Decadal modulation of the relationship between intraseasonal tropical variability and ENSO

Abstract

The El Nino Southern Oscillation (ENSO) amplitude is modulated at decadal timescales, which, over the last decades, has been related to the low-frequency changes in the frequency of occurrence of the two types of El Nino events, that is the Eastern Pacific (EP) and Central Pacific (CP) El Ninos. Meanwhile ENSO is tightly linked to the intraseasonal tropical variability (ITV) that is generally enhanced prior to El Nino development and can act as a trigger of the event. Here we revisit the ITV/ENSO relationship taking into account changes in ENSO properties over the last six decades. The focus is on two main components of ITV, the Madden–Julian Oscillation (MJO) and convectively coupled equatorial Rossby waves (ER). We show that the ITV/ENSO relationship exhibits a decadal modulation that is not related in a straight-forward manner to the change in occurrence of El Nino types and Pacific decadal modes. While enhanced MJO activity associated to EP El Nino development mostly took place over the period 1985–2000, the ER activity is enhanced prior to El Nino development over the whole period with a tendency to relate more to CP El Nino than to EP El Nino. In particular the relationship between ER activity and ENSO was particularly strong for the period 2000–2015, which results in a significant positive long-term trend of the predictive value of ER activity. The statistics of the MJO and ER activity is consistent with the hypothesis that they can be considered a state-dependent noise for ENSO linked to distinct lower frequency climate modes.