Friday, September 14, 2018

An El Nino Watch is in Effect

The odds of an El Nino this winter have gone up slightly -- the September outlook for ENSO now has an El Nino Watch, according to the CPC/IRI* outlook, which calls for "a 50-55% chance of El Niño development during fall, rising to 65-70% for winter 2018-19. An El Niño watch is in effect."

*Climate Prediction Center/International Research Institute for Climate and Society at Columbia University

These back-to-back (-to-back-to-(maybe) back) nonneutral years aren't as uncommon as I thought. In fact, 47 of 68 (69%) ENSO seasons since 1950-51 (July-June) have been classified as a nonneutral year, according to the ONI index. NOAA considers a season to be an El Nino if there are 5 or more consecutive months of a +0.5°C Nino3.4 Index (the temperature anomaly in the Nino3.4 region), and a La Nina if there are 5 or more consecutive months with an anomaly of -0.5°C or less.

This page explains more, including how they classify weak, moderate and strong seasons.

There's basically no trend in the annual average Nino3.4 Index since 1950-51: it's -0.02°C/decade.

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