Modeling the Tropical Teleconnections to Antarctica: Interactions between Flavors of El Niño and the SAM

Abstract:

Tropical teleconnections associated with the El Niño – Southern Oscillation (ENSO) impact the high-latitudes of the Southern Hemisphere through Rossby wave propagation, changes to the sub-tropical jet stream, and the modulation of the eddy-driven mean meridional circulation. However, the Southern Annular Mode (SAM) modulates this ENSO teleconnection through transient eddy interaction, reinforcing or weakening atmospheric circulation anomalies in the South Pacific Ocean. ENSO teleconnections also depend on the location of the heating/cooling anomalies in the tropical Pacific Ocean, otherwise known as ENSO flavors. Numerical simulations using the National Center for Atmospheric Research Community Atmosphere Model (CAM) have been performed based on tropical forcing of El Niño flavors with warm sea surface temperature anomalies in the central (CP) or eastern (EP) tropical Pacific. CAM captures well the wide spatial and temporal variability associated with the SAM with both types of El Niño forcing. Autocorrelation analysis reveals a higher degree of SAM persistence in the EP simulations, supporting the expectation that EP events effectively influence SAM- events through the propagation of eddy momentum to the high latitudes. However, CAM only simulates accurately the impacts on atmospheric circulation in the high latitudes when the observed SAM phase is matched, especially for CP events. Evaluation of the transient eddies supports this conclusion, as eddy-propagation to the high latitudes is more robust during in-phase events (El Niño/SAM-) while out-of-phase events (El Niño/SAM+) inhibit eddy behavior in the mid-latitudes and limit the propagation of the tropical teleconnection. A regional perspective shows that this interaction takes place primarily over the Pacific Ocean, with high latitude circulation variability being a product of both tropical and high-latitude forcing.