ENSO Cycle: Recent Evolution, Current Status and Predictions

1
ENSO Cycle Recent Evolution, Current Status and
Predictions
Update prepared by Climate Prediction Center /
NCEP September 4, 2007
2
Outline

• Overview
• Recent Evolution and Current Conditions
• Oceanic Niño Index (ONI) Revised 1 March
  2004
• Pacific SST Outlook
• U.S. Seasonal Precipitation and
• Temperature Outlooks
• Summary

3
Overview

• La Niña conditions are developing across the
  tropical Pacific.
• SST anomalies have become more negative in the
  eastern equatorial Pacific, and remain positive
  in the western equatorial Pacific.
• Most dynamical models predict the strengthening
  of La Niña in the next couple of months, while
  half of the statistical models indicate
  ENSO-neutral conditions.
• Recent equatorial Pacific SST observations and
  model forecasts indicate La Niña conditions will
  continue to develop during the next several
  months.

4
Recent Evolution of Equatorial Pacific SST
Departures (oC)
Between May 2006 and December 2006, positive SST
anomalies increased across the equatorial Pacific
between 160ºE and the South American coast. The
SST anomalies decreased rapidly in January 2007
everywhere east of the Date Line.
Time
Over the past several months, below average SST
anomalies have shifted westward and are mostly
below average east of the Date Line.
Longitude
5
Niño Region SST Departures (oC) Recent Evolution
0C
The latest weekly SST departures are -1.2ºC in
the Niño 3 region and -2.3ºC in the Niño 12
region. SST departures are -0.6ºC in the Niño
3.4 region and 0.0ºC in the Niño 4 region.
-0.6C
-1.2C
-2.3C
6
SST Departures (oC) in the Tropical Pacific
During the Last 4 Weeks
Equatorial SSTs during the last four weeks were
more than 1ºC below average in areas between
135ºW and the South American coast, and more than
0.5oC above average in portions of the western
Pacific.
7
Global SST Departures (oC)
The equatorial SSTs are below average in the
eastern Pacific Ocean and north of Australia, and
remain above average in the western Pacific and
the Indian Ocean. Positive anomalies dominate
the middle latitudes in the South Pacific Ocean
and northernmost latitudes of the Pacific and
Atlantic Oceans.
8
Weekly SST Departures (oC) for the Last Four
Weeks

• During August 2007 below-average SSTs persisted
  in the eastern equatorial Pacific Ocean.
• Over this 4-week period equatorial SST anomalies
  decreased throughout much of the central and
  eastern equatorial Pacific.

9
Upper-Ocean Conditions in the Eq. Pacific

• The basin-wide equatorial upper ocean (0-300 m)
  heat content is greatest prior to and during the
  early stages of a Pacific warm (El Niño) episode
  (compare top 2 panels) and least prior to and
  during the early stages of a cold (La Niña)
  episode.
• The slope of the oceanic thermocline is least
  (greatest) during warm (cold) episodes.
• The values of the upper-ocean heat anomalies
  (positive) and the thermocline slope index
  (negative) during the last half of 2006 indicated
  weak-to-moderate El Niño conditions.
• Recent values indicate ENSO-neutral conditions.

Cold Episodes Warm Episodes
Monthly thermocline slope index, calculated as
the difference in the anomalous depth of the 20ºC
isotherm between the western Pacific
(160ºE-150ºW) and the eastern Pacific (90º-140ºW).
10
Central Eastern Pacific Upper-Ocean
(0-300 m) Weekly Heat Content Anomalies
Between early April 2006 and early January 2007
the heat content was above average, but showed
intraseasonal fluctuations related to the MJO
(see following slides). Since January 2007 the
upper ocean heat content anomalies have been
negative, indicating subsurface conditions are
favorable for the development of La Niña.
11
Sub-Surface Temperature Departures (oC) in the
Equatorial Pacific

• During July August 2007 equatorial sub-surface
  temperatures remained mostly below average in the
  region east of the date line (180o), while
  positive temperature departures were confined to
  the western Pacific during the period.
• The most recent period (below) shows negative
  anomalies between the surface and 150 m depth
  across the eastern half of the equatorial Pacific
  Ocean with the largest departures between 120
  and 90W.

