Alaska Coastal Current Research Articles

Spatial variability in the partial pressures of CO2 in the northern Bering and Chukchi seas

Abstract In the summers of 1999 and 2003, the 1st and 2nd Chinese National Arctic Research Expeditions measured the partial pressure of CO2 in the air and surface waters (pCO2) of the Bering Sea and the western Arctic Ocean. The lowest pCO2 values were found in continental shelf waters, increased values over the Bering Sea shelf slope, and the highest values in the waters of the Bering Abyssal Plain (BAP) and the Canadian Basin. These differences arise from a combination of various source waters, biological uptake, and seasonal warming. The Chukchi Sea was found to be a carbon dioxide sink, a result of the increased open water due to rapid sea-ice melting, high primary production over the shelf and in marginal ice zones (MIZ), and transport of low pCO2 waters from the Bering Sea. As a consequence of differences in inflow water masses, relatively low pCO2 concentrations occurred in the Anadyr waters that dominate the western Bering Strait, and relatively high values in the waters of the Alaskan Coastal Current (ACC) in the eastern strait. The generally lower pCO2 values found in mid-August compared to at the end of July in the Bering Strait region (66–69°N) are attributed to the presence of phytoplankton blooms. In August, higher pCO2 than in July between 68.5 and 69°N along 169°W was associated with higher sea-surface temperatures (SST), possibly as an influence of the ACC. In August in the MIZ, pCO2 was observed to increase along with the temperature, indicating that SST plays an important role when the pack ice melts and recedes.

The effect of oceanographic variability and interspecific competition on juvenile pollock (Theragra chalcogramma) and capelin (Mallotus villosus) distributions on the Gulf of Alaska shelf

Abstract Results from this study suggest that small-scale variability in the Alaska Coastal Current (ACC) and competition between juvenile pollock and capelin are potential mechanisms affecting the distribution and abundance of fishes in the Gulf of Alaska (GOA). Fish distributions in Barnabus Trough, off the east coast of Kodiak Island, were assessed using acoustic data collected with a calibrated echosounder during August–September 2002 and 2004. Trawl hauls were conducted to determine the species composition of the fish making up the acoustic backscatter. Oceanographic data were collected from moorings, conductivity–temperature–depth (CTD) probes, trawl-mounted microbathythermographs (MBT) and expendable bathythermographs (XBT). National Centers for Environmental Prediction (NCEP) reanalysis data were used to assess area winds, and information on regional transport was derived from current meters deployed on moorings north and south of Kodiak Island. The distribution of water-mass properties and fish during 2002 showed variability at the temporal scale of weeks. Juvenile pollock (age-1 and age-2) were initially most abundant in warm, low-salinity water on the inner shelf, whereas capelin were distributed primarily on the outer shelf in cool, high-salinity waters. During a 2-week period juvenile pollock distribution expanded with the offshore expansion of warm, low-salinity water, and capelin abundance in outer-shelf waters decreased. We hypothesize that wind-driven pulsing of the ACC resulted in increased transport of warm, low-salinity water through the study area. In 2004, warm, low-salinity water characterized the inner shelf and cool, high-salinity water was found on the outer shelf. However, the distribution of water-mass properties did not show the weekly scale variability observed in 2002. Area winds were consistently toward the southwest during 2004, such that we would not expect to see the wind-driven pulsing of ACC water that occurred in 2002. Age-1 and age-2 pollock were not observed in Barnabus Trough in 2004. Instead, the midwater acoustic backscatter was composed of capelin mixed with age-0 pollock, and these capelin were not restricted to the outer-shelf waters, but were found primarily in warm, low-salinity inner-shelf waters that had been previously occupied exclusively by age-1 and age-2 pollock. We suggest that this is consistent with inner-shelf waters being preferred foraging habitat for juvenile pollock and capelin. Further study of the mechanisms linking climate change with variability in the ACC is needed, as are studies of the potential for competition between juvenile pollock and capelin.

