The low-frequency intraseasonal oscillations (ISOs) dominate the subseasonal variability of the Indian summer monsoon and involve rainbands propagating northward from the equator. These oscillations modulate the active–break cycle of the monsoons and have two distinct regimes, one where rainfall maximum is located near the equator and the second one in which it is located around central India. The interaction of vorticity and divergence is an important feature for the northward propagation of ISOs. With a correlation study, we show that the low-frequency modes in vorticity and divergence are coupled in the boundary layer and upper troposphere. We use multichannel singular spectrum analysis to extract the low-frequency oscillatory modes in vorticity and divergence. The examination of the spatiotemporal structure of these modes reveals that vorticity has coherent northward propagation at all pressure levels. In contrast, divergence only shows propagation in the boundary layer and the upper troposphere. The vorticity low-frequency mode has a near barotropic structure, while the divergence mode has a baroclinic structure. We point out the contrasting features of vorticity and divergence for the two regimes of the ISO. The barotropic vorticity leads the rainfall for the equatorial region, whereas, for the central Indian region, the vorticity lags the rainfall. Meanwhile, the rainfall is in phase with baroclinic divergence for both regimes. These findings are relevant to understanding the initiation and propagation of ISOs and can contribute to the further development of simple models of these phenomena.

In response to north-south pressure gradients set by the annual march of the Sun, a cross-equatorial flow that turns to become a low-level Somali jet at around $10^{\circ}$ N is established in the lower troposphere over the Indian ocean. This flow plays a fundamental role in the Indian monsoon. A mechanistic understanding of drivers of this flow is lacking. Here we present a seasonal-mean analysis of the Kinetic Energy (KE) budget of the low-level flow using high spatiotemporal resolution ERA5 reanalysis to identify sources and sinks of KE. We find that the largest KE generation occurs around east African orography where the Somali jet forms while a significant KE is also generated over western Ghats and the Madagascar Island (‘hot spots’). These regions are distant from core monsoon precipitation regions, suggesting that local circulations driven by condensation do not directly produce the bulk of KE during monsoons. A unique KE balance supports the generation of Somali jet, with KE generation balanced by nonlinear KE advection as it forms. Over oceans, KE generation occurs mainly due to cross-isobaric meridional winds in the boundary layer. In contrast, over east African highlands and western Ghats KE generation maximizes just above the boundary layer and mainly occurs due to interaction of flow with orography. We propose a simple decomposition of lower tropospheric KE generation into contributions from surface pressure, orography and free-tropospheric gradients that corroborates the important role played by surface pressure gradients once adjusted for effects of orography.