PNA nonlinearity and ENSO transition asymmetry weaken PMM before La Niña onset

Abstract The Pacific Meridional Mode (PMM) is one of dominant coupled modes in the northeastern tropical Pacific (NETP), characterized by a strip-like sea surface temperature (SST) anomalies spanning from Baja California to the central equatorial Pacific. While the majority of the El Niño events follow a positive PMM, only a few La Niña events are preceded by a negative PMM. Such an asymmetric activity of PMM before the onset of El Niño-Southern Oscillation (ENSO) was previously attributed to the inherent nonlinear response of the wind-evaporation-SST (WES) feedback to trade winds in NETP. Through data analysis and coupled model experiments, we pointed out that PMM is in fact a highly symmetric phenomenon, and the asymmetry of PMM before ENSO onset thus must be associated with ENSO. On the one hand, the nonlinear response of deep convection over the equator to symmetric ENSO forcing in the central equatorial Pacific permits a stronger Pacific North America (PNA) pattern in El Niño years than in La Niña years. On the other hand, since the majority of La Niña events are preceded by a sharp decay of an El Niño, the warm equatorial SST anomalies associated with the preceding El Niño provides another source to trigger PNA before La Niña onset. The two mechanisms modulate the trade winds and heat fluxes in NETP more heavily before La Niña onset than the El Niño onset, and equally contribute to PMM asymmetry before ENSO onset.