An Overview of the 200708 La Nia

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An Overview of the 2007-08 La Niña
Michelle LHeureux
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Outline

• The seasonal mean
• Variability within the season
• 3. Summary

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Overview of the 2007-08 La Niña

• The 2007-08 La Niña episode began in JAS 2007
  and ended during AMJ 2008.
• The cooling began in the eastern Pacific during
  March 2007 and slowly extended westward across
  the equatorial Pacific throughout 2007.
• The peak of the event occurred in late January
  2008 when the Niño-3.4 reached a weekly value of
  -2.2C.
• 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 2008.
• 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.

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Evolution of the Equatorial SST Departures 2007-08
La Niña conditions occurred during August 2007
and lasted through June 2008. The episode
reached peak amplitude during late January 2008.
Time
Longitude
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Historical Niño-3.4 Index

• The Official Oceanic Nino Index (ONI)
• ERSST.v3 3-mth Average (Jan 1950- present)
• Peak on DJF 2007/08 -1.5C

-1.5C exceeded 6 times since 1950
ERSST.v3 1-mth Average (Jan 1950-
present) Peak on Jan. 2008 -1.7C
-1.7C exceeded 6 times since 1950
OI.v2 1-week Average (Nov 1981-
present) Peak centered on Jan. 23, 2008
-2.2C
-2.2C exceeded only once since 1981
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Seasonal Sea Surface Temperature (C) and
Precipitation (mm/day) Anomalies
OND 2007
JFM 2008
SSTs
Precip.
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Seasonal 500-hPa GPH (m) and Zonal Wind (m/s)
Anomaly is shown by color shading.
OND 2007
JFM 2008
500-hPa GPH
200-hPa u-wind
Red shading Anomalous westerlies Blue shading
Anomalous easterlies
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December-March Temperature (C) and Precipitation
(mm/day) Observations and CPC Forecast
Gridded NCDC Co-op Temperature Anomalies
Gridded Unified Precipitation Anomalies
Observations
CPC Forecast (for DJF/JFM)
Now lets look at the variation within the
season..
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200-hPa Zonal Wind ? 3 regimes
Why do we look at the Pacific jet? Because of its
downstream impacts over the United States.
Using daily NCEP/NCAR Reanalysis from Dec. 1st,
2007 until March 31st, 2008 (122 days
total) Shown here are the total 200-hPa zonal
wind and the 500-hPa Geopotential Height Anomalies
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)
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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)
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3 regimes and their impact on Precipitation
Anomalies
Precipitation Anomalies
(with the seasonal mean removed)
Retracted Jet (33 days)
Remaining Days (57 days)
Extended Jet (32 days)
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Variability within the season related to the
active MJO?
Positive anomalies (brown shading) indicate
unfavorable conditions for precipitation.
Negative anomalies (green shading) indicate
favorable conditions for precipitation.
Moderate-to-strong MJO activity was present
during November 2007 February 2008.
Time
Longitude
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The relationship between two jet regimes
(extended vs. retracted) and the MJO
This analysis includes half of the days in the
Dec. 2007 -Mar. 2008 period (60 out of 120
days)
Blue Extended Jet days (32 days) Maroon
Retracted Jet days (33 days)
No. of Days
Phase of the MJO
Convection over Western Pacific and Western
Hemisphere
Convection over Indian Ocean Maritime Continent

• Retracted jet was associated with MJO-related
  convection over the Eastern Hemisphere.
• Extended jet was associated with MJO-related
  convection over the Western Hemisphere.

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Summary

• The 2007-08 La Niña episode began in JAS 2007
  and ended during AMJ 2008.
• The peak of the event occurred in late January
  2008 when the Niño-3.4 reached a weekly value of
  -2.2C.
• 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 2007 -February 2008, there was
  moderate-to-strong MJO activity.
• During Dec 2007 -Mar 2008, the extensions and
  retractions of the jet (and corresponding
  downstream impacts) were related to the phase of
  the MJO.