Time
Most recent pentad analysis
Longitude
12
Tropical OLR and Wind AnomaliesDuring the Last
30 Days
Positive OLR anomalies (suppressed convection and
precipitation, red shading) were observed in the
west-central equatorial Pacific extending into
the eastern Pacific, especially between the
equator and 10N. Negative OLR anomalies were
present over portions of Southeast Asia and
Indonesia.
Low-level (850-hPa) easterly wind anomalies
(stronger-than-average easterly winds) continue
to be observed in the west-central Pacific.
Upper-level (200-hPa) westerly wind anomalies
were observed over the east-central Pacific and
easterly anomalies were observed over the eastern
Pacific.
13
Atmospheric Circulation over the North Pacific
North America During the Last 60 Days
500-hPa Height Anoms.
925-hPa Temp. Anoms. (oC)
200-hPa Wind
In the first half of July, a 500-hPa ridge over
the west-central United States led to above
average temperatures over the West. In the
second half of July, below average temperatures
remained over Texas, while above average
temperatures accompanied an anomalous ridge
across the north-central United States. During
the first half of August, the anomalous ridge
flattened over the northern United States
contributing to above average temperatures over
much of the country. In the second half of
August, the broad flat ridge persisted over the
United States leading to above-normal
temperatures over much of the country.
14
U.S. Temperature and Precipitation Departures
During the Last 30 and 90 Days
Last 30 Days
30-day (ending 1 Sep 2007) temperature departures
(degree C)
30-day (ending 2 Sep 2007) of average
precipitation
Last 90 Days
90-day (ending 2 Sep 2007) of average
precipitation
90-day (ending 1 Sep 2007) temperature departures
(degree C)
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Intraseasonal Variability

• Intraseasonal variability in the atmosphere (wind
  and pressure), which is often related to the
  Madden-Julian Oscillation (MJO), can
  significantly impact surface and subsurface
  conditions across the Pacific Ocean.
• Related to this activity
• significant weakening of the low-level easterly
  winds usually initiates an eastward-propagating
  oceanic Kelvin wave.
• Several Kelvin waves have occurred during the
  last year (see next slide).

16
Weekly Heat Content Evolution in the Equatorial
Pacific

• El Niño conditions and anomalously high heat
  content (red) were present during the latter part
  of 2006.
• Month-to-month variability in heat content during
  May 2006 January 2007 is due to Kelvin wave
  activity.
• During February-May 2007, temperature departures
  were persistently negative in the eastern
  equatorial Pacific. Below-average subsurface
  temperatures are usually found prior to La Niña.
• Recently, subsurface temperature anomalies have
  been affected by weak Kelvin wave activity.

Time

• Kelvin waves have alternating warm and cold
  phases. The warm phase is indicated by dashed
  lines and the cold phase is indicated by dotted
  lines. Down-welling and warming occur in the
  leading portion of a Kelvin wave, and up-welling
  and cooling occur in the trailing portion.

Longitude
17
Low-level (850-hPa) Zonal (east-west) Wind
Anomalies (m s-1)
Westerly wind anomalies (orange/red shading).
Easterly wind anomalies (blue shading).
Easterly wind anomalies have persisted since
January 2007 over the equatorial Pacific in areas
between 160ºE and 150ºW, while low-level westerly
anomalies have generally prevailed in the eastern
Pacific.
Time
Longitude
18
200-hPa Velocity Potential Anomalies (5ºN-5ºS)
Positive anomalies (brown shading) indicate
unfavorable conditions for precipitation.
Negative anomalies (green shading) indicate
favorable conditions for precipitation.
During May - July, weak-to-moderate MJO activity
was observed. Recently, MJO activity has been
weak.
Time
Longitude
19
Outgoing Longwave Radiation (OLR) Anomalies
Drier-than-average conditions (orange/red shading)
Wetter-than-average conditions (blue shading)
Since February, convection has been suppressed
across the eastern half of the equatorial
Pacific. Convection has occasionally been
enhanced in the western equatorial Pacific and
central Indian Ocean.
Time
Longitude
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Oceanic Niño Index (ONI)

• The ONI is based on SST departures from average
  in the Niño 3.4 region, and is a principal
  measure for monitoring, assessing, and predicting
  ENSO.
• Defined as the three-month running-mean SST
  departures in the Niño 3.4 region. Departures are
  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
• NOAAs operational definitions of El Niño and La
  Niña are keyed to the ONI index.

