La NiĆ±a is essentially the below normal cooling of sea surface temperatures in the eastern tropical Pacific Ocean. La NiĆ±a usually follows an El NiĆ±o event, but not always. La NiĆ±a events, like El NiĆ±o events, will vary year to year and with each event. La NiĆ±a can be thought of as opposite to El NiĆ±o. Some scientists use El Viejo (Spanish for "old man") instead of La NiĆ±a in their work.
|Figure A: El NiĆ±o and La NiĆ±a events that occurred from 1950 to 2009.|
The eastern tropical Pacific Ocean cools during a La NiĆ±a event. This occurs because the easterly trade winds around the equator strengthen as the differences in pressure between the eastern and western tropical Pacific increase. Upwelling of colder ocean water in the eastern tropical Pacific from below the surface increases and more of this colder water moves to the west, pushed by the strong easterly trade winds. This causes the precipitation usually found over the western and central tropical Pacific to move farther to the west. This cools the average sea surface temperatures from the normal in the eastern tropical Pacific.
|Image from NOAA|
The polar jet stream during the winter in a La NiĆ±a event splits into two over the Pacific Ocean. This is because a persistent high pressure area resides south of Alaska and drives the upper portion of the polar jet stream to the north, while the lower portion swings around the high pressure from the south. Over Canada and the northern Great Plains, colder than normal conditions persist with the location of the upper portion of the polar jet. The southern portion of the polar jet brings warm, moist air into the northwestern United States causing much of this area to be wet. During the La NiĆ±a event, the southern states surrounding the Gulf of Mexico tend to remain warm and dry. Along the east coast of the United States, warm temperatures persist during the winter months.
Figure C shows the typical weather conditions felt across the US during the winter months in a La NiĆ±a event. This is typical of a moderate La NiĆ±a event. As seen from the map, much of the United States is affected by the cooling of the Pacific Ocean.
|Figure C: Changes in Atmospheric Circulations During Winter|
|Image from NOAA|