Idealized three‐dimensional modeling of seasonal variation in the Alaska Coastal Current

Seasonal variation of the buoyancy‐ and downwelling‐wind‐forced Alaska Coastal Current (ACC) and the fate of freshwater contained in it is considered using idealized analytical and numerical models of the ACC formed from a half‐line source of buoyant inflow. The coastal current initially develops two‐dimensionally but becomes three‐dimensional from a balance between coastal influx of buoyancy and its downstream transport, which leads to a coastal current depth limit Hmax = ()1/2, where x is along‐shelf distance, Q is the line source strength for unit length, f is the Coriolis frequency, and g′ is the reduced gravity of the buoyant inflow. This limit is unchanged under downwelling wind stress and is reached on timescales of less than 1 month for the ACC. The coastal current width is roughly constant in x and increases in time at the same rate as the two‐dimensional solution. Imposition of a downwelling wind stress τ results in an approximate balance among wind stress and along‐ and cross‐shelf momentum advection so that the current width is reduced to Ywind ≈ LD ()1/2, where LD is the Rossby radius of deformation, τ is the wind stress and ρ0 is a reference density. Waves/eddying motions eventually grow in the half‐line source coastal current with wavelengths proportional to the coastal current width and with a downstream phase speed slower than the maximum current speed. These features cause an offshore flux of buoyant water, a broader coastal current, and further accumulation of buoyancy on the shelf. Increasing downwelling wind stress reduces the effects of the instabilities. Continual accumulation of buoyancy on the shelf occurs during all model runs but is nearly absent under maximum winter downwelling wind stress. It is suggested that freshwater accumulation on the shelf during spring, summer, and fall may be largely lost downstream during winter.

Freshwater variability and predictability in the Alaska Coastal Current

Abstract We constructed the annual cycle of the baroclinic, geostrophic component of the mass and freshwater transports and freshwater content (FWC) in the Gulf of Alaska's Alaska Coastal Current (ACC) from CTD data to assess the processes controlling the freshwater budget of the ACC. In most months the coastal freshwater discharge is balanced by the along-shelf freshwater transport (FWT). Freshwater is trapped to the ACC because offshore eddy freshwater fluxes appear to buffer onshore Ekman transport of low-salinity surface waters due to the cyclonic winds. The annual average FWT is 880 km 3 yr −1 , which compares favorably with the annually averaged runoff of 760 km 3 yr −1 . The barotropic component of the ACC might transport an additional 380 km 3 yr −1 of freshwater, with the imbalance due to underestimates in runoff, the neglect of freshwater influx from the British Columbian shelf, or the reference salinity (33.8). The mean FWC is 540 km 3 , so that the freshwater flushing time over the 1500 km portion of the ACC considered is ⩽1 yr. Our conclusions are tentative because of uncertainties in discharge estimates, structures of the cross-shelf flow and along-shelf winds, neglect of flow-topography interactions, and the barotropic transport. If our results hold, they imply that the ACC is an important freshwater source for the Bering Sea shelf and Arctic Ocean. Mass transports and FWT in late winter 1998 (El Nino) were nearly twice as large as in winter 1999 (La Nina), and the springtime onset of stratification was earlier in 1998 than in 1999. These differences were due to the winter 1997–98 being subject to anomalously large runoff and strong downwelling over the coastal Northeast Pacific Ocean. The comparison suggests that climate changes leading to warmer, rainier winters will advance the onset of springtime stratification on the inner shelf. November–May anomalies of ACC mass transport, FWT, and FWC are significantly correlated with runoff anomalies and the latter are significantly correlated with anomalies of Ketchikan–Seward atmospheric sea level pressure gradient. These results lead to a 20th century runoff record that permits retrospective examination of the ACC. It appears that the wettest (driest) decade was the 1920s (1900s) during which time ACC transports decreased (increased). While the Pacific Decadal Oscillation is correlated with this “runoff” time series, it explains

