James D. Schumacher

Characteristics and variability of the inner front of the southeastern Bering Sea

The inner front of the southeastern Bering Sea shows marked spatial variability in frontal characteristics created by regional differences in forcing mechanisms. Differences in forcing mechanisms (sea ice advance/retreat and storm strength and timing) and early spring water properties result in strong interannual variability in biological, chemical, and physical features near the front. We have developed a simple model based on surface heat flux and water-column mixing to explain the existence of cold belts (Cont. Shelf Res. 19(14) (1999) 1833) associated with such fronts. Hydrography, fluorescence and nutrient observations show that pumping of nutrients into the euphotic zone occurs, and this can prolong primary production at the inner front. The effectiveness of this process depends on two factors: the existence of a reservoir of nutrients in the lower layer on the middle shelf and the occurrence of sufficient wind and tidal energy to mix the water column.

Correlations between seabirds and ocenic fronts around the Pribilof Islands, Alaska

Abstract Located on the extensive continental shelf of the Bering Sea, the Pribilof Islands, Alaska are the site of one of the largest breeding colonies of seabirds in the northern hemisphere. During summer these islands are surrounded by a front that separates vertically homogeneous waters from well stratified waters farther seaward. We studied the front with hydrographic data and the bird distributions with concurrent counts during summer 1977 and spring, summer and fall 1978. Murres (Uria lomvia and U. aalge) sitting on the water aggregated near the front during summer 1977 and probably during summer 1978. Other species, such as northern fulmars (Fulmarus glacialis) and auklets (Aethia pusilla and A. cristatella) were unaffected by the front. We hypothesize that the aggregation of the murres was related to an enhanced availability of their food near the front.

The Alaska coastal current : continuity of transport and forcing

Current moorings were deployed on the continental shelf at 13 locations in the northwest Gulf of Alaska. They measured the Alaska Coastal Current at three positions along the coast during April–October 1991. The strongest currents were in the confined region of Shelikof Strait. Mean daily transport was as large as 2.5×106 m3 s−1. Mean transport over the 6-month period ranged from 0.85×106 m3 s−1 at the easternmost line (off Gore Point) to 0.64×106 m3 s1− at the westernmost section (Shelikof Strait), indicating significant transport to the south of Kodiak Island. The transports were well correlated (r > 0.8) and in phase with each other and were also correlated with wind (r ∼ 0.6). Data suggest that in Shelikof Strait and off Gore Point, baroclinic flow is ∼75% and <40%, respectively, of total transport.

Low-Frequency Current Regimes over the Bering Sea Shelf

Abstract Using direct current measurements made during the period 1975–81, we describe the general circulation over the southeastern Bering Sea and differentiate it by regimes related to depth and forcing mechanisms. Three regimes are present, delineated by water depth (z): the coastal (z ≤ 50 m), the middle shelf (50 < z < 100 m), and the outer shelf (z ≥ 100 m). These are nearly coincident with previously described hydrographic domains. Statistically significant mean flow (∼1 to 10 cm s−1) exists over the outer shelf, generally directed toward the northwest, but with a cross-isobath component. Flow of similar magnitude (1–6 cm s−1) occurs in the coastal regime, paralleling the 50 m isobath in a counterclockwise sense around the shelf. Mean flow in the middle shelf is insignificant. Kinetic energy at frequencies < 0.5 cycle per day (cpd) is greater over the outer shelf than in the other two regimes, suggesting that oceanic forcing is important there but does not affect the remainder of the shelf. Kinetic...

Volume transport in the Alaska Coastal Current

Abstract Nine moorings were deployed in three sections in the Shelikof Strait/Semidi Islands region of the Alaskan continental shelf during the period of August 1984 to July 1985. Analysis of the resulting current and bottom pressure data, together with surface wind, provides a new understanding of transport in the Alaska Coastal Current. Using current observations, mean volume transport through the Shelikof sea valley was computed to be 0.85 × 106 m3 s−1, which is in good agreement with estimates of transport obtained from hydrographic data. Approximately 75% of this flux flowed seaward through the Shelikof sea valley, with the remainder flowing along the Alaska Peninsula. Data showed the expected increase of volume transport concomitant with maximum freshwater discharge in autumn. The greatest monthly mean transport, however, occurred in winter and was related to wind forcing. On time intervals of days, fluctuations in transport were often large (up to 3.0 × 106 m3 s−1), and generally geostrophic (r = 0.79). Some of these fluctuations resulted from convergence of flow caused by the complex interaction of storms with orography. Approximately half of the fluctuations in volume transport were accounted for by the alongshore wind.

