This study seeks to identify thermally driven sources of ENSO amplitude and uncertainty, as they are relatively unexplored compared to wind-driven sources. Pacific meridional modes are argued to be wind triggers for ENSO events. This study offers an alternative role for the South Pacific meridional mode (SPMM) in ENSO dynamics, not as an ENSO trigger, but as a coincident source of latent heat flux (LHF) forcing of ENSO SSTA that, if correctly (incorrectly) predicted, could reduce (increase) ENSO prediction errors. We utilize a coupled model simulation in which ENSO variability is perfectly periodic and each El Niño experiences identical wind stress forcing. Differences in El Niño amplitude are primarily thermally driven via the SPMM. When El Niño occurs coincidentally with positive phase SPMM, the positive SPMM LHF anomaly counteracts a fraction of the LHF damping of El Niño, allowing for a more intense El Niño. If the SPMM phase is instead negative, the SPMM LHF amplifies the LHF damping of El Niño, reducing the event’s amplitude. Therefore, SPMM LHF anomalies may either constructively or destructively interfere with coincident ENSO events, thus modulating the amplitude of ENSO. The ocean also plays a role, as the thermally forced SSTA is then advected westward by the mean zonal velocity, generating a warming or cooling in the ENSO SSTA tendency in addition to the wind-forced component. Results suggest that in addition to wind-driven sources, there exists a thermally driven piece to ENSO amplitude and uncertainty that is generally overlooked. Links between the SPMM and Pacific decadal variability are discussed.
Keywords: South Pacific Ocean; Atmosphere-ocean interaction; ENSO; Interannual variability; Pacific decadal oscillation
Sarah M. Larson, Kathy V. Pegion, Ben P. Kirtman. The South Pacific Meridional Mode as a Thermally Drive Source of ENSO Amplitude Modulation and Uncertainty. Published Online 5 June 2018. American Meterological Society Journal of Climate July 2018 https://doi.org/10.1175/JCLI-D-17-0722.1