Meteorology and oceanography of the Northern Gulf of Alaska

The Gulf of Alaska shelf is dominated by the Alaska Coastal current (ACC), which is forced by along-shore winds and large freshwater runoff. Strong cyclonic winds dominate from fall through spring, and substantial runoff occurs from late spring through fall with annual distributed freshwater discharge greater than that of the Mississippi River. We examine the ACC from Icy Bay to Unimak Pass, a distance of over 1500 km. Over this distance, the ACC is a nearly continuous feature with a marked freshwater core. The annual mean transport, as measured from current meters, is approximately 1.0×10 6 m 3 s −1 along the Kenai Peninsula, with transport decreasing as the ACC travels westward. Even though the coastal GOA is a predominately downwelling system, it supports a productive ecosystem. Macro nutrients from the basin are provided to the coastal system through a number of processes including topographic steering, eddies, upwelling in response to horizontal shear in the barrier jets, and during winter the on-shelf flux in the surface Ekman layer. Micronutrients (e.g., iron) are supplied from mechanisms such as resuspension of shelf sediments and river discharge. While strong seasonal cycles and interannual variability are dominant scales in atmospheric forcing and the oceanic response, there is also forcing on ENSO and decadal time scales.

Interannual variability and sensitivity study of the ocean circulation and thermohaline structure in Prince William Sound, Alaska

The interannual variability and sensitivity of ocean circulation and the thermocline structure of Prince William Sound, Alaska were examined using a 3D circulation model. A 4-year (1995–1998) simulation compared well with field observations of circulation and monthly mean sea surface temperature at NOAA Station 46060. Seasonal circulation regimes were characterized by an anticyclonic gyre in the central sound in January–April, and a strong cyclonic gyre in the central sound in September to December, while summer was the transition period of the two circulation regimes. The size, position and strength of the gyres and thermohaline depth in the central sound showed small interannual variability. Freshwater displayed a very strong seasonal cycle in the sound with minimum in April–May and maximum in October with a spatial distribution of more in the northwest sound and less in the other areas. The simulated freshwater thickness in the whole sound showed up to 20% interannual variability related to wind. The numerical oil spill drift experiments also showed a large interannual variability of possible oil spill trajectories. Sensitivity studies showed the relative importance of each model forcing: (1) wind has more impact on the surface circulation and mixed layer depth. Without wind, the surface current became weaker and the thermocline became shallower; (2) tidal current is a major current in the sound and important to surface and bottom mixing. Without tide, the thermocline depth became shallower; (3) the magnitude of the Alaska coastal current (ACC) inflow determined the outflow current through Montague Strait. Doubled ACC inflow could change the cyclonic circulation pattern in October in case of normal or less ACC inflow into a northward jet with a small accompanying anticyclonic gyre and strongly flush the west sound in addition to the central sound. Also, double ACC inflow would increase the mix-layer depth significantly; and (4) the surface T, S restoring is critical to maintain T, S seasonal cycle and surface circulation patterns. Salinity was the most important factor determining the central sound circulation patterns.

Numerical simulations of the seasonal circulation patterns and thermohaline structures of Prince William Sound, Alaska

AbstractA three‐dimensional, primitive‐equation ocean circulation model was applied to Prince William Sound, Alaska (3D‐PWS circulation model), under forcing of an ocean tide, freshwater runoff, surface heat flux, Alaska Coastal Current (ACC) throughflow (inflow/outflow), and daily (synoptic), spatially varying winds. The 3D structures and seasonal cycles of the circulation patterns, temperature, salinity (density), and mixed layer are examined. Freshwater runoff significantly contributes to the basin‐scale cyclonic circulation, which was not addressed in the previous simulations. Two typical circulation regimes, cyclonic and anticyclonic, characterize the complex flow patterns that depend on the intensities of the ACC thoughflow, freshwater discharge, and the synoptic wind. The spring (April–May) circulation pattern is characterized by a weak (maximum current 0.1 ms−1) anticyclonic flow in the central Sound, while the autumn (September–October) circulation is dominated by a basin‐scale, cyclonic gyre (maximum current 0.2 ms−1) due to the increase of the ACC throughflow and the maximum freshwater influence. During the summer, the circulation includes the cyclonic and anticyclonic gyres. During the winter, the circulation pattern is controlled by the basin‐scale cyclonic gyre and surface drift driven by the strong north‐easterly (south‐westward) wind forcing. The seasonal cycles of temperature (T) and salinity (S) vs. depth compare well with the observations. The simulated spring and autumn surface circulation patterns compare qualitatively well with the towed ADCP (acoustic Doppler current profilers) flow patterns and dynamic height patterns in the central Sound. An application of this model to zooplankton overwintering is discussed.

