A Frontal Challenge for Climate Models

An unusually detailed portrait of an ocean front off Japan could help improve climate predictions. The ocean surface is filled with a convoluted web of “fronts” that separate waters of different temperatures and salinities (see the figure). Just as thin ducts in the lung called alveoli facilitate the rapid exchange of gases when breathing, fronts are the ducts through which heat, carbon, oxygen, and other climatically important gases enter into the deep ocean. A lack of observations, however, has hampered progress in understanding the dynamics of fronts, which can be as narrow as hundreds of meters and as wide as tens of kilometers. Global satellite measurements of ocean-surface velocities and air-sea fluxes, for instance, are only available at resolutions of a few hundred kilometers. Although shipboard researchers can sample vertical ocean profiles down to centimeter scales, only rarely do they collect profiles that are less than 100 km apart. On page 318 of this issue, D'Asaro et al. (1) present a breakthrough in observing ocean fronts, providing direct observations of the workings of the Kuroshio front off Japan on scales from kilometers to millimeters. This detailed and wide-ranging portrait was made possible by the development of towed instruments that continuously sample the waters behind a steaming ship, the deployment of freely drifting instruments that follow ocean currents, and the exercising of a great deal of ingenuity in keeping all these tools along the front for a few weeks in 2007.