- The Northern Indian Ocean has been experiencing an increasing trend of severe cyclones and extreme rainfall with significant impacts on its densely populated coasts and elsewhere.
- Ocean warming, cyclones and extreme and erratic rainfall are connected to events and processes at different timescales, near and as far as the Pacific and the poles.
- Some of these phenomena involve natural climate variability, but others are intensified due to climate change.
Global warming has led to oceans also warming over the past century – more in some locations and less in others. Between 1901 and 2020, sea surface temperatures (SST) rose at an average rate of 0.14 degrees Fahrenheit (equivalent to a rate of 0.08 degrees Celsius) per decade, remaining consistently higher during the past 30 years than at any other time, since observations became available from 1880. As the Intergovernmental Panel on Climate Change’s (IPCC) sixth assessment report notes “it is virtually certain that the global upper ocean (0-700 m) and very likely that the global intermediate ocean (700-2000 m) warmed substantially from 1971 to the present.”
The Indian Ocean however, has warmed faster than the global average. Climate models show a non-uniform warming trend, with hotspots in the Arabian Sea and the southeastern Indian Ocean. This process has implications for sea level rise, storm patterns and rainfall events, especially in the coastal areas as (IPCC) studies show.
The Northern Indian Ocean (NIO) specifically, has been experiencing an increasing trend of severe cyclones and extreme rainfall with significant impacts on its densely populated coasts and elsewhere. Ocean warming, cyclones and extreme and erratic weather are connected to events and processes at different timescales, near and as far as the Pacific and the poles. Some of these phenomena involve natural climate variability, but others are due to climate change. These events and processes have serious implications on human security, livelihoods and development.
Newer analyses show an increasing trend of extremely severe cyclones (ESCS, maximum winds above 168 kmph) and call for better policies and planning for defending coastal flood risk and associated impacts.
What is Tropical Warm Pool? How does it influence the climate?
The Indo-Pacific Warm Pool (or Tropical Warm Pool) is a region located in the eastern Indian Ocean, where the sea surface temperature (SST) remains above 28 degrees Celsius throughout the year. The warm pool has been warming and expanding over the past century, while its warmest part has almost doubled in size. This warm pool is the largest expanse of warm SST with high rainfall, that significantly influences global atmospheric circulation and the water cycle. When compared to the warm pool area, the western Indian Ocean is cooler.
The Indo-Pacific Warm Pool has also experienced the world’s highest rates of sea-level rise in recent decades with impacts on islands here. As the IPCC notes, “Thermal expansion explained 50% of sea level rise during 1971–2018, while ice loss from glaciers contributed 22%, ice sheets 20% and changes in land-water storage 8%.” This expanding warm pool affects the global climate system by enhancing tropical convection (the transport of heat and moisture).
What is the Madden Julian Oscillation?
Meanwhile, as the tropical warm pool expands, it also affects Madden Julian Oscillation, a massive disturbance involving clouds, rainfall, winds, and pressure that influences rain patterns across the world.
If El Niño (the event that involves warming of the ocean surface temperatures in the central and eastern tropical Pacific Ocean) is a static system, the Madden Julian Oscillation (MJO) is a moving system. It traverses the tropics eastward for thousands of kilometres before returning to its initial starting point in 30 to 60 days. Once this atmospheric disturbance sets in, its features last several seasons over the Pacific Ocean basin, raising and lowering convection over specific regions. This oscillation, eastward and back, leads to severe rainfall and storms.
Apart from the Arctic amplification (rapid Arctic warming) and sea ice melting playing important roles in influencing monsoon in several ways, including through ENSO and Hadley Circulation, MJO is also a key influence that triggers extreme rainfall within seasons over tropical regions. It is also the key trigger for tropical cyclone formation in the northern Indian Ocean. Over 80 percent tropical cyclones over this ocean part during 1979-2008 occurred during the active phase of the MJO. Higher SST in the Central Pacific ENSO events, as opposed to eastern Pacific El Nino events, can also lead to more tropical cyclones, as scientists note.
How does atmospheric circulation affect rainfall?
Another key mechanism of the ocean’s impact on global weather is through air circulation. Air warmed by the sun swells up and rises, letting colder, denser air sink in. Air from the warm tropics air rises and moves out poleward and cooler air from mid-latitudes moves in, forming a circulation called the Hadley cell.
The Earth’s rotation drives winds east to west along the equator. One such movement over the Pacific involves the Walker cell. It forms as high pressure off the coast of South America and pushes air over the Pacific westward to Indonesia, where it rises and circles back to the east to finish the loop. Rising air in the west and sinking air in the east keep the loop closed. This movement drives westerly trade winds. As the warm air rises and cools, it sheds the moisture as rain over the western Pacific.
