Targeted artificial ocean cooling to weaken tropical cyclones would be futile

Reducing the hazards of tropical cyclone (TC) landfalls is a major societal concern. Efforts focus on improving forecasts, warning systems, and infrastructure. Geoengineering methods, such as artificial weather modification, have also been proposed. Past attempts, like Project STORMFURY, aimed at weakening TCs through direct intervention, yielded no conclusive results. More recently, with increasing global warming, solar radiation management (SRM) and lower-tropospheric cloud seeding have been explored, though these are large-scale interventions with potential for unintended consequences. Artificial cooling of the upper ocean, achieved through vertical mixing of cooler subsurface waters, is another proposed method, rooted in the understanding that warm surface waters fuel TC intensity. This approach capitalizes on the connection between TC intensity and air-sea heat and momentum fluxes, as described by WISHE and MPI theories. While natural wind-driven cooling can create cold wakes, mitigating TC intensity, this natural feedback might be insufficient in environments with high ocean heat content or inverted temperature gradients. This research investigates whether targeted artificial cooling could effectively weaken an approaching TC.

The literature extensively discusses various geoengineering techniques to mitigate TC impacts. Project STORMFURY, an early attempt at weather modification, failed to produce consistent results. Solar radiation management (SRM), though proposed for large-scale climate modification, also faces concerns about unintended consequences and uneven regional effects. Studies exploring lower-tropospheric cloud seeding show varied results. Artificial ocean cooling through vertical mixing has received attention, with several patent applications focusing on this method, although most have been abandoned. The theoretical basis for these approaches lies in the fundamental relationship between TC intensity and air-sea heat exchange, as detailed by wind-induced surface heat exchange (WISHE) and maximum potential intensity (MPI) theories. These theories show that warm surface waters are crucial for TC intensification, and cooling them could potentially weaken the storms. However, existing models often simplify the complexities of TC intensification and ocean interactions, warranting a more comprehensive assessment of the effectiveness of artificial cooling.

This study uses two approaches to evaluate the effectiveness of artificial ocean cooling in weakening TCs: (1) modifying the MPI theory to account for TC-induced ocean mixing (OPI model), and (2) conducting highly realistic, idealized three-dimensional mesoscale simulations using the WRF model. The OPI model incorporates the feedback between TC winds and SST cooling through vertical mixing. This allows assessing the effectiveness of artificial cooling across various environmental conditions (SST, mixed layer depth (MLD), and TC translation speed). The simulations, however, consider realistic factors such as the finite size of the cooled region and gradual exposure of the TC to the cooled patch, accounting for factors ignored by simplified models like OPI. The simulations employed varying sizes of artificially cooled patches along a coastline, and two different TC translation speeds (4 m/s and 8 m/s) were considered to explore sensitivity to these parameters. Minimum central pressure (*P*_min) and integrated kinetic energy (*I*_KE^TS) were used as key metrics for TC intensity, since these are better correlated with damage than wind speeds alone. The simulations assessed the changes in intensity following exposure to the artificially cooled regions, comparing against a no-cooling control simulation. The ocean heat content extracted from the cooled patches was calculated to quantify the scale of the intervention.

The OPI model indicated that artificial cooling could significantly weaken TCs, particularly for high SST, deep MLD, and fast translation speeds – conditions where natural cooling is less effective. However, mesoscale simulations revealed that substantial weakening at landfall requires massive artificially cooled regions. Even the largest cooled region (4.5 × 10¹⁹ kJ of extracted ocean heat, equivalent to the solar energy striking the Earth in 10 minutes), only caused a 15% reduction in intensity relative to the control for the fast-moving TC case. For slower TCs, the effect of artificial cooling was less significant because TC-induced cooling was already substantial. While absolute reductions in *P*_min were larger for slow-moving TCs, the percentage reductions were larger for faster-moving TCs, where natural cooling is less effective. The simulations showed that weakening began a few hours before the TC reached the cooled patch, but the effect was short-lived once the storm moved out of the cooler water. Using integrated kinetic energy (*I*_KE^TS) as a metric, the percentage reduction in intensity was even smaller than that obtained using *P*_min, further suggesting the limited impact of artificial cooling.

The findings demonstrate a significant discrepancy between theoretical predictions (OPI) and the results from realistic simulations. While the OPI model suggests a potential for substantial TC weakening under ideal conditions, the simulations highlight the impracticality of such a large-scale intervention. The massive energy requirements to create significant cooling make artificial ocean cooling an infeasible approach for TC mitigation. The study's limitations, such as the assumption of an initially quiescent ocean (ignoring boundary currents and eddies), and ideal atmospheric conditions, likely contribute to an overestimation of artificial cooling's effectiveness. Real-world scenarios would involve additional challenges posed by natural horizontal temperature gradients and currents. Furthermore, the potential ecological consequences of artificial ocean mixing remain largely unknown, and there might be unintended negative effects on the marine ecosystem. Considering these limitations, directing resources towards improved forecasting, robust infrastructure, and enhanced observational capabilities would be far more effective than pursuing large-scale geoengineering techniques.

This study conclusively demonstrates the futility of using artificial ocean cooling to significantly weaken approaching TCs. The massive scale and energy requirements, along with the potential for unforeseen ecological consequences, render this geoengineering approach impractical. Investments in improved forecasting, infrastructure resilience, and advanced observational technologies provide far more promising avenues for mitigating TC risks. Future research should focus on refining TC intensity prediction models and developing strategies for community resilience.

The study's idealized simulations assumed a simplified ocean environment lacking boundary currents and mesoscale eddies. The simulations also assumed ideal atmospheric conditions, which may not always be present in real-world situations. Furthermore, the potential ecological consequences of large-scale ocean cooling were not fully explored. These limitations suggest that the findings likely represent an overestimation of the effectiveness of artificial cooling in real-world TC weakening.