La Nina/ El Nino

Both El Niño and La Niña are driven by water temperature and atmospheric conditions. These two are coupled: the warm oceanic phase, El Niño, accompanies high air surface pressure in the western Pacific, while the cold phase, La Nina, accompanies low air surface pressure in the western Pacific.Mechanisms that cause the oscillation remain under study.

La Nina

During La Niña years, the trade winds are unusually strong due to an enhanced pressure gradient between the eastern and western Pacific. As a result, upwelling is enhanced along the coast of South America, contributing to colder than normal surface waters over the eastern tropical Pacific and warmer than normal surface waters in the western tropical Pacific. This enhances normal levels of fish stocks as the upwelling of deep waters are nutrient rich.

El Nino
The trade winds do not replenish and sometimes even reverse direction to blow from west to east. This causes warm surface waters from the east to move eastward. Beneath the surface, the thermocline along the equator flattens as the warm waters at the surface effectively cap preventing the colder, deeper waters from upwelling. As a result, the large central and eastern Pacific regions warm up into an El Niño with waters warming by 3° to 5°F.

Below i have added a table that i created to summarise the difference between the two climatic conditions:

Below i have listed some websites with more information as i found it quite difficult to explain the two conditions:

http://weather.about.com/

This website has quite a lot of information with some interesting diagrams

http://ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/eln/home.rxml

I have used this website quite a bit during the blog as i always find the explanations interesting and easy to understand

http://earthobservatory.nasa.gov/Features/ElNino/

NASA website also has some in depth explanations