ИСТИНА

El Niño Southern Oscillation (ENSO) is one of the most prominent patterns of climate variability at interannual scale. The ENSO evolution and mechanisms have been widely investigated, nevertheless key questions remain unresolved, in particular regarding its irregularity and strong diversity of the events. Here the focus is on the atmospheric disturbances of intraseasonal timescale (ITV) and its relationship with the two types of El Niño, the Eastern Pacific (EP) or conventional El Niño and Central Pacific (CP) or Modoki El Niño. The dominant intraseasonal mode in tropics – the Madden-Julian Oscillation (MJO) and convectively coupled equatorial Rossby waves (ER) were in particular shown to be tightly related to ENSO through their relationship to episodes of westerlies that can trigger downwelling intraseasonal Kelvin waves, a precursor to El Niño onset. The ENSO/ITV relationship exhibits a marked seasonal dependence: MJO induces the oceanic Kelvin wave in March-April preceding El Niño in the western Pacific while the ER intensification in June-July in the central Pacific serves the maintenance for Kelvin wave dissipating along its way through the Pacific. It is shown that MJO and ER activity have a distinct characteristics during the two types of El Niño. The most striking difference is that during conventional El Niño the MJO and equatorial Rossby waves activity are precursor signals, acting as a trigger of the event while during Modoki El Niño they contribute also to its persistence once it has appeared. The ENSO/ITV relationship does not only demonstrate a drastic seasonal dependence, it is also sensitive to state of the tropical Pacific, which has implication for ENSO seasonal forecasts. Our results suggest that ER and MJO predictive score relative to the both types of El Niño drastically changes on the decadal timescale, with ER/ENSO relationship experiencing less low-frequency modulation than MJO. The predictive score of MJO relative to EP El Niño (to lesser extent to CP El Niño) is strongly correlated to the PDO and IPO index, suggesting the former is closely related to the mean zonal SST gradient across the equatorial Pacific: reduced (enhanced) SST gradient associated to positive (negative) IPO and PDO favors (decreases) the MJO contribution to the development of El Niño (Fig.1). ER predictive score relative to ENSO is less dependent on decadal modulation of Pacific mean state, but ER/CP relationships is likely to be modulated by the meridional mode in the Pacific.