La Niña prospects beyond early 2011?Klaus Wolter

University of Colorado, CIRES & NOAA-ESRL PSD 1, Climate Analysis Branchklaus.wolter@noaa.gov

• La Niña is on a (steam-)roll(er), especially in MEI

• ENSO forecasts for next six to nine months

• What does La Niña mean for the SE U.S.?

• What about 2011-12?

NIDIS Mtg.18 XI 2010, Albany

El Niño event peaked early this year, followed by a breath-taking transition to a strong La Niña; MEI is now just about as low as it can get; since 1950, only 1955 dropped lower than this year’s MEI, and 1975 was close.

Current state of ENSO (bottom) compared to June (top): La Niña became established about 5 months ago in the Niño 3.4 region (top), reaching SST anomalies of up to and over -2C (bottom); easterly wind anomalies are focused west of the dateline, showing enhanced trade winds exactly where they typically appear with La Niña.

Niño 3.4

The European model’s February 2010 forecast (left) had the right idea about a fairly rapid transition towards at least ENSO-neutral conditions this summer – got overtaken by speed of transition in summer;

Every one of the 50 ensemble members stays below -1C through January 2011. This model has been doing quite well compared to others over the last decade. It will be updated by early next week.

ENSO forecasts from 15 dynamical & 8 statistical forecast models (below) vs. last June’s (left). While the statistical models were more hesitant to anticipate La Niña earlier this year, they have now fallen in line with dynamical models in predicting a strong La Niña winter. For two years in a row, the dynamical models have had the ‘edge’ compared to statistical models.

Note: The PDO never got above +0.8 last winter, and switched to negative values by June 2010, at -1.6 by September. It will not ‘challenge’ typical ENSO impacts this winter…

Models have a hard time predicting ENSO events beyond about 1 year; nevertheless, the majority shows negative anomalies below -1C through FMA, and some show renewed strengthening of La Niña by next summer.

Summer ENSO correlation plots Summer (left) correlation map of seasonal precipitation with MEI; observed anomalies in June-August 2009 (El Niño; below) and JJA 2010 (La Niña, bottom)

Fall ENSO correlation plots Fall (left) correlation map of seasonal precipitation with MEI; observed anomalies in September-November 2009 (El Niño; below) and last 90 days in 2010 (La Niña, bottom)

Winter ENSO correlation plots Winter (left) correlation map of seasonal precipitation with MEI; observed anomalies in December-February 2009-10 (El Niño; below)

Spring ENSO correlation plots Spring (left) correlation map of seasonal precipitation with MEI; observed anomalies in March-May 2010 (fading El Niño; below)

La Niña winters (Top 7 Fall/Winter cases)

Composite temperature (left) and precipitation (right) anomalies in seven strong La Niña cases (based on fall-winter MEI). Anomalies of 0.5 standard deviations or higher are considered significant (such as warmth and dryness from Texas to Mid-Atlantic States). Note: PDO negative in all seven cases; AMO positive in 1955-56 and 2007-08, not otherwise.

La Niña springs (following Fall/Winter cases)

Composite temperature (left) and precipitation (right) anomalies in seven strong La Niña cases (based on fall-winter MEI). Anomalies of 0.5 standard deviations or higher are considered significant (such as dryness in parts of Florida).

Post-La Niña summers (following same cases)

Composite temperature (left) and precipitation (right) anomalies in seven strong La Niña cases (based on fall-winter MEI). Anomalies of 0.5 standard deviations or higher are considered significant (such as coolness in Florida).

La Niña precipitation with positive AMO phase

Composite precipitation anomalies in five moderate+ La Niña cases with a warm North Atlantic. In the southeastern U.S. winters were drier than with other composites, spring seasons similar, and subsequent summers were drier.

How long will La Niña last? <MEI.ext: 1871-2005>

2010 started in June-July: expected demise between Jan-Feb 2012 and Nov-Dec 2013

How long will La Niña last? <last 60 years>

2010 started in June-July: expected demise between Sep-Oct 2011 and June-July 2013

How long will La Niña last?

One year analog cases, plus climatology and persistence thru next April (green line); 4 of 6 one-year analog predictions show persistent La Niña into 2012

How long will La Niña last?

Two year analog cases, plus climatology and persistence thru next April (green line): 5 of 6 show two-year analog predictions show persistent La Niña into 2012

What is difference for Year 1 vs. Year 2 Las Niñas?

For southeastern U.S., second winter season (right) tends to be less dry than first winter season (left) in prolonged La Niña scenario. This is based on seven La Niña cases with a tendency to continue into following winter. I have verified this for a few stations in northern Georgia (Canton, Clayton, Gainesville). More could be done with station data.

Executive Summary (18 November 2010)

1. A major La Niña is occurring right now, with high potential to linger into 2012.

2. The ‘hyper-active’ Hurricane season of 2010 is winding down, with a very unusual juxtaposition of a high count of storms that did not make landfall in the U.S. – this represents a lost opportunity for building up a moisture surplus before heading into a typically dry La Niña winter season in the southeastern U.S.

3. While the majority of international ENSO forecast models show this strong La Niña to continue into early 2011, they all ease up on its amplitude as we go forward into spring. Niño 3.4 anomalies tend to peak in early (boreal) winter and are typically minimal in April/May (standard deviation drops by almost 50%). This shows that the published models are well trained on historical data. They actually have little proven skill in breaking the ‘spring predictability barrier’.

4. I will start issuing MEI forecasts next month that cover the next 6-24 months. Current indications are that this will be an event that has a good chance of lasting well beyond spring 2011 (75%). In the southeastern U.S., 2nd year La Niña winters do not show a tendency towards worsening drought (in contrast to Western U.S.).