Jane Fleischbein

Currents and water masses of the Coastal Transition Zone off northern California, June to August 1988

In summer 1988, we made repeated mesoscale surveys of a grid extending 200 km offshore between 37°N and 39°N in the coastal transition zone off northern California, obtaining continuous acoustic Doppler current profiler data and conductivity-temperature-depth data at standard stations 25 km apart on alongshore sections 40 km apart. All surveys showed a baroclinic equatorward jet, with core velocities of >50 cm s−1 at the surface decreasing to about 10 cm s−1 at 200 m, a width of 50–75 km, and a baroclinic transport of about 4 Sv. The core of the jet lay between the 8.6 and 9.4 m2 s−2 contours of geopotential anomaly (relative to 500 dbar). Three current meter moorings, deployed at 25-km separation across the jet at the beginning of the survey sequence, provided time-series of the velocity; throughout the 37-day deployment, at least one mooring was within the core defined by the 8.6 and 9.4 m2 s−2 contours. The jet flowed southwestward across the grid from late June until mid-July 1988, when the jet axis moved offshore in the north and onshore in the southern portion of the grid. Temperature-salinity analysis shows that jet waters can be distinguished from both the freshly upwelled coastal waters and the offshore waters. Isopycnal maps indicate alongshore advection of relatively fresh, cool water from farther north, as well as small-scale patchiness not resolved by our survey grid. The baroclinic jet observed here may be continuous with the core of the California Current off central California. The later surveys clearly showed a poleward-flowing undercurrent adjacent to the continental slope, with core velocities up to 20 cm s−1 at depths of 150–250 m. Its baroclinic transport (relative to 500 dbar) increased from 1.0 Sv between late June and early August 1988. Within the survey grid, there was a definite onshore gradient in the characteristics of North Pacific Intermediate Water. The subsurface waters adjacent to the continental margin were warmer and more saline than those offshore, indicating net northward advection by the California Undercurrent over the inshore 100 km and equatorward advection farther from shore.

Cold halocline, increased nutrients and higher chlorophyll off Oregon in 2002

Received 24 March 2003; accepted 18 June 2003; published 31 July 2003. [1] Observed changes in the nutrient levels in the halocline of the California Current during 2002 indicated a natural eutrophication that was accompanied by increased chlorophyll and oxygen in surface water. Decreased oxygen in the lower water column over the shelf indicated that much of the phytoplankton production was respired rather than passedontohighertrophiclevels.In2002thehaloclinewater was >1� C colder than usual and 0.5� C colder than any previous observation. Four transect lines off the coast of Oregonshowa50%increaseinnitrate,phosphateandsilicate at 33 psu in 2002 compared to 1998–2001. The increase in nutrients resulted in a 2-fold increase in chlorophyll standing stocks during the summer of 2002 compared with the preceding four years. A significant portion of the increased production was subsequently respired resulting in low oxygen water over the shelf. INDEX TERMS: 4215 Oceanography: General: Climate and interannual variability (3309); 4845 Oceanography: Biological and Chemical: Nutrients and nutrient cycling; 4283 Oceanography: General: Water masses. Citation: Wheeler, P. A., A. Huyer, and J. Fleischbein, Cold halocline, increased nutrients and higher chlorophyll off Oregon in 2002, Geophys. Res. Lett., 30(15), 8021, doi:10.1029/ 2003GL017395, 2003.

