[A-CG34] Climate Variability and Predictability on Subseasonal to Multidecadal Timescales

[ACG34-11] An atmospheric wavenumber-4 pattern in the Southern subtropics and its linkage to rainfall during austral summer

*BALAJI SENAPATI¹、Pranab Deb¹、Mihir K. Dash¹、Swadhin K. Behera² (1.Indian Institute of Technology Kharagpur、2.JAMSTEC Application Laboratory)

A stationary barotropic atmospheric wavenumber-4 (AW4) is revealed in the Southern mid-latitudes (30°S-60°S) during austral summer using empirical orthogonal function analysis. The generation mechanism and its linkage to the Southern Hemisphere climate are explored using a linear response model and composite analysis. The AW4 pattern originates by a Rossby wave source in the upstream region of the upper-tropospheric westerly waveguide. The anomalous convection over the Tasman Sea adjacent to the westerly jet emits Rossby wave train due to vortex stretching around mid-November. Later, this disturbance gets trapped in the southern westerly waveguide and circumnavigate the globe, which turns out as a well-established AW4 pattern in early December (15-25 days later) over the southern mid-latitudes. Further, correlation analysis suggests the AW4 pattern is independent of other natural variabilities such as El Niño/Southern Oscillation, Southern Annular Mode, and Indian Ocean Dipole. The AW4 pattern is found to influence the rainfall over different parts of South America and Australia by modulating upper-level divergence.