Long-term measurements of flow near the Aleutian Islands

In summer 1995, the Alaskan Stream at 173.5W was very intense; the peak geostrophic speed was ≈125 cm s−1, and the computed volume transport above 1000 db, referred to 1000 db, was 9 × 106 m3 s−1). Flow north of the central Aleutians was shallow, convoluted and weak (2– 3 × 106 m3 s−1). A sequence of CTD casts across Amukta Pass, spaced irregularly in time during 1993–1996, showed a mean northward (southward) geostrophic transport of 1.0 (0.4) × 106 m3 s−1, for a net flow into the Bering Sea of 0.6 × 106 m3 s−1. The source of this flow was the Alaskan Stream except in 1995, when it was Bering Sea water. Results from two 13-month current moorings west and east of the pass were quite different. To the west, flow was weak and variable and appeared to have a barotropic component. To the east, flow was stronger and unidirectional eastward

Flow along and across the Aleutian Ridge

During a synoptic hydrocast survey in September 1993 near the Aleutian Islands, net northward flow of Alaskan Stream water occurred through deep passes near 180 and 172W. This inflow (≈4 × 106 m3 s−1) was the source of the eastward flow in the Bering Sea north of the islands. The eastward flow, however, was weaker and more convoluted than the stream flow (≈7 × 106 m3 s−1, referred to 1000 db) south of the islands

The recent return of the Alaskan Stream to Near Strait

A hydrocast survey was conducted near the western Aleutian Islands and in the Bering Sea in September 1992. Presence of the Alaskan Stream was indicated by temperatures \u3e 4°C to depths in excess of 200 m. Geopotential topography showed the Alaskan Stream flowing through Near Strait into the Bering Sea, with branches also flowing northward through Amchitka and Buldir passes. Satellite-tracked drifter paths were in general agreement with these features. Transport through Near Strait was ≈5 × 106 m3 s−1. Previous research indicated that the Stream had been absent from the Strait for more than a year. Data from three current moorings (13-mo duration), however, suggested that the Alaskan Stream started flowing through Near Strait in October 1991 and continued into September 1992. This inflow had periods of both steady and variable flow

Summer Transport Estimates of the Kamchatka Current Derived As a Variational Inverse of Hydrophysical and Surface Drifter Data

The quasistationary summer Bering Sea circulation is reconstructed as a variational inverse of the hydrographic and atmospheric climatologies, transport estimates through the Bering Strait, and surface drifter data. Our results indicate the splitting of the Kamchatka Current in the vicinity of the Shirshov Ridge. This branching is in agreement with independent ARGO drifter observations. It was also found, that transport of the Kamchatka Current gradually increases downstream from 14 Sv in the Olyutorsky Gulf to 24 Sv in the Kamchatka Strait, which is twice higher than previous estimates

Summer Transport Estimates of the Kamchatka Current Derived As a Variational Inverse of Hydrophysical and Surface Drifter Data

The quasistationary summer Bering Sea circulation is reconstructed as a variational inverse of the hydrographic and atmospheric climatologies, transport estimates through the Bering Strait, and surface drifter data. Our results indicate the splitting of the Kamchatka Current in the vicinity of the Shirshov Ridge. This branching is in agreement with independent ARGO drifter observations. It was also found, that transport of the Kamchatka Current gradually increases downstream from 14 Sv in the Olyutorsky Gulf to 24 Sv in the Kamchatka Strait, which is twice higher than previous estimates

