Cyclone Arnab and Murasu: What explains two successive cyclones in seven days?

On October 28, extremely severe Cyclone Arnab made landfall along the Andhra Pradesh coast. Before the region could recover from its impact, another cyclone, Murasu, struck the same coastline in the early hours of November 3, just seven days later. Two powerful cyclones hitting the same region within such a short interval is an unusual occurrence. Meteorologists describe this as a ‘climatological anomaly’. But is this merely a coincidence or is there a scientific explanation behind this unusual sequence of events?

According to data from the India Meteorological Department (IMD), Cyclone Arnab originated as a low-pressure system on October 24 and intensified into a cyclonic storm within just 36 hours. It subsequently strengthened rapidly, making landfall along the Andhra Pradesh and Yanam coasts with wind speeds of 125–135 kmph. Reports have offered varying estimates of the death toll. While some put the number at three, others report five or even nine fatalities. The damage to agriculture was extensive. In Andhra Pradesh alone, between 2.24 lakh and 3.25 lakh hectares of farmland were affected. Before the cyclone struck, large numbers of people had been evacuated to safer locations across Andhra Pradesh and Odisha.

Five days after Arnab made landfall, another cyclonic system, Murasu, developed over the southeast Bay of Bengal on November 1. It made landfall near Ongole in Andhra Pradesh on the morning of November 3, with maximum wind speeds of 100–110 kmph. Although considerably weaker than its predecessor, Murasu triggered flooding through heavy rainfall and strong winds. At least two people died across Guntur, Bapatla and Palnadu districts while 52,459 hectares of agricultural land suffered crop damage.

The science behind back-to-back cyclones

Cyclone formation generally requires sea surface temperatures exceeding 26.5°C, with sufficiently warm water extending to a depth of around 10 metres. However, when a powerful cyclone moves slowly across the Bay of Bengal, its intense winds churn the upper layers of the ocean, bringing colder water from below to the surface. This phenomenon is known as a ‘cold wake’.

As a result, the affected stretch of ocean needs time to regain its heat before it can support another developing cyclone. If a second storm forms too soon, it encounters these relatively cooler waters and may be deprived of the thermal energy required to intensify.

A mature cyclone also draws enormous quantities of moisture and heat from the surrounding atmosphere, redistributing them into the upper troposphere. This process alters regional atmospheric pressure patterns and can produce ‘subsidence’, a downward movement of air in neighbouring areas that suppresses convection and inhibits the formation of new clouds.

Cyclone development is also influenced by vertical wind shear. After a cyclone passes, it can leave behind atmospheric conditions that are unfavourable for another system to develop, weakening emerging low-pressure formations.

How could two cyclones develop in such quick succession?

According to meteorologists, the principal explanation lies in the unusually ‘over-energised’ state of the Bay of Bengal.

Arnab and Murasu originated in different parts of the basin. While Arnab developed over the east-central Bay of Bengal, Murasu emerged over its southeastern waters. Given the vast expanse of the central Bay, a cyclone developing in one region does not necessarily disrupt the conditions prevailing elsewhere.

The solar heat accumulated during the monsoon season has turned much of the Bay of Bengal into an enormous reservoir of thermal energy. Even when a cyclone brings colder water to the surface, the overall temperature of the basin remains sufficiently high to provide the energy needed for another storm to develop and intensify.

The impact of climate change

Although the overall number of cyclones forming in the Bay of Bengal has been declining, their intensity is increasing. This trend is increasingly supported by scientific research.

In September this year, global sea surface temperatures were 1.10°C above normal, marking a fourth consecutive month of record-high temperatures. Warmer oceans supply cyclones with greater amounts of heat and moisture, enabling them to undergo ‘rapid intensification’ — a process in which a storm strengthens dramatically within a relatively short period.

Statistics indicate that since 2000, around 38 per cent of cyclones forming over the North Indian Ocean have undergone rapid intensification. The corresponding figure for the Northwest Pacific Ocean stands at 22 per cent.

The El Niňo factor

A strong El Niño is currently active in the Pacific Ocean and has influenced India's monsoon patterns. El Niño conditions can generally reduce the frequency of cyclones in the Bay of Bengal, even as the storms that do develop may become more intense and larger in scale.

According to meteorologist K. J. Ramesh:

"Above-normal sea surface temperatures and the possible influence of El Niño may have contributed to Arnab's intensity. Although climate change does not cause El Niño, the underlying rise in global temperatures adds extra heat to the system and amplifies the impact of extreme weather events."

The challenges ahead

Over the past decade, India has made considerable progress in strengthening its cyclone preparedness. Doppler radars, satellite monitoring and advance warning systems have helped reduce casualties and limit damage to a significant extent. However, when two powerful cyclones strike in quick succession, the pressure on the entire disaster management infrastructure multiplies.

Experts point out that the impact of cyclones is no longer confined to coastal regions. Their effects are increasingly felt deep inland as well. This makes climate-resilient housing, stronger electricity grids, flood-resistant roads and improved crop insurance mechanisms essential components of long-term preparedness.

The seven-day interval between Arnab and Murasu was more than an unusual meteorological occurrence. It offers a glimpse into the changing dynamics of the Bay of Bengal, where rising ocean temperatures and climate change are reshaping the conditions under which cyclones develop.

As sea surface temperatures continue to rise, rapidly intensifying storms and back-to-back cyclonic events could become increasingly common. Advances in science and technology may help improve forecasting and disaster response, but there is no substitute for sustained, long-term planning to confront the growing fury of nature.