The coastal ocean off Oregon and northern California during the 1997-8 El Nino

Abstract The evolution and decay of El Nino 1997–8 was observed in coastal waters off Oregon in a sequence of cruises along 44.6°N from the coast to more than 150 km offshore. Hydrographic observations were made during eleven cruises between July 1997 and April 1999 at stations on the Newport Hydrographic Line, which had been occupied regularly from 1961 to 1971. The data from the earlier decade provide a basis for defining ‘normal’ conditions and allow comparisons with the recent El Nino in terms of T, S, spiciness and geostrophic velocity. Independent of El Nino, the ocean in July 1997 was already anomalously warm offshore of 50 km and above 100 m. By September 1997 there were unambiguous indications of El Nino: isotherms and isohalines sloped down toward the coast indicating poleward flow over shelf and slope, and anomalously spicy water was present at the shelf-break. In November 1997 and February 1998 shelf-break waters were even warmer, and there was strong poleward flow inshore of 100 km, extending to depths greater than 200 m. The April 1998 section closely resembled that of April 1983 (another El Nino year) but by June 1998 the anomalies were mostly gone. November 1998 was near normal and the sections from subsequent cruises resemble the mean sections from 1961–1971. Four cruises between November 1997 and November 1998 included sampling at several latitudes between 38° and 45°N. As expected, these sections show significant alongshore gradients, but also a surprising degree of homogeneity in the anomalous features associated with El Nino (in the temperature, salinity, spiciness and geostrophic velocity fields). The anomalous signature of El Nino was stronger at its winter peak in 1998 than in 1983, but the signature in the temperature and spiciness fields, and in coastal sea level, did not persist as long as in 1983. By April 1999, the coastal ocean from 38°N to 45°N was significantly colder than it had been in April 1984.

Two coastal upwelling domains in the northern California Current system

A pair of hydrographic sections, one north and one south of Cape Blanco at 42.9N, was sampled in five summers (1998‐2000 and 2002‐2003). The NH line at 44.6N lies about 130 km south of the Columbia River, and spans a relatively wide shelf off Newport, Oregon. The CR line at 41.9N off Crescent City, California, lies 300 km farther south and spans a narrower shelf. Summer winds are predominantly southward in both locations but the southward winds are stronger on the CR line. Sampling included CTD/rosette casts (to measure temperature, salinity, dissolved oxygen, nutrients, chlorophyll), zooplankton net tows and continuous operation of an Acoustic Doppler Current Profiler. We summarize and compare July-August observations from the two locations. We find significant summer-season differences in the coastal upwelling domains north and south of Cape Blanco. Compared to the domain off Newport, the domain off Crescent City has a more saline, cooler, denser and thicker surface mixed layer, a wider coastal zone inshore of the upwelling front and jet, higher nutrient concentrations in the photic zone and higher phytoplankton biomass. The southward coastal jet lies near the coast (about 20‐30 km offshore, over the shelf) on the NH line, but far from shore (about 120 km) on the CR line; a weak secondary jet lies near the shelf-break (35 km from shore) off Crescent City. Phytoplankton tend to be light-limited on the CR line and nutrientlimited on the NH line. Copepod biomass is high (15 mg C m 3 ) inshore of the mid-shelf on both NH and CR lines, and is also high in the core of the coastal jet off Crescent City. The CR line shows evidence of deep chlorophyll pockets that have been subducted from the surface layer. We attribute these significant differences to stronger mean southward wind stress over the southern domain, to strong small-scale wind stress curl in the lee of Cape Blanco, and to the reduced influence of the Columbia River discharge in this region.

The coastal ocean off Oregon from 1961 to 2000: is there evidence of climate change or only of Los Niños?

Abstract A zonal hydrographic section along 44.65°N, from the coast of Oregon to 300 km offshore, was occupied regularly (at least seasonally) from 1961 to 1971 and then sporadically until recently. Regular monitoring of this section to 160 km offshore resumed in July 1997 as part of the GLOBEC Long Term Observational Program; the recent data provide observations in Oregon coastal waters of El Nino 1997–98 and La Nina conditions that followed. The complete seasonal data from the decade 1961–1971 provide a basis for comparison with the recent temperature and salinity sections, steric height profiles, geostrophic velocity, and water mass characteristics. These data, and sporadic observations in intervening years, allow us to compare conditions during several ENSO events with the recent event and to search for evidence of climate change. The PFEL Coastal Upwelling Index, sea level from the University of Hawaii Sea Level Center, the Multivariate ENSO Index (MEI), the Pacific Decadal Oscillation (PDO), and outflow from the Columbia River are used to distinguish local and remote causes of variability in physical oceanographic conditions off Oregon. The sequence of El Nino/La Nina/El Nino in 1963–66, during a cool phase of PDO, provides a comparison to El Nino/La Nina of 1997–2000. El Nino in 1982–83 and 1997–98, during a warm phase of PDO, caused the largest oceanographic anomalies in the 40 years. The comparison indicates warming of the coastal ocean off Oregon and suggests a modulation of ENSO effects by PDO. Such modulation would mask evidence for secular climate change in our 40-year oceanographic data series.