The role of larval retention and transport features in mortality and potential gene flow of walleye pollock

This paper reviews the physical oceanography in the range of walleye pollock that influences transport and mortality of eggs and larvae, and summarizes the current status of stock structure studies of pollock. A synthesis is presented that links the genetic stock structure and the potential for gene flow due to transport. In the eastern North Pacific Ocean, walleye pollock Theragra chalcogramma consistently spawn in several locations, with the major deposition of planktonic eggs in Shelikof Strait in the Gulf of Alaska and in the eastern Bering Sea, near Unimak Pass. In the Gulf of Alaska, eddies are common features where pollock larvae tend to be concentrated; eddies in the region of Shelikof Strait retain larvae in the region, where they are protected from transport offshore in the Alaska Coastal Current (ACC). The ACC is vigorous in Shelikof Strait (20–50 cm s−1), but in the downstream region along the Alaska Peninsula it flows over the continental shelf and is more sluggish (10 cm s−1). There is equivocal information on the condition and mortality of larvae inside and outside eddies; there may be seasonal and interannual differences in the direct role of eddies on larval mortality. In some years with extremely high storm activity, resulting in a very energetic ACC, eddies are less prevalent and pollock larvae are swept offshore into the swift flowing Alaskan Stream (50–100 cm s−1). In the Alaskan Stream the nutritional condition and growth of larvae is poor, leading to high mortality. Satellite‐tracked drifter trajectories indicate that Alaskan Stream water can enter the Bering Sea through deep Aleutian passes, the nearest (easternmost) being Amutka Pass. Larvae surviving in the Alaskan Stream and carried into the Bering Sea through Amutka Pass would find themselves in the Aleutian Basin, where feeding conditions are poor and where larval mortality has been high. Therefore, geographical barriers (such as the Alaska Peninsula and shallow sills at passes between the Gulf of Alaska and Bering Sea), retention features, and high mortality rates of vagrants are likely to impede gene flow due to larval drift between the Gulf of Alaska and eastern Bering Sea. Recent findings using restriction fragment length polymorphism (RFLP) analysis of mtDNA variation, further supported by differences in nuclear DNA microsatellite loci, indicate distinct genetic stocks in the Bering Sea and Gulf of Alaska. Within the Bering Sea, several geographically separate pollock spawning grounds result in larvae occupying a variety of basin, slope and shelf habitats. Larvae can be concentrated in eddies in slope waters, but on the eastern Bering Sea shelf eddies are uncommon and a persistent mean northwestward flow exists along the 100‐m isobath. Thus, the potential for gene flow through larval drift is much higher. Genetically distinct stocks have not been identified in the eastern Bering Sea, although there is some evidence for their existence.

The plankton of Shelikof Strait, Alaska: standing stock, production, mesoscale variability and their relevance to larval fish survival

ABSTRACTPhysically‐mediated variations in production, standing stock and distribution of plankton have a significant impact on the growth and survival of larval fishes in Shelikof Strait. We integrate descriptions of mechanisms that control the distribution and regional production of planktonic organisms with mechanisms that influence survival of early life history stages of marine fish, especially the locally abundant walleye pollock, Theragra chalcogramma.The timing of the spring phytoplankton bloom is more variable than the regular appearance of pollock larvae, and is affected by variability in the winter to spring storm season transition, stratification in the Alaska Coastal Current (ACC), and cloudiness. The spring bloom occurs first in the ACC, fuelling local production of copepod nauplii, the main prey item of early larval pollock. Shelikof Strait zooplankton standing stock is higher in the ACC than the surrounding Coastal Water (CW) throughout the spring. This is the result of local production as well as regional production which occurs upstream of the strait. Mesoscale features associated with the ACC (fronts, meanders, and eddies) determine the distribution of plankton through physical convergence, and reduced dispersion and transport. Evidence for enhancement of planktonic production in these features is lacking. Thus the ACC plays a strong role in determining plankton production, standing stock, and distribution in the Shelikof region.The strength of the relationship among plankton, planktonic production, larval pollock growth and survival, and fisheries recruitment is variable. The plankton is only one of several key variables that affect eventual recruitment to the pollock fishery.

