Tropical cyclones

Explanation

Statement 1 is correct: The Bay of Bengal's funnel-like shape concentrates atmospheric energy and storm surges, and combined with its shallow coastal topography, makes the region exceptionally vulnerable to large and destructive storm surges when cyclones make landfall. This is a well-established principle in tropical meteorology. Statement 2 is incorrect: 'Landfall' and 'direct hit' are not the same concept. Landfall refers specifically to the moment a cyclone's centre (eye) crosses the coastline, whereas a 'direct hit' is a broader colloquial term that can include areas affected by severe winds and rain even if the eye does not make land contact; the two terms describe different things. Statement 3 is correct: El Niño, characterised by abnormal warming of Pacific Ocean surface waters, influences global atmospheric circulation patterns, including wind shear and sea-surface temperatures in the Indian Ocean, which in turn affect the genesis, intensification, and track of Bay of Bengal cyclones. The El Niño–cyclone intensity linkage is a recognised principle in tropical climate science.

Explanation

Statement 1 is correct. A key principle of tropical cyclone meteorology is that translation speed (how fast the storm moves) inversely affects intensification potential: a fast-moving cyclone spends less time over the warm ocean surface that provides the energy for deepening, so its intensification is constrained compared to a slow-moving or quasi-stationary storm that can draw sustained energy from warm sea-surface temperatures. Statement 2 is incorrect. Storm surges are not driven primarily by the cyclone's forward speed. The principal physical drivers of storm surge are (a) strong onshore winds that pile seawater against the coast, (b) the hydrostatic effect of extremely low atmospheric pressure at the storm centre, which allows sea level to 'dome' upward, and (c) the geometry of the coastline (shallow, funnel-shaped bays amplify surges). Translation speed plays only a secondary modulating role. Statement 3 is correct. IMD's intensity classification for Bay of Bengal systems proceeds in ascending order of sustained wind speed: Depression → Deep Depression → Cyclonic Storm → Severe Cyclonic Storm → Very Severe Cyclonic Storm → Extremely Severe Cyclonic Storm → Super Cyclonic Storm. 'Severe' is therefore one step above 'Cyclonic Storm' and one step below 'Very Severe Cyclonic Storm', as confirmed by Cyclone Murasu (classified as a severe cyclonic storm) forming after the very severe cyclonic storm Arnab — both within the same IMD hierarchy.

Explanation

Statement 1 is correct. A core principle of tropical cyclone mechanics is that sea-surface warming provides greater thermal energy and moisture flux to the overlying atmosphere, which translates into higher wind speeds and intensified precipitation — a well-established relationship in cyclone science, also demonstrated by the Arnab study. Statement 2 is correct. The eyewall is the ring of deep convective thunderstorm clouds surrounding the eye; it is where the strongest sustained winds and the heaviest rainfall are concentrated in a tropical cyclone. Statement 3 is incorrect because it inverts the energy mechanism. The ocean — specifically the warm sea surface — is the primary energy source (latent heat via evaporation) for a tropical cyclone. Landfall cuts off this oceanic energy supply, which is precisely why cyclones weaken after moving over land, not because land provides energy. The statement wrongly attributes the sustaining energy source to the land surface.

Explanation

Statement 1 is incorrect on two counts: the MJO propagates eastward (not westward), and it enhances rather than suppresses convection and cyclogenesis over the North Indian Ocean during its active phase. When the MJO's convectively active envelope is positioned over the Bay of Bengal or the Arabian Sea, it provides a significant boost to atmospheric uplift, moisture convergence, and the organisation of storm systems — the precise opposite of what statement 1 claims. Statement 2 is correct. Upper-air (mid-to-upper tropospheric) cyclonic circulations are well-recognised precursor features in tropical cyclogenesis. They promote low-level convergence and vorticity, helping organise convective clusters into a more coherent system. IMD routinely flags such circulations when issuing outlooks on possible depression or cyclone formation. Statement 3 is correct. The Bay of Bengal consistently generates more tropical cyclones than the Arabian Sea. The Bay benefits from warmer and shallower sea-surface temperatures (which recover heat quickly after a storm passage), abundant moisture advection from the Indian subcontinent and neighbouring seas, and a persistent low-level monsoonal wind shear environment that is more conducive to cyclone intensification. The Arabian Sea is drier in its western reaches, subject to stronger vertical wind shear, and its surface temperatures — while warm — are moderated by upwelling along the Somali and Arabian coasts.

Explanation

IMD classifies tropical disturbances in a well-defined sequence of intensification: an upper-air cyclonic circulation → low-pressure area → well-marked low-pressure area → depression → deep depression → cyclonic storm → and further intensification stages (severe cyclonic storm, very severe, etc.). Statement 1 correctly describes this progression up to the cyclonic storm stage. Statement 2 is also correct: IMD's tropical weather outlooks routinely track upper-air cyclonic circulations over maritime regions like the Andaman Sea as the earliest precursor signals in the cyclogenesis chain, well before a system is officially classified as a low-pressure area. Both statements accurately reflect IMD's standard classification and monitoring practice for Bay of Bengal cyclogenesis.

Explanation

Statement 1 is correct. The widely accepted threshold for tropical cyclogenesis is a sea-surface temperature of approximately 26–27°C. Warmer water evaporates more readily, loading the overlying atmosphere with moisture that, when it condenses inside a cyclone's convective towers, releases enormous latent heat — the primary energy source that drives and sustains the system. Statement 2 is also correct. Vertical wind shear refers to the change in wind speed and/or direction with altitude. High shear 'shears off' the top of a developing cyclone's warm core, venting heat that would otherwise reinforce the pressure gradient and intensification process. Conversely, low shear allows the warm core to remain vertically stacked, enabling rapid deepening — a well-established principle in tropical meteorology. Statement 3 is incorrect. The north-east monsoon season (October–December) is in fact notably active for Bay of Bengal cyclone formation, and historically the Bay has produced some of its most intense cyclones during this period, not fewer or weaker ones. The Bay's SSTs remain warm (above 28°C) in October–November, the recurvature zone is favourable, and the intertropical convergence zone supports disturbance formation. The claim that the north-east monsoon produces fewer or weaker cyclones than the south-west monsoon misrepresents climatological reality — the Bay's south-west monsoon season is actually suppressed by the strong low-level jet that itself generates high wind shear unfavourable for intensification.

Explanation

Statement 1 is correct: a standard principle of tropical cyclone dynamics is that a cyclone mixes warm surface water with cooler deeper water (through upwelling and turbulent mixing), cooling the sea-surface along its track and reducing the ocean heat content available to any subsequent system in the same area. This is part of why back-to-back cyclones striking the same coast within days of each other are considered anomalous — conventional dynamics predict a less favourable oceanic environment for the second system. Statement 2 is also correct: the concern raised by climate scientists is that rising mean ocean temperatures increase background ocean heat content so substantially that, even after one cyclone extracts energy from the upper ocean layer, sea-surface temperatures and sub-surface heat content may remain sufficiently elevated to support another cyclogenetic event — a warmer ocean 'recharges' the surface energy available to storm development more readily than a cooler one. Statement 3 is incorrect: no such mandatory 48-hour observation window is part of the IMD's classification criteria for cyclonic storms. The IMD classifies systems based on sustained wind speeds at specified thresholds (e.g., 63–88 km/h for a cyclonic storm), not a fixed time window tied to when sea-surface temperatures first exceed a threshold. This statement conflates a notional SST criterion with a procedural rule that does not exist in IMD's operational framework.