Beyond Beta Drift: Multi-level Steering and Deep Vortex Structures in Anomalous Monsoon Depression Propagation

The dominant northwestward propagation of Monsoon Depression (MD) has been well established by the existing theoretical framework that is analogous to Beta Drift theory; however, rare northeastward-moving cases remain unexplored. We investigate six northeastward-moving systems (NEMS) that occur over the Bay of Bengal and the Indian subcontinent, while comparing them with northwestward-moving systems (NWMS) to identify their distinctive structures and the mechanisms driving atypical propagation.

Structural analysis reveals that NEMS possess a substantially deeper relative vorticity core at the mid and upper troposphere, along with higher rainfall to the east of the depression center. Vorticity equation diagnosis reveals that horizontal vorticity advection, specifically the asymmetric advection of symmetric vorticity (AASV) term, dominates the vorticity tendency and exhibits a persistent dipole structure for both NEMS and NWMS, although towards different directions at different pressure levels. This highlights a multi-level steering, particularly prevalent in NEMS cases, which is effective in understanding track variabilities.

Further analysis reveals distinctive negative geopotential anomalies (centered at ~37°N) at the upper troposphere extending from the extratropics into the subtropics for NEMS and eventually interacting with these depressions to modulate their trajectories. These anomalies are significantly stronger and quasi-stationary, resulting in large-scale impacts on overall track directions. The previous theory fails due to the assumption of a single pressure level primarily impacting depression propagation. This work establishes that understanding and predicting monsoon depression tracks requires explicit representation of multi-level steering and deep vortex structures.