An eddy‐resolving model of circulation on the western Gulf of Alaska shelf: 1. Model development and sensitivity analyses

An eddy‐resolving primitive equation model was used to simulate circulation on the shelf in the western Gulf of Alaska. This paper describes the development and sensitivity analysis of the model, while a companion paper [Stabeno and Hermann, this issue] demonstrates correspondence with measured currents in specific years. The model is initialized with a salinity field derived from local conductivity‐temperature‐depth surveys and forced with an idealized alongshore wind stress pattern. Buoyancy forcing is achieved by freshening the salinity field along a portion of the coastal wall. The model exhibits many of the patterns evident in observations, including mean flows and mesoscale eddies associated with the Alaska Coastal Current (ACC). Sensitivity studies suggest a strong dependence of the barotropic flux of the ACC on local winds and weak dependence on upstream buoyancy forcing. Both wind and buoyancy forcing significantly affect the generation of eddies. Strong northeasterly winds enhance cross‐shelf flow of the ACC to the west of Shelikof Strait, where isobaths veer southward. Strong buoyancy forcing also favors cross‐shelf flow in that region and enhances the formation of eddies.

Glacial meltwater input to the Alaska Coastal Current: Evidence from oxygen isotope measurements

Results of a study of the oxygen isotopic composition of coastal, pelagic, and fresh waters from the northern Gulf of Alaska region are presented. This study was undertaken to investigate whether isotopic tracers could be of use in determining the important freshwater inputs to the Alaska Coastal Current (ACC) and whether they could confirm the presence of the ACC in coastal waters west of Kodiak Island. The Alaska Coastal Current, the major coastal circulation feature of the northern Gulf of Alaska, can be distinguished from oceanic waters on the basis of its lower salinity at least as far west as Kodiak Island. This study adds significantly to the small amount of oxygen isotopic information available for the waters of this region. The isotopic results suggest that in late summer, glacial meltwater may provide a substantial portion of the total freshwater runoff into the ACC, and that the ACC does extend as far to the west as Unimak Pass.

On the Alaska Coastal Current in the western Gulf of Alaska

Analyses of wind, adjusted sea level, temperature, salinity, and current data show that the Alaska Coastal Current (ACC) flows generally westward from Shelikof Strait to the Shumagin Islands. The seasonal nature of the ACC in the western Gulf of Alaska is similar to that in the northern gulf, but minimum salinity, maximum current speeds, and changes in baroclinic transport are less extreme to the west. Southwest of Shelikof Strait the ACC apparently is bifurcated by the bathymetry. One branch continues flowing along the Alaska Peninsula, and the other flows south. The southward flowing branch may rejoin the westward flowing one in the sea valley just east of the Shumagin Islands.

Sources and dispersal patterns of clay minerals in surface sediments from the continental-shelf areas off Alaska

The Three mineral suites were recognized in the Gulf of Alaska. The glaciomarine sediments of fjords and the eastern shelf consist typically of the primary phyllosilicates, micas, and chlorites, derived locally from argillite, graywacke, and metavolcanics under conditions of mild chemical weathering. More glycol-expandable clay minerals (GECM) in the north Gulf substantiate clay supply to that region primarily from the Yakataga and Poul Creek Formations, and to a lesser extent from the Copper River. The northern Gulf clay-mineral suite is transported to central Prince William Sound and northeast Kodiak Shelf by the Alaska coastal current. In the western Gulf, clay minerals adjacent to the Alaska Peninsula are derived principally from the Susitna River via Cook Inlet, whereas those of the outer shelf presumably have their provenance in Kodiak Island and the Katmai Ash submarine deposit. In the southeast Bering Sea, a progressive northwestward decrease in GECM, from 65% to 11%, is caused by a gradually decreased sediment export from the Alaska Peninsula and the Aleutians, with a possible minor increase in illite from the Kuskokwin River. The Yukon River is the predominant clay source for the north Bering Sea, as signified by an abrupt increase in that area of GECM (20% to 35%) and of kaolinite/chlorite ratios. Clay mineralogy indicates that the central Chukchi Sea is the major repository of the Yukon clays, suggesting that clay-mineral stratigraphic studies have implications for interpretation of the Quaternary paleogeography of “Beringia.” In the Beaufort Sea nearshore, influence of different fluvial inputs and their dispersals by currents are sharply defined. The lack of a definite pattern to the distribution of clay minerals in the outer continental shelf may be due to haphazard transport of clays by ice rafting and/or reworking and redistribution of sediments resulting from ice gouging of the sea substrate. Movement of sediment to the west in the Alaskan portion of the inner shelf of Beaufort Sea is evident. An apparent seaward increase in illite/GECM ratio in the Colville Delta is tentatively ascribed to regeneration of degraded illite-micas by K + fixation in saline waters. An isolated area of sediment rich in expandable clay minerals in the central Beaufort Shelf presumably is relict.