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NOAA Operational Definitions of El Niño and La
Niña Conditions
El Niño characterized by a positive ONI greater
than or equal to 0.5C. La Niña 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 (oC) Evolution since 1950
The most recent ONI value (May July 2007) is
-0.1oC.
El Niño
neutral
La Niña
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Historical El Niño and La Niña Episodes Based on
the ONI computed using ERSST.v2
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Historical Pacific warm (red) and cold (blue)
episodes based on a threshold of /- 0.5 oC for
the Oceanic Nino Index (ONI) 3 month running
mean of ERSST.v2 SST anomalies in the Nino 3.4
region (5N-5S, 120-170W), calculated with
respect to the 1971-2000 base period. For
historical purposes El Niño and La Niña episodes
are defined when the threshold is met for a
minimum of 5 consecutive over-lapping seasons.
 
     
25
Historical Pacific warm (red) and cold (blue)
episodes based on a threshold of /- 0.5 oC for
the Oceanic Nino Index (ONI) 3 month running
mean of ERSST.v2 SST anomalies in the Nino 3.4
region (5N-5S, 120-170W), calculated with
respect to the 1971-2000 base period. For
historical purposes El Niño and La Niña episodes
are defined when the threshold is met for a
minimum of 5 consecutive over-lapping seasons.
     
   
26
Historical Pacific warm (red) and cold (blue)
episodes based on a threshold of /- 0.5 oC for
the Oceanic Nino Index (ONI) 3 month running
mean of ERSST.v2 SST anomalies in the Nino 3.4
region (5N-5S, 120-170W), calculated with
respect to the 1971-2000 base period. For
historical purposes El Niño and La Niña episodes
are defined when the threshold is met for a
minimum of 5 consecutive over-lapping seasons.
0.5
0.2
0.7
0.8
0.9
0.9
0.9
0.2
0.2
0.3
0.4
0.8
0.4
0.3
0.2
0.0
-0.2
-0.4
-0.7
0.6
0.5
0.3
0.4
0.5
0.0
0.0
0.1
0.4
0.7
0.9
1.1
-0.8
-0.7
-0.4
-0.2
0.3
1.1
0.8
0.3
0.1
-0.1
0.0
-0.1
     
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Pacific Niño 3.4 SST Outlook

• Most model forecasts indicate slightly
  below-average SSTs in the Niño 3.4 region during
  the remainder of the year. Most statistical
  models show ENSO-neutral conditions persisting
  through the end of 2007, while most dynamical
  models indicate the development of La Niña during
  the next three months.

Figure provided by the International Research
Institute (IRI) for Climate and Society (updated
20 August 2007).
28
SST Outlook NCEP CFS Forecast Issued 3 September
2007
The CFS ensemble mean (heavy blue line) indicates
a gradual strengthening of La Niña conditions
during September - December.
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U. S. Seasonal OutlooksSeptember - November 2007
Temperature
Precipitation
Outlooks combine long-term trends and
soil-moisture effects, along with typical ENSO
impacts when appropriate.
30
Summary

• La Niña conditions are developing across the
  tropical Pacific.
• SST anomalies have become more negative in the
  eastern equatorial Pacific, and remain positive
  in the western equatorial Pacific.
• Most dynamical models predict the strengthening
  of La Niña in the next couple of months, while
  half of the statistical models indicate
  ENSO-neutral conditions.
• Recent equatorial Pacific SST observations and
  model forecasts indicate La Niña conditions will
  continue to develop during the next several
  months.