Dynamical Mechanisms for the Teleconnection between ENSO and NAO in Late Winter

Abstract

The dynamical mechanisms for the late-winter teleconnection between El Nino-Southern Oscillation (ENSO) and the North Atlantic Oscillation (NAO) are examined using output from two global climate models and various reanalysis datasets.

During El Nino winters, the intensified transient disturbances along the equatorward shifted North Pacific storm track extend their influences farther downstream, thereby leading to eastward extension of eddy vorticity forcing to the North Atlantic region. Such eddy forcing induces negative geopotential height tendencies along the southern lobe of the NAO, thus favoring more occurrences of negative NAO events. It is further demonstrated that these transient eddy effects can be reproduced in atmospheric GCM integrations subjected to ENSO-related SST anomalies in the tropical Pacific region.

Analysis of the persistent anomalous circulation episodes in the North Pacific-North America-North Atlantic sector further demonstrate the contributions of downstream eddy development to the ENSO/NAO teleconnection. These episodes are characterized by a strengthened Pacific subtropical jet stream and an equatorward-shifted Pacific storm track. The wave packets that populate the storm tracks travel eastward through downstream development. The pursuant barotropic forcing of the embedded synoptic-scale eddies is conducive to the formation of the negative phase of the NAO. The more frequent and higher persistence of those episodes during El Nino winters contribute to the prevalence of negative NAO conditions.

A higher frequency of weak (strong) stratospheric vortex events for El Nino (La Nina) condition is generated in a climate model with a more realistic upper atmosphere. During El Nino events, the enhanced tropospheric stationary wave-1 driving contributes to the increased frequency of stratospheric weak vortex events. The easterly wind anomalies induced by Eliassen-Palm (EP)-flux convergence over the polar cap propagate downward to the lower stratosphere/upper troposphere. Anomalous westerly in the subtropics cannot be explained by the stratospheric planetary-wave-mean-flow interaction. Rather, it is driven by the upper tropospheric eddy momentum flux convergence, which in turn results from strong poleward wave refraction. This upper-tropospheric westerly anomaly is linked to the lower-troposphere via the eddy-driven tropospheric overturning circulation, and the resulting SLP anomaly response resembles the negative NAO in the North Atlantic region.