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.