Shifting Hadley Cells Steer the Swell Energy That Reaches India’s Coasts

Every winter, long lines of swell roll quietly across the equator and deliver a large share of the wave energy that shapes the coasts of the North Indian Ocean. A new study published in Ocean Dynamics shows that the year-to-year fate of that energy is governed, at least in part, by one of the atmosphere’s most fundamental engines: the Hadley circulation. By tracking the north-south movements of the Southern Hemisphere Hadley cell over the Indian Ocean between 1980 and 2022, researchers found a coherent link between the cell’s expansion and contraction and the amount of swell power arriving in the North Indian Ocean during boreal winter.

The Hadley cell is a vast overturning circulation in which air rises near the equator, flows poleward aloft, descends in the subtropics, and returns to the equator near the surface. Its poleward edge is not fixed. From one winter to the next, the boundary of the Southern Hemisphere branch over the Indian Ocean can shift noticeably toward or away from the pole, and climate scientists have documented a longer-term tendency for tropical circulations to widen as the planet warms. Because the descending branch of the cell helps anchor the subtropical winds and storm tracks of the Southern Indian Ocean, any movement of that edge has the potential to rearrange where storms generate waves and where those waves travel.

To pin down that connection, the team, led by F R Thanvi of the Department of Physical Oceanography at Cochin University of Science and Technology, together with R Sajeev, L. Sheela Nair of the National Centre for Earth Science Studies, and J K Lix, needed a reliable way to measure the cell’s position. They used outgoing longwave radiation, or OLR, the infrared heat emitted by the Earth and atmosphere as observed from satellites. Deep convective clouds near the equator emit low OLR values, while the clear, dry, subsiding air of the subtropics emits high values. By applying an OLR threshold, the researchers could determine the latitude of the poleward edge of the Southern Hemisphere Hadley cell over the Indian Ocean for each winter, and then classify individual years as expansion years, when the edge moved poleward, or contraction years, when it retreated equatorward.

With those years identified, the team turned to wave power, the flux of energy carried by ocean waves, which depends on wave height and period. They drew on the ERA5 reanalysis produced by the European Centre for Medium-Range Weather Forecasts, a state-of-the-art reconstruction of past weather and ocean conditions, accessed through the Copernicus Climate Data Store. Crucially, they separated the wave field into two components: wind waves, generated locally by winds blowing over the North Indian Ocean, and swell, the long-period waves that were generated by distant storms and have traveled far from their birthplace. Composite analysis, in which the wave fields of expansion years and contraction years are averaged separately and compared, revealed that the two components respond very differently to Hadley cell shifts.

The swell response proved far more pronounced than the wind-wave response, and it was not uniform across the basin. The eastern North Indian Ocean, including the waters off India’s east coast and the Bay of Bengal, emerged as the clear hotspot of swell variability tied to the cell’s movements. In years when the Southern Hemisphere Hadley cell expanded, swell power in this region was typically enhanced; in contraction years, it was reduced. That asymmetry matters for practical reasons. The eastern North Indian Ocean hosts busy shipping lanes, fishing fleets, ports, and a growing interest in wave energy conversion, so knowing which large-scale atmospheric pattern favors energetic swell winters could improve seasonal planning and coastal hazard assessment.

But identifying a correlation is only the first step. To understand the physical chain of cause and effect, the researchers deployed three complementary techniques: composite analysis, regression analysis, and swell-source back-tracing. Back-tracing is a diagnostic method that follows swell waves backward along their great-circle propagation paths, effectively asking where the energy arriving at a given point came from. The results were unambiguous in their broad strokes: the swell that reaches the eastern North Indian Ocean during boreal winter predominantly originates in the central and eastern Southern Indian Ocean, thousands of kilometers away, where strong winter storms and the westerly wind belt generate long-period waves that race northward with remarkably little loss of energy.

Yet the back-tracing also revealed an important subtlety. Although the central-eastern Southern Indian Ocean is the climatological source region, the exact location of the generating storms varies from year to year. In certain winters, the swell sources are displaced from their usual position, and that displacement modifies the typical anomaly pattern that would otherwise be expected from the Hadley cell shift alone. In other words, the Hadley cell sets the stage, but the precise geography of the Southern Ocean storms determines the final script. This interplay between large-scale circulation shifts and the year-to-year wanderings of storm tracks helps explain why some individual winters depart from the average relationship.

The study also uncovered a role for one of the most famous climate oscillations on the planet. During expansion years, the swell response was further modulated by the El Niño-Southern Oscillation, specifically by La Niña conditions, the cool phase of the tropical Pacific marked by unusually cold sea surface temperatures in the central and eastern equatorial Pacific. The relationship between Hadley cell expansion and enhanced swell power was most clearly expressed when expansion coincided with La Niña. This makes physical sense, because ENSO is known to influence the extent and intensity of the Hadley circulation, and La Niña tends to strengthen the trade winds and reorganize convection across the Indo-Pacific region, with consequences that ripple into the wind fields of the Southern Indian Ocean where the swell is born.

Contraction years told a different story. Rather than a single dominant modulation, the changes in swell power during these winters were associated with contrasting wind conditions over distinct source regions. When the cell’s edge retreats equatorward, the wind patterns over the various swell-generating areas of the Southern Indian Ocean diverge from one another, and the net swell arriving in the north depends on the balance of these regional wind anomalies. This finding underscores that the Southern Hemisphere winter atmosphere is not a monolith: different patches of the storm belt can behave in opposite ways, and the swell climate of the North Indian Ocean integrates all of them.

Taken together, the results suggest a coherent dynamical linkage running from the meridional position of the Southern Hemisphere Hadley cell, through the remote wind forcing over the Southern Indian Ocean, to the displacement of swell source regions, and finally to the swell energy delivered to the eastern North Indian Ocean. The authors frame Hadley cell variability as a contributing large-scale dynamical control on the swell climate of the region, one that operates alongside better-known influences such as the Indian Ocean Dipole and ENSO. For engineers assessing wave-energy potential along the Indian coast, for forecasters anticipating winter swell events, and for coastal planners weighing long-term changes in wave conditions, the message is that the answer may lie as far away as the descending branch of a tropical circulation over the southern Indian Ocean. As the tropical belt continues to broaden under greenhouse warming, understanding how these circulation shifts translate into wave power will only grow in importance, and this study provides a template for tracing that connection from the top of the atmosphere down to the breaking waves on a distant shore.

Subject of Research: Influence of Southern Hemisphere Hadley cell shifts on winter swell wave power variability in the North Indian Ocean

Article Title: Atmospheric circulation driven variability of wave power in the North Indian Ocean: the role of Hadley cell shifts

Article References: Atmospheric circulation driven variability of wave power in the North Indian Ocean: the role of Hadley cell shifts. (n.d.). https://doi.org/10.1007/s10236-026-01868-7

Image Credits: AI Generated

DOI: 10.1007/s10236-026-01868-7

Keywords: Hadley circulation, swell waves, wave power, North Indian Ocean, Southern Indian Ocean, boreal winter, La Niña, ENSO, outgoing longwave radiation, ERA5 reanalysis, swell back-tracing, coastal wave climate