Results of sea surface mapping in the Peru upwelling system1

Sea surface maps of temperature, salinity, nitrate, silicate, phosphate, and fluorescence were produced almost daily during a 6-week cruise (PISCO) in the Peru upwelling system in the austral fall, 1969. The maps were produced by a shipboard computer from data gathered with the ship underway. The maps show persistent “plumes” of freshly upwelled water associated with coastal irregularities and prominent bathymetric features. The information on the location, orientation, and extent of these plumes was used to locate stations where primary productivity experiments were carried out. The definition of persistent plumes has led to the development of successful two-dimensional simulation models of the ecosystem. One of the primary difficulties in studies of time-varying mesoscale phenomena in the ocean, such as coastal upwelling, is that large surface gradients and the rapid response of the upper ocean to changes in wind stress make it impossible to synthesize a synoptic picture from discrete stations taken over many days or weeks. More truly synoptic sampling is required. Remote sensing has shown considerable promise for such mapping of temperature and chlorophyll, but the use of a shipboard system has the advantage that many more variables can be determined (Kellcy 1975). We developed such a shipboard system for the study of marine ecosystems and used it during a cruise of the RV Thomas G. Thompson to the upwelling area off the coast of Peru near 15’S in 1969. During ’ Contribution No. 841 from the Department of Oceanography, University of Washington. This research was supported by the International Biological Program (NSF GB-35880X-Upwelling Biome) and the Office of Naval Research (N-0001467-A-0103-0014). The preparation of this paper was supported in part by National Science Foundation GX 33502 [Coastal Upwelling Ecosystems Analysis ( NSF-IDOE ) I. ’ Current address: School of Natural Sciences, San Francisco State University, San Francisco, California 94132. ” Current address: Division of Oceanographic Sciences, Brookhaven National Laboratories, Upton, New York 11973. 4 Curent address: Bigelow Laboratory of Marine Science, West Boothbay Harbor, Mainc 04575. this cruise (PISCO), 20 March-3 May 1969, data were acquired and digitized by a shipboard data system and analyzed by shipboard computer to produce sea surface maps of the concentrations of variables important to the understanding of the ecosystem. Maps of the sea surface, temperaturc, salinity, nitrate, nitrite, silicate, phosphate, and fluorescence were produced on a grid 70 nautical miles ( 130 km) long in the longshore dimension and 15 nautical miles (28 km) wide in the offshore dimension ( Fig. 1). Since much of the station work on the cruise was confined to the daylight hours because sunlight was necessary for the experiments conducted, the night was used for underway mapping of the relevant sea surface properties. Typically about a third of the total grid was covered on a given night. Maps were concentrated in the central portion of the grid, and occasionally the entire grid was covered by continuous steaming for 36 h. With the aid of the shipboard computer, maps from a night’s run could bc plotted in the early morning and used to plan the location of the day’s stations. Thus, the sampling program could bc adapted to conditions measured only a few hours before. We wish to thank L. and R. Olund for their assistance during the cruise and J. Glass and M. Friebertshauser for their assistance in analysis of data. LIMNOLOGY AND OCEANOGRAPHY 784 SEPTEMBER 1975, V. 20( 5) Surface maps of Peru upwelling 785 UNDERWAY SAMPLING DATA FLOW