The upward propagation of the ENSO signal towards the stratosphere: An Overview

Abstract

The study of El Niño-Southern Oscillation (ENSO) phenomenon above the troposphere has traditionally been difficult because of the lack of global observations at high altitudes and also because of the presence of other sources of variability whose signals are difficult to disentangle from ENSO effects. Recent works with general circulation models that isolate the ENSO signal have demonstrated its upward propagation into the stratosphere. Here we use a recent version of the Whole Atmosphere Community Climate Model to illustrate the propagation and mechanisms whereby the signal manifests itself in the stratosphere. The ENSO signal propagates upwards to about 40 km at middle latitudes in Northern Hemisphere boreal winter months by means of large-scale Rossby waves. The ENSO signal is also identified at tropical latitudes in the form of a zonally homogeneous cooling during strong El Niño events and at polar latitudes in the Northern Hemisphere winter as a zonally homogeneous warming accompanied by weaker winds during a strong El Niño event and winter boreal months. This signal is the result of the intensification of the stratospheric meridional circulation forced by anomalous propagation and dissipation of Rossby waves at middle latitudes during strong El Niño events.