Tropical cyclones (TCs) are among nature's most powerful and destructive forces, threatening lives and properties on both sea and land. Understanding how they interact with the ocean is crucial for improving forecasting accuracy and mitigating their impacts. Recent research led by Bishakhdatta and Devang, in collaboration with researchers from the University of Melbourne (UniMelb) and the Indian Institute of Science (IISc), powered by National Computational Infrastructure (NCI)’s supercomputer Gadi, provides new insights into the complex ocean dynamics that influence cyclone intensity. Using high-resolution simulations and real-world observations, this study explores the intricate relationship between wind stress, surface temperature, salinity, and ocean mixing during a cyclone’s passage. This research was recently published in the prestigious journal Geophysical Research Letters (GRL).
Figure: Turbulence resolving large-eddy simulations, conducted on NCI's Gadi supercomputer showing convective and shear instabilities in the upper ocean during the passage of a tropical cyclone.
What are Tropical Cyclones?
Tropical cyclones are low-pressure systems that form over warm ocean waters (27°C or warmer) and gain strength from the heat they extract from the sea surface. As these storms move over the ocean, they impact the upper ocean layers, altering sea surface temperatures (SST) and salinity (SSS), which in turn affect the cyclone’s development. This study helps us understand the integration of ocean convection and shear-driven processes, which drive crucial changes in ocean mixing during cyclones.
Understanding Ocean Mixing During Cyclones
A key challenge in predicting cyclone intensity is accurately simulating ocean mixing. During a cyclone, two primary processes—shear-induced turbulence and buoyancy-driven convection—drive mixing in the upper ocean. Shear turbulence occurs when surface wind exerts stress on the ocean’s surface, creating velocity differences in the water layers that become unstable, generating turbulence. Convection, on the other hand, arises from surface cooling and evaporation, changing water densities and enabling ocean mixing. Both processes contribute to high ocean mixing, but quantifying these effects is challenging, leading to the rarity of accurate ocean models. This study addresses these challenges by using high-resolution Large Eddy Simulations (LES) and mooring observations (located 200km from the storm centre) to investigate upper ocean turbulence and mixing.
How Gadi Supercharged the Research
To study these processes in detail, the research team employed Large Eddy Simulations (LES), a computational technique to model turbulent fluid flow. Powered by NCI’s Gadi supercomputer, the team conducted simulations with exceptional precision, modelling interactions between wind stress, ocean stratification, and mixing. These simulations provided insights into how shear and convection contribute to ocean dynamics during cyclone events, offering a level of detail that traditional observational methods couldn't achieve.
The team used mooring data to establish initial conditions for their LES, with temperature and salinity profiles setting the stage for the simulations. They also faced the challenge of resolving the millimetre-scale sublayer at the air-ocean interface—a computationally expensive task due to the wide range of time and spatial scales involved. Thanks to Gadi’s immense computing power, the simulations captured ocean sea-state evolution and predicted sea surface temperature (SST) and sea surface salinity (SSS) over the following week, regularly validated against mooring data. This allowed the team to explore ocean mixing and its response to SST and SSS, critical factors in understanding cyclone intensity and evolution.
The Bigger Picture: Improving Cyclone Forecasting
Focusing on Cyclone Phailin as it passed over the Bay of Bengal, the study used high-resolution simulations to model the ocean's response at different stages of the cyclone. By examining how shear-driven turbulence and buoyancy-driven convection interacted, the team observed how these processes deepened the ocean’s mixed layer and redistributed temperature and salinity. During the cyclone's peak, wind stress and surface buoyancy loss were at their highest, leading to intense mixing, while intermittent rain events further altered the ocean’s stratification, influencing the mixing processes.
The research revealed that both shear-induced turbulence and convective processes play a vital role in governing ocean mixing during cyclones. Shear turbulence deepens the mixed layer and transports cooler, fresher water downward, while convective processes help redistribute heat and salt in the upper ocean. These combined effects create a highly dynamic and variable environment, making it challenging to predict cyclone intensity accurately.
This breakthrough in understanding ocean mixing during cyclones is crucial for improving cyclone intensity forecasting. By gaining a better understanding of how the ocean’s upper layers respond to cyclonic forces, scientists can develop more accurate models for predicting the behaviour of future storms. As climate change increases the frequency and intensity of tropical cyclones, this research offers vital insights for anticipating the impacts of these extreme weather events.
NCI's Crucial Role in the Research and Paving the Way for Future Cyclone Research
The success of this research underscores the essential role that high-performance computing plays in advancing our understanding of complex environmental processes. NCI’s supercomputer Gadi provided the computational power necessary to run the high-resolution simulations, enabling the team to model the ocean’s behaviour with unprecedented accuracy. Without such resources, the detailed insights into ocean mixing and cyclone behaviour would not have been possible.
This study is a significant step forward in improving tropical cyclone forecasting. By leveraging advanced simulations and supercomputing resources, the research team has uncovered new details about the ocean’s response to cyclones, with implications for future predictions and climate research. As the frequency and intensity of tropical cyclones continue to rise, this work paves the way for better forecasting tools and disaster preparedness strategies, with NCI’s computational resources playing a pivotal role in shaping the future of cyclone research.
If you would like to know more, you will find the research paper available here:
https://agupubs.onlinelibrary.wiley.com/doi/full/10.1029/2024GL111925
