Monitoring the Coastal Ocean off Oregon: El Niño and Beyond

(A. Huyer, R. L. Smith, P. M. Kosro, J. A. Barth, Jane Fleischbein)
College of Oceanic and Atmospheric Sciences
Oregon State University

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Monitoring the Coastal Ocean off Oregon: El Niño and Beyond

As part of the GLOBEC North East Pacific program, oceanographers at Oregon State University are making repeated observations along a section extending 150 km out to sea from Newport, Oregon. This work is funded by NOAA and NSF, through two projects: "Pilot Monitoring Cruise off Oregon for Climate Change Studies in the Eastern North Pacific", and "Supplementary Monitoring of ENSO Signals in the Eastern North Pacific" (see project summaries below). Since stations along the Newport Hydro Line were sampled repeatedly between 1961 and 1969, observations made now can be compared with historical monthly means. The section has been sampled three times in 1997 (on 28-30 July, 19-20 Sept. and 15-16 Nov.), and five times so far in 1998 (1-2 Feb., 4-5 April, 3 June, 6-7 Aug., and 24-25 Sept.). WOCE-type drifters, with socks at 15m, have been deployed on the 1998 cruises and continue to be tracked by ARGOS.

The section in July 1997 showed the classical upwelling structure, with all isotherms > 9° C and isohalines < 33.5 bending up toward the coast, and intersecting the sea surface over the continental shelf (Figure 1a). Temperatures at all depths above 500 m were warmer than normal; offshore surface waters were 2-3° C warmer than normal, while inshore and subsurface waters were about 0.0-1.0 °C warmer (Figure 2a). Monthly mean sea level data from west coast tide gages, low-passed sea level data from Tofino, B. C., and adjusted sea level data from the IGOSS Sea Level Program in the Pacific at the University of Hawaii showed that there had been persistent positive anomalies at Crescent City and Neah Bay since May 1997. The steric height of the sea surface relative to 500 dbar also indicated the sea surface was nearly 10 cm above normal (Figure 3).

In September 1997, the surface layer temperature was over 17° C along the entire section (Figure 1b). This water was of low salinity, indicating that the Columbia River plume had spread over the entire coastal ocean in the absence of significant upwelling. The subsurface water over the continental margin was significantly warmer than usual: 25 nautical miles off the coast, the water at 100 m is normally cooler than 8° C - but in September 1997, water warmer than 8° C extended to the bottom at 300 m. These results suggest that the anomalous conditions off Newport in September were the result of both the regional anomalous weather, unfavorable for upwelling, and the advection of warmer water from the south, both presumably because of El Niño.

In November 1997, the surface layer was still 1-2° C warmer than normal along the entire section (Figure 2a), with offshore surface temperatures >14° C (Figure 1c). Subsurface temperatures over the continental margin were even more anomalous than in September (Figure 2a). For example, the 8° C isotherm intersects the bottom at a depth of 370 m, about 150 m deeper than normal, and the waters near the shelf-break are >1° C warmer than normal down to depths of 400 m. Steric height rose steeply toward shore over an offshore distance of ~80 km (Figure 3). These steep gradients in surface elevation and the steeply sloping temperature contours are indicative of strong northward flow over the slope, whose presence is verified by observations from the ship-borne acoustic doppler current profiler (Figure 4c). Sections at other latitudes (off Coos Bay, Crescent City, and the Russian River) were surprisingly similar.

