El Nino and La Nina

El Nino and La Nina are two phenomena that concern pressure differences over the Pacific Ocean and have effects all over the world, most specifically on North and South America. El Nino and La Nina conditions are defined by the pressure of the air above the northeastern Pacific Ocean. During an El Nino, a low pressure system is situated over this region, and during a La Nina, a high pressure is situated over this region. Although low and high pressure systems come and go, some areas of the world generally have a low pressure or high pressure over them. One example of this is the Bermuda high, which is a high pressure over the Bermuda area during the summer months. When this high pressure is weaker, it allows tropical cyclones to curve off the east coast of the United States and not impact land, but when it is strong, it acts as a barrier, and tropical cyclones are pushed into making landfall along the Atlantic coast.



An example of a weak Bermuda high. Tropical cyclones are able to curve eastward without affecting land



An example of a strong Bermuda high. Tropical cyclones are pushed into the U.S. This condition was present during the 2004 and 2005 seasons, and these were some of the worst and most active in history.

Minor El Nino and La Nina conditions are common and usually only last a few months. But a long term event, or episode, occurs every five to seven years. An El Nino has the effect of letting a stronger Jet Stream enter the United States, which causes wet weather in the Midwest and South, and cool weather in the north. During a La Nina, the high pressure system in the Pacific severely weakens the Jet Stream and prevents moisture from reaching the Midwest and South. Therefore, there is dry weather in this region. El Nino and La Nina conditions also affect tropical cyclone formation. The strong Jet Stream during an El Nino causes a strong west to east wind along the tropics, causing intense wind shear (for the effects of a strong Jet Stream, see The Dagger of Death) which rips tropical cyclones apart. During a La Nina event, the lack of wind shear allows more tropical cyclones to form. A recent example of tropical cyclone formation hindered by an El Nino was 2006, when only 10 storms formed. Also, due to the effect of the Jet Stream on the Bermuda High, the only strong hurricanes of this season didn't affect land.



The effects of El Nino and La Nina.

El Nino and La Nina also affect water temperature, and therefore fish migrations. The fish migrations, in turn, affect the fishing business and therefore the economy. Although the results of El Nino and La Nina seem minor, they start many chains of events that change things in many different topics in many different parts of the world.

Sources: http://svs.gsfc.nasa.gov/vis/a010000/a010000/a010069/index.html (images), wikipedia (some information and image)