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

Abstract

The 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′/\(\overline {KE} \)) 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.