John Bane Department of Marine Sciences, University of North Carolina at Chapel Hill
Twenty-seven flights with an instrumented aircraft were made to observe the structure and evolution of the ocean and lower atmosphere over the Oregon continental margin during the COAST summer 2001 field program. Flights, which began in mid-May and continued through August, were scheduled to coordinate with and extend measurements made by the COAST ship and moored instrumentation efforts. The aircraft repeatedly measured sea surface temperature, oceanic subsurface temperature down to 500m, upper-ocean color, and atmospheric wind, temperature, humidity and pressure.
Atmospheric structure varied throughout the summer on periods ranging from diurnal to several days (the atmospheric synoptic scale), and an atmospheric temperature inversion typically, though not always, developed during episodes of northerly winds. An inversion rarely accompanied southerly winds. The main contributors to the surface wind field in the COAST region were waves in the jet stream, the relative positions of the North Pacific high pressure anticyclone and transient low pressure systems that passed through the Gulf of Alaska towards the northwestern U.S., and the strength of the thermal trough inland over Oregon and northern California. One remnant tropical cyclone and one coastally trapped southerly surge added spice to the variable atmospheric conditions during COAST.
The principal oceanic response to atmospheric forcing was the onset of coastal upwelling during sustained northerly wind events. The short time series of aircraft-measured sea surface temperatures shown in Figure 1 demonstrates the spin-up of an upwelling event driven by a northerly wind episode in late July. Winds during the three days prior to July 18 were southerly and weak, with speeds measured at the COAST meteorological buoy (45.0N, 124.1W) averaging less than 3 m/s. As a result, the ocean surface temperature field on the 18th was warm except for a small area of cool, nearshore water that remained from an earlier upwelling event. Northerly winds with speeds varying between 1 and 8 m/s occurred between July 18 and 21, and persistent northerlies at speeds exceeding 6 m/s were measured between July 21 and 24. The development of two upwelling centers in response to these winds is apparent in Figure 1, one north of Newport (from about 44.5 to 45.1N) and one extending south from 44.3N. As the northern upwelling center evolved, its southward-flowing coastal upwelling jet separated from the coastline and continued towards the southwest due to topographic steering by Stonewall and Heceta Banks. The chlorophyll field on July 24, as indicated by aircraft-sensed upper-ocean color data (Figure 2), generally followed the cool water patterns. Small, nearshore regions of elevated chlorophyll concentrations were also seen, and these were related to terrestrial effects such as outflows from coastal rivers.
The persistence of upwelled conditions for a number of days after the demise of northerlies (and on occasion the change to southerlies) was observed in ocean temperature and color fields. The nearshore upwelling band and separated coastal upwelling jet over Heceta Bank were clearly delineated in the oceanic temperature field after such wind changes.
The extensive, high quality data sets gathered from the COAST ships, aircraft and instrument arrays will reveal further details of this complex coastal system as our analyses continue to progress.
Figure 1. Ocean surface temperature field in the COAST study region as measured by aircraft on three days in July, 2001. The evolution of two upwelling centers is evident. The cool water band extending southwestward from the coast near Newport, Oregon (NWP) on July 24 is upwelled water flowing in a coastal upwelling jet that has separated from the coastline. The arrow in the lower right of each panel shows the buoy-measured wind stress averaged over the three-day period immediately preceding that aircraft flight.
Figure 2. Upper-ocean chlorophyll content on July 24, as determined by the aircraft's hyperspectral color radiometers. The false-color scale used here shows the relative levels of the upward radiance wavelength ratio 510nm/555nm, which may be taken as an indicator of chlorophyll concentration. Note the high chlorophyll levels in the area of the separated coastal upwelling jet and in the cool nearshore regions. One exception to this is immediately offshore of Newport, where the cool water is likely so recently upwelled that phytoplankton levels have not had sufficient time to develop.
Acknowledgments: We are grateful for the support provided for this study by the National Science Foundation through grant number OCE-9907919 to the University of North Carolina at Chapel Hill. I thank Sara Haines and Melanie Meaux for their continuing, superb efforts in data collection, management and analysis. Meredith Sessions was instrumental in the success of this flight program.