Hiroji Onishi

On Seasonal and Year to Year Variation in Flow of the Alaskan Stream in the Central North Pacific

AbstractThe Alaskan Stream is the westward boundary current of the North Pacific subarctic gyre. In the central region of the North Pacific, the Alaskan Stream serves as a connection between the Alaskan gyre, Western subarctic gyre and Bering Sea gyre. Its volume transport is very important in estimating the magnitude of the subarctic circulation in the North Pacific. In order to clarify its seasonal and interannual variation, we conducted observations along a north-south section at 180° during June from 1990 to 1997. Moorings were deployed from 1995 to 1997. Hydrographic casts were made at intervals of ∼37 km to a depth of 3000 m. Moorings were set between CTD stations, with Moor1 (Moor2) at the center (southern edge) of the Alaskan Stream. Geostrophic volume transport (referred to 3000 m) revealed large interannual variability in the Alaskan Stream. Average volume transport over the 8 years was 27.5 × 106 m3s-1 with a standard deviation of 6.5 × 106 m3s-1. Maximum transport was 41.0 × 106 m3s-1 (1997) and minimum was 21.7 × 106 m3s-1 (1995). Stable westward flows were observed at Moor1 1500 m (259°, 11.7 cm s-1) and 3000 m (240°, 3.7 cm s-1, 1996–1997 year average). The ratio of eddy to mean kinetic energy (KE′/

Spatial and Temporal Variability in a Vertical Section Across the Alaskan Stream and Subarctic Current

\overline {KE}

Seabird densities and species and hydrographic features across Amchitka Pass, Aleutian Islands

) was very small (<0.6) throughout the year. A relatively weak and unstable westward flow was observed at Moor2 at 3000 m depth. Conversely, the average flow direction at Moor2 5000 m was eastward.

Evaluation of the fall rates of the present and developmental XCTDs

Abstract Anticyclonic eddies propagating southwestward in the Alaskan Stream (AS) were investigated through analysis of altimetry data from satellite observations during 1992–2006 and hydrographic data from profiling float observations during 2001–06. Fifteen long-lived eddies were identified and categorized based on their area of first appearance. Three eddies were present at the beginning of the satellite observations; another three formed in the eastern Gulf of Alaska off Sitka, Alaska; and four were first detected at the head of the Gulf of Alaska near Yakutat, Alaska. The other five eddies formed along the AS between 157° and 169°W, and were named AS eddies. While the eddies that formed in the Gulf of Alaska mainly decayed before exiting the Gulf of Alaska, the AS eddies mostly crossed the 180° meridian and reached the western subarctic gyre. Four of five AS eddies formed under negative or weakly positive wind stress curls, which possibly caused AS separation from the coast. Comparison of eddy propag...

Seasonal and interannual variability of temperature inversions in the subarctic North Pacific

In the central North Pacific Subarctic Gyre, CTD hydrographic measurements were carried out yearly in late June from 1990 to 1998 at 9 stations along 180° meridian from 48°N to 51.2°N. Vertical sections of 9-year means, anomalies for each year and others of potential temperature, salinity, potential density and geostrophic velocity (referred to 3000 m) were calculated based on this data set. Empirical Orthogonal Function (EOF) analysis was adopted in the investigation of spatial characteristics and its temporal variation in vertical sections. The spatial distribution of the 1st mode EOF of velocity shows the westward Alaskan Stream and the eastward Subarctic Current. This mode explains 37.6% of the total variance. Two positive maxims appear in its amplitude in 1991 and 1997, which is similar to the variation in volume transport of the eastward Subarctic Current. These variations are closely related to the vertical movement of Ridge Domain deep water.

Impact of Alaskan Stream eddies on chlorophyll distribution in the North Pacific

Dalls porpoise (Phocoenoides dalli) is a small toothed cetacean, widely inhabiting the North Pacific Ocean and adjacent seas, between about 30 and 62°N; however, only limited studies of its ecology have been made in nearshore areas. A cetacean sighting survey lasting 60 days was conducted during the 2012 summer cruise of the T/S Oshoro Maru (Hokkaido University, Japan) in the North Pacific Ocean and Bering Sea. Based on this data, the distribution of Dalls porpoises and the factors controlling it in the pelagic habitat were investigated. A total of 808 individual Dalls porpoises in 166 groups were sighted during a total of 469.6 hr and 4946.6 nm observations. The cruise consisted of three legs and the average porpoise group size was significantly larger in Leg 1. The sightings were concentrated at water depths of less than 1000 m and near eastern Aleutian passes. Sighting clusters were found on the 200 m isobath of the southeastern Bering Sea continental slope. There was a peak in sightings where the sea surface temperature (SST) was relatively cold, between 5 and 7°C. Although similar track routes were taken in Leg 1 and Leg 3, the number of sightings per unit effort was larger in Leg 1. This difference may have arisen from the significant rise in SST as the season progressed. Relatively large group size found in this study might relate with prey abundance along the Aleutian Islands.

Formation and variation of temperature inversions in the eastern subarctic North Pacific

To examine mesoscale physical and biological factors affecting seabird communities, boat-based seabird censuses were conducted during a single cruise from the North Pacific to the Bering Sea across Amchitka Pass, central Aleutian Islands, during summer. Acoustic surveys of potential prey biomass and currents, and conductivity-temperature-depth profiler measurements for vertical profiles of the temperature, salinity, and fluorescence, were conducted simultaneously. The seabird density and number of species were highest in Amchitka Pass (283 birds km−2, 17 species), where the acoustically determined biomass was the highest, followed by the Bering Sea basin (100 birds km−2, 13 species) and North Pacific basin (4 birds km−2, seven species). Crested auklets Aethia cristatella were dominant (95% in number) in Amchitka Pass, while short-tailed shearwaters Ardenna tenuirostris were dominant (92% in number) in the Bering Sea basin. Surface fluorescence was higher in the Bering Sea basin (median: 1.3 volts) than in Amchitka Pass (0.9) and the North Pacific basin (0.6). Within the pass, the number of crested auklets was highest over the northern part of the sill, where the water column was well mixed, and the northward-flowing tidal currents increased the concentration of acoustically determined biomass in the subsurface layer. Thus, we hypothesize that small alcids were attracted to an aggregation of prey that resulted from tidal currents impinging on the sill.