How interference between the North Atlantic Oscillation and the tropical Indo-Pacific convection modulates wave trains along the subtropical jet: Impacts on the Asian winter climate

Abstract

The Rossby wave train along the wintertime subtropical jet over the Asian continent extends from Western Europe to Japan, and it significantly influences the East Asian climate. Two primary excitation sources of the subtropical wave train are the European vorticity anomaly related to the North Atlantic Oscillation (NAO) and tropical anomalous convection associated with the Indo-Pacific Walker circulation. The wave trains excited solely by the European vorticity anomaly exhibit large amplitudes near Europe but diminish downstream. However, as they reach the Asian domain, the waves can amplify through vorticity advection linked with convective activity in the Indo-Western Pacific. The coherency of the NAO-related European vorticity and the anomalous Indo-Pacific Walker circulation plays a key role in the excitation of the subtropical wave train. To investigate their interference, statistical analyses were carried out based on reanalysis data, and additional numerical experiments were performed using a linear baroclinic model. Results show that when the cyclonic vorticity injection associated with the NAO in Western Europe coincides with the anomalous meridional divergent wind over South China associated with the enhanced Indo-Pacific Walker circulation, constructive interference occurs, leading to a significant low-pressure anomaly in the Far East and colder winters. On the other hand, when the NAO-related vorticity forcing and tropical convective activity are out of phase, destructive interference arises, suppressing the propagation of Rossby waves downstream of South Asia and ensuing a weaker impact on the East Asian climate. The findings reveal that the phase combination of the NAO-related European vorticity and Indo-Pacific Walker circulation determines the eastern extent of the wave train along the subtropical jet and its associated climate impacts on the broader Asian region.