Monsoons, Tropical Convergence and Tropical Cyclones

Monsoon low pressure and tropical convergence

A monsoon low pressure system, or monsoon depression, is a synoptic-scale cyclonic disturbance that forms within the monsoon trough, the elongated low-pressure axis stretching from northwest India across the Gangetic plain to the head of the Bay of Bengal during the summer monsoon. Such systems form most frequently over the northern Bay of Bengal in July and August, where cyclonic vorticity and moist convection in the trough are concentrated. They then move west-northwest across central India and are a major source of monsoon rainfall over the core monsoon region.

In the tropics, the trade winds of the two hemispheres converge near the intertropical convergence zone and force air to rise. Ascending air expands at lower pressure and cools adiabatically. In a convergence zone the tropospheric heat budget is closed mainly by condensational heating from latent heat release in deep clouds, while radiative cooling plays a secondary role.

At 200 hPa over the Tibetan Plateau in boreal summer, the seasonal mean flow is dominated by the South Asian (Tibetan) High, an upper-level anticyclone. Outflow from this high makes the flow divergent, with positive horizontal divergence \nabla\cdot\mathbf{V}>0. Its rotational part is anticyclonic, with negative relative vorticity, so the flow is neither cyclonic nor irrotational.

The monsoon involves seasonal reversals of temperature gradients. The reversal of the north–south temperature gradient is a tropical feature, whereas the reversal of the east–west gradient is linked to mid-latitude land–sea contrast. Surface cooling and deepening of the mixed layer in winter are most pronounced over the Arabian Sea. With the monsoon wind reversal, the South Equatorial Current in the Indian Ocean moves northward across the equator.

The Indian summer monsoon requires a land–sea temperature contrast, a seasonal reversal of winds, and a meridional pressure gradient. El Niño is not a necessary condition for its formation, although it modulates the strength of the monsoon from year to year.

• monsoon trough — the seasonal low-pressure axis over the Indo-Gangetic plain and the Bay of Bengal. Ex) Depressions often travel along the trough once they form.
• adiabatic cooling — cooling of a rising parcel caused by expansion at lower pressure, with no exchange of heat. Ex) Ascent in a convergence zone cools the air even without loss of heat to space.
• condensational heating — heating released when water vapour condenses into cloud droplets. Ex) Latent heat release balances adiabatic cooling in the tropical troposphere.
• South Asian High — the upper-tropospheric anticyclone centred over the Tibetan Plateau in summer. Ex) Its outflow produces divergence at 200 hPa.
• irrotational flow — flow with zero relative vorticity. Ex) A flow with anticyclonic circulation is rotational, not irrotational.

Tropical cyclone formation and structure

The Coriolis parameter, f = 2\Omega\sin\phi, measures the planetary vorticity that tropical cyclones need to organise and sustain a cyclonic circulation. It vanishes at the Equator and stays small within a few degrees of it. Tropical cyclones therefore rarely form within about 5 degrees of the Equator, even though the sea surface there is warm and the vertical wind shear is weak.

Northern Hemisphere tropical cyclones are warm-core systems, because latent heat released in deep convection keeps the upper troposphere warmer than its surroundings. The Coriolis force turns low-level inflow counterclockwise, which gives cyclonic low-level circulation. Upper-level divergence removes mass from the column, sustains low-level convergence, and keeps the central pressure low. Strong vertical wind shear is unfavourable because it tilts and disrupts the vortex.

During the summer monsoon, strong low-level southwesterly flow combined with upper-level easterlies produces large vertical wind shear over the Bay of Bengal. This shear separates the developing warm core from the convection, so it is the dominant reason cyclogenesis is suppressed there. Low-level vorticity and mid-tropospheric humidity matter too, but they are secondary.

• Coriolis parameter — f=2\Omega\sin\phi, which equals zero at the Equator. Ex) Cyclonic rotation is weak in a band close to the Equator.
• warm core — a central region of the cyclone that is warmer than its environment at upper levels. Ex) The warm core is maintained by condensational heating.
• vertical wind shear — change of horizontal wind with height. Ex) Large shear tilts the vortex and inhibits intensification.