On 1-2 February 1998, all of the water over the inner half of the shelf was warmer than 12° C (Figure 1d), about 2.5° C warmer than the February average (Figure 2a). As is usual because of winter runoff from numerous coastal streams and rivers, waters over the inner shelf were strongly stratified in salinity (Figure 1d): at the most inshore station (over the 30 m isobath, 3 km from shore) the surface salinity was less than 30 psu while the bottom salinity was about 31.5 psu. Over and beyond the outer shelf, the surface mixed layer was much deeper than normal, and both temperature and salinity were nearly uniform at about 12° C and 32.8 psu between the sea surface and the bottom at 130 m (Figure 1d). Sea surface temperature decreased with distance from shore, from 12.5° C inshore to 10.3° C at a location 150 km from the coast; these offshore waters were about 1° C warmer than the February average. The surface mixed layer was about 80 m deep offshore, and even deeper over the shelf break. The downward slope of the 6-10° C isotherms over the upper continental slope was not as steep as it had been in November, but it extended farther offshore (to 150 km from the coast). Similarly, the upward gradient in steric height was not as steep and extended farther from shore (Figure 3). The sloping isotherms and steric height are indicative of continued strong northward flow over the shelf, which was observed directly both by the shipboard ADCP and by satellite-tracked drifters (Figure 4,5). All drifters deployed on this cruise (Figure 5a, * indicates deployment position, + indicates last reported position) initially went northward and stayed in coastal waters; only one of the drifters was south of its launch position by early June.

On 5 April 1998, isotherms, isohalines, and steric height were all approximately flat (Figures 1e, 3), though temperatures in the upper 100 m remained nearly 2° C above normal (Figure 2b), similar to those observed on 18 April 1983. Local winds had been weakly favorable for upwelling for several days preceding this section. Low-passed sea level at Tofino had fallen abruptly to near-normal values in early March, and the steric height off Newport on 5 April was only about 5 cm above normal (Figure 3); temperature anomalies at depths below 150 m were <0.5° C. Monthly mean sea levels at Neah Bay and Crescent City remained near normal in April and May. Drifters were launched between Eureka CA and Newport OR; all but one was south of its launch position by early summer and several have moved far offshore (Figure 5b).

The 3 June 1998 section showed isotherms and isohalines sloping up toward the coast less steeply than normal (Figure 1f). Water over the inner shelf was substantially warmer (by 1-3° C) and fresher than normal, consistent with very weak upwelling in May. Beyond the shelf, temperatures continued to be significantly above normal (by >1° C) in the 30-150 m layer between the seasonal and permanent haloclines (at 32.5 and 33.8 psu). Surface salinities over the outer shelf were above normal, probably because of weaker-than-normal southward advection of Columbia River waters. Three drifters were launched; all are moving southward and offshore (Figure 5c).

By June 1998, sea surface temperatures in the eastern equatorial Pacific were returning to normal and sea level at Callao, Peru, was already below normal. Though sea surface temperatures off Oregon and California remained a few degrees higher than normal, seasonal upwelling was apparently established.

By early August 1998, the classical summer upwelling regime was fully established off Newport (figure1g), with subsurface isotherms and isohalines sloping upward over the shelf, surface temperature decreasing (and surface salinity increasing) monotonically toward shore across the continental margin, and the core of the Columbia River Plume (S<31.0 psu) lying > 100 km offshore. Temperature anomalies (Figure 2b) were generally small (<1.5° C) and probably insignificant, and the steric height was only a few centimeters above the historical average (Figure 3). The only apparently anomalous feature remaining was a northward countercurrent or undercurrent whose core was about 100 km offshore, detached from the continental margin (Figure 5). The same or a similar poleward current was observed off Coos Bay, Eureka and along the CODE Central Line; off Crescent City, the poleward undercurrent lay adjacent to the shelf-break as expected. One of the six satellite-tracked drifters deployed on the Newport Line was initially caught in this poleward current, travelling northward for several days before turning offshore. The other five drifters all migrated generally equatorward and offshore: of these the drifter released at NH-25 travelled the most rapidly: about 500 km in the 40-day period beginning 7 August (Figure 5d).

Figure 1. Contemporary and historical-average temperature and salinity distributions along the Newport Hydro Line at 44° 40’N (a) for 28-30 July 1997 and July 1962-69, (b) 19-20 Sept. 1997 and Sept. 1962-69, (c) 15-16 Nov. 1997 and Nov. 1963-69, (d), 1-2 Feb. 1998 and Feb. 1962-69, (e) 5 April 1998 and April 1962-69, (f) 3 June 1998 and June 1962-69, (g) 6-7 August 1998 and August 1961-71, and (h) 24-25 Sept. 1998 and Sept. 1962-69.

