ENSO Cycle: Recent Evolution, Current Status and Predictions

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ENSO Cycle Recent Evolution, Current Status and
Predictions
Vernon E. Kousky and R. Wayne Higgins Climate
Prediction Center / NCEP October 2004
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Synopsis

• Warm episode (El Niño) conditions are expected
  to continue through early 2005.
• Expected impacts over the U. S. during this
  winter include wetter- and cooler-than-average
  conditions over Texas, and portions of the Gulf
  Coast and Southeast, drier-than-average
  conditions over the Pacific Northwest and the
  Ohio and Tennessee Valleys, warmer-than-average
  conditions over most of the West, and over the
  northern and central Great Plains.

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Recent Evolution of Equatorial Pacific SST
Departures

• Since early 2004 SSTs have been
  cooler-than-average in the eastern equatorial
  Pacific and warmer-than-average in the western
  and central equatorial Pacific.
• Departures increased substantially in the
  central equatorial Pacific during July.
• The warmth in the central equatorial Pacific has
  persisted and expanded eastward in recent months.

Time
Longitude
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SST Departures in the Tropical Pacific Last 4
Seasons
During the last year positive SSTA in the western
eq. Pacific decreased while positive SSTA in the
central eq. Pacific increased.
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Average SST Departures in the Tropical Pacific
Last 4 Weeks
Equatorial ocean surface temperatures greater
than 0.5C (1F) above average are found at
most locations between 160E and 120W.
Departures greater than 1C are found between
165E and 130W. Slightly cooler-than-average
ocean surface temperatures are found near the
South American coast, and in the region of
Indonesia. 12 September-9 October 2004
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Niño Indices Recent Evolution
SST anomalies greater than 0.5C are observed in
the Niño 4 and Niño 3.4 regions. In recent months
negative SST anomalies have steadily decreased in
magnitude in the Niño 12 region, with the latest
weekly SSTs being near average. The persistent
warmth in the central equatorial Pacific (Niño 4
region) and recent expansion of this warmth
eastward into the Niño 3.4 and Niño 3 regions
indicate the early stages of a warm (El Niño)
episode.
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Evolution of SST Departure Patterns in the Last 4
Weeks
During mid-September through early October 2004
positive SST anomalies persisted in the central
and east-central equatorial Pacific, where
departures greater than 1C were observed.
Negative anomalies decreased in areal extent and
magnitude in the eastern equatorial Pacific. The
recent evolution indicates the early stages of a
warm (El Niño) episode.
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Subsurface Conditions in the Eq. Pacific Last 4
Seasons

• During the last year the upper-ocean heat
  content gradually increased in the central
  equatorial Pacific.
• The build-up of the upper ocean heat content in
  the central equatorial Pacific is a precursor of
  warm (El Niño) episodes.

Time
Longitude
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Subsurface Conditions in the Eq. Pacific
September 2004

• During September 2004 the upper-ocean heat
  content was greater than average throughout most
  of the equatorial Pacific.
• The recent eastward expansion of subsurface
  temperature anomalies east of 180W is associated
  with an oceanic Kelvin wave initiated in late
  August.
• Basin-wide the upper ocean heat content is
  greater than average, which is a feature usually
  found during the early stages of warm (El Niño)
  episodes.

Time
Longitude
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MJO Intraseasonal Variability

• MJO activity in recent months has resulted in
  significant variability in the atmosphere (wind
  and pressure) and Pacific Ocean (surface and
  subsurface temperature).
• Related to this activity
• a significant weakening of the low-level
  easterly winds occurred over the equatorial
  Pacific during late June-early July 2004 and
• an eastward propagating Kelvin wave was
  initiated, which contributed to an increase in
  SST anomalies and upper ocean heat content in the
  central and east-central equatorial Pacific
  during August.
• the easterlies over the equatorial Pacific
  weakened again during late August-early
  September, initiating another Kelvin wave.

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Low-level (850-hPa) Zonal (east-west) Wind
Anomalies
Weaker-than-average easterlies (orange/red
shading). Stronger-than-average easterlies (blue
shading).
In early July, the easterlies weakened
substantially throughout the equatorial Pacific.
Time
In late August, the easterly winds again became
weaker than average over most of the equatorial
Pacific.
Recently, the easterlies weakened substantially
over the western equatorial Pacific.
Longitude
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Outgoing Longwave Radiation (OLR) Anomalies
Drier-than-average conditions (orange/red shading)
Wetter-than-average conditions (blue shading)
MJO activity was more regular from late May to
late August 2004 had a period of about 40-50 days.
Time
Wetter-than-average conditions have persisted
over the western equatorial Pacific since early
September.
Longitude
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Oceanic Niño Index (ONI)

• Based on the principal measure for monitoring,
  assessment, and prediction of ENSO (SST
  departures from average in the Niño 3.4 region)
• Three-month running-mean values of SST departures
  from average in the Niño 3.4 region, based on a
  set of improved homogeneous historical SST
  analyses (Extended Reconstructed SST ERSST.v2).
  The methodology is described in Smith and
  Reynolds, 2003, J. Climate, 16, 1495-1510.
• Used to place current conditions in historical
  perspective
• NOAA operational definitions of El Niño and La
  Niña are keyed to the index.

