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Cyclone Amphan

North Indian Ocean cyclone in 2020

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Super Cyclonic Storm Amphan over the Bay of Bengal on May 18, 2020
The National Aeronautics and Space Administration (NASA) · Public domain

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Through East India, Bangladesh and Indian Ocean

The first tropical cyclone of the 2020 North Indian Ocean cyclone season, Amphan originated from a low-pressure area persisting a couple of hundred miles (300 km) east of Colombo, Sri Lanka, on 13 May 2020. Tracking northeastward, the disturbance organized over exceptionally warm sea surface temperatures; the Joint Typhoon Warning Center (JTWC) upgraded the system to a tropical depression on 15 May while the India Meteorological Department (IMD) followed suit the following day. On 17 May, Amphan underwent rapid intensification and became an extremely severe cyclonic storm within 12 hours. On 18 May, at approximately 12:00 UTC, Amphan reached its peak intensity with 3-minute sustained wind speeds of 240 km/h (150 mph), 1-minute sustained wind speeds of 270 km/h (165 mph), and a minimum central barometric pressure of 920 mbar (27.17 inHg). The storm began an eyewall replacement cycle shortly after it reached its peak intensity, but the continued effects of dry air and wind shear disrupted this process and caused Amphan to gradually weaken as it paralleled the eastern coastline of India. On 20 May, 12:00 UTC, the cyclone made landfall in West Bengal. At the time, the JTWC estimated Amphan's 1-minute sustained winds to be 175 km/h (110 mph). Amphan rapidly weakened once inland and dissipated shortly thereafter.

Coastal areas in West Bengal comprising West Midnapore, East Midnapore, North 24 Parganas, South 24 Parganas, Kolkata, Hooghly and Howrah were heavily affected by the cyclone. It also caused significant destruction in Bangladesh, bringing rain and strong winds to Rajshahi and Rangpur. It created havoc in the metro city of Kolkata.

Meteorological history

During 13 May 2020, an area of low pressure developed over the Southeastern Bay of Bengal about 1020 km (635 mi) to the southeast of Visakhapatnam in the Indian state of Andhra Pradesh. The area of low pressure was located within a favorable environment for further development with good equator-ward outflow, exceptionally warm sea surface temperatures between 32 and 34 °C recorded in the basin, and low vertical wind shear. Over the next couple of days, the system became more marked as it gradually consolidated further, with bands of deep atmospheric convection wrapping into the system's low-level center. During 16 May, the India Meteorological Department (IMD) reported that the area of low pressure had developed into a depression and designated it as BOB 01 while it was located about 1,100 km (685 mi) to the south of Paradip in the Indian state of Odisha.

Moving northwards, the depression continually organised and became a cyclonic storm a few hours later, receiving the name Amphan. Intensification was temporarily impeded as moderate easterly wind shear disrupted the system's cloud pattern. On 17 May, conditions for significant intensification became more conducive as upper-level winds improved. As a result, Amphan underwent explosive intensification, with the U.S.-based Joint Typhoon Warning Center (JTWC) assessing an increase in winds from 140 km/h (85 mph) at 12:00 UTC to 215 km/h (135 mph), the equivalent to a Category 4 hurricane on the Saffir–Simpson scale (SSHWS), just six hours later. Furthermore, the IMD upgraded Amphan to an extremely severe cyclonic storm on their cyclone intensity scale. At 10:30 UTC, the IMD assessed Amphan as peaking as a super cyclonic storm with 3-minute sustained winds of 240 km/h (150 mph) and a minimum pressure of 920 hectopascals (27.17 inHg), while the JTWC set the peak about eight hours later with 1-minute sustained winds of 270 km/h (165 mph), or Category 5-equivalent intensity. The broad storm was characterized by a cloud shield extending more than 1,110 km (690 mi) and a sharply-outlined 10 nautical mile-wide eye.

Early on 18 May, microwave imagery depicted two distinct concentric eyewalls, a telltale sign of an eyewall replacement cycle that is typical for very intense cyclones. Through the day, the effects of wind shear and dry air hampered this internal core change, causing the eyewall to erode by late on 18 May. Around 5:30 p.m. IST (12:00 UTC), Amphan made landfall as a Very Severe Cyclonic storm near Bakkhali, West Bengal with winds of 155 km/h (95 mph). As it moved further inland, Amphan rapidly weakened. Just six hours after landfall, the JTWC downgraded the storm to a Category 1-equivalent cyclone and issued its final warning on the system, as it became disorganized. On 21 May, Amphan degenerated into a well-marked low pressure area.

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