Observations and modeling of Lagrangian transport in the Gulf of Maine

Abstract:

Field observations of 24 surface drifters deployed for 5 weeks from 25 June – 31 July 2020 are used with numerical modeling to explore the spatial patterns of larval lobster dispersal in the southern Gulf of Maine near the coast of New Hampshire. Lateral dispersion coefficients for coastal currents are estimated from clusters of six drifters deployed 4, 11, 15, and 20 km from shore along a cross-shelf transect passing by the Isles of Shoals. Dispersion O(10¹m²/s) found in shallow coastal regions of O(5-20 m) water depths is an order of magnitude larger than O(10^-1-10⁰ m²/s) found in deeper water offshore of O(100 m). Observed drifter trajectories are simulated with Lagrangian particle tracking methods utilizing estimated dispersion coefficients and driven by three-hour nowcast surface velocity fields from NOAA’s Gulf of Maine Operational Forecast System (GOMOFS). Model trajectory patterns and dispersal agree reasonably well with observed drifter tracks, including resolving spatial differences in drifter patterns that depend on initial location with distance from shore. Drifters initially deployed closer to shore head southward following a coastal current around Cape Ann into Massachusetts Bay and around Cape Cod. Drifters deployed greater distances from shore head seaward, with the furthest heading northward towards Downeast Maine. Results suggest that simple parameterizations for the unresolved sub-grid scale dispersion for GOMOFS velocity fields applied to the Lagrangian particle methods can be used to estimate dispersal patterns of passively drifting surface particles including early stage lobster larvae. This work was funded by the National Sea Grant American Lobster Research Initiative.