El Niño Composites

A lot of attention has been given to the consequences of the latest strong El Niño event. People often talk about meteorological phenomena as El Niño (or La Niña) conditions, but what are these, and how do we come about our notions of what is a ’typical’ El Niño event? How consistent do we expect the effects of this phenomena to be, especially when these ’signature effects’ occur thousands of kilometers away from the Pacific Ocean? Often understanding about the typical effects of large scale climate variations are derived from composites. This is a common statistical method where elements are classified into groups based on some external consideration, and then the properties of each group is expressed by the average of all the elements it contains. This can be a very efficient way to visualize large data sets, but it can also imply more consistency within groups than is actually the case. This post goes over some of mechanics of creating composites, and ways to explore to what degree they can be taken at ’face value’.



Big Picture

Just briefly, what is the El Niño Souther Oscillation (ENSO)? Typically, strong winds off the coast of Equatorial South America move surface water away from the coast, which is replaced from below with cold water from the ocean depths [this colder deep water also carries nutrients which are important for sustaining marine life]. If these winds weaken, upwelling decreases, and weather patterns [as well as fish stocks] are altered by the presence of this atypically warm surface water in the Central East Pacific [\ref{fig:NOAA_DecSat}]. This is known as an El Niño event and the oscillation between El Niño and the opposing La Niña states is termed ENSO.

\label{fig:NOAA_DecSat}Satillite estimates of mean Sea Surface Temperature anomalies for the month of December 2015, as provided by the United States National Oceanic and Atmospheric Administration

There are many ways that people attempt to quantify the degree to which an El Niño state is occurring. These include average sea surface temperatures over various subregions of the Pacific Ocean, differences in air pressure between different locations, and more involved metrics that combine these and other factors. An overview of the more common approaches can be found here. An El Niño event does not manifest in the same way every time it occurs, but can have many different realizations or “flavors” (Hoell 2014). Which is why, as stressed in the linked discussion, there tend to be many approaches to quantifying ENSO, each of which highlights different aspects of the variation. Looking at documented values for some of the most common indices [\ref{fig:enso_indices}] shows similar general patterns, even though the different time series have noticeably different properties.