Mechanisms of Linear and Non-Linear Pacific Meridional Mode/ENSO Interactions

This study investigates the interaction between the Pacific Meridional Mode (PMM) and El Niņo / Southern Oscillation (ENSO) using a combination of NCAR's Community Earth System Model (CESM1.2) and an intermediate coupled model that simulates ENSO. An ensemble of CESM simulations, in which the Community Atmospheric Model is coupled with a slab ocean model (CAM5+SOM), is run with imposed surface heat flux anomalies associated with the atmospheric North Pacific Oscillation (NPO). The CAM+SOM model architecture includes the necessary physics for the PMM, but not ENSO. The surface wind stress anomalies from the CAM+SOM then forces the Battisti (1988) intermediate coupled model (ICM), which has the necessary physics for ENSO, but not the PMM. The combination of models is capable of reproducing the connection between the NPO and ENSO although large variability between ensemble members suggests high sensitivity to natural variability. Of note is a nonlinear ENSO response (despite the ICM being in a linearly stable regime) to positive and negative NPO / PMM forcing. Sensitivity experiments using the ICM determine the physical processes responsible for the development of ENSO, including the non-linear response. It is shown that oceanic Kelvin waves are critical for connecting the PMM to ENSO. Sources of the nonlinear response will be discussed, including the impact of PMM wind anomalies on wind stress, non-linearity in the atmospheric coupling, and non-linearity in the thermocline / SST interactions.