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Predictability of El Niño-Southern Oscillation
Principal Investigator:
Chris Landsea
Collaborating Scientist:
John Knaff (CIRA/Colorado State University)
Objective:
To provide a baseline method for prediction of the El Niño-Southern
Oscillation phenomena and to assess the skill of real-time
predictions
Method:
A statistical prediction method is developed for the El
Niño
-Southern
Oscillation (ENSO) phenomena which is based entirely on the optimal
combination of persistence, month-to-month trend of initial conditions
and climatology. The selection of predictors is by design intended to
avoid any pretense of predictive ability based on "model physics" and
the like, but rather is to specify the optimal "no-skill" forecast as
a baseline comparison for more sophisticated forecast methods.
Multiple least squares regression using the method of leaps and bounds
is employed to test a total of fourteen possible predictors for the
selection of the best predictors, based upon 1950-1994 developmental
data. A range of zero to four predictors were chosen in developing
twelve separate regression models, developed separately for each
initial calendar month. The predictands to be forecast include the
Southern Oscillation (pressure) Index (SOI) and the Niño 1+2, Niño 3,
Niño 4 and Niño 3.4 SST indices for the equatorial eastern and central
Pacific at lead times ranging from zero seasons (0 - 2 months) through
seven seasons (18 - 20 months). Though hindcast ability is strongly
seasonally dependent, substantial improvement is achieved over simple
persistence wherein largest gains occur for two to seven season (6 to
21 months) lead times. For example, expected maximum forecast ability
for the Niño 3.4° SST region, depending on the initial date, reaches
92, 85, 64, 41, 36, 24, 24 and 28 percent of variance for leads of
zero to seven seasons. Comparable maxima of persistence only forecasts
explain 92, 77, 50, 17, 6, 14, 21 and 17 percent, respectively.
Details on the paper can be found in Knaff and Landsea (1997).
The very strong 1997-98 El Niño was the first major event in which
numerous forecasting groups participated in its real-time prediction.
A previously developed simple statistical tool - the El Niño-Southern
Oscillation CLImatology and PERsistence (ENSO-CLIPER) model - is
utilized as a baseline for determination of skill in forecasting this
event. Twelve statistical and dynamical models were available in real-
time for evaluation. Some of the models were able to outperform ENSO-
CLIPER in predicting either the onset or the decay of the 1997-98 El
Niño, but none were successful at both for a medium-range two season
(6-8 months) lead time. There were no models, including ENSO-CLIPER,
able to anticipate even one-half of the actual amplitude of the El
Niño's peak at medium-range (6-11 months) lead. In addition, none of
the models showed skill (i.e. lower root mean square error than ENSO-
CLIPER) at the zero season (0-2 months) through the two season (6-8
months) lead times. No dynamical model and only two of the statistical
models (the canonical correlation analysis [CCA] and the constructed
analog [ANALOG]) outperformed ENSO-CLIPER by more than 5% of the root
mean square error at the three season (9-11 months) and four season
(12-14 months) lead time. El Niño impacts were correctly anticipated
by national meteorological centers one half year in advance, because
of the tendency for El Niño events to persist into and peak during the
Boreal winter. Despite this, the zero to two season (0-8 month)
forecasts of the El Niño event itself were no better ENSO-CLIPER and
were, in that sense, not skillful - a conclusion that remains unclear
to the general meteorological and oceanographic communities. Details
on this analysis are found in Landsea and Knaff (2000).
A continuing effort is being made to periodically revise the
assessment of skill in real-time predictions of El Niño-Southern
Oscillation by groups from around the world. An evaluation of the
1998-2001 La Nina event and the onset of the 2002-03 El Niño event
is forthcoming.
Accomplishments:
- ENSO-CLIPER program, documentation and archive of forecasts
are available (as of 2000)
on the web.
- Real-time forecasts from ENSO-CLIPER were published in the
Experimental Long-Lead Forecast Bulletin quarterly
(1997-2001).
- Real-time forecasts from ENSO-CLIPER are being published
quarterly in the Climate Diagnostics Bulletin (2002-03).
References:
Knaff, J. A. and C. W. Landsea, 1997: An El Niño-Southern Oscillation
Climatology and Persistence (CLIPER) forecasting system. Wea.
Forecasting, 12, 633-652.
Landsea, C. W. and J. A. Knaff, 2000: How much skill was there in
forecasting the very strong 1997-98 El Niño? Bull. Amer.
Meteor. Soc., 81, 2107-2119.
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Last modified: 02/21/2003
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