Weather forecasting

Explanation

All three pairs are correctly matched as understood through the mechanics of the north-east monsoon and synoptic-scale weather forecasting. A trough is an elongated low-pressure extension that connects two circulation systems and acts as a channel along which moisture, cloud cover and rainfall are organised — distinct from a closed low-pressure system. A cyclonic circulation is a rotational wind pattern around a low-pressure centre; over warm ocean surfaces such as the Bay of Bengal or the Arabian Sea, such circulations can intensify progressively into a low-pressure area, a depression, a deep depression, and eventually a cyclonic storm. A wind convergence zone is the region where two airstream branches meet; converging surface winds force air aloft, which cools, condenses, and produces convective rainfall — making such zones a reliable rainfall-producing mechanism during both the south-west and north-east monsoon seasons. These three mechanisms together explain how the IMD diagnoses impending heavy rainfall events over the South Peninsula during the north-east monsoon.

Explanation

All three pairs are correctly matched. A low-pressure area (often abbreviated 'low') is a region of below-normal atmospheric pressure into which surrounding air converges; over the warm waters of the Bay of Bengal, such systems can deepen and intensify into depressions and ultimately cyclones. A cyclonic circulation is a smaller, rotating wind feature at lower atmospheric levels that frequently acts as a seed for low-pressure development or merges with an existing low to reinforce it — exactly the mechanism described when an Arabian Sea cyclonic circulation merged with the Bay of Bengal low in this season. The seasonal anticyclone is a persistent large high-pressure system, typically positioned over the Bay of Bengal region in the northeast monsoon season, in which winds rotate clockwise; its position and strength are one of the key steering influences that can deflect an intensifying storm northeastward toward the Odisha–West Bengal coast rather than allowing it to make landfall on the Tamil Nadu–Andhra Pradesh coast.

Explanation

Statement 1 is correct. The IMD classifies tropical disturbances in ascending order of intensity: low-pressure area → depression → deep depression → cyclonic storm → severe cyclonic storm → very severe cyclonic storm → extremely severe cyclonic storm → super cyclonic storm. This is the standard intensification ladder used in operational forecasting. Statement 2 is incorrect for two substantive reasons. First, the Madden-Julian Oscillation (MJO) is not a seasonal annual cycle — it is an intra-seasonal oscillation with a period of roughly 30–60 days, meaning it can occur multiple times a year and is distinct from the annual monsoon cycle. Second, the MJO's primary influence on cyclogenesis is through enhancing atmospheric convective activity and low-level wind convergence, thereby favouring both the genesis and intensification of cyclones — not merely through sea-surface temperature modulation. The MJO's 'active phase' over the Indian Ocean creates an environment conducive to cyclone formation, which is why IMD tracks its phase when issuing extended-range outlooks. Statement 3 is correct. Under the North Indian Ocean naming protocol coordinated through IMD (the RSMC for the region), member countries — including Bangladesh, India, Iran, Maldives, Myanmar, Oman, Pakistan, Qatar, Saudi Arabia, Sri Lanka, Thailand, UAE, and Yemen — contribute names in rotation. Critically, a disturbance is given a name only when it intensifies to the threshold of a cyclonic storm (≥ 3-minute sustained winds of 63 km/h); before that it carries only a tracking identifier (such as the model tag '98B') and a numbered designation as a depression.

Explanation

Statement 1 is correct. Vertical wind shear — the change in wind speed or direction with altitude — tilts a cyclone's structure and disrupts the coupling between the lower-level inflow and upper-level outflow, effectively weakening or preventing intensification. Lower shear permits the storm to remain vertically stacked and organised, enabling it to strengthen. Statement 2 is also correct. A tropical cyclone functions as a heat engine whose 'exhaust' must be expelled aloft. Upper-level outflow acts as the ventilation mechanism for the towering thunderstorms in the storm's core. If outflow is inhibited — even with warm sea-surface temperatures — latent heat cannot be efficiently exported, and intensification is suppressed. This is why a 'sufficiently coherent upper-level outflow' is identified as a separate enabling factor alongside warm seas. Statement 3 is incorrect. A slower translational speed — not a faster one — is associated with greater potential intensification, because a slow-moving system spends more time over warm water, extracting more heat and moisture. A faster-moving storm exits the favourable warm-water environment more quickly, reducing the time available for intensification. Slower translation also prolongs heavy rainfall over affected coastal areas.

