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T H E U.S. NAVY expends a considerable effort to determine the locations and properties of most major ocean frontal systems across the globe. This synoptic picture, called a nowcast, is partly constructed using expendable bathythermographs dropped from ships (XBTs) and airplanes (AXBTs). Such in situ measurements can give very detailed information, but the cost limits their utility to relatively localized and short-lived surveys. Fortunately, given an adequate database of earlier measurements, simply knowing the surface location of a front is often sufficient to reconstruct an accurate picture of the three-dimensional thermal structure of the water column to depths of thousands of meters. Obtaining that surface information is not a trivial task however. Satellite infrared-radiometer (IR) images provide locations of important mesoscale features over large areas, but clouds can obscure important fronts for long periods, and the surface thermal structure is not always an accurate guide to the actual location of the predominant currents. An unknown geoid and other problems not withstanding (Mitchell et al.. 1990), satellite altimetry can provide accurate locations of currents and related features. In many areas of interest, the time and space scales on which the oceans evolve are not adequately sampled, however (Hurlburt. 1984: Kindle, 1986; Thompson, 1986). For example, the U.S. Navy's Geodetic Earth Orbiting Satellite (GEOSAT) (Born et al., 1987) with its 17-day repeat cycle and equatorial track separation of less than 150 km was adequate to sample regions such as the Gulf of Mexico, but was not sufficient to provide a synoptic picture for the evolution of the Gulf Stream. One approach to the sampling problem is to combine computer models of the oceans with data assimilation methods to extract the most infor-

Characterizing Major Frontal Systems: A Nowcast/Forecast System for the Northwest Atlantic