Active geologic processes in Barrow Canyon, northeast Chukchi Sea

Circulation patterns on the shelf and at the shelf break appear to dominate the Barrow Canyon system. The canyon's shelf portion underlies and is maintained by the Alaska Coastal Current (A.C.C.), which flows northeastward along the coast toward the northeast corner of the broad Chukchi Sea. Offshelf and onshelf advective processes are indicated by oceanographic measurements of other workers. These advective processes may play an important role in the production of bedforms that are found near the canyon head as well as in processes of erosion or non-deposition in the deeper canyon itself. Coarse sediments recovered from the canyon axis at 400 to 570 m indicate that there is presently significant flow along the canyon. The canyon hooks left at a point north of Point Barrow where the A.C.C. loses its coastal constriction. The left hook, as well as preferential west-wall erosion, continues down to the abyssal plain of the Canada Basin at 3800 m. A possible explanation for the preferential west-wall erosion along the canyon, at least for the upper few hundred meters, is that the occasional upwelling events, which cause nutrient-rich water to flow along the west wall would in turn cause larger populations of burrowing organisms to live there than on the east wall, and that these organisms cause high rates of bioerosion. This hypothesis assumes that the dominant factor in the canyon's erosion is biological activity, not current velocity. Sedimentary bedforms consisting of waves and furrows are formed in soft mud in a region on the shelf west of the canyon head; their presence there perhaps reflects: (a) the supply of fine suspended sediments delivered by the A.C.C. from sources to the south, probably the Yukon and other rivers draining northwestern Alaska; and (b) the westward transport of these suspended sediments by the prevailing Beaufort Gyre which flows along the outer shelf.

A long range seismic-recording buoy

The transport of Pacific-origin water across the eastern Chukchi Sea during summer is studied using shipboard hydrographic and Acoustic Doppler Current Profiler data from multiple surveys. The study identifies two major transport pathways in this region. The well-known Alaskan Coastal Current (ACC) flows poleward along the Alaska coast, while a more recently discovered and slower current flows northward through the Central Channel between Herald and Hanna Shoals. The two currents separate past Point Hope and appear to merge again in Barrow Canyon via previously unresolved pathways flowing west to east across the Chukchi Sea. The collective flow that transports Pacific water from Bering Strait to Barrow Canyon is termed the Bering to Barrow Current System (BBCS). The coastal branch of the BBCS, which encompasses the ACC, is a weakly baroclinic flow with a maximum speed in excess of 80 cm/s inside Barrow Canyon. The Central Channel branch of the BBCS is considerably weaker, with a maximum flow speed under 20 cm/s. A portion of the Pacific water that flows towards Herald Canyon to the west may also contribute to the BBCS, the amount, but the amount is within the margin of error of our data. The difference in the advective time scales of the various branches allows water masses of different seasonal characteristics to be transported simultaneously by the BBCS. Summer water masses occupy most of the southern and central Chukchi Sea while winter water masses occupy Hanna Shoal and shelfbreak regions in the northern Chukchi Sea during summer. The total BBCS volume transport in summer is measured at four different locations in the eastern Chukchi Sea and found to be generally consistent. The overall mean transport is 0.86 Sv, compared to the summer Bering Strait transport of 1.08 Sv. This study suggests that the BBCS is the main transport pathway delivering Pacific water masses into the western Arctic Basin.