Quantifying connectivity in the coastal ocean with application to the Southern California Bight

Abstract

[1] The quantification of coastal connectivity is important for a wide range of real-world applications ranging from assessment of pollutant risk to nearshore fisheries management. For these purposes, coastal connectivity can be defined as the probability that water parcels from one location have advected to another site over a given time interval. Here we demonstrate how to quantify connectivity using Lagrangian probability-density functions (PDFs) based on numerical solutions of the coastal circulation of the Southern California Bight (SCB). Ensemble mean dispersal patterns from a single release site show strong dependencies on particle-release location, season, and year, reflecting annual and interannual circulation patterns in the SCB. Mean connectivity patterns are heterogeneous for the advection time of 30 days or less, due to local circulation patterns, and they become more homogeneous for longer advection times. However, connectivity patterns for a single realization are highly variable because of intrinsic eddy-driven transport and synoptic wind-forcing variability. In the long term, mainland sites are good sources while both Northern and Southern Channel Islands are poor sources, although they receive substantial fluxes of water parcels from the mainland. The predicted connectivity gives useful information to ecological and other applications for the SCB (e.g., designing marine protected areas and predicting the impact of a pollution event) and demonstrates how high-resolution numerical solutions of coastal ocean circulations can be used to quantify nearshore connectivity.

Lagrangian assessment of simulated surface current dispersion in the coastal ocean

“…The two-dimensional (2-D) velocity fields at 1 m are used for consistency with drifter observations. The 2-D modeled trajectories differ from the fully three-dimensional (3-D) trajectories described by Mitarai et al [2009]. Fixed-depth tracking can lead to "beaching" compared with 3-D flows (particles that reach the shore in the surface layer continue their motion vertically in 3-D to satisfy continuity).…”

“…Lagrangian PDFs of modeled dispersion are not expected to be isotopic or spatially homogeneous as with Gaussian velocity distributions [Mitarai et al, 2009]. The focus here is on the K-S test statistic as a quantitative metric for assessing agreement in the spatial distribution of the ending positions of modeled and observed trajectories.…”

“…All drifter position records contained within each of Mitarai et al's [2009] launch sites during the 1996 through 1999 period are first identified. For each individual drifter that passes through a launch site, the position record closest to the center time of all position records within the launch site is identified as the starting record.…”

“…[8] Launch locations (located within 10 km of the coast), the scheme for launching modeled trajectories, and computation of Lagrangian PDFs from positions along surface current trajectories after some integration time, are all described in detail by Mitarai et al [2009]. Lagrangian PDFs are computed for 1, 2, 3, and 4 day advection times allowing trajectories to terminate through beaching.…”

“…Trajectories computed from numerical circulation models must therefore be relied on. In a recent study, Mitarai et al [2009] determine Lagrangian PDFs with millions of trajectories computed from the Eulerian output of Dong and McWilliams' [2007] Southern California Regional Ocean Modeling System (ROMS) simulations. Results of the work by Mitarai et al [2009] are a part of the planning process for designation of Marine Protected Areas in Southern California.…”