Climate model biases in the eastern tropical oceans: Causes, impacts and ways forward

Abstract

The eastern boundaries of the tropical and subtropical oceans are regions of high biological productivity that support some of the world's largest fisheries. They also feature extensive stratocumulus cloud decks that play a pivotal role in the response of the climate system to greenhouse gas forcing. Global climate models experience great difficulties simulating eastern boundary regions, with one of the most notable shortcomings being warm sea-surface temperature biases that often exceed 5 K. These model biases are due to several reasons. (1) Weaker than observed alongshore winds lead to an underrepresentation of upwelling and alongshore currents and the cooling associated with them. (2) Stratocumulus decks and their effects on shortwave radiation are underpredicted in the models. (3) The offshore transport of cool waters by mesoscale eddies is not adequately represented by global models due to insufficient resolution. (4) The sharp vertical temperature gradient separating the warm upper ocean layer from the deep ocean is too diffuse in the models. More work will be required to assess the relative importance of these error sources and to find ways of mitigating them. Coordinated multi-model experiments are vital to achieve this goal, as are enhanced ocean and atmosphere observations of the eastern boundary regions. To what extent eastern ocean biases compromise the models' ability to produce accurate seasonal predictions, and climate change projections should be another focus of research efforts.