Figure 1: (!Click on the thumbnail to see the complete version of the Figure!)

Figure 2. Distributions of temperature anomalies from the historical average (1961-1971 for August, 1962-1969 for all other months) along the Newport Hydro Line at 44° 40’N (a) July, Sept., Nov. 1997, and Feb. 1998, (b) April, June, August and September 1998.

Figure 2: (!Click on the thumbnail to see a bigger version of the Figures!)

Figure 3. Offshore profiles of the steric height of the sea surface (relative to 500 dbar), calculated from historical-average (left) and contemporary (right) density data along the Newport Hydro Line. The fill-color of the contemporary profiles indicates whether the section was during (red) or after (green) the 1997-8 El Niño.

Figure 3: (!Click on the thumbnail to see a bigger version of the Figure!)

Figure 4. Distributions of alongshore current (positive northward) measured with the ADCP on the Newport Hydro Line (a) 28-30 July 1997, (b) 19-20 Sept. 1997, (c) 15-16 Nov. 1997, (d) 1-2 Feb. 1998, (e) 5 April 1998, (f) 6-7 August 1998, and (h) 24-25 Sept. 1998.

Figure 4: (!Click on the thumbnail to see the complete version of the Figure!)

Figure 5.Trajectories of satellite-tracked drifters deployed during the project: a) 31 Jan. to 11 Oct. 1998, (b) 3 April to 11 Oct. 1998, (c) 3 June to 11 Oct 1998, (d) 7 Aug. to 11 Oct. 1998, (e) 24 Sept. to 11 Oct. 1998, (f) Summary of drifters deployed along Newport Hydro Line, 31 Jan. to 11 Oct. 1998.

Figure 5: (!Click on the thumbnail to see a bigger version of the Figures!)

 

Pilot Monitoring off Oregon for Climate Change Studies
in the Eastern North Pacific

(PI: R. L. Smith, A. Huyer, P. A. Wheeler, W. T. Peterson, P. M. Kosro, J. A. Barth)

Project Summary

To understand the effects of climate variability on marine life in the eastern North Pacific requires that we know what the oceanic environment is, how it is changing, and what the oceanic environment was in the past. There is evidence that the ecosystems over the continental margins of the eastern North Pacific have changed since a "climate shift" occurred about 1976. It is obvious to fishermen, and newspaper reporters, that salmon production is different since 1976, and that interannual changes in the coastal ocean affect the productivity and survival of biota. The limited hydrographic data obtained off Oregon in the notoriously anomalous years of 1983 and 1992 show that the subsurface oceanic conditions off central Oregon were unique when compared with data from the 1960s; no years in the 1960s had as warm temperatures at depth (50 to 200 m) as those in 1983 and 1992. Seasonal monitoring had been conducted for the decade 1961-71, but with very limited hydrographic data since 1976, we aren’t able to say whether the mean conditions for non-anomalous years since 1976, i.e., since the "climate shift", are different than earlier years. In addition, the regulation of the Columbia has been a major anthropogenic change from pre-1970 conditions and has had a major effect on salmon; how the change is manifested in the oceanic environment is uncertain since the plume region has not been studied systematically since the 1960s. Without monitoring we will not be able to say whether future process studies are representative of past, or "typical", oceanic conditions. Fortunately, a good data base exists for the 1960s. Comparable measurements are needed now.

