Presenter: Fernando Carvalho Pacheco (Hawaii Ocean Time-series (HOT))

Description:
Time-series observations provide critically needed data for understanding how physical, atmospheric, biological, and biogeochemical processes impact marine ecosystems and how they change over time. The Hawaii Ocean Time-series (HOT) is now 33 years long, allowing us to better understand interannual to decadal variations in water properties from the near-surface layer to the bottom (~4800 m) at Station ALOHA (22°45'N, 158°W), approximately 100 km north of the Hawaiian Island of Oahu. Near-monthly ship-based sampling is conducted by HOT, during which physical and biogeochemical properties are measured with hydrocasts at 3-hour intervals over 36 hours, covering 3 semidiurnal tidal cycles and 1 inertial period (~31h). Despite energetic cruise-to-cruise variations associated with mesoscale eddies, low-frequency variability of temperature, