Multi-scale Interactive Genesis of TC05A (2011) over the Equatorial Indian Ocean: An Observational Analysis

The Indian Ocean (IO) Tropical Cyclone 05A (TC05A) formed close to the equator (3.5°N, 80.0°E) on 24 November 2011 during the Dynamics of the Madden-Julian Oscillation (DYNAMO) field experiment. The atmospheric processes leading to the unusual near-equatorial TC genesis are analyzed with the NCEP operational final analysis (FNL) and satellite data sets. It is found that the genesis of TC05A is a result of multi-time-scale interactions involving a synoptic-scale mixed Rossby gravity (MRG) vortex and an active phase of the Madden-Julian oscillation (MJO) events that was modulated by the 2011–2012 La Niña.

The 2011–2012 La Niña episode enhances the MJO development and slows down its eastward propagation over the central Indian Ocean. The MJO convection emerges in the southwestern Indian Ocean convergence zone and subsequently propagates northeastward slowly toward the south tip of India, which sets up a favorable large-scale condition for TC formation: the low-level convergence, high humidity at the mid-to-low-level, and weak easterly vertical wind shears (VWSs). Prior to the formation of the TC05A, an anticlockwise rotating MRG vortex is peculiarly intensified and shifted northwestward, providing a strong cyclonic vorticity source for spinning up a tropical depression.

The sudden intensification of the MRG vortex, which serves as a seedling for TC05A genesis, is due to enhanced low-level wind convergence between the anomalous easterlies and westerlies located in the front edge of the MJO convective complex. The MJO reaches its peak phase immediate after TC05A rapidly intensified, suggesting that the synoptic scale eddy can contribute to MJO development. It is also suggested that the impact of MJO on TC genesis depend critically on the location of the seedling MRG disturbance relative to the spatial and temporal phases of MJO.