	0.4	-0.1	-0.4	-0.3	-0.1	0.1	0.2	0.3	0.3	0.3
2004	0.2	0.1	0.0	0.0	0.0	0.0	0.3	0.6	0.7	0.6	0.6	0.6
2005	0.5	0.4	0.2	0.2	0.1	0.1	0.1	0.0	-0.1	-0.2	-0.4	-0.7
2006	-0.8	-0.7	-0.6	-0.4	-0.2	-0.1	0.1	0.3	0.5	0.8	0.9	0.9
2007	0.6	0.2	-0.1	-0.3	-0.4	-0.5	-0.5	-0.7	-1.1	-1.4	-1.6	-1.6
2008	-1.7	-1.6	-1.4	-1.0	-0.9	-0.6	-0.3	-0.2	-0.3	-0.4	-0.6	-0.7
2009	-0.8	-0.7	-0.6	-0.3	0.0	0.3	0.5	0.6	0.7	1.0	1.3	1.5
YR	DJF	JFM	FMA	MAM	AMJ	MJJ	JJA	JAS	ASO	SON	OND	NDJ
2010	1.4	1.2	0.9	0.4	-0.2	-0.7	-1.1	-1.3	-1.5	-1.6	-1.7	-1.6
2011	-1.5	-1.3	-1.0	-0.8	-0.6	-0.4	-0.5	-0.6	-0.8	-0.9	-1.0	-1.0
2012	-0.9	-0.7	-0.6	-0.4	-0.3	0.0	0.3	0.5	0.4	0.3	0.1	-0.1
2013	-0.3	-0.3	-0.2	-0.3	-0.3	-0.4	-0.4	-0.3	-0.3	-0.1	-0.2	-0.2
2014	-0.4	-0.4	-0.2	0.1	0.2	0.2	0.1	0.1	0.3	0.5	0.6	0.7
2015	0.6	0.5	0.5	0.7	0.8	1.0	1.4	1.7	2.0	2.3	2.5	2.6
2016	2.4	2.1	1.5	0.9	0.3	-0.1	-0.4	-0.5	-0.6	-0.7	-0.7	-0.5
2017	-0.3	-0.1	0.1	0.2	0.3	0.3	0.1	-0.1	-0.4	-0.6	-0.8	-0.9
2018	-0.9	-0.8	-0.7	-0.5	-0.3	-0.1	0.1	0.2	0.4	0.6	0.8	0.8
2019	0.7	0.7	0.7	0.7	0.5	0.4	0.3	0.2	0.2	0.3	0.5	0.5
YR	DJF	JFM	FMA	MAM	AMJ	MJJ	JJA	JAS	ASO	SON	OND	NDJ
2020	0.4	0.4	0.3	0.1	-0.2	-0.4	-0.4	-0.6	-1.0	-1.2	-1.3	-1.2
2021	-1.0	-0.9	-0.7	-0.6	-0.5	-0.4	-0.4	-0.4	-0.7	-0.8	-1.0	-1.0
2022	-0.9	-0.9	-1.0	-1.1	-1.1	-0.9	-0.9	-0.9	-1.0	-1.0	-1.0	-0.9
2023	-0.8	-0.5	-0.2	0.1	0.4	0.7	1.0	1.3	1.4	1.6	1.7	1.8
2024	1.7	-	-	-	-	-	-	-	-	-	-	-

Tables 6-9. ENS ONI Index rounded to the nearest 0.1°C (1850 - Feb 2024)

ACKNOWLEDGEMENTS

I want to provide a special thanks to my peers at UNC-Charlotte, my advisor Dr Brian Magi (@brianmagi), and Terrell Wade (@Thunderstorm381), and so many others for their support, advice, and providing me with the necessary tools and coding skill sets to conduct this project

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