A La Nina's Strength & Duration Is Deeply Dependent On What Time Of The Year It Starts.
La Niņa events are operationally defined using the Oceanic Niņo Index (ONI, Click Here to See ONI Index), which is the three-month running-mean values of sea surface temperature departures from average in the Niņo 3.4 region of the central Pacific (bounded by 5N-5S, 120-170W). NOAA defines La Niņa as the condition whereby the ONI is less than or equal to -0.5 degrees C. This definition was adopted by the U.S. and 25 other countries in North and Central America and the Caribbean in April 2005. Source: NOAA
Plus La Nina 2007-200X Forecast...
How is a La Nina defined in scientific terms?
The Statistics of the Oceanic Nino Index show that when a La Nina Starts Early in the Year or a Late forming LA Nina has a MJJ(May-June-July) period linkage to early year negative sea surface temperatures anomalies it will persist for longer & have greater strength. I'll explain the "MJJ linkage" in a bit. As shown in the graphs below.
"Year" are the years in which the La Nina Event occured.
"Start" is the part of the year which the La Nina started. "Start" can be defined by either "1st" or "2nd".
"1st" is defined as the trimonthly period of ONI from (DJF to MJJ) or the 1st part of the Year in which the La Nina Event Started, The 1st 6 months of the Year.
"2nd" is defined as the trimonthly period of ONI from (JJA to NDJ) or the 2nd part of the Year in which the La Nina Event Started, The 2nd 6 months of the Year.
"Peak" is defined as the Lowest Temperature(C) Anomaly Reading in any given trimonthly period within the La Nina. Example, the 1954-1957, 73-74, & 88-89 La Nina's Had The Coldest Trimonthly Reading Of All Time, -2.0 Degrees Celcius below average.
"Peak Rank" is nothing more then each La Nina Event Ranked by it's strength, or lowest temp anomaly reading. Example, The La Nina Event of 2000-2001 only had a peak of -0.7C, which makes it the weakest La Nina since 1949 and weakest of the 12 Events.
"Start Period" is the trimonthly period in which the La Nina Event Started, for example MAM 54 translates to March-April-May 1954.
"Start P #" is the Start Period Number, All periods from (DJF to NDJ) Are given numbers in order from (1 to 12). All this did was help me classify "2nd" and "1st" "Start" designations. All "1st's" are 1 to 6, and all "2nd's" are 7 to 12.
"End Period" Is the last trimonthly period the La Nina Existed.
"Duration" is designated as the total number of trimonthly periods in which the La Nina event Existed.
"Dur. Rank" is a Ranking of Duration, meaning La Nina 2000-2001 only lasted 5 trimonthly periods, it was the shortest existing classified La Nina event on Record along with 1967-1968's La Nina event. While The 1954-1957 Event Lasted 34 Trimonthly periods or just about 3 years, #1 in Duration Rank.
Now take a look at all the La Nina Events Shaded in Pink, All these have something in common They All have the Lowest Nina Trimonthly Temperature anomalies, the longest Durations, and Start Early in the Year, AKA "1st" designations.
If you look at the La Nina Events shaded in Green they all start late in the year, have the shortest durations, and the weakest peak negative Sea surface temperature anomalies per their best trimonthly period, all start Late in the year, and are designated as "2nd"s.....
Now your probably asking yourself , "Why are some late starting La Nina's classified in with the Strongest and Longest Duration crowd.... AKA the "2nd"s with the *-star Marked next to them"?, these are special cases. These special cases can be used to predict/forecast upcoming La Nina events, Like the current one that started in 2008. As you will read later.
In order to Classify these as "2nd*" La Nina's as special cases, all Late Year Starting Nina's (Regular "2nd" must pass the MJJ Linkage Test.(May-June-July Period Link Test)
The MJJ Linkage Test * Indicates all periods between the MJJ period before the start period and the Start Period Itself of the La Nina have an ONI of -0.1 or below AND the ASO period (if applicable) must attain an ONI of -0.5 or below.
This MJJ linkage is suited to a Later Year development of a La Nina, which means it shares the qualities of a Beginning of the Year La Nina rather then a Later Year starting La Nina. This means its as long in duration and as cold as an Early Year Starting La Ninas. The Red Boxes are Mislinks, Automatic Test Failure. The Whitish Boxes are Links, all must Link up to the coresponding green "Start" periods on the right.
In Short, The MJJ linkage is about a "2nd" Late Starting Nina Event to getting some link to early year continous negative sea surface temperature anomalies. As you see below only the 98-2000, 70-72, and 74-76 Nina Events fit the shoe, as does the 2007-200X La Nina.
2008-200X La Nina Forecast:
Now Using MJJ Link "2nd*" La Nina climatology we gain 3 analogs to work with in Forecasting the rest of the 2007-200X La Nina. Typically, these peak out at High End - Moderate Events or Low End - Strong LA Nina's (-1.4 C to -1.8C Peak) and last 18-24 trimonthly periods in duration or 1 1/2 to 2 years. This La Nina will be a Multiyear event, as it will last till sometime into 2009.