12.7 OCEAN FRONTS AROUND ALASKA FROM SATELLITE SST DATA Igor M. Belkin*, Peter Cornillon and David Ullman University of Rhode Island, Narragansett, Rhode Island ABSTRACT Long-term time series of satellite observations are used to survey ocean surface thermal fronts in the Gulf of Alaska, Bering, Chukchi and Beaufort Seas as well as in the upstream region of the Alaskan Current and farther south along the North American shelf to include British Columbia waters and the Columbia River Plume area, thus covering 45°N-75°N, 160°E-120°W. The Cayula-Cornillon algorithms for front detection and cloud screening were applied to the Pathfinder twice-daily 9-km resolution SST images from 1985-1996. A number of new frontal features have been identified in the Alaskan Seas; some previously known fronts have been systematically studied for the first time. Frontal frequency maps are provided for each of four Alaskan Seas. In the Gulf of Alaska and Bering Sea the SST fronts are best defined in spring (May) and fall (November), while being masked by surface heating in summer. In the Chukchi and Beaufort Seas the SST fronts are best seen in summer (August-September), when both seas are typically ice-free. Seasonal evolution of SST fronts is noted off the Oregon- Washington coasts and Vancouver Island, in Hecate Strait and Dixon Entrance. INTRODUCTION Ocean fronts are relatively narrow zones of enhanced horizontal gradients of physical, chemical, and biological properties that separate broader areas of different vertical structure (stratification). The fronts are crucial in various processes that evolve in the ocean and at the ocean interfaces with the atmosphere, sea ice and ocean bottom (Belkin, 2003a, b). Until the 1970s, frontal studies were based on ship data (Fedorov, 1986). The main problem in using ship data for climatological purposes is extremely non-uniform, patchy, and mostly sparse coverage provided by ship data. On the contrary, satellite data, at least, in principle, are spaced regularly and allow a fairly dense global coverage to be attained. The satellite data sets extend back to the early 1980s, thus encompassing nearly two decades and allowing a *Corresponding author address: Igor M. Belkin, Univ. Rhode Island, Grad. School of Oceanography, 215 S. Ferry Road, Narragansett, RI 02882, email: ibelkin@gso.uri.edu long-term variability of fronts to be studied. Satellite-retrieved sea surface temperature (SST) data have been widely used for regional frontal studies since the 1970s (e.g. a global survey of Legeckis, 1978). Earlier studies (see extensive bibliographies in Belkin et al., 2003a,b,c) have been focused mainly on fronts associated with western boundary currents such as the Gulf Stream, Kuroshio, Brazil-Malvinas Confluence, Agulhas Current, and East Australian Current; fronts associated with eastern boundary currents and coastal upwellings have been studied in such areas as the California Current, Peru-Chile Current, Canary Current and Northwest African upwelling, Benguela Current, and Leeuwin Current; in the open ocean, the North Atlantic Subtropical Front and the North Pacific Subtropical Front received most attention, while high-latitude fronts masked by persistent cloudiness have nonetheless been studied in the Nordic Seas and the Southern Ocean. Satellite observations of surface fronts in high- latitude seas are hampered by seasonal ice cover and persistent cloudiness. Nonetheless, several studies have demonstrated the great potential of remote sensing, including infrared imagery, in observing surface manifestations of oceanic phenomena (fronts, eddies, upwelling etc.) such as the Warm Coastal Current in the Chukchi Sea (Ahlnäs and Garrison, 1984), coastal upwelling off St. Lawrence and St. Matthew islands in the Bering Sea (Saitoh et al., 1998), the St. Lawrence Island Polynya (SLIP; Lynch et al., 1997), and spring blooming in the Bering Sea (Maynard and Clark, 1987; Walsh et al., 1997). The above studies, being very important in elucidating physics and geography of individual fronts, didn’t amount however to a regional synthesis, which requires a unifying approach to be consistently applied to a long-term data set of thoroughly calibrated measurements. The present work summarizes the most important results of such a project undertaken at the University of Rhode Island (URI), where advanced algorithms for front detection and cloud screening have been developed earlier, described in the next section. The present work is essentially an exploratory study. The main goal is to describe all the robust (persistent) frontal features noticeable in the data, regardless of the spatial scale of the features, and