What occurs over the continental margin off Oregon has an impact on the oceanic conditions off California. Drifters launched in spring and summer over the Oregon continental shelf populate the California Current region off California while those launched far offshore in the West Wind Drift remain seaward of the continental margin off Oregon and California. These observations are consistent with satellite altimetry and model simulations. It is likely that the summer oceanic conditions off central California are more affected by phenomena over the continental margin from Oregon to Vancouver Island than by the transport of the West Wind Drift. The covariance of conditions over the continental margin in the California Current System and those in the Coastal Gulf of Alaska are likely the result of the covariance in the large-scale meteorological forcing, i.e., the relative strength and positions of the North Pacific High and the Aleutian Low, which determine the winds that directly affect both coastal regions.

This research project initiates a pilot monitoring program along two transects off the central Oregon coast (44.6° and 43.2°N) where regular hydrographic sampling programs have existed in the past. We plan five cruises per year to monitor the hydrography, nutrients, chlorophyll, and zooplankton species composition and abundance at biologically-important times of year: Winter (Dec-Feb) when winds are normally favorable for downwelling and shelf currents are normally northward, chlorophyll is low, and the zooplankton Calanus target species are in diapause or just emerging from diapause and have not begun their seasonal population increase; Early Spring (March/April), at or soon after the spring transition that marks the seasonal onset of upwelling, and before juvenile salmon enter the ocean; Late Spring (May/June), when the southward flow is fully established, the phytoplankton is likely to be in full bloom, euphasiid abundances normally begin their seasonal increase and salmonids are entering the ocean; Summer (July/August), near the height of the upwelling season and the peak of copepod abundance; and Autumn (Sept/Oct), when seasonal heating has ended, coastal convergence has begun, coastal copepods are still abundant, and Calanus species are preparing for diapause.

This monitoring will provide both a basis for retrospective analysis of coastal ocean conditions in the Northeast Pacific and the temporal context for subsequent process studies, i.e., enable the variability of physical and biological processes observed for only one or two years to be related to the phase of the ENSO cycle. The monitoring off central Oregon will allow separation of local from large-scale or remote forcing when the data is combined with satellite data, gridded meteorological fields, and similar oceanographic measurements made off Vancouver Island in the Canadian GLOBEC program near 49°N, and monitoring programs "downstream" off California.

 

 

Supplementary Monitoring of ENSO Signals in the Eastern North Pacific

(PI: R. L. Smith, A. Huyer, P. M. Kosro, P. A. Wheeler)

Project Summary

To understand the effects of climate variability on marine life in the eastern North Pacific requires that we know what the oceanic environment is, how it is changing, and what the oceanic environment was in the past. In recent weeks and months there has been mounting evidence of the onset of a major El Niño in the eastern equatorial Pacific Ocean. The relatively scarce observations from past El Niño episodes show that the coastal ocean along the west coast of North America can be significantly affected, manifesting higher sea surface temperatures and coastal sea levels, lower primary and secondary productivity, a deeper thermocline, and enhanced poleward flow over the shelf. However, the scant evidence, from different locations at different times, also suggests that the extent, intensity and nature of the effects may differ among different episodes. The present onset of El Niño, combined with the beginning of the GLOBEC Northeast Pacific Pilot Monitoring effort, provides a unique opportunity to gather a more complete data set. Under the CoOP-GLOBEC Program, we have already been funded to sample the hydrography, nutrients, chlorophyll and zooplankton distributions along two cross-margin sections off Oregon five times in 1998. In this supplementary program, we are initiating the Oregon monitoring sooner, and extending the coverage to include waters off Northern California.

This program initiates the GLOBEC pilot monitoring program along two transects off the central Oregon coast (44.6º and 43.2ºN) in November 1997, several months earlier than planned under NOAA funding, and extends the monitoring in November 1997, February and August 1998 to include northern California sections (41.9ºN and 38.7ºN) where regular hydrographic sampling programs have existed in the past two decades. This work augments five cruises under NOAA funding in 1998. These cruises are monitoring the hydrography, subsurface currents derived from ADCP measurements, nutrients, chlorophyll, and zooplankton species composition and abundance. If the present ENSO event becomes a full El Niño, occupations of these lines in November 1997, February and August 1998 will help determine the extent, duration, and evolution of the anomalous conditions.

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