Seasonal climatology of hydrographic conditions in the upwelling region off northern Chile

Over 30 years of hydrographic data from the northern Chile (18 degreesS-24 degreesS) upwelling region are used to calculate the surface and subsurface seasonal climatology extending 400 km offshore. The data are interpolated to a grid with sufficient spatial resolution to preserve cross-shelf gradie nts and then presented as means within four seasons: austral winter (July-S eptember), spring (October-December), summer (January-March), and fall (Apr il-June). Climatological monthly wind forcing, surface temperature, and sea level from three coastal stations indicate equatorward (upwelling favorabl e) winds throughout the year, weakest in the north. Seasonal maximum alongs hore wind stress is in late spring and summer (December-March). Major water masses of the region are identified in climatological T-S plots and their sources and implied circulation discussed. Surface fields and vertical tran sects of temperature and salinity confirm that upwelling occurs year-round, strongest in summer and weakest in winter, bringing relatively fresh water to the surface nearshore. Surface geostrophic flow nearshore is equatorwar d throughout the year. During summer, an anticyclonic circulation feature i n the north which extends to at least 200 m depth is evident in geopotentia l anomaly and in both temperature and geopotential variance fields. Subsurf ace fields indicate generally poleward flow throughout the year, strongest in an undercurrent near the coast. This undercurrent is strongest in summer and most persistent and organized in the south (south of 21 degreesS), A s ubsurface oxygen minimum, centered at similar to 250 m, is strongest at low er latitudes. Low-salinity subsurface water intrudes into the study area ne ar 100 m, predominantly in offshore regions, strongest during summer and fa ll and in the southernmost portion of the region. The climatological fields are compared to features off Baja within the somewhat analogous California Current and to measurements from higher latitudes within the Chile-Peru Cu rrent system.