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NOAA Operational Definitions for El Niño and La
Niña
El Niño characterized by a positive ONI greater
than or equal to 0.5C. La NiZa characterized
by a negative ONI less than or equal to
?0.5C. To be classified as a full-fledged El
Niño or La Niña episode these thresholds must be
exceeded for a period of at least 5 consecutive
months.
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ONI Evolution since 1950
The most recent ONI value (0.8C for
July-September 2004) exceeds the threshold
(0.5C) for El Niño conditions.
El Niño
neutral
La Niña
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Niño 3.4 evolution Comparison for El Niño
episodes
Recent Niño 3.4 values, derived from ERSST.v2,
lie in the middle of the distribution of
historical El Niño episodes since 1950.
Most recent Niño 3.4 values (heavy black line)
compared to values for 13 historical El Niño
episodes. On the time axis year 0001 is the first
year of a warm episode.
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Niño 4 evolution Comparison for El Niño episodes
Recent Niño 4 values, derived from ERSST.v2, lie
near the top of the distribution of historical El
Niño episodes since 1950.
Most recent Niño 4 values (heavy black line)
compared to values for 13 historical El Niño
episodes. On the time axis year 0001 is the first
year of a warm episode.
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Niño 3 evolution Comparison for El Niño episodes
Recent Niño 3 values, derived from ERSST.v2, lie
near the bottom of the distribution of historical
El Niño episodes since 1950.
Most recent Niño 3 values (heavy black line)
compared to values for 13 historical El Niño
episodes. On the time axis year 0001 is the first
year of a warm episode.
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Niño 12 evolution Comparison for El Niño
episodes
Recent Niño 12 values, derived from ERSST.v2,
lie near the bottom of the distribution of
historical El Niño episodes since 1950.
Most recent Niño 12 values (heavy black line)
compared to values for 13 historical El Niño
episodes. On the time axis year 0001 is the first
year of a warm episode.
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Pacific Niño 3.4 SST Outlook

• Statistical and coupled model forecasts range
  from neutral conditions to moderate El Niño
  conditions during the last half of 2004 and early
  2005. About 80 of the forecasts indicate that El
  Niño conditions will continue through early 2005.

Figure provided by the International Research
Institute (IRI) for Climate Prediction (updated
10 September 2004).
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SST Outlook NCEP Coupled Forecast System
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Behind the Forecasts

• Tools
• Tropical Pacific SST forecasts (mid-Pacific,
  weak-to-moderate warm episode)
• ENSO composites based on ONI

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DJF Composite Patterns
ONI 0.5 to 0.8
ONI 1.6 to 2.4
ONI 1.0 to 1.3
500-hPa
Temp.
Precip.
Years 63-64, 69-70, 76-77, 77-78, 87-88
Years 65-66, 68-69, 86-87, 94-95, 02-03
Years 57-58, 72-73, 82-83, 91-92, 97-98
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DJF Composite Patterns

• As the ONI increases the U.S. as a whole becomes
  increasingly warmer and wetter.
• For DJF 2004-05 the expected ONI lies in the
  range 0.5 to 1.3.
• For the 10 historical events in this range, 8
  featured colder-than-average conditions in the
  Southeast, 7 featured drier-than-average
  conditions over portions of the mid-Mississippi,
  Tennessee and Ohio Valleys and 5 were dry in
  California and 6 were wet in Florida.

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DJF US-Precip Patterns Range of ONI 0.5-0.8
1963-64
1969-70
1976-77
1987-88
Composite
1977-78
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DJF US-Precip Patterns Range of ONI 1.0-1.3
1965-66
1968-69
1986-87
1994-95
2002-03
Composite
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U. S. Seasonal OutlooksDecember 2004-February
2005
Temperature
Precipitation
Outlooks combine long-term trends and
soil-moisture effects, with typical ENSO cycle
impacts, when appropriate.
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Summary

• Persistent warmth in the central equatorial
  Pacific and recent expansion of this warmth
  toward the east indicate that warm episode (El
  Niño) conditions have developed.
• The most recent value of the ONI is 0.8 (for
  the period July-September 2004), which satisfies
  the NOAA operational definition for El Niño. The
  most recent 5-month running mean value of the
  Southern Oscillation Index (SOI) is 0.5, which
  is consistent with the development of warm
  episode conditions.
• Based on statistical and coupled model forecasts
  and the recent evolution of oceanic and
  atmospheric conditions in the tropical Pacific,
  it seems most likely that SST anomalies in the
  Niño 3.4 region will remain positive, at or above
  0.5C, through early 2005.
• Expected impacts over the U. S. during this
  winter include wetter- and cooler-than-average
  conditions over Texas, and portions of the Gulf
  Coast and Southeast, drier-than-average
  conditions over the Pacific Northwest and the
  Ohio and Tennessee Valleys, warmer-than-average
  conditions over most of the West, and over the
  northern and central Great Plains.