A New Pathway of Tropical Cyclone–Trough Interaction as Illustrated by the Over-Land Re-Intensification of Oswald (2013)

Tropical Cyclone (TC) Oswald (2013) significantly impacted Australia with extensive rainfall and prolonged circulation over land, largely influenced by two mid-latitude troughs. Unlike other documented studies, Oswald's interaction with the two troughs occurred in the mid-troposphere, not the upper troposphere. Under the high vertical wind shear, the upper TC circulation was greatly weakened. However, in the middle levels between 400 and 600 hPa, high cyclonic potential vorticity (PV) air, was transported from the troughs to Oswald's mid-layer circulation, replenishing its outer circulation. With the inner circulation, PV redistribution between the inner core and outer core was observed over the southeastern quadrant. This process enhanced mid-to-lower updrafts and boundary-layer convergence, supporting the downshear reformation of mesovortices. Hence, despite sustained unfavorable strong shear and the absence of a warm ocean surface, the lower half of Oswald's circulation persisted and reorganized over land, significantly extending its impact after landfall.