Zonal evolution of Alaskan Stream structure and transport quantified with Argo data

Abstract

The Alaskan Stream (AS) flows west‐southwestward along the south side of Alaska and the Aleutian Island Arc; a western boundary current at the northern edge of the North Pacific subpolar gyre. The Argo float array has improved sampling of the Gulf of Alaska, allowing quantification of the AS's zonal evolution from 140°W to 175°W. Geostrophic alongshore transport of the AS in the upper 1000 dbar referenced to an assumed level of no motion at 1000 dbar shows little change from east to west. However, alongshore absolute geostrophic transports in the top 2000 dbar (obtained by combining mean absolute 1000‐dbar velocities from float displacements with the geostrophic velocity fields) generally increase to the west. We estimate full‐depth transports by fitting a barotropic and the first two baroclinic modes calculated from a climatology to the absolute geostrophic velocities in the upper 2000 dbar and applying the velocities from these fits from 2000 dbar to the seafloor. Flowing west from its formation region at 140°W–145°W the full‐depth AS becomes stronger, more barotropic, and also narrower once it reaches ∼160°W, with along‐shore transports increasing from −16.4 ± 4.9 Sv (1 Sv = 106 m3 s−1) at 140°W to −32.6 ± 5.2 Sv at 175°W. Mean concentrations of relatively warm, salty, oxygen‐poor, and nutrient‐rich Pacific Equatorial Water (PEW) in the AS decrease from 17.8% ± 0.3% to 8.5% ± 0.5% between 140°W and 175°W. However, the volume transport of PEW by the AS exhibits little change over the PEW density range between these longitudes.

Variability and Trends of the Alaska Gyre From Argo and Satellite Altimetry

“…With this definition, we find mean transport above 1,500 m for the Alaska Gyre of 14.2 Sv (from altimetry) and 13.0 Sv (based on Argo, integrated vertically 0-1,500 m). These numbers fall in line with other estimates of the Alaska Gyre intensity that put it in the 15-to 21-Sv range (Logan & Johnson, 2017;Reed, 1984;Stabeno & Hristova, 2014). Our Argo estimate is very close to the mean 0-to 1,500-m transport readily inferred from the depth-integrated steric height H from Cummins and Masson (2018).…”