Tidal energy in the Bering Sea

Tidal harmonics computed from TOPEX/POSEIDON altimetry are assimilated into a barotropic, finite element model of the Bering Sea whose accuracy is evaluated though comparisons with independent bottom pressure gauges. The model is used to estimate energy fluxes through each of the Aleutian Passes and Bering Strait and to construct an energy budget for the major tidal constituents. The finite element model does not conserve mass locally and this is shown to give rise to an additional term in the energy budget whose contribution is significant for the prior model, but which is reduced substantially with the assimilation technique. Though the M2 constituent is estimated to have the largest net energy flux into the Bering Sea at 31.2 GW, the K1 constituent is not far behind at 24.9 GW and the sum for the three largest diurnal constituents is found to be greater than the sum for the largest three semi-diurnals. Samalga and Amutka Passes are found to be the primary conduits for influx of semi-diurnal energy while Amchitka Pass is the primary conduit for diurnal energy. A significant portion of the diurnal energy is seen to exist in the form of continental shelf waves trapped along Bering Sea slopes.The effect of the 18.6-year nodal modulation is estimated and found to cause basin-wide variations of approximately 19% in the net incoming tidal energy flux. Larger variations in the dissipation occur in subregions that are strongly dominated by the diurnal constituents, such as Seguam Pass and south of Cape Navarin. These variations should correlate with tidal mixing and may have important consequences for biological productivity, similar to those previously found for Pacific halibut recruitment (Parker et al., 1995) and shrimp, capelin, herring, cod, and haddock biomass in the Barents Sea (Yndestad, 2004)

Under-ice observations of water column temperature, salinity and spring phytoplankton dynamics: Eastern Bering Sea shelf

The inundation of two moored platforms by sea ice in late winter and early spring of 1995 provided unique time series of water column temperature, salinity, estimated chlorophyll-a, and phytoplankton fluorescence under advancing and retreating sea ice. One platform was located at 72 m in the weakly advective middle shelf regime. Here, chlorophyll-a concentrations began increasing shortly after the arrival of the ice (arch) during the period of weak stratification and continued to increase while wind actively mixed the water column to \u3e60 m. Changes in water column structure and properties resulted from an event of strong advection rather than vertical fluxes. During winter, such advective events can replenish the nutrients required to support the rich blooms that occur over the middle shelf during spring. The advancing ice was associated with the coldest waters and a deep (\u3e50 m ) mixed layer. The ice melt enhanced the two-layer system previously established by advection. A second mooring was located at the 120 misobath on the more advective outer shelf. The ice reached this site on April 6, and chlorophyll-a concentrations increased as the sea ice melted. At the third mooring, located on the shelf farther south beyond the range of ice, the spring bloom began on ~ May 9

Fine-scale variability at 140°W in the Equatorial Pacific

In November-December 1984 we carried out an intensive 12-day upper ocean sampling program on the equator at 140°W as part of the Tropic Heat Experiment. From our observations we constructed hourly averaged profiles of temperature, salinity, σ₁, turbulent kinetic energy dissipation rate, and horizontal velocity. These data were used to examine the correspondence between hydrographic and velocity fields and to compare the measured turbulent dissipations with the calculated Richardson numbers. We found that the core of the Equatorial Undercurrent tracked a density surface (σ₁ = 25.25) on times as short as 1 hour. The variability in both hydrographic and velocity fields was greatest at the semidiurnal frequency. The supertidal energy was not significantly different from the Garrett-Munk mid-latitude level once latitudinal scaling was removed from the Garrett-Munk model parameters. Horizontal velocity spectra were found to be contaminated by displacement of the background shear. Turbulent dissipation was dominated by a dirunal cycle, with high values of dissipation occurring at night above the undercurrent core. Shear and buoyancy frequency, calculated over 12-m vertical scales, were observed to track each other above the core and were dominated by a diurnal period above 40 m and by a semidiurnal period below 40 m. When shear and buoyancy frequency were combined to form a Richardson number, neither diurnal nor semidiurnal cycles were present. Above the undercurrent core, the Richardson numbers were uniformly small (0.3 to 0.6)

Late pleistocene sedimentation history of the Shirshov Ridge, Bering Sea

The analysis of the lithology, grain-size distribution, clay minerals, and geochemistry of Upper Pleistocene sediments from the submarine Shirshov Ridge (Bering Sea) showed that the main source area was the Yukon–Tanana terrane of Central Alaska. The sedimentary materials were transported by the Yukon River through Beringia up to the shelf break, where they were entrained by a strong northwestward-flowing sea current. The lithological data revealed several pulses of ice-rafted debris deposition, roughly synchronous with Heinrich events, and periods of weaker bottom-current intensity. Based on the geochemical results, we distinguished intervals of an increase in paleoproductivity and extension of the oxygen minimum zone. The results suggest that there were three stages of deposition driven by glacioeustatic sea-level fluctuations and glacial cycles in Alaska