An examination of mixed layer sensitivity in the northeast Pacific Ocean from July 2001–July 2005 using the general ocean turbulence model and Argo data

Abstract

The mixed layer depth (MLD) at Ocean Station Papa (OSP) in the northeast Pacific Ocean has been shoaling for the past 50 years, with the shallowest maximum MLDs ever recorded occurring in the winters of 2002–03 and 2003–04. We use the General Ocean Turbulence Model (GOTM) to estimate the MLD and to determine how various atmospheric forcings from 2001–05 at four stations in the northeast Pacific affect the MLD. Three of the stations are in the Alaskan Gyre: OSP (50°N, 145°W) in the south; S16 (49°17'N, 134°40'W) at the southeastern edge; CAG (55°N, 145°W) in the centre; and one, NSG (40°N, 145°W), is south of the Alaskan Gyre. Interpolated Argo temperature and salinity, for both initial conditions and restoring, and atmospheric inputs (heat fluxes and wind stress) from the National Centers for Environmental Prediction/National Center for Atmospheric Research (NCEP/NCAR) reanalysis were used. Experiments showed that in the winter, the MLD was most sensitive to increased winds at all stations and also to decreased heat fluxes everywhere except at OSP. In summer, MLD was most sensitive to changes in the shortwave radiation. A combination of effects from the 2002–03 El Niño (i.e., decreased outgoing heat fluxes, cloud cover and wind speeds) and a strong stratification created by a subsurface cold water anomaly caused the MLD to be shallow in the Gulf of Alaska during the winter of 2002–03. Finally, our results show that for the modelled period (2001–05), the MLD at OSP behaved differently than at other stations in the Gulf of Alaska and therefore OSP may not be the best location to represent the region.