A Lagrangian Perspective of the Tropical Cold Bias in Climate Models

The presence of too cold sea surface temperatures (SST) in the equatorial band of the tropical Pacific cold tongue and its central Pacific extension represents a persistent bias of many state-of-the-art climate models. A systematic analysis of the simulations of the Climate Model Intercomparison Project (CMIP5) points to extra-tropical latitudes as a potential source of this cold bias, mediated through the wind-driven subtropical overturning cells. Using the output of the IPSL-CM5A coupled climate model, which displays a pronounced cold bias in the tropics, we investigate the origins of this bias by tracking Lagrangian particles backwards in time from the ocean equatorial mixed layer. We outline the dominant pathways, geographical origins, relevant heat budgets and temporal dependence of the tropical cold bias in the model, which originates primarily in the mixed layer of the subtropical gyre interior and follow the shallow overturning cells southwards before upwelling in the equatorial Pacific.