Quantifying β-Effect–Induced Drift of Tropical Cyclones over the Indian Ocean and Its Implications for Track Forecasting

Tropical cyclone (TC) translation is governed by large-scale environmental steering and by the interaction between the vortex circulation and the planetary vorticity gradient (β-effect). This interaction generates asymmetric secondary gyres that induce a systematic poleward and westward drift (β-drift), which can persist even under weak steering and substantially modify TC trajectories. Despite its recognized dynamical importance, β-drift remains poorly quantified over the Indian Ocean and is rarely treated explicitly in operational track prediction systems. In this research, we present a basin-wide assessment of β-effect–induced TC drift over the Indian Ocean during pre-monsoon and post-monsoon seasons. Zonal and meridional winds and relative vorticity from ERA5 reanalysis at 850–200 hPa are collocated with IBTrACS best-track data to compute vertically averaged environmental steering velocities. Residual translation vectors, obtained by removing the steering component from observed TC motion, are interpreted as β-drift within a barotropic dynamical framework. The analysis reveals a statistically significant increase in β-drift magnitude with latitude, consistent with planetary vorticity gradient control. A non-linear regression model applied to multi-storm residual motion identifies dominant predictors of β-drift and yields an empirical parameterization of β-effect–induced translation for Indian Ocean cyclones. The results demonstrate that β-drift contributes substantially to TC motion variability, particularly under weak-steering regimes, and represents a systematic source of track forecast error. Incorporating this parameterization into forecast systems offers a pathway to improve operational TC track predictability over the Indian Ocean.

Keywords: Indian Ocean, Tropical Cyclones, β-Drift, Track Predictability, Weak Steering Flow, ERA5 Reanalysis