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Anthony G. Barnston, Lisa Goddard, Simon Mason Steve E. Zebiak, Bradfield Lyon,

Evaluation of the ENSO Forecasts of Three Institutions Issued Prior to and During the Onset of the 2002 El Nino. Anthony G. Barnston, Lisa Goddard, Simon Mason Steve E. Zebiak, Bradfield Lyon, Chester F. Ropelewski and M. Neil Ward. International Research Institute for Climate Prediction

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Anthony G. Barnston, Lisa Goddard, Simon Mason Steve E. Zebiak, Bradfield Lyon,

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  1. Evaluation of the ENSO Forecasts of Three Institutions Issued Prior to and During the Onset of the 2002 El Nino Anthony G. Barnston, Lisa Goddard, Simon Mason Steve E. Zebiak, Bradfield Lyon, Chester F. Ropelewski and M. Neil Ward International Research Institute for Climate Prediction Earth Institute of Columbia University 61 Route 9W, Palisades, NY 10964 e-mail: tonyb@iri.columbia.edu

  2. Introduction A moderate El Nino occurred in 2002-03. It began in late May, and lasted throughout 2002 and into 2003. This work evaluates the verbally expressed ENSO forecasts issued by three institutions: The NOAA/NWS Climate Prediction Center (CPC) of the U.S., the International Research Institute for Climate Prediction (IRI), and the Bureau of Meteorology (BOM) of Australia. We examine the language used, and the implied quantitative probability forecast of whether or not an El Nino would develop during 2002. The El Nino, as reflected in the montly SST anomalies in the NINO3.4 region during 2002, was observed as follows with dollar signs denoting the El Nino period: Jan Feb Mar Apr May Jun Jul Aug Sep Oct Nov Dec -.02 .25 .17 .26 .39 .94 .90 1.08 1.19 1.46 1.75 1.62 $$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$==>

  3. Monthly ENSO Forecast Statements from NOAA/CPC, IRI and BMRC December 2001 – October 2002 The following few pages highlight the verbally expressed ENSO outlooks of the three Institutions on a month by month basis. Excerpts are shown that express the main content and thoughts in the outlook.

  4. NOAA/CPC Gradual transition to warm episode conditions. Warm episode conditions likely to develop in 3-6 mo. Seems most likely that warm episode (El Niño) conditions will develop in next 3 months. Seems likely that warm-episode (El Niño) conditions will develop during next 3 months. IRI - - - - - - enhanced likelihood of an El Niño (55%)…there is considerable uncertainty enhanced likelihood of an El Niño (65%)…it would not be among very strongest El Ninos. BMRC Expect neutral cond-itions next 3 months. Expect neutral cond-itions next 3 months. 50% chance of an El Niño this year. 50% chance of an El Niño this year. Dec Jan Feb Mar

  5. NOAA/CPC Seems likely that warm episode conditions will continue to develop. Appears most likely that a gradual warming will continue…weak to moderate El Niño conditions likely by the end of 2002. Is likely that further development towards El Niño will continue…weak to moderate El Nino. El Niño conditions are expected to continue… IRI Enhanced likelihood of an El Niño (70%)…it would be weak to moderate. 55% probability of an El Niño…clearly leaves un- certainty…weak or lower portion of moder-ate…max SST anomaly in Nino3.4 likely less than 1.5 deg C. 75% probability of an El Niño…most likely weak. 90% probability of an El Niño…1/3 to 1/2 the strength of 1997-98 event. BMRC 60% chance of an El Niño this year. 50% chance of an El Niño this year. El Niño clearly more likely than not. El Niño very likely. Apr May Jun Jul

  6. CPC Weak to moderate El Niño conditions are expected to continue. Moderate El Niño conditions are expected to continue. Moderate El Niño conditions are expected to continue. IRI 97% probability of an El Niño…most likely weak to moderate. 99% probability of an El Niño…more likely moderate than weak. Greater than 99% probability of an El Niño…most likely moderate. BMRC …the currently devel-oping El Niño… …the current El Niño… …the current El Niño… Aug Sep Oct

  7. The verbal outlooks above were subjectively converted to a probability that El Nino conditions would develop during year 2002. These probabilities follow, for the common period of February to October. Month Probability of an El Nino CPC IRI BoM Comments Feb 80 55 50 Warming along S.Am. coast Mar 70 65 50 El Nino hints present Apr 70 70 60 El Nino hints continue May 60 55 50 El Nino hints weakened Jun 75 75 75 El Nino state clearly observed Jul 95 90 90 El Nino continues Aug 99 97 97 El Nino strengthens further Sep 100 100 100 El Nino strengthens further Oct 100 100 100 El Nino strengthens further

  8. RPSS for Probabilistic El Nino Forecasts, 2002, derived from the probabilities from the last page. CPC IRI BOM Feb .93 .63 .53 Mar .84 .78 .53 Apr .84 .83 .69 May .71 .63 .53 Jun .89 .89 .88 Jul 1.00 .98 .98 Aug 1.00 1.00 1.00 Sep 1.00 1.00 1.00 Oct 1.00 1.00 1.00 AVG .91 .86 .80

  9. Findings: A moderate El Nino developed during 2002. Out of the three institutions, CPC was consistently most confident that it would occur before it first appeared. The IRI’s forecasts were less confident than CPC’s, and BoM’s forecasts were still less confident. Hence, when scoring the forecasts with the ranked probability skill score for this event of 2002, CPC’s score was highest, followed by IRI, and then BoM. However, one might ask whose implied probabilities were actually most appropriate. Were CPC’s the highest because CPC is better at ENSO forecasting than the other two institutions? That is a possibility. Another possibility is that CPC’s probabilities were more “bullish” than they should have been, and although they scored well this time, if the same early-year ENSO setup could be repeated 100 times, an El Nino would have developed in a lower proportion of the 100 cases than the probabilities implied in CPC’s statements. We do not know if this would be the case or not. There will only be a handful of El Ninos during the remainder of the time that the present key scientists remain at these institutions, so that the sample of cases over which to test this question will be inadequate. All we know right now is that CPC exhibited impressive apparent skill in anticipating the El Nino of 2002 several months before it appeared. It may have been luck, or it may have been great insight. (Or, perhaps there was some combination of both.)

  10. Features of IRI’s ENSO Forecasts IRI does not have its own ENSO forecast model. It depends on observations, the collective indications of other institutions’ models, and its own expertise. IRI’s ENSO statements contain material intended to inform, and be educational for, non-technical audiences. . The forecasts are expressed verbally, pictorially, and probabilistically. Probabilities are explicitly stated. For the 2002-03 El Niño prior to the late May onset, the IRI’s forecasts were more cautious than CPC’s, and slightly less cautious than BoM’s.

  11. Issues for Future ENSO Forecasts Homogenization of forecast styles -is quantitative definition of probabilistic words desirable? (“likely”, “most likely”, etc.) -is explicit use of probabilities desirable? -should ENSO forecast statements include statement of likely climate impacts? Alignment of definitions -ENSO condition vs. ENSO event or episode? -ENSO strength categorization

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