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North and South Pacific Meridional Modes (NPMM and SPMM) are known precursors of El Nino-Southern Oscillation (ENSO) and Tropical Pacific decadal variability (TPDV). However, the relative importance of these precursors and the timescale on which they impact the tropics remain unclear. Using a 30-member ensemble of the Community Earth System Model as the control climate, we generate two additional members where the NPMM and SPMM are selectively suppressed. We find that both meridional modes energize the tropical variance independently on different timescales. The absence of NPMM leads to a significant reduction of the tropical interannual variability (similar to 35%), while the absence of the SPMM has no appreciable impact on ENSO but significantly reduces the TPDV (similar to 30%). While the relative importance of the NPMM and SPMM may be model dependent, the stochastic atmospheric variability in the extratropics that energizes the meridional modes emerges as a key source of TPDV.

Plain Language Summary El Nino-Southern Oscillation (ENSO) and the Tropical Pacific decadal variability (TPDV) exert a strong influence over the climate of large parts of the world. To understand and predict tropical Pacific variability, it is necessary to study all those oceanic and atmospheric processes that act as triggers and energizers of ENSO and TPDV, also referred to as the precursor dynamics. The North and South Pacific Meridional Modes (NPMM and SPMM) are well-known precursors of ENSO and TPDV; however, their relative importance and the timescale on which they exert their influence remain unclear. We use a climate model to perform simulations where either NPMM or SPMM variability are selectively suppressed. We find that the absence of NPMM leads to a significant reduction of ENSO (similar to 35%), while the absence of the SPMM has no appreciable impact on ENSO but significantly reduces the TPDV (similar to 30%). The NPMM and SPMM modes impact the tropical variance independently on interannual and decadal timescale, respectively.