Treffer: Drifting Along: A Global Validation of Climatologies of Numerical Dispersal Over the Continental Shelf Using Trajectories From the Global Drifter Program.

The distance over which planktonic larvae are dispersed and the variability within that dispersal distance are important for understanding gene flow and species persistence in the coastal ocean. The breadth of spatial and temporal scales that are important to dispersal in shelf seas makes direct observations difficult—instead, we often use numerical simulations of circulation to estimate the statistics of larval dispersal. However, meroplanktonic life histories are most common in coastal regions where drifter‐based estimates of circulation are sparsely distributed, making validation of these numerical simulations quite difficult. We use a novel technique to validate climatological mean and standard deviation of dispersal distance at a global scale by drawing on the tens of thousands of sparsely distributed drifter observations on the shelf. Numerical dispersal estimates were made using Lagrangian particle trajectories calculated with circulation fields from a 1/12° global physical model and were validated against data from the Global Drifter Program (GDP), an international program that observes ocean circulation using drifters. The median dispersal distance of a climatological ensemble of numerical drifters released from a single location were found to match GDP drifter estimates quite well (with a mean deviation of 0.2%), whereas model estimates of dispersal were shown to underestimate the diffusivity of GDP drifters by 30%–50%. Our results indicate that although global numerical estimates of dispersal statistics provide a close approximation of median dispersal distance in the coastal ocean, these numerical simulations underestimate the overall variation in dispersal distance of drifters in the coastal ocean. Plain Language Summary: Understanding how organisms with planktonic larvae, such as crabs, barnacles, and reef fish, disperse in the coastal ocean is important for management, conservation, and understanding gene flow and species distribution. Because the ocean is exceedingly large and planktonic larvae are quite small, we often use computer‐based simulations to explore patterns of larval dispersal. In the coastal ocean, where many species disperse as planktonic larvae, observations of circulation that can be used to ground‐truth our simulations are very sparse. This work simulates the dispersal of drifters, floating instruments that follow ocean currents. By rotating and scaling dispersal pathways relative to the average motion of the simulated drifters, we can combine global observations for both real drifters and simulated trajectories. By combining many observations around the globe into a single distribution, we can overcome the sparseness of observations in the coastal ocean to compare computer simulations to real‐world observations. We find that the median motion of simulated drifters matched that of real‐world drifters to within 1%. The variation in dispersal was underestimated in the model, indicating that simulated planktonic larvae will spread by 30%–50% less than their real‐ocean counterparts. Key Points: Numerical drifters were validated against Global Drifter Program observations in shelf seasThe vector mean component of dispersal of numerical drifters was within 0.2% of observations (on average)Numerical drifters underestimated the stochastic component of dispersal by around 30%–50% [ABSTRACT FROM AUTHOR]

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