Explanation

Statement 1 is correct. Tropical cyclone intensification is fundamentally driven by sea-surface temperatures (SSTs): warm ocean water (typically above 26–27°C) supplies latent heat and water vapour through evaporation. Higher SSTs, such as the ~30°C observed in the west-central Bay of Bengal, allow cyclones to intensify more rapidly and reach higher peak intensities — this is the core thermodynamic mechanism behind Bay of Bengal cyclone behaviour. Statement 2 is incorrect. The IMD North Indian Ocean cyclone classification scale has several categories above 'Very Severe Cyclonic Storm.' The hierarchy runs: Depression → Deep Depression → Cyclonic Storm → Severe Cyclonic Storm → Very Severe Cyclonic Storm → Extremely Severe Cyclonic Storm → Super Cyclonic Storm. 'Very Severe' is therefore a mid-tier, not the top, of the IMD scale. Statement 3 is correct. A cyclone that moves slowly over warm oceanic waters has a longer 'linger window' over the energy-rich surface, allowing sustained evaporation, moisture uptake, and intensification before reaching the coast. Rapid-moving cyclones spend less time over the warm ocean and thus have less opportunity for deep intensification. This slow-movement-over-warm-waters dynamic is a well-established principle in tropical meteorology.

Explanation

Statement 1 is correct. Vertical wind shear refers to the variation in wind speed and direction across different altitudes of the atmosphere. High vertical wind shear disrupts the vertical column of organised thunderstorm activity (the 'storm tower') that sustains a cyclone, thereby weakening or preventing intensification. This is a well-established principle in tropical meteorology. Statement 2 is incorrect. In IMD's tropical cyclone intensity scale, 'extremely severe cyclonic storm' (with sustained winds of 89–117 knots, roughly 165–220 km/hr) is a HIGHER category than 'very severe cyclonic storm' (sustained winds of 48–63 knots, roughly 89–117 km/hr). The ascending order of IMD categories is: Depression → Deep Depression → Cyclonic Storm → Severe Cyclonic Storm → Very Severe Cyclonic Storm → Extremely Severe Cyclonic Storm → Super Cyclonic Storm. The statement inverts this order. Statement 3 is correct. Tropical cyclones derive their energy through the evaporation of warm ocean water, which fuels latent heat release and drives the storm's convective engine. When a cyclone moves over cooler sea-surface temperatures (SSTs), the available energy supply diminishes, causing the storm to weaken. This is why cyclones commonly lose intensity as they approach coastlines where cooler upwelled water or shallower shelf waters are present.

Explanation

Statement 1 is correct. When a tropical cyclone makes landfall, it is cut off from the warm ocean waters that supply heat and moisture to sustain it. As a result, it weakens in stages — from a cyclonic storm to a deep depression and eventually to a low-pressure area or remnant circulation. This staged weakening is a well-established feature of the tropical cyclone life cycle, consistently reflected in IMD forecasting bulletins. Statement 2 is incorrect on two counts. An upper-air cyclonic circulation is a precursor weather feature, not a named cyclonic storm. IMD classifies and names a system as a cyclonic storm only after it reaches a defined wind-speed threshold. Moreover, at the stage of an upper-air circulation or nascent low-pressure area, the future track and intensity are inherently uncertain, and IMD typically cautions that it is 'too early to forecast' the system's influence on specific regions — which is precisely the caveat issued in operational forecasts for such developing systems. Statement 3 is correct. The northeast monsoon (also called the retreating monsoon) is the primary rainfall mechanism for peninsular India — especially Tamil Nadu, coastal Andhra Pradesh, and to a lesser extent Kerala — between October and December. During this season, low-pressure systems that develop over the Bay of Bengal track westward or northwestward and can bring significant rainfall to these states, helping to compensate for any deficiency accumulated during the southwest monsoon.

Explanation

Both statements reflect established tropical meteorology. First, low-pressure systems over warm ocean waters — like the Bay of Bengal — act as reservoirs of moisture-laden air. When positioned near a cyclone, they channel warm, humid air into the cyclone's circulation, fuelling convection and intensification; this is the classical 'moisture feed' mechanism by which peripheral disturbances strengthen a parent cyclone. Second, the Fujiwhara effect (also called binary interaction of vortices) describes the tendency of two proximate cyclonic vortices to rotate around their common centre of mass and, if one is sufficiently stronger, to absorb the weaker system entirely. Both mechanisms are standard content in tropical cyclone dynamics and are part of IMD's operational understanding of cyclone genesis and intensification in the Bay of Bengal.