Abyssal Stratification Decrease in the Southwest Pacific Basin: Implications for Heat Fluxes and Internal Tidal Generation

Helen Zhang1, Caitlin B Whalen1, Nirnimesh Kumar2 and Sarah G Purkey3, (1)Applied Physics Laboratory University of Washington, Seattle, WA, United States, (2)University of Washington, Department of Civil & Environmental Engineering, Seattle, WA, United States, (3)Scripps Institution of Oceanography, University of California San Diego, San Diego, CA, United States
Density stratification sets the environment for many physical processes in the ocean including mixing, heat flux, and internal wave generation and propagation. Changing stratification in the abyssal ocean impacts the upwelling branch of the overturning circulation, which has potential implications for the global overturning ocean circulation and oceanic heat budget. Historical repeat hydrography data from Global Ocean Ship-Based Hydrographic Investigations Program (GO-SHIP) and its predecessors provide a unique dataset to study decadal trends in the open ocean. In this study, observations between 1990s and 2010s are used to calculate the change in buoyancy frequency (N2) along each survey section within the abyssal Southwest Pacific Basin. Consistent with previously observed bottom intensified warming, we find an increased separation between the 0.8 °C and 0.6 °C isotherms, which corresponds to a reduction in N2 by a factor of about 0.8. This net decrease in the abyssal ocean N2 is significant with a 95% confidence. In addition, we find that the regional diffusive heat flux per unit area decreases by around 0.01 W/m2 within the same time period. The associated change in internal wave energy generation due to the reduction in bottom stratification is also quantified. Finally, the implications of these changes on global ocean circulation and heat budget are qualitatively discussed.