Scientists used GPS data collected from ocean drifters during Hurricane Isaac with a coupled atmosphere-wave-ocean model to better understand how hurricanes affect upper ocean circulation. The researchers found that hurricane-induced Stokes drift (wind-wave-driven water mass transport) created a cyclonic rotational flow to the storm’s left and an anticyclonic rotational flow to its right. Stokes drift accounted for more than 20% of the average current’s velocity and changed its direction up to 90 degrees, significantly enhancing shoreward upper ocean transport on the storm’s right side. The team estimated that the spread rate for these surface flows was 6 times larger than before the storm, a significant deviation from recognized measurements of lateral dispersion during non-hurricane conditions. The scientists published their findings in Geophysical Research Letters: Hurricane-induced ocean waves and stokes drift and their impacts on surface transport and dispersion in the Gulf of Mexico.
Previous research has demonstrated the importance of Stokes drift in local upper ocean circulation. However, Stokes drift’s effect on storm-scale surface currents has never been studied using in situ observations and coupled modeling. Because hurricanes produce extremely high winds and waves that significantly impact upper ocean circulation and water mass transport, it is important to better understand and predict their influence during events like the 2010 Deepwater Horizon oil spill.
The Grand Lagrangian Deployment (GLAD) field campaign released nearly 300 surface drifters in July 2012 near the Deepwater Horizon site and transmitted GPS data that tracked the transport and dispersion dynamics of submesoscale and mesoscale ocean flow for several months. Hurricane Isaac entered the Gulf of Mexico and traveled through the drifter experiment site at the end of August. The timing and the coincidence of the two events offered an unexpected opportunity to look into how hurricanes affect water movement. Hurricane Isaac subjected the drifters to strong winds and waves, and the drifters captured unprecedented high-spatial and temporal resolution measurements of near-surface Lagrangian velocity, which researchers used to improve wind, waves and ocean circulation forecasts. This study helps further improve coupled models and contributes to better forecasts of ocean transport.
GLAD drifter measurements confirmed that the Eulerian current typically used in ocean circulation models without coupling to the wind and waves underestimates near-surface water mass transport under strong wind conditions. These findings indicate that Stokes drift should be included in upper ocean transport model predictions in the event of an oil spill, especially under high-wind conditions.
Data are publicly available through the Gulf of Mexico Research Initiative Information & Data Cooperative (GRIIDC) at doi:10.7266/N7VD6WC8, doi:10.7266/N7KW5CX6, doi:10.7266/N7G44N66, and doi:10.7266/N7BG2KWS.
This research was made possible in part by a grant from the Gulf of Mexico Research Initiative (GoMRI) to the Consortium for Advanced Research on Transport of Hydrocarbon in the Environment II (CARTHE II). Other funding sources included the Ofﬁce of Naval Research under the National Oceanography Partnership Program (N0001401010162).
The Gulf of Mexico Research Initiative (GoMRI) is a 10-year independent research program established to study the effect, and the potential associated impact, of hydrocarbon releases on the environment and public health, as well as to develop improved spill mitigation, oil detection, characterization and remediation technologies. An independent and academic 20-member Research Board makes the funding and research direction decisions to ensure the intellectual quality, effectiveness and academic independence of the GoMRI research. All research data, findings and publications will be made publicly available. The program was established through a $500 million financial commitment from BP. For more information, visit http://gulfresearchinitiative.org/.