The Southern Hemisphere as a Thermodynamic Modulator of ENSO Amplitude

Abstract:

Variability in the South Pacific Meridional Mode (SPMM) has previously been argued to be a “trigger” for ENSO events. In this presentation, we offer an alternative linkage between the SPMM and ENSO, not as a trigger for ENSO but as a coincident modulator of ENSO amplitude. A collection of carefully designed coupled model experiments demonstrates that when El Niño events occur coincidentally with +SPMM, the events grow larger and exhibit a larger meridional extent. If instead an El Niño event occurs coincidentally with -SPMM, the event is weaker and is confined to the equatorial region. Analyses reveal that SPMM modulates ENSO amplitude through the thermodynamic coupling between the atmosphere and ocean. The usual atmospheric damping of an El Niño event is counteracted by a latent heat flux warming in the eastern equatorial Pacific during +SPMM, allowing the El Niño event to remain stronger. If instead a -SPMM is present, the latent heat flux cooling associated with -SPMM amplifies the atmospheric damping of the El Niño event, causing the event to further decay. We argue that whether seasonal climate prediction models can skillfully predict SPMM will determine if SPMM is a source of predictability or uncertainty in ENSO predictions. If models do not predict SPMM skillfully, the SPMM will act as a source of uncertainty in ENSO predictions due to incorrect predictions of the eastern equatorial Pacific thermodynamics.