In natural cycles (without the impacts of climate change), the Walker circulation and trade winds can gain or lose strength. However, trade winds weaken during a phase called El Niño and gain strength during La Niña.
El Niño significantly influencing global weather patterns. During an El Niño event, warmer water extends out to the east, warming the air, and causing it to rise and form lower pressure. In turn, there is less rising motion, and therefore higher pressure near Indonesia, as a result of relatively cooler waters and overlying air. The recurring climate pattern involving La Niña, El Niño and the neutral phases is referred to as El Niño Southern Oscillation (ENSO).
Read more: What are El Niño and La Niña? How do they affect the Indian monsoon?
The Pacific basin covers a third of the planet. Therefore, the wind and humidity changes associated with El Niño get transmitted around the world, disrupting circulation patterns. Such weather changes occurring in widely separated regions of the globe are called teleconnections.
El Niño is often associated with a weaker Indian summer monsoon, and La Niña a stronger monsoon. This year, scientists have associated El Niño with a weak monsoon involving long breaks and an outlook for a possible drought. Analysing the rather long monsoon break from 6 – 19 August, National Centre for Earth Science Studies monsoon expert Madhavan Nair Rajeevan posted on the X platform (formerly known as Twitter), “Convection over India was suppressed by heavy sinking motion. On the other hand, over the central Pacific enhanced convection occurred due to ascending air. This resembles the classical impact of an El Niño.” Subsidence denotes the downward movement of air as it cools down and becomes denser.
Why is the Arabian Sea becoming a hotspot for intense storms?
Global warming leads to an increase in SST and changes in atmospheric conditions, and an increase in the Potential Intensity, or the modelled maximum speed limit, of tropical cyclones. Globally, severe-category storms have been increasing. In the Northern Indian Ocean, such storms have been increasing in recent decades, most of them forming in May. Summer monsoon winds of June-September influence NIO circulation, and that leads to peaking of sea surface temperature and Potential Intensity during April-May and later after the monsoon. Projected changes in SST and a significant weakening of the wind speed difference between the upper and lower troposphere appear to be responsible for the increase in ESCSs. Scientists conclude that global warming has increased the probability of post-monsoon ESCSs over the Arabian Sea.
Besides, summer monsoon circulation has been weakening. The intensity of the cyclone does not only depend on sea surface temperature, but more importantly the volume of warm water in the ocean, scientists note. Cyclones gain more intensity due to ocean heat content from the surface down to the depth of the 26℃ isotherm (line linking points of the same temperature), as recent studies show.
There are different storm categories based on the intensity of maximum sustained wind speeds. Recent extremely severe cyclones in the NIO during the month of May include Mocha (2023), Tauktae (2021), Amphan (2020), Fani (2019), and Mekunu (2018). Of these, Tauktae and Mekunu formed over the Arabian Sea.
The Arabian Sea, located in the generally cooler western part of the NIO, has been so far considered cyclonically less active with close to two storms in a year or 2% of global storm frequency during 1979–2015. However, it is now emerging as a hotspot in the changing climate in the background of the warming of the Western Indian Ocean.
Post-monsoon events are gaining intensity of late, with the first post-monsoon ESCS in 2014, as recent studies show. Cyclone Nilofar in October 2014 was followed by two back-to-back ESCSs, Chapla and Megh in 2015. ESCS Maha coexisted briefly with the Super Cyclonic Storm (above 222 km per hour) Kyar in the 2019 post-monsoon season, in an unprecedented event. In 2023, Mocha was closely followed by another extremely severe cyclone, Biparjoy, in June.
There have been unusual storm tracks and rapid intensification of cyclones. The case of the very severe cyclone Ockhi (2017) is an example. On its unusually long 2,500 km track, it rapidly gained strength close to Sri Lanka and South India. Madden–Julian Oscillation and warm oceanic conditions provided favourable conditions for Ockhi.
Along the densely populated Indian Ocean coasts increasing intense storms, and other phenomena such as sea level rise and extreme rain pose great challenges in terms of human and habitat safety. Scientists have therefore called for more effective forecasts, dissemination, and disaster preparedness.
Read more: The Arabian Sea faces an increased frequency and duration of marine heatwave days, finds study
Banner image: A rainy day in Mumbai. As compared to other oceans, the Indian Ocean is warming at a much higher rate with pronounced warming in the Arabian Sea. Photo by Saumitra Shinde/Mongabay.