Asymmetric impacts of tropical Pacific decadal variability on the frequency, duration, and amplitude of El Nino and La Nina

The El Nino-Southern Oscillation (ENSO) simulated in a 1300-year preindustrial control run of the Community Climate System Model version 4 (CCSM4) exhibits distinct modulation of its frequency, duration, and amplitude in association with decadal changes in the tropical Pacific background state. The leading mode of tropical Pacific decadal variability (TPDV), which resembles the interdecadal Pacific oscillation (IPO) in observations, is linked to changes in the frequency of El Nino events. When the IPO is in the positive phase, decadal deepening of the equatorial thermocline increases the occurrence of El Ninos in boreal spring-summer by twofold compared to the negative phase. In addition, weak El Nino events become more persistent during the positive phase of IPO. The onsets of La Nina events are, on the other hand, controlled more by the preceding El Ninos and associated equatorial ocean waves. During the negative phase of IPO, decadal shoaling of the equatorial thermocline tends to increase the occurrence of La Ninas, but this effect is nearly counteracted by the decrease in the number of La Ninas following El Ninos.

The amplitude of ENSO is closely related to the second leading mode of TPDV, consistent with previous modeling studies. This decadal mode is characterized by a zonal dipole pattern of sea surface temperature (SST) anomalies and a meridional shift of the intertropical convergence zone (ITCZ) in the eastern tropical Pacific. When the eastern tropical Pacific is warmer and the ITCZ is displaced southward, El Ninos develop more rapidly through boreal summer-fall compared to those occurring during the opposite phase of TPDV. These strong El Nino events are terminated by large upwelling Kelvin waves propagating from the western equatorial Pacific in the following winter-spring, leading to strong La Nina events. The strong La Nina events tend to persist for multiple years. In addition to large ocean heat content anomalies initiating the events, precipitation and surface wind anomalies associated with strong La Nina events extend further westward compared to weak events, making the Pacific atmospheric circulation anomalies more insensitive to remote forcing from the Indian Ocean.

The patterns of TPDV associated with decadal ENSO modulation in CCSM4 have resemblance to those simulated by its atmospheric component model coupled to a slab ocean. It is suggested that TPDV induced by stochastic atmospheric variability interacts with the ENSO dynamics. The ENSO changes associated with TPDV are not symmetrical between El Nino and La Nina and thus are not likely to occur solely due to random variability. The decadal modulation of ENSO may, in turn, contribute to decadal changes in the tropical Pacific background state. In particular, the asymmetry in the patterns of El Nino and La Nina becomes more pronounced during the period of strong ENSO, which would weaken the zonal contrast of tropical Pacific SSTs.