Accelerating Salinity Trends at Station ALOHA in the North Pacific Subtropical Gyre

Determining oceanic salinity cycles and trends is important to understanding the hydrological cycle of Earth's climate and for anticipating future changes. There are very few long oceanic time-series suitable for making reliable estimates of linear trends that are not affected by decadal and multi-decadal variations. Hydrographic data for the region of Station ALOHA, north of Oahu, are available from 1950 to 1988 from the Hydrobase dataset. Since 1988, high accuracy and high vertical resolution CTD profiles are made nearly monthly as part of the Hawaii Ocean Time-series (HOT). Since mid-2004, high temporal resolution moored CTD measurements were also made in the upper 155 m from the WHOTS mooring. We compare the linear regression trends of salinity in two potential density layers (upper pycnocline = 24.1-25 σ_θ and mid-pycnocline = 25-26 σ_θ). This eliminates internal wave noised and highlights salinity variations that may be associated with changes of the hydrological cycle, including net surface freshwater fluxes and advection of salt in the ocean. Although the quality and quantity of salinity data from 1950 to the 1980s is much less than during the HOT project, it appears that increasing salinity in the upper pycnocline has accelerated, with (0.011, 0.050, 0.102)/decade for (1950-2015, 1988-2015, and 2004-2015). Mid-pycnocline freshening appears to be nearly zero for the 1950-2015 record, but -0.08/decade for 1988-2015. This freshening appears to have increased during the 2004-2015 period when interannual variations are accounted for. The changing rates of salinity trends in these two layers suggest the influence of multi-decadal variations in rainfall and/or gyre circulation, or the more recent impacts of climate change.