US GLOBEC Newsletter 14: LTOP In the NEP

| Physical Oceanography: GLOBEC's LTOP in the NEP by Robert Smith, Michael Kosro, Adriana Huyer and Jack Barth
Also in This Issue .... LTOP in the NEP Zooplankton in OR Coastal Zone El Niño Affects Marine Nekton GLOBEC Studies in the CGOA GoMex Workshop - Summary Cod & Climate Change Program Notes From the Field .... Sea Scallop Fishery Closures
Regular sampling of the marine ecosystem off Oregon under the pilot GLOBEC Northeast Pacific Long-Term Observation Program (LTOP) began in November 1997. The cornerstone of this monitoring effort is a section, extending due west from Newport, Oregon along latitude 44.6 °N for 160 km, with a historical database from 1961. As a result of the prompt detection of a major El Niño in the spring of 1997, and with the help of colleagues, we were able to "jump-start" observations (CTD casts and ADCP profiles) along this section in July 1997. By July 1999, we had made 12 cruises, most of which also included sections off southern Oregon and northern California. A compact overview of our observations of the evolution of El Niño is provided by profiles of the dynamic height of the sea surface extending offshore from Newport along the east-west section (Fig. 1). The variations in dynamic (steric) height represent the variation in sea level due to density differences integrated over the upper 500 m of the water column; the dynamic height has been extrapolated into shallower water by the method of Reid and Mantyla (1976; JGR 81: 3100). The slope of the sea surface across the east-west section is proportional to the north-south (along coast) geostrophic current at the surface; the steeper the slope up (down) toward the coast, the faster the northward (southward) current. FIGURE 1. Offshore profiles of the steric height of the sea surface relative to 500 dbar, off Newport, OR. Heavy lines were observed during the indicated month; thin lines represent the long term average (1961-1971) for the same month. Changes in dynamic height at a location indicate changes in temperature and salinity of the underlying water. The historical data base contains at least six sections for each month during 1962-1971, allowing us to compare our observations (thick lines) with the mean during "normal" times (thin lines). The monthly sea level anomalies measured at the coast (from tide gauge at 42 °N) were above normal from May 1997 to July 1998, with maximum anomalies of 15 and 16 cm in November 1997 and February 1998, in very good agreement with those inferred from our dynamic height computations. FIGURE 2. Shipboard acoustic Doppler measured northward currents over the continental margin in November, 1987 off Newport, OR (44.6 °N). The surface layer off Oregon was already warmer than normal when we began in July 1997; the largest anomalies (> 4 °C) were more than 60 km offshore, but upwelling had been occurring and the southward currents were typical for the upwelling season. In September 1997, the largest anomalies were inshore of 60 km and the surface layer was everywhere warm (> 17 °C); the sea surface sloped up toward the coast indicating northward current. In November 1997 the subsurface waters over the slope were even warmer than in September and the steric height rose steeply toward the shore over 60 km, indicating strong northward flow over the shelf and slope. Sections were also made at several latitudes between Newport and San Francisco in November 1997; these sections all showed the steric height of the sea surface rising steeply toward shore (~20 cm in ~60 km), giving a northward geostrophic current of about 30 cm/sec along the continental margin. The directly measured currents from the acoustic Doppler profiler showed the northward flow extending to at least 500 m depth, and from at least 38.5 °N off California to Newport at 44.6 °N (Fig. 2). This northward flow maintained the relatively warm ocean temperatures through the Winter of 1997-8. FIGURE 3. Temperatures over the continental margin off Newport, OR observed in April 1998 (El Niño), 1999, 1983 (El Niño) and the historical average (1962-1971). El Niño conditions continued in the eastern tropical Pacific into early May 1998 and upwelling favorable winds returned to Oregon in June, but subsurface waters off Oregon remained anomalously warm through September 1998. "Normal" conditions were observed in November 1998, and El Niño was over. The April temperature sections (Fig. 3) tell the story and provide the clearest contrast: April 1998 closely resembled April 1983, the year of the previous major El Niño, and both were considerably warmer than April 1999, which resembles the mean of seven Aprils during 1962-1971. Cruises from July, September and November 1999 will help to answer the question: What will La Niña bring? Finally, one may ask whether the section off Newport really is "connected" to the larger California Current. To elucidate the connection we deployed satellite-tracked drifters (drogue at 15 m) on the January, April, June, August and September 1998 cruises. The drifters were released between 18 and 120 km from the coast; trajectories of the drifters from their release until Spring 1999 are shown in Fig. 4. The eight drifters released off Newport in January 1998 initially moved coherently to the north, with strongest northward flow (60 cm/sec) near the continental shelf break, consistent with the ADCP measurements and geostrophic current estimates during Winter 1997-8. Five of these drifters eventually beached between 47 and 56 °N, two not until September 1998 and January 1999; the other three turned southward in spring and were south of 35 °N in March 1999. FIGURE 4. Trajectories of satellite-tracked WOCE-type drifters deployed off Newport, OR in January 1998 (left panel); April, June, and August 1998 (center panel); and late September 1998 (right panel). Some of the drifters released in April initially moved northward, but from then through September 1998 most drifters moved southward and offshore, delineating the meandering jets of the California Current. By October, some drifters from the January, April, June and August releases were between 35 and 39 °N. The drifters released in late September 1998 initially moved offshore and southward but, with the onset of fall storms, moved back toward the coast and all eventually beached; the last came onto the continental shelf in severe storms during February 1999 near 43.5 °N and moved northward over the mid- and inner-shelf for more than 100 km before beaching near 44.5 and 46.5 °N. The drifter observations in 1998 were consistent with the limited previous drifter studies in 1994 and 1995 (Barth & Smith, 1998; S. Afr. J. Mar. Sci. 19: 5-14). Drifters released over the continental margin off Oregon during the upwelling season (April-September) move offshore and southward into the California Current, but drifters released too late to move south of about 40 °N before fall and winter storms eventually reach the beaches of Oregon, Washington, and British Columbia. For a more complete discussion and additional figures, please check out the Monitoring the Coastal Ocean off Oregon: El Niño and Beyond web site at: http://www.oce.orst.edu/po/coastal.html The GLOBEC-processed CTD files are on our public ftp site. The ftp site address is: ftp.oce.orst.edu (login as anonymous, using your e-mail address as your password), then 'change directory' (cd) to: /dist/globec/ -- the files are under their respective cruise names. The contact is Jane Fleischbein. Last updated: 14 October, 1999 Please address any comments, suggestions or questions to: GLOBEC Webmaster