Content area
Full Text
Kevin E. Trenberth
National Center for Atmospheric Research,* Boulder, Colorado
ABSTRACT
A review is given of the meaning of the term "El Nino" and how it has changed in time, so there is no universal single definition. This needs to be recognized for scientific uses, and precision can only be achieved if the particular definition is identified in each use to reduce the possibility of misunderstanding. For quantitative purposes, possible definitions are explored that match the El Ninos identified historically after 1950, and it is suggested that an El Nino can be said to occur if 5-month running means of sea surface temperature (SST) anomalies in the Nino 3.4 region (5 deg N-5 deg S, 120 deg -170 deg W) exceed 0.4 deg C for 6 months or more. With this definition, El Ninos occur 31% of the time and La Ninas (with an equivalent definition) occur 23% of the time. The histogram of Nino 3.4 SST anomalies reveals a bimodal character. An advantage of such a definition is that it allows the beginning, end, duration, and magnitude of each event to be quantified. Most El Ninos begin in the northern spring or perhaps summer and peak from November to January in sea surface temperatures.
1. Introduction
The term "El Nino" has evolved in its meaning over the years, leading to confusion in its use. Because the phenomenon involving El Nino has become very visible in recent years as a dominant source of interannual climate variability around the world, there is a need to provide a more definitive meaning. However, this works only if it is accepted by everyone, and past attempts have not achieved such a consensus. This article arose from a query to the World Climate Research Programme's CLIVAR (Climate Variability and Predictability) program expressing the view that the definition of El Nino needed to be clarified. A draft response was published in the CLIVAR newsletter Exchanges for commentary, and this article provides the final outcome after taking the comments received into account. A brief review is given of the various uses of the term and attempts to define it. It is even more difficult to come up with a satisfactory quantitative definition. However, one such definition is explored here,...