The Ocean That Fed the Storm

It's also warm enough to build a cyclone.

This is the elegant and slightly horrifying physics of tropical storms: warm water evaporates, warm air rises, the atmosphere spirals into the hole that rising air leaves behind, and what began as thermodynamic fidgeting becomes organized kinetic violence. The ocean provides the fuel. The wind, the rotation, the storm surge that crests over fishing villages — that's all exhaust.

Cyclone Roanu formed from a low-pressure smear over the Bay of Bengal on May 14, 2016. By May 17, it was a depression. By May 19, a named storm. On May 21, it made landfall northwest of Chittagong, Bangladesh, dragging a storm surge two meters above the tidal baseline into a coast that sits barely above sea level to begin with. Wind shear and proximity to land kept Roanu from organizing into something more dramatic on paper — maximum sustained winds of 85 km/h, minimum pressure of 983 hPa, not a record-setter by any metric. The numbers don't tell the story. The story is 135 deaths and 99 people still listed as missing.

What made it lethal wasn't peak intensity. It was geography and thermodynamics conspiring without malice.

The Bay of Bengal is shaped like a funnel. Bangladesh is at the narrow end. The bay is shallow enough — average depth around 2,600 meters, compared to the deep basins of the open Pacific — that its surface stays warm even as storm activity stirs the water below. Deep cold water doesn't get pulled up fast enough to cool the fuel supply. So the ocean keeps giving and the storm keeps taking, all the way to the coast.

There's something difficult to hold about the indifference of this mechanism. The sea surface doesn't decide to generate a cyclone. It just provides the enthalpy budget for one to exist. The storm doesn't choose Bangladesh. It follows pressure gradients. The two-meter surge doesn't target the delta farmers who built on silt the Ganges deposited over a millennium. It just flows downhill. All of it is physics. None of it is personal. Which is, somehow, harder to sit with than malice would be.

The Bay of Bengal has been warming for decades, tracking with global mean temperatures in a relationship that is not mysterious and cannot be argued away. Warmer sea surfaces don't guarantee stronger cyclones — the relationship is complicated by wind shear and atmospheric factors that can tear storms apart before they fully organize. But they expand the ceiling on possible intensity, the theoretical maximum a storm can achieve given the thermal fuel beneath it. Roanu worked within one set of constraints. The next storm with the same track and a warmer bay beneath it starts with more to spend.

This is also not mysterious. The physics of the greenhouse effect has been understood for over a century. What remains is not a knowledge problem.

Why do a hundred million people live in a floodplain at the narrow end of an ocean funnel? Mostly: rice. The Ganges-Brahmaputra-Meghna delta is one of the most extraordinarily fertile agricultural systems on the planet, precisely because the dynamics that make it flood-prone also make the soil capable of feeding dense human populations. River sediment. Seasonal inundation. Nutrient-rich alluvial deposits. The ocean feeds the storm. The river feeds the delta. The delta feeds the people. The people stay.

The Bay of Bengal was thirty-one degrees on May 21, 2016. It is warmer now. The physics doesn't change.

i · sources

• Cyclone Roanu — Wikipedia, 2016-05-21