Little Buoys, Big Ocean: Observations of Wave-Driven Transport of Buoyant Objects in the Nearshore and Marginal Ice Zone

Abstract

A variety of objects drift on the ocean surface, including plastics, sea ice, search and rescue targets, oil, and marine organisms. Accurately predicting their trajectories is essentialfor both scientific and operational applications. The processes that drive the transport of these objects, particularly on shorter time and space scales, are not fully understood. This work addresses this problem, particularly investigating the wave-driven transport mechanisms in the nearshore and marginal ice zone. These regions are dynamically complex, and wave transformation is a dominant feature of both regions. This work primarily uses in situ observations to investigate buoyant object transport. The observations are from two large field experiments: the US Coastal Research Program funded During Nearshore Events Experiment (DUNEX) and NASA's Salinity and Stratification at the Sea Ice Edge (SASSIE) campaign. The following work consists of the development and nearshore deployments of a small-scale free-drifting wave buoys called microSWIFTS, observations of surfing transport from the microSWIFTs, and observations of transport driven by wind and waves in the Arctic marginal ice zone. The microSWIFT is a small buoy equipped with a GPS module to measure the buoy'sposition and horizontal velocities and an Inertial Measurement Unit (IMU) to directly measure the buoy's rotation rates, accelerations, and magnetic heading. Measurements were collected over a 27-day field experiment (DUNEX) in October 2021 at the US Army Corps of Engineers Field Research Facility in Duck, NC. The microSWIFTs were deployed as a series of coherent arrays, meaning they all sampled simultaneously with a common time reference, leading to a rich spatial and temporal dataset during each deployment. Measure- ments spanned offshore significant wave heights ranging from 0.5 to 3 meters and peak wave periods ranging from 5 to 15 seconds over the entire experiment. Observations of surfing transport are made using the data collected from the microSWIFTsas part of DUNEX. Surfing events are observed in the drift trajectories of the buoys as 'jumps' in the time series of cross-shore position. There are 3,172 surfing events observed, with a median jump amplitude of 8.3 meters and a median duration of 2.5 seconds. The buoy's trajectories are simulated using three models of increasing physics complexity: 'Wind-Only," 'Wind and Waves," and 'Wind, Waves, and Surfing." The surfing process is represented using a probabilistic parameterization. The accuracy of the simulations is significantly improved when surfing is included. Sea ice in the marginal ice zone is generally considered to be in "free-drift" and acts like adrifter following the wind, waves, and currents. The marginal ice zone is the dynamic region of sea ice between 15% - 80% sea ice area coverage. A dominant feature of the marginal ice zone is the attenuation of waves propagating from the open ocean, making this region analogous to the surf zone. SWIFT drifters were deployed in the marginal ice zone, and their drift speeds were measured. A combination of wave radiation stress gradient-driven currents, direct windage on the drifters, and Ekman transport accounts for their observed drift speeds. These results suggest that wave-driven transport mechanisms cause a significant portion of the observed drift at the outer edges of the ice pack. Wave-driven transport in the marginal ice zone may play an important role in shaping the future evolution of the sea ice edge, as earlier melting and later refreezing create more open ocean and allow larger waves to interact with the ice edge.