El Nino and La Nina have become the latest meteorological catch phrases, but what do they really mean for Florida's weather?

     El Nino, the unusual warming of the ocean surface in the eastern pacific along the equator, usually begins in the summer and reaches its peak in late winter.  The extra heat released into the atmosphere causes stronger than normal temperature differences between the equator and the high latitudes.  As a result the jet stream, the river of air 5-7 miles above the earth, is much stronger than normal and further south, often right over Florida and blowing at more than 120 mph.

     The jet stream has a major influence on Florida weather.  Low pressure systems generally develop and track along the jet stream, so Florida experiences more frequent and stronger low pressure systems from late fall into early spring during strong EL Nino's.  This increased storminess brings slightly cooler than normal temperatures, a greater chance of heavy rain and flooding, and severe weather such as tornadoes and damaging wind storms.

  The image above compares the mean monthly SST anomalies for February 1989 on the left during strong La Nina conditions (cold phase) with the anomalies for February 1998 on the right during strong El Nino conditions (warm phase).

     La Nina, the unusual cooling of the ocean surface along the equator, causes weaker temperature differences and results in the average position of the jet stream being much further north, away from Florida.  Strong La Nina's typically bring fewer low pressure systems to Florida in the fall and winter.  Weather conditions are much drier and slightly warmer than during EL Nino's.  La Nina's bring an increased chance of drought and wildfires and, surprisingly,  a greater chance of freezing weather. The wintertime storm track is well north of Florida increasing the chances that strong low pressure systems will pull cold Canadian air into the deep south behind them.

     La Nina and El Nino generally reach their greatest strength during the fall and winter and their effects are  most evident in the dry season in Florida from November through April.  NWS Melbourne has launched a new El Nino - La Nina Homepage along with an experimental dry season forecast for storminess and rainfall at:
http://www.srh.noaa.gov/mlb/enso/mlbnino.html .  There is a tremendous amount of detailed information about El Nino and La Nina available on this page.

     Recent research has also indicated that La Nina and El Nino can impact the hurricane season as well.  If a strong El Nino is building during hurricane season the stronger than normal upper level westerly winds can cause increased  wind shear that can inhibit hurricane formation.  Likewise strong La Nina conditions can lessen westerly shear and provide a more favorable environment for hurricane formation in the Atlantic Basin. However, it only takes one hurricane to make a terrible season - if it hits you.  The 1992 hurricane season was under the influence of a moderately strong El Nino and hurricane activity was greatly reduced in the Atlantic, but we all remember what Hurricane Andrew did in August 1992.

     La Nina and El Nino favor certain types of hazardous weather, but it is important to remember that dangerous weather can occur in any year, and you should always be prepared.