Two cyclones in a week: why the Bay of Bengal keeps sending storms at Andhra Pradesh

Why is the Bay of Bengal so prone to cyclones?

Because its water is unusually warm, and stays that way.

A tropical cyclone is a heat engine. It draws energy from warm sea water, which evaporates, rises, cools and releases heat as it condenses into cloud. For that to happen, scientists say the sea surface needs to be at least 26.5°C, with warm water running about 60 metres deep. Surface water in the Bay of Bengal usually sits at about 29 to 31°C, comfortably above that line.

The Bay also has more fuel stored below the surface than the Arabian Sea. A 2021 study of two storms that formed at the same time in October 2018, Titli in the Bay and Luban in the Arabian Sea, found the Bay’s heat content was far higher, one reason Titli grew stronger. Over the long run the Bay sees about four times as many cyclones as the Arabian Sea.

What do rivers have to do with it?

Fresh water from the Ganga, Brahmaputra and other rivers, along with heavy rain, makes the Bay unusually layered.

The Bay receives more rain and river water than it loses to evaporation, and its surface is at least 3 units less salty than the Arabian Sea. That lighter fresh water floats on top and the layering is at its strongest in October, just as the post-monsoon storm season begins. A sea layered like this mixes less, which helps a warm surface layer stay in place for storms to feed on.

Why now, in late October and November?

This is one of the two cyclone seasons in India’s seas. The north Indian Ocean has two peaks: April to June, before the monsoon, and October to December, after it. May and November are usually the most favourable months.

During the monsoon itself, strong winds blowing in different directions at different heights tear young storms apart. Weather scientists call this vertical wind shear. Once the monsoon withdraws, the shear drops below about 10 metres a second over large parts of the Bay, and low-pressure systems are left free to organise and spin up. The Bay also gets help from the east: the remains of storms from the Pacific often drift across Myanmar and seed new disturbances over its warm waters.

So why two storms within a week?

Because the conditions that made Arnab did not go away when Arnab did.

One big influence is the Madden-Julian Oscillation, or MJO, a band of clouds and rain that travels eastward around the tropics every 30 to 60 days. When its wet phase sits over the Indian Ocean, it strengthens the rising air and moisture that storms need, and depressions can grow into cyclones. When it moves on, the Bay often goes quiet. A favourable spell can last weeks, so one storm often follows another while the window is open.

A cyclone does cool the sea behind it. Titli cooled the water along its track by about 1°C as it passed, and up to 3°C afterwards. But the Bay is large, and Murasu formed and travelled further south than Arnab, over water that had not been churned up. Arnab was forecast to come ashore further north, between Bapatla and Visakhapatnam, while Murasu began crossing near Kothapatnam, close to Ongole, before dawn on Tuesday.

Two storms in a fortnight

1. 21 OctA low-pressure area forms off the Tamil Nadu coast; another is expected days later.
2. 24 OctA depression forms about 820 km east-south-east of Visakhapatnam.
3. 28 OctCyclone Arnab crosses the Andhra Pradesh coast; south Odisha floods. At least seven people die.
4. 1 NovA deep depression further south strengthens into Cyclone Murasu.
5. 2 NovMurasu becomes a severe cyclonic storm; signal 10 at Nizampatnam port.
6. 3 NovMurasu's eye reaches the coast near Kothapatnam, close to Ongole, with winds up to 120 km/h.

Is this unusual?

Not for this sea. Of the roughly 80 tropical cyclones that form around the world each year, about five form in the north Indian Ocean, and most of those come from the Bay. Back-to-back storms in the same season are a familiar pattern for coastal Andhra Pradesh, Odisha and Tamil Nadu. What makes this fortnight hard is that Murasu struck a state still clearing up after Arnab, with fields already flooded and power lines already down.

Could there be more?

The season is not over. The post-monsoon cyclone window runs through December, and November is usually its most active month. The India Meteorological Department tracks every low-pressure area that forms over the Bay and issues bulletins as it grows, and its early warnings, together with evacuations, are the main reason cyclones in India now kill far fewer people than they once did. For farmers and fishing families along the coast, though, the bigger question after two storms in a week is how quickly relief and repairs can follow.