The Alaskan Stream: Zonal Evolution of Structure and Transport Quantified with Argo Data

The Alaskan Stream flows west and slightly south along the south side of Alaska and the Aleutian Island Arc, a western boundary current at the northern edge of the Pacific subpolar gyre. The Argo array has improved sampling of temperature, salinity, and velocity in the Gulf of Alaska, allowing quantification of zonal variations in the structure of this current, which we analyze at 5° longitude intervals from 140°W to 175°W. The geostrophic along-shore (roughly westward) transport of the current in the top 1000 db referenced to an assumed level of no motion at 1000 db has a peak of 6.3 Sv at 155°W, with values of 3.5 Sv at 140°W and 4.1 Sv at 175°W. Combining estimates of 1000-dbar velocities from float displacements with the geostrophic velocity fields yields absolute velocities in the upper 2000 dbar, for which along-shore transports in the top 2000db generally increase to the west from 14 Sv at 140°W to 21 Sv at 175°W. We estimate transports below 2000 db by fitting a barotropic and the first two baroclinic modes calculated from full-depth World Ocean Atlas 2013 climatological values to the absolute geostrophic velocities in the upper 2000 dbar and applying the velocities and transports from these fits from 2000 dbar to the ocean bottom. The implied full-depth Alaskan Stream along-shore transports generally increase from 17 Sv at 140°W to 33 Sv at 175°W. West of its formation region at 140°W-145°W the current becomes stronger, more barotropic, and also narrower once it reaches ~160°W. Peak concentrations of nutrient-rich Pacific Equatorial Water (PEW) in the Alaskan Stream approach 25% at 140°W and still exceed 10% as far west as 175°W. We quantify the volume transport of PEW by the Alaskan Stream within the potential density range of 26.2–27.2 kg m^-3, finding generally decreasing values from around 2.3 Sv at 140°W to 1.5 Sv at 175°W. This water-mass is important for primary productivity and other ecosystem dynamics in the region.