An Overview of the La Niña Michelle L’Heureux

Outline 1. 1.The seasonal mean 2. 2.Variability within the season 3. Summary

Overview of the La Niña The La Niña episode began in JAS 2007 and ended during AMJ The cooling began in the eastern Pacific during March 2007 and slowly extended westward across the equatorial Pacific throughout The peak of the event occurred in late January 2008 when the Niño-3.4 reached a weekly value of -2.2°C. By early February 2008, above-average SSTs emerged in the far eastern Pacific and extended westward over next several months, bringing the La Niña episode to an end during June The circulation over the North Pacific sector was highly variable within the boreal winter season, and led to downstream impacts on temperature and precipitation across the United States.

Evolution of the Equatorial SST Departures Longitude Time La Niña conditions occurred during August 2007 and lasted through June The episode reached peak amplitude during late January 2008.

Historical Niño-3.4 Index The “Official” Oceanic Nino Index (ONI) - -ERSST.v3 3-mth Average (Jan present) Peak on DJF 2007/08: - 1.5°C ERSST.v3 1-mth Average (Jan present) Peak on Jan. 2008: -1.7°C OI.v2 1-week Average (Nov present) Peak centered on Jan. 23, 2008: -2.2°C -1.5°C exceeded 6 times since °C exceeded 6 times since °C exceeded only once since 1981

Seasonal Sea Surface Temperature (°C) and Precipitation (mm/day) Anomalies OND 2007JFM 2008 SSTs Precip.

Seasonal 500-hPa GPH (m) and Zonal Wind (m/s) OND 2007JFM hPa GPH 200-hPa [u]-wind Anomaly is shown by color shading. Red shading: Anomalous westerlies Blue shading: Anomalous easterlies

December-March Temperature (°C) and Precipitation (mm/day) Observations and CPC Forecast Now let’s look at the variation within the season….. Gridded NCDC Co-op Temperature Anomalies Gridded Unified Precipitation Anomalies Observations CPC Forecast (for DJF/JFM)

200-hPa Zonal Wind  3 regimes Retracted Jet (33 days) Remaining Days (57 days) Extended Jet (32 days) Why do we look at the Pacific jet? Because of its downstream impacts over the United States. 200-hPa [u]-wind variance Time series created by taking the average of the [u]-wind in the gray shaded region (area of the highest variance) Using daily NCEP/NCAR Reanalysis from Dec. 1 st, 2007 until March 31 st, 2008 (122 days total) Shown here are the total 200- hPa zonal wind and the 500-hPa Geopotential Height Anomalies

Retracted Jet (33 days) Remaining Days (57 days) Extended Jet (32 days) 3 regimes and their impact on Temperature Anomalies Temperature Anomalies (with the seasonal mean removed)

Retracted Jet (33 days) Remaining Days (57 days) Extended Jet (32 days) 3 regimes and their impact on Precipitation Anomalies Precipitation Anomalies (with the seasonal mean removed)

Negative anomalies (green shading) indicate favorable conditions for precipitation. Positive anomalies (brown shading) indicate unfavorable conditions for precipitation. Longitude Time Moderate-to-strong MJO activity was present during November 2007 – February Variability within the season related to the active MJO?

The relationship between two jet regimes (extended vs. retracted) and the MJO Phase of the MJO No. of Days Convection over Indian Ocean & Maritime Continent Convection over Western Pacific and Western Hemisphere Blue: Extended Jet days (32 days) Maroon: Retracted Jet days (33 days) Retracted jet was associated with MJO-related convection over the Eastern Hemisphere. Extended jet was associated with MJO-related convection over the Western Hemisphere. This analysis includes half of the days in the Dec Mar period (~60 out of 120 days)

Summary The La Niña episode began in JAS 2007 and ended during AMJ The peak of the event occurred in late January 2008 when the Niño-3.4 reached a weekly value of -2.2°C. The circulation over the North Pacific sector was highly variable within the boreal winter season, and led to downstream impacts on temperature and precipitation across the United States. During November February 2008, there was moderate-to-strong MJO activity. During Dec Mar 2008, the extensions and retractions of the jet (and corresponding downstream impacts) were related to the phase of the MJO.