Kolkata: Two cyclones — Arnab and Murasu — forming and hitting the same area in less than a week is a ‘climatological anomaly’ that has rarely happened along the Bengal-Odisha-Andhra coast, said weather scientists. The rapid succession of Cyclone Arnab (which struck the Andhra coast on Oct 28) and Cyclone Murasu (due to make a landfall between Monday midnight and Tuesday morning) within a span of less than seven days is an exceptionally rare meteorological event, they said.Usually, when one storm forms, it alters the atmospheric and ocean heat profiles, making immediate secondary cyclogenesis right behind it a statistical anomaly — though instances of overlapping satellite-tracked systems or successive depressions have occurred in hyperactive seasons (such as late 2020 with Nivar and Burevi, though spaced slightly further apart).Tropical cyclones are giant atmospheric heat engines. For a back-to-back formation to happen, the environment must overcome several strict thermodynamic and dynamic barriers like an oceanic heat depletion. “A tropical cyclone requires sea surface temperatures (SSTs) of at least 26.5°C extending to a depth of several tens of meters. When a powerful cyclone tracks slowly across the Bay of Bengal, its violent winds churn the upper ocean layer, upwelling deep, colder water from below. This effectively leaves a ‘cold wake’ in the ocean. For a second cyclone to form right behind it, the ocean needs time to recover and regain solar heat. If a storm follows too closely, it runs into this cooler water and is starved of the thermal energy it needs to intensify,” said a senior weather scientist.He added that a mature cyclone acts as a massive atmospheric vacuum, drawing in vast quantities of moisture and heat, and releasing immense latent heat into the upper troposphere. “This process fundamentally alters regional pressure patterns and causes widespread subsidence (sinking air) in the surrounding periphery. Sinking air suppresses convection (thunderstorm formation), creating a temporary ‘recovery window’ where the atmosphere is stabilised and unable to easily spawn a new low-pressure core,” he added.Cyclones require low vertical wind shear — the winds at the surface and high up in the atmosphere must blow at similar speeds and directions so they don’t tilt or tear apart the storm’s vertical vortex. A passing cyclone drastically alters regional wind fields. Often, the trailing tail of a weather system leaves behind high wind shear or disruptive upper-level troughs that tear apart fledgling disturbances nearby.Even though two cyclones in quick succession are rare, a cyclone over Bay of Bengal has often been followed closely by lesser systems in the past, like the 2013 season when Cyclone Phailin made a severe landfall in Odisha in Oct 2013, followed closely by subsequent cyclonic systems (like Helen and Lehar) over the following weeks.The areas of origin of the two cyclones are different. “While Arnab originated over east-central Bay, Murasu formed over south-east Bay. The conditions are different in the two areas. Central Bay of Bengal is a vast ocean and a storm in one area may not affect the conditions in another part of the sea,” said HR Biswas, deputy director-general of meteorology.Back-to-back systems on the east coast could be triggered by an ‘over-energised’ Bay of Bengal, said meteorologists. “Late in the post-monsoon season, the accumulated solar heating from the entire summer has baked the Bay of Bengal, creating a massive pool of high thermal energy. Even after one storm upwells cooler water, the background temperature of the basin remains high enough to fuel another.”