Characteristics of an eddy over a continental shelf: Shelikof Strait, Alaska

During May 1990, satellite-tracked buoy, water property, and ichthyoplankton observations were collected over the Shelikof sea valley in the western Gulf of Alaska. These observations revealed the presence of an anticyclonic mesoscale eddy (radius 10–12 km). Characteristics of the eddy included a relatively warm, low-salinity core and high concentrations of walleye pollock larvae. The eddy remained nearly stationary for several weeks before moving westward into shoaler waters. During translation, the eddy maintained its characteristics. Speeds from geopotential topography and direct measurements agreed. Current speeds generally increased with distance from the center of the eddy; maximum speeds (>30 cm s−1) existed near the perimeter. Analysis of water properties indicates little or no exchange of mass occurred with adjacent waters. The lack of dispersion permits us to estimate larval mortality (≤4.7% d−1) based only on biological factors. Eddy formation occurred through baroclinic instability upstream within Shelikof Strait proper.

Observation of a Baroclinic Eddy: An Example of Mesoscale Variability in the Bering Sea

Abstract Drift buoys with shallow (17 m) drogues, released during May 1977 and tracked by satellite, delineated an eddy in the southeastern Bering Sea. Located above complex topography having a depth range of 200 to 3000 m, the eddy had a diameter of about 150 km. Mean rotational speeds ∼50 km from the eddys center were 20 cm s−1, but speeds up to 50 cm s−1 were measured. A CTD survey during July defined the eddy from 200 to 1500 m depth in temperature and salinity distributions, but no hydrographic evidence for the eddy existed at the surface. A geostrophic calculation relative to 1500 m agreed qualitatively with drifter data, but was ∼5 cm s−1 less than mean drifter speeds. Examination of the T-S correlation showed that water masses at the eddys core were the same as those at its periphery, in contrast with a cyclonic ring observed nearby in July 1974. The last drifter left the eddy in October, and a second CTD survey in February 1978 showed that the eddy had either dissipated or moved. An earlier STD...

Wave‐current interaction study in the Gulf of Alaska for detection of eddies by synthetic aperture radar

High resolution ERS-I Synthetic Aperture Radar (SAR) images are used to detect a mesoscale eddy. Such features limit dispersal of pollock larvae and therefore likely influence recruitment of fish in the Gulf of Alaska. During high sea states and high winds, the direct surface signature of the eddy was not clearly visible, but the wave refraction in the eddy area was observed. The rays of the wave field are traced-out directly from the SAR image. The ray pattern gives information on the refraction pattern and on the relative variation of the wave energy along a ray through wave-current interaction. These observations are simulated by a ray-tracing model which incorporates a surface current field associated with the eddy. The numerical results of the model show that the waves are refracted and diverge in the eddy field with energy density decreasing. The model-data comparison for each ray shows the model predictions are in good agreement with the SAR data.

A census of mesoscale eddies in Shelikof Strait, Alaska, during 1989

Over a 5-month period in the spring and summer of 1989, 12 mesoscale eddies were identified crossing a section of moored current meters in the Shelikof sea valley. This represents the first census of mesoscale eddies on the northwestern Gulf of Alaska continental shelf. The majority of eddies (7) were observed in May and June. Anticyclonic eddies outnumbered cyclonic eddies two to one, but all had characteristics similar to eddies previously observed in the region. Analysis of the current meter time series revealed that the mean speeds, volume transport, and the structure of the flow at the present location were similar to current meter and hydrographic measurements during other years in the sea valley. We suggest that 1989 was a typical year in terms of eddy formation as well. The proliferation of mesoscale eddies in this region in spring has an impact on the larval dispersal and eventual recruitment of walleye pollock.

Circulation in Shelikof Strait, Alaska

Abstract Extensive hydrographic surveys were conducted in Shelikof Strait in March and October 1985. The data are used to describe circulation and property distributions and the changes that occurred. The upper layer flows to the southwest throughout the year, but greatest speeds occur in the fall when surface waters are least saline because of a maximum in freshwater discharge. The deep water has its source to the south, and the properties seem to result from vertical mixing of this southern water. Thus Shelikof Strait has an estuarine-like circulation with a northward, deep inflow. Property distribution showed that isolines were usually deepest on the right side of the channel looking to the southwest; greatest baroclinic speeds were often there also. Differential Ekman pumping may contribute to the development of this structure and its changes. Volume transport estimates varied considerably. In October the southwest flow bifurcated, with part continuing along the Alaska Peninsula and the rest exiting t...