Demographics of Mesoscale Eddies in an Eddy-Permitting Ocean Model and Reanalysis

TL;DR

This study evaluates the realism of mesoscale eddies in ocean models by comparing them with satellite observations, revealing significant discrepancies in eddy count, size, lifespan, and strength.

Contribution

It provides a comprehensive comparison of observed and modeled eddy characteristics, highlighting limitations in current eddy-permitting ocean models.

Findings

Reanalysis and model data miss nearly 30% of eddy trajectories compared to satellite data.

Eddies in models tend to be larger, longer-lived, and weaker than observed eddies.

Significant spatial and global differences in eddy statistics are documented.

Abstract

Ocean mesoscale eddies can be thought of as the "weather" of the ocean and strongly influence the ocean's physics, chemistry, and biology; they influence other components of the Earth system via air-sea and sea-ice interactions, and are crucial drivers of marine heat waves. Thus, proper modeling of eddies in both historical and future climates is crucial to accurately capturing the Earth system. Climate projections using global coupled models with eddying ocean components are only recently starting to be more widely used. Despite their critical role in understanding and forecasting climate characteristics, these so-called eddy-permitting models have not been explored to verify that resolved eddies are realistic, and thus any downstream scientific testing of hypotheses in biogeochemistry, ocean physics or other associated Earth systems impacted by eddies hinge on this critical assumption. This paper compares observed eddies with lifetimes longer than 6 weeks present in $1/4^\circ$ satellite altimetry data with observed eddies in $1/4^\circ$ reanalysis data and ocean model output. When compared to eddies observed in satellite altimetry data, eddies in reanalysis data and ocean model output are missing almost 30% of the number of eddy trajectories. In addition to missing eddy trajectories, the characteristics of eddies in reanalysis data and ocean model output differ from eddies observed in satellite altimetry data. At a high level, eddies in reanalysis data and ocean model output tend to live longer, are larger, and are weaker than eddies in observed altimetry data. This paper presents a variety of statistics describing these differences both spatially and in global aggregate.