Section 10.4 El Nino - Southern Oscillation (ENSO)
The El Nino - Southern Oscillation (ENSO) is an irregular periodic variation in winds and sea surface temperatures over the tropical eastern Pacific Ocean. These variations shift the degree of cold upwelling that occurs along the South American coast, and are connected to larger cycles of periodic warming events connected to global climate.
ENSO conditions are assessed by a range of methods, including
SOI (Southern Oscillation Index): the difference in pressure between Tahiti and Darwin, Australia;
the SST and SSTA patterns in the equatorial eastern Pacific
the release of latent heat and outgoing radiation as precipitation patterns shift
monitoring of sea level variations across the equatorial Pacific basin
Normal ("cold") phase (+SOI)
upwelling along South American coast
strong trade winds in tropics
high pressures in east-central Pacific (Tahiti > Darwin or +SOI)
cold water moving westward across equatorial Pacific
piling up of water in the western Pacific
dry conditions over east-central Pacific, wet conditions in western Pacific
La Nina can be considered an unusually intense manifestation of this cold phase
El-Nino ("warm") phase (-SOI)
downwelling along South American coast
weakened trade winds in tropics (or even some low-level westerly flow)
low pressures in east-central Pacific (Tahiti < Darwin or -SOI)
warm water moving eastward across equatorial Pacific
piling up of water in the eastern Pacific
the entire Walker cell weakens and most eastward
wetter conditions over east-central Pacific, dry conditions in western Pacific
Hadley circulation strengthens; stronger subtropical jet stream
Global climate and teleconnection
the changes above influence weather and climate conditions over very large distances. These affects are sometimes referred to as climate teleconnections.
El Nino conditions strengthen the subtropical jet and weaken the polar jet stream. This tends to lead to milder winters in the upper Midwest of the United States, and wetter conditions in the American southwest.
La Nina conditions tend to strengthen the polar jet stream, tending to yield colder and more intense winters in the upper Midwest (i.e. we may find ourselves more often north of the polar front).