The Pacific Equatorial Undercurrent in Three Generations of Global Climate Models and Glider Observations.

The Equatorial Undercurrent (EUC) is a vital component of the coupled ocean‐atmosphere system in the tropical Pacific. The details of its termination near the Galápagos Islands in the eastern Pacific have an outsized importance to regional circulation and ecosystems. Subject to diverse physical processes, the EUC is also a rigorous benchmark for global climate models (GCMs). Simulations of the EUC in three generations of GCMs are evaluated relative to recent underwater glider observations along 93°W. Simulations of the EUC have improved, but a slow bias of ~36% remains in the eastern Pacific, along with a dependence on resolution. Additionally, the westward surface current is too slow, and stratification is too strong (weak) by ~50% above (within) the EUC. These biases have implications for mixing in the equatorial cold tongue. Downstream lies the Galápagos, now resolved to varying degrees by GCMs. Properly representing the Galápagos is necessary to avoid new biases as the EUC improves. Plain Language Summary: The Equatorial Undercurrent (EUC) is a swift current that flows eastward along the equator in the Pacific Ocean, about 100 m below the surface. This current is just as challenging to observe as it is to simulate with models—after all, it was only discovered in the 1950s. One of the interesting aspects of the Undercurrent is how it is diverted by the Galápagos Islands when it encounters them in the eastern Pacific. The resultant upwelling is responsible for the remarkably productive and diverse ecosystem of the Galápagos. This paper takes advantage of a unique set of observations from a recent, successful field campaign using underwater gliders to measure the Undercurrent just before it reaches the Galápagos, in order to evaluate how the latest generations of global climate models simulate this current and its neighboring features. Models have steadily improved, but they still struggle to capture the high speed of the EUC. Models are also being run at finer spatial resolution, which enables islands like the Galápagos to be included in some of the model grids. A sampling of islands in different models, and how they interact with the EUC, demonstrates the importance of a proper representation of the Galápagos in models. Key Points: A recent glider campaign offers a unique opportunity to evaluate model simulations of equatorial circulation in a key region for climateGlobal climate model simulations of the Equatorial Undercurrent have improved, but a slow bias of ~36% remains in the eastern PacificDetails of the encounter of the Equatorial Undercurrent with the Galápagos are impactful; resolving them well is demonstrably important [ABSTRACT FROM AUTHOR]