Tropical cyclones are among the most powerful and destructive storms on our planet. They bring fierce winds, torrential rains, and surges of water that can cause widespread flooding and dramatic damage to coastal cities and surrounding areas. Cyclone Fani, which hit Odisha in 2019, claimed 89 lives and caused around 8.1 billion in damages. As coastal communities grow larger and more sophisticated, understanding how these storms form—and possibly strengthening our ability to predict and prepare for them—becomes more urgent than ever before.
In most discussions about how tropical cyclones develop, attention has traditionally focused on factors like temperature at the ocean’s surface, low-level spinning winds, wind shear, or humidity in the middle layers of the atmosphere. While sea surface temperatures and atmospheric parameters have long been recognised as key elements, new research suggests a crucial but often overlooked player in this story: the subsurface ocean.
In a study published in Nature, a collaborative team of researchers from Princeton University, Shanghai Jiao Tong University, Chinese Academy of Sciences, National Taiwan University, Ocean University of China, University of Hawaii, and Indian Institute of Technology (IIT) Bombay, provides a new perspective on hurricane formation, highlighting the crucial role of the subsurface ocean. Specifically, variations in the 26 °C isothermal depth known as D26, which represents the (Ocean Mean Temperature), appear to be surprisingly influential in the birth of tropical cyclones.
The researchers examined the early stages of 2,032 tropical cyclones worldwide over a 25-year span from 1998 to 2022. Their goal was to see whether the subsurface ocean differed from more familiar surface conditions and might play a part in cyclone birth. They defined “pre-genesis” storms as those that had reached about 30 knots (roughly 15 meters per second) but not yet surpassed the 35-knot threshold that officially classifies them as tropical cyclones.
For each of these developing storms, the research team collected data on temperature profiles in the upper ocean. They used a product called Global Ocean Physics Reanalysis (GLORYS12v1), a detailed reanalysis that combines different sources of ocean observations with model simulations. This dataset provided daily snapshots of how water temperatures were arranged from the surface down to hundreds of meters below. Specifically, the scientists tracked where the 26 °C isotherm sat in the water column. They compared the position of this layer during the days before the storm formally formed (Days −10 to −4 in their timeline) to its location on the first day of the pre-genesis stage (Day 0).
In addition, they analysed wind and wind curl - how wind speed and direction vary over an area. Even though a storm at this early stage would not yet have the extremely high wind speeds of a mature hurricane, the research showed that the wind stress and its curl reached strengths an order of magnitude greater than typical background winds. This was enough to churn and mix the water thoroughly or propel deeper water upward, changing not only the thermal structure but also creating meaningful shifts in sea surface temperature.
One of the interesting findings of the research is that even in the early or “pre-genesis” stages, the ocean below the surface can be dramatically impacted by winds. Previous assumptions held that these early-stage winds were too “weak” to significantly alter what happens deep in the water. Yet the data show that these winds, although technically not as strong as those in a mature hurricane, are still more than powerful enough to stir the upper layers of the ocean. In fact, they can cause the 26 °C isothermal depth (D26) to move up or down by as much as tens of meters. In some cases, it deepens by over 50 meters; in others, it becomes shallower by nearly that amount.
The 26 °C isothermal depth serves as a measure of how much heat is available in the ocean’s upper layers. Tropical cyclones feed off warm ocean waters like an engine, so any drop or rise in this layer can make the difference between a storm that fizzles out and one that strengthens. When the ocean’s subsurface warms—or the warm layer thickens—it provides extra energy. On the other hand, if cool water is pushed closer to the surface, potential storms can lose some of the fuel they need to form or grow. The research team linked these D26 changes directly to the development of tropical cyclones, revealing that birth locations and initial intensities can shift based on the presence or absence of deeper, warmer ocean layers.
Understanding that subsurface ocean dynamics play a direct role in how tropical cyclones start can improve our preparedness, providing an earlier and clearer picture of a storm’s potential strength and path. Recognising that wind stress in the early stage already influences ocean temperatures can guide us toward better-equipped observation networks.
Although the results are compelling, there are certain constraints that need further exploration. For one, the scientists emphasise that factors like vertical wind shear or middle-level moisture in the atmosphere are still major contributors to a storm’s genesis, and the ocean is only one aspect of a complicated picture. The researchers also mention that variations in salinity, or how salty the water is, can influence how the water layers mix. In some places, a stratified layer of water influenced by low salinity might limit cooling in the pre-genesis stage—and potentially fuel storm formation more than simple temperature readings would suggest.
Additionally, the data come primarily from observational and reanalysis sources, possibly introducing uncertainties in how well the model captures fine-scale interactions between wind, waves, and ocean depth. Researchers also note that improving how climate models reproduce the correct depth and variation of D26 in the future is critical if these models are to improve predictions of where and how new cyclones will emerge.
All in all, this research offers an eye-opening reminder that “weak” winds and hidden ocean layers matter far more than once believed. Far from being a passive backdrop, the ocean’s inner layers are an active participant in tropical cyclone formation, constantly pushed and pulled by swirling wind systems even before a cyclone’s official birth. By recognising how shifting subsurface temperatures intersect with wind and atmospheric conditions, scientists can refine how we predict these storms and, hopefully, help protect lives and property in regions threatened by their destructive power.
The article was edited to correct the name of Ocean University of China. The error is regretted
This research article was written with the help of generative AI and edited by an editor at Research Matters.