Guidance) 3/05/13 15 38: IRI ENSO forecast summary part 1 MJJ 2013 ASO 2013 NDJ 2013 Dynamical models -0.1 0.1 0.2 Statistical models -0.4 -0.3 -0.3 All -0.2 0 0 Technical ENSO Update 18 April 2013 Recent and Current Conditions After a brief period approaching borderline La Niña SST conditions in January and February 2013, the SST anomaly in th Nino3.4 region returned to more clearly neutral levels during March and has remained neutral through mid-April 2013. Fo March 2013 the Nino3.4 SST anomaly was -0.22 C, indicative of cool-neutral ENSO conditions, and for January-March th anomaly was -0.34 C. The most recent weekly SST anomaly in the NINO3.4 region was 0.0 C, indicating neutral ENSO conditions in the tropical Pacific; this is not far from the -0.22 C level observed in March. Expected Conditions The most recent official diagnosis and outlook was issued earlier this month in the NOAA/Climate Prediction Center ENSO Diagnostic Discussion, produced jointly by CPC and IRI; it called for a high likelihood of neutral ENSO conditions endurin through the second quarter of 2013, with probabilities of El Niño or La Niña each less than 25% through northern summer 2013 The latest set of model ENSO predictions, from mid-April, is now available in the IRI/CPC ENSO prediction plume, discusse below. Currently, SSTs have been in the middle of the ENSO-neutral range (anomaly of about -0.3 to 0.1 C), and SST is slightl above average in the far western part of the basin. Subsurface temperatures across the equatorial Pacific average close to th climatological average, but are above average in the western quarter of the basin and near average in the east-central tropica Pacific. In the east-central part of the basin, a large pocket of below-average sea temperature at depth is currently overlain by 50-meter deep layer of near to slightly above average sea temperature, protecting the surface (SST) from the anamalously colde water beneath. In the atmosphere, the basin-wide sea level pressure pattern (e.g. the SOI), has been mainly to slightly abov average and the low-level zonal winds have also been near average. Anomalous convection (as measured by OLR) has generall been negative in the central tropical Pacific, and positive in the far western part of the basin. Together, these features collectivel reflect ENSO-neutral conditions, and the tendency toward cooler than average ENSO conditions has weakened over the las month. As of mid-April, 20% of the set of dynamical and statistical models models predicts weak La Niña SST conditions for the Apr Jun 2013 season, none predicts El Niño conditions, and 80% indicates neutral ENSO. For the Jul-Sep season, among models tha do use subsurface temperature information, 85% predicts ENSO-neutral SSTs, 5% predicts El Niño conditions and 10% predict La Niña conditions. For all model types, the probability for neutral ENSO conditions is 65% or greater from Apr-Jun to the en of the forecast period in northern winter 2013/14. (Note 1). Caution is advised in interpreting the distribution of mode predictions as the actual probabilities. At longer leads, the skill of the models degrades, and skill uncertainty must be convolve with the uncertainties from initial conditions and differing model physics, leading to more climatological probabilities in th long-lead ENSO Outlook than might be suggested by the suite of models. Furthermore, the expected skill of one model versu another has not been established using uniform validation procedures, which may cause a difference in the true probabilit distribution from that taken verbatim from the raw model predictions. An alternative way to assess the probabilities of the three possible ENSO conditions shows probabilities for La Niña at 16% fo Apr-Jun 2013, 24% for May-Jul, and also 24% for Jun-Aug 2013, remaining between 20% and 25% throughout 2013. Mode probabilities for ENSO-neutral conditions are 83% for Apr-Jun 2013, 69% for May-Jul, and 64% for Jun-Aug 2013, decreasin to between 55% and 65% from Jul-Sep through the calendar year. Probabilities for El Niño are 1% for Apr-Jun 2013, 7% fo May-Jul, rising to approximately 15% from Jun-Aug through the rest of 2013. In words, the models collectively favor neutra ENSO conditions through to the remainder of 2013; La Niña is very slightly favored over El Niño during all of the period. Th probabilities derived from the 24 or more models on the IRI/CPC plume describe, on average, maintenance of neutral ENSO conditions during the coming months, continuing through into the second half of 2013. Uncertainty exists, because we are in th period of typically most likely new ENSO event evolution from now until about late June. The model forecast spread expresse that undertainty in the spread between weak La Nina and borderline El Nino conditions, even though the majority of the forecast is in the neutral range. 40: Summary of main seasonal ENSO model resu NIWA’s National Climate Centre Global powerpoint Dynamical Statistical
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