The seasonal relationship between intraseasonal tropical variability and ENSO in CMIP5

The El Niño–Southern Oscillation (ENSO) is tightly linked to the intraseasonal tropical variability (ITV) that contributes to energise the deterministic ocean dynamics during the development of El Niño. Here, the relationship between ITV and ENSO is assessed based on models from the Coupled Model Intercomparison Project (CMIP) phase 5 (CMIP5) taking into account the so-called diversity of ENSO, that is, the existence of two types of events (central Pacific versus eastern Pacific El Niño). As a first step, the models' skill in simulating ENSO diversity is assessed. The characteristics of the ITV are then documented revealing a large dispersion within an ensemble of 16 models. A total of 11 models exhibit some skill in simulating the key aspects of the ITV for ENSO: the total variance along the Equator, the seasonal cycle and the characteristics of the propagation along the Equator of the Madden–Julian oscillation (MJO) and the convectively coupled equatorial Rossby (ER) waves. Five models that account realistically for both the two types of El Niño events and ITV characteristics are used for the further analysis of seasonal ITV ∕ ENSO relationship. The results indicate a large dispersion among the models and an overall limited skill in accounting for the observed seasonal ITV ∕ ENSO relationship. Implications of our results are discussed in light of recent studies on the forcing mechanism of ENSO diversity.

The relationship between intraseasonal tropical variability and ENSO simulated by the CMIP5

This study evaluates the simulation of relationship between intraseasonal tropical variability (ITV) and El Niño Southern Oscillation (ENSO) in 23 models from the Coupled Model Intercomparison Project (CMIP) phase 5 (CMIP5) in the Intergovernmental Panel on Climate Change (IPCC) Fifth Assessment Report (AR5). As a first step, the models' skill in simulating ENSO diversity is assessed, which indicates that 16 models among 23 are able to simulate realistically the statistics of the relative size of two types of El Niño. The characteristics of the ITV are then documented revealing that only five models (CMCC-CM, CCSM4, BNU-ESM, INMCM4 and MIROC5) simulate realistically the parameters crucial for proper reproducing of ITV contribution to the El Niño, in particular the total variability, seasonal cycle and propagation along the equator of Madden-Julian oscillation (MJO) and convectively coupled equatorial Rossby waves (ER). At last step the ITV/ENSO relationship in the models are analyzed and compared to observation. It is shown that the key aspects of this interaction such as phase lag between ITV peak activity and El Niño peak and longitude localization of maximum correlation between ITV and ENSO is realistically simulated by CMCC-CM and MIROC5 for MJO and CMCC-CM and INMCM4 for equatorial Rossby waves. These models are capable to reproduce the distinct MJO and ER behavior associated to the two El Niño flavors. Aforementioned models may be used for the investigation of the sensitivity of the ITV/ENSO seasonal dependence to global warming.

The relationship between intraseasonal tropical variability and ENSO and its modulation at seasonal to decadal timescales

The intraseasonal tropical variability (ITV) patterns in the tropical troposphere are documented using double space-time Fourier analysis. Madden and Julian oscillations (MJO) as well as equatorial coupled waves (Kelvin and Rossby) are investigated based on the NCEP/NCAR Reanalysis data for the 1977–2006 period and the outputs of an intermediate ocean-atmosphere coupled model named LODCA-OTCM. A strong seasonal dependence of the ITV/ENSO relationship is evidenced. The leading relationship for equatorial Rossby waves (with the correlation of the same order than for the MJO) is documented; namely, it is shown that intensification of Rossby waves in the central Pacific during boreal summer precedes by half a year the peak of El Niño. The fact that MJO activity in spring-summer is associated to the strength of subsequent El Niño is confirmed. It is shown that LODCA-QTCM is capable of simulating the convectively coupled equatorial waves in outgoing long wave radiation and zonal wind at 850 hPa fields with skill comparable to other Coupled General Circulation Models. The ITV/ENSO relationship is modulated at low frequency. In particular the periods of low ENSO amplitude are associated with weaker MJO activity and a cancellation of MJO at the ENSO development phase. In opposition, during the decaying phase, MJO signal is strong. The periods of strong ENSO activity are associated with a marked coupling between MJO, Kelvin and equatorially Rossby waves and ENSO; the precursor signal of MJO (Rossby waves) in the western (central) Pacific is obvious. The results provide material for the observed change in ENSO characteristics in recent years and question whether the characteristics of the ITV/ENSO relationship may be sensitive to the observed warming in the central tropical Pacific.