Many web resources are available for understanding El Niño and La Niña. Here I give a brief introduction with some suggestions for navigation through the online sites.
What is El Niño?
El Niño is a somewhat unusual behavior (Figure 1) of the ocean circulation in the tropical Pacific Ocean that leads to changes in weather patterns in that region and, through this ocean-atmosphere linkage, affects weather worldwide (Figure 2). For an overview on El Niño, go to the El Niño homepage of the Pacific Marine Environmental Laboratory (PMEL) of the National Oceanic and Atmospheric Administration.
What is La Niña?
A closely related phenomenon in this same region is La Niña. The PMEL site also has a good description of La Nina .
Animated visualizations of El Niño provide a 3-dimensional perspective of the changes in temperature of the ocean surface and subsurface over the lifetime of the El Niño and La Niña events.
Global Impacts of El Nino
An overview of global impacts of El Nino show that many extreme weather events worldwide can be traced to El Nino conditions.
Predictions of future El Ninos
Columbia University's International Research Institute for Climate Prediction (IRI) provides additional information on El Niño and provides a summary listing of current El Nino forecasts
Current measurements of El Niño
For the latest measurements showing the presence or absence of an El Niño go to PMEL's current information page.
There is a wealth of online materials relating to El Nino, but if you start with just the pages indicated above, you will gain a good overview of the topic. From these pages you will be able to follow your curiosity in a number of directions.
Be sure to watch the PMEL (TAO) animation that shows the deployment of the measurement network and data retrieval