Extratropical Forcing of Tropical Synoptic-Scale Waves over the Indian Ocean during Austral Summer.

This study investigates the dominant structure, characteristics, and origins of the tropical synoptic-scale waves over the Indian Ocean during austral summer. An extended empirical orthogonal function-based composite analysis is performed on 2-8-day filtered atmospheric circulation fields during December-February 1979/80-2015/16. The analysis results reveal the structure and evolution of the tropical synoptic-scale waves and associated extratropical-tropical interactions. The composite wave pattern exhibits a modified mixed Rossby-gravity wave-like structure which is coupled with deep convection in the Indian Ocean intertropical convergence zone. The waves propagate westward at approximately 11 m/s, with zonal wavelengths of about 6000-7000 km. Eastward amplification of wave troughs and ridges due to group propagation occurs along the monsoon westerly flow. The eastward group speed is estimated to be 7m/s. A two-dimensional (horizontal) wave energy flux (WEF) formulation derived by Aiki et al. (2017, PEPS) is used to diagnose the downstream amplification of the waves along the mean monsoon westerly flow. The new WEF vectors are considered to be useful to assess energy dispersion characteristics of various types of waves in the tropics. The WEF diagnostics verify that wave energy propagation facilitates the downstream development of the synoptic-scale wave train, suggesting that the mean monsoon westerly flow acts as a tropical waveguide. Extratropical forcing mechanisms responsible for the development of the tropical synoptic-scale waves are further explored. Two types of extratropical forcing of the tropical synoptic-scale waves are identified over the southwest Indian Ocean. One type is southerly surges originated in the southern midlatitude that could intensify the tropical synoptic-scale wave trough. The southerly surges are induced by the baroclinic development of the midlatitude synoptic-scale waves. The other type is direct propagation of midlatitude synoptic-scale waves toward the tropics that induces the growth of the tropical waves through the Rossby wave energy dispersion from the subtropics to the tropics. These two mechanisms trigger the development of the tropical waves along the monsoon westerly flow.