Genesis, structure and propagation of synoptic systems over the Indian Ocean during the Northeast Monsoon

The Northeast Monsoon (NEM) in South Asia, occurring from October to January, plays a pivotal role in precipitation, often giving rise to extreme weather events. This study aims to elucidate the diverse synoptic systems responsible for rainfall during the NEM and track their origins. Specifically, using a synoptic system tracking algorithm, we identify and characterise the genesis locations, propagation, and structures of these synoptic systems.  

Our findings reveal a seasonally evolving latitude dependence in genesis locations, with a bimodal distribution that shifts southwards and becomes more meridionally confined as the season progresses. These genesis locations coincide with regions of high relative vorticity and column-integrated Moist Static Energy (MSE).  Based on the pressure level at which maximum vorticity is observed at genesis, we classify these systems into three categories: Lower Tropospheric Cyclones (LTCs), Mid-Tropospheric Cyclones (MTCs), and Upper Tropospheric Cyclones (UTCs).  Each category exhibits an evolving preference for genesis location, generally evolving southwards and eastwards, as the season advances. The UTCs are further categorised into two subtypes: one forming near the equator (up to 15°N/S) and another of subtropical origin (poleward of 15°N/S). Composites of near-equatorial UTCs display westward tilt with height, warm temperature anomalies at upper levels, and cold anomaly below, with vorticity maximum near 400 mb. This structure resembles that of MTCs, which exhibit a similar westward tilt and warm-over-cold core structure, but with maximum vorticity near 600 mb. In contrast, LTCs exhibit an upright structure with a warm core aloft and vorticity maximum centred around 800 mb. The joint distribution of MSE and relative vorticity at genesis indicates that LTCs are typically associated with stronger values of both variables, whereas UTCs and MTCs each appear in two distinct regimes: one with higher values of MSE and vorticity and another with lower values of these variables.  

UTCs account for 14% of all systems, MTCs 44%, and LTCs 42%. Despite being fewer, on average a UTC produces rainfall of comparable magnitude to an MTC. UTCs predominantly generate precipitation over the Bay of Bengal shifting to the southwest Indian Ocean in January. MTCs generate significant rainfall over the Arabian Sea, Bay of Bengal, and South China Sea until December, and over Indo-Pacific region and the tropical South Indian Ocean in January. LTCs produce the largest rainfall, mainly over the Bay of Bengal and South China Sea, throughout the season and over the tropical South Indian Ocean as the season progresses. Lastly, while cyclonic propagation trajectories show overall westward movement for all categories, there are important differences: LTCs tend to have a more meridional motion towards northwest, while MTCs and UTCs exhibit a comparatively more zonally directed motion. Given the structural differences between systems, especially MTCs and LTCs, and their potential to morphologically evolve (e.g., MTC transitioning to LTC and vice versa), our study focussing on the genesis of these systems offer valuable insight into their formation mechanism.