Important Note on ONI Revisions<\/h4>\n
The CPC has implemented its latest climatological base-period update, shifting the reference climatology to 1996\u20132025<\/strong> as part of its routine five-year revision cycle. Adopted in January 2026, this change has materially altered the historical ENSO record<\/strong>\u2014not by changing the ocean, but by changing how anomalies are defined.<\/p>\n The Oceanic Ni\u00f1o Index (ONI) is calculated as sea-surface temperature anomalies in the Ni\u00f1o-3.4 region relative to a rolling 30-year \u201cnormal.\u201d Each forward shift of this baseline redefines what constitutes normal<\/em> conditions. While such updates often result in a warmer baseline, the latest revision has cooled the Ni\u00f1o-3.4 climatological mean<\/strong>, as clearly reflected in the historical SST climatology.<\/p>\n As a direct consequence, the period October\u2013December 2019 through February\u2013April 2020<\/strong>, previously classified as ENSO-neutral under the 1991\u20132020<\/strong> baseline, now satisfies the formal threshold for a weak El Ni\u00f1o<\/strong>. This reclassification effectively creates a historical El Ni\u00f1o (2019-2020) event that was never identified in real time<\/strong>.<\/p>\n Crucially, the underlying oceanic conditions have not changed<\/strong>. What has changed is the statistical reference against which those conditions are measured. This episode demonstrates that ENSO classifications are not absolute physical truths; they are diagnostic constructs<\/strong> that depend explicitly on the chosen climatological baseline.<\/p>\n For researchers, forecasters, and communicators, this serves as a clear caution: ENSO \u201chistory\u201d is not immutable. When the yardstick is recalibrated, past classifications can\u2014and do\u2014change.\u00a0 <\/p>\n A Historical El Ni\u00f1o (2019-2020) Created by a Climatology Update 9 January 2026 Important Note on ONI Revisions The CPC has implemented its latest climatological base-period update, shifting the reference climatology to 1996\u20132025 as part of its routine five-year revision Read More …<\/a><\/p>\n","protected":false},"author":1,"featured_media":0,"parent":0,"menu_order":0,"comment_status":"closed","ping_status":"closed","template":"","meta":{"footnotes":""},"class_list":["post-45972","page","type-page","status-publish","hentry"],"_links":{"self":[{"href":"https:\/\/www.gujaratweather.com\/index.php?rest_route=\/wp\/v2\/pages\/45972","targetHints":{"allow":["GET"]}}],"collection":[{"href":"https:\/\/www.gujaratweather.com\/index.php?rest_route=\/wp\/v2\/pages"}],"about":[{"href":"https:\/\/www.gujaratweather.com\/index.php?rest_route=\/wp\/v2\/types\/page"}],"author":[{"embeddable":true,"href":"https:\/\/www.gujaratweather.com\/index.php?rest_route=\/wp\/v2\/users\/1"}],"replies":[{"embeddable":true,"href":"https:\/\/www.gujaratweather.com\/index.php?rest_route=%2Fwp%2Fv2%2Fcomments&post=45972"}],"version-history":[{"count":1,"href":"https:\/\/www.gujaratweather.com\/index.php?rest_route=\/wp\/v2\/pages\/45972\/revisions"}],"predecessor-version":[{"id":45973,"href":"https:\/\/www.gujaratweather.com\/index.php?rest_route=\/wp\/v2\/pages\/45972\/revisions\/45973"}],"wp:attachment":[{"href":"https:\/\/www.gujaratweather.com\/index.php?rest_route=%2Fwp%2Fv2%2Fmedia&parent=45972"}],"curies":[{"name":"wp","href":"https:\/\/api.w.org\/{rel}","templated":true}]}}![\"\"](\"https:\/\/www.gujaratweather.com\/wp-content\/uploads\/2026\/01\/30yrbaseperiods_Nino34._v5-scaled.png\")
<\/a><\/p>\n![\"\"](\"https:\/\/www.gujaratweather.com\/wp-content\/uploads\/2026\/01\/1991-2020-ONI.jpg\")
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\nThe above CPC ONI Table shows the ENSO Neutral conditions for OND 2019 to FMA 2020
\n<\/strong><\/h5>\n<\/h5>\n
![\"\"](\"https:\/\/www.gujaratweather.com\/wp-content\/uploads\/2026\/01\/1996-2025-ONI.jpg\")
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\nThe above CPC ONI Table shows a weak El Ni\u00f1o for OND 2019 to FMA 2020<\/strong><\/h5>\n<\/figure>\n<\/div>\n<\/div>\n","protected":false},"excerpt":{"rendered":"