Influences of ENSO-induced Indo-Western Pacific SST anomalies on extratropical atmospheric variability during the boreal summer

The causes for the observed occurrence of anomalous zonally symmetric upper-level pressure ridges in the midlatitude belts of both hemispheres during the year after warm El Nino-Southern Oscillation (ENSO) events have been investigated. Sea surface temperature (SST) anomalies in the Indo-western Pacific (IWP) sector were simulated by allowing an oceanic mixed layer model for that region to interact with local atmospheric changes forced remotely by observed ENSO episodes in the eastern/central tropical Pacific. The spatiotemporal evolution of these SST conditions through a composite ENSO cycle was then inserted as lower boundary conditions within the IWP domain in an ensemble of atmospheric general circulation model (GCM) integrations. This experimental setup is seen to reproduce zonally symmetric geopotential height anomalies with maximum amplitudes being attained over the extratropics in the boreal summer after the peak phase of ENSO. The model evidence hence supports the notion that these global-scale atmospheric changes are primarily responses to SST perturbations in IWP, which are in turn linked to ENSO variability in the equatorial Pacific by the "atmospheric bridge" mechanism. Experimentation with a stationary wave model indicates that the Eastern Hemisphere portion of the aforementioned atmospheric signals are attributable to forcing by tropical heat sources and sinks associated with precipitation anomalies in the IWP region, which are closely related to the underlying SST changes. Diagnosis of the output from the GCM integrations reveals that these circulation changes due to diabatic heating are accompanied by alterations of the propagation path and intensity of the high-frequency eddies at locations farther downstream. The geopotential tendencies associated with the latter disturbances bear some resemblance to the anomalous height pattern in the Western Hemisphere. Such local eddy-mean flow feedbacks hence contribute to the zonal symmetry of the atmospheric response pattern to forcing in the IWP region. Analysis of zonally averaged circulation statistics indicates that the mean meridional circulation induced by divergence of anomalous transient eddy momentum fluxes in ENSO events could also generate zonally symmetric perturbations in midlatitudes. The model-simulated precipitation and surface temperature anomalies in the North American sector in response to SST changes in IWP suggest an increased frequency of droughts and heat waves in that region during the summer season after warm ENSO events.