{"status":"ok","message-type":"work","message-version":"1.0.0","message":{"indexed":{"date-parts":[[2025,5,14]],"date-time":"2025-05-14T12:58:07Z","timestamp":1747227487747,"version":"3.40.5"},"posted":{"date-parts":[[2021,3,4]]},"group-title":"pico","reference-count":0,"publisher":"Copernicus GmbH","content-domain":{"domain":[],"crossmark-restriction":false},"short-container-title":[],"abstract":"<jats:p>&amp;lt;p&amp;gt;El Ni&amp;amp;#241;o Southern Oscillation (ENSO) index has been shown as a non-Gaussian and nonlinear stochastic process. Here we assess the statistical significance of non-Gaussianity and non-linearity through the analysis of third-order statistics of El Ni&amp;amp;#241;o 3.4 index in the period 1870&amp;amp;#8211;2018, namely the bicovariance (lagged third-order moments) and bispectrum (its 2D Fourier transform). The analysis of bicovariance reveals a tendency for extreme (weak) ENSO signal in the Boreal Spring to be followed by la Ni&amp;amp;#241;as (El Ni&amp;amp;#241;os) in the forthcoming Boreal Winter, thus contributing for a nonlinear attenuation of the ENSO Spring Predictability Barrier. The bispectrum provides a spectral decomposition of skewness in a similar way of the spectral decomposition of variance. &amp;amp;#160;Positive and negative real bispectrum values identify triadic phase synchronizations (at frequencies f1, f2 and f1+f2, mostly in the period range 2&amp;amp;#8211;6 years) contributing respectively to extreme El Ni&amp;amp;#241;os and La Ni&amp;amp;#241;as. The known positive ENSO skewness and the main features of the ENSO bicovariance and bispectrum are shown to be well reproduced by fitting a bilinear stochastic model where the influence of non-observed variables is simulated by a delayed multiplicative noise, being able to generate non-Gaussianity and non-linearity. The model shows improved forecasts, with respect to benchmark linear models, up to four trimesters ahead, especially of the amplitude of extreme El Ni&amp;amp;#241;os. The authors would like to acknowledge MISU (Meteorological Institute at Stockholm University) and the financial support FCT through project&amp;amp;#160; &amp;amp;#160;UIDB\/50019\/2020 &amp;amp;#8211; IDL and project JPIOCEANS\/0001\/2019 (ROADMAP: &amp;amp;#8217;The Role of ocean dynamics and Ocean&amp;amp;#8211;Atmosphere interactions in Driving cliMAte variations and future Projections of impact&amp;amp;#8211;relevant extreme events&amp;amp;#8217;).&amp;lt;\/p&amp;gt;<\/jats:p>","DOI":"10.5194\/egusphere-egu21-8439","type":"posted-content","created":{"date-parts":[[2021,3,4]],"date-time":"2021-03-04T04:22:22Z","timestamp":1614831742000},"source":"Crossref","is-referenced-by-count":0,"title":["Bicovariance and Bispectrum of ENSO index and its impact in nonlinear predictability&amp;#160;"],"prefix":"10.5194","author":[{"ORCID":"https:\/\/orcid.org\/0000-0002-1700-6607","authenticated-orcid":false,"given":"Carlos","family":"Pires","sequence":"first","affiliation":[]},{"ORCID":"https:\/\/orcid.org\/0000-0002-8255-5186","authenticated-orcid":false,"given":"Abdel","family":"Hannach","sequence":"additional","affiliation":[]}],"member":"3145","container-title":[],"original-title":[],"deposited":{"date-parts":[[2021,3,4]],"date-time":"2021-03-04T04:37:31Z","timestamp":1614832651000},"score":1,"resource":{"primary":{"URL":"https:\/\/meetingorganizer.copernicus.org\/EGU21\/EGU21-8439.html"}},"subtitle":[],"short-title":[],"issued":{"date-parts":[[2021,3,4]]},"references-count":0,"URL":"https:\/\/doi.org\/10.5194\/egusphere-egu21-8439","relation":{},"subject":[],"published":{"date-parts":[[2021,3,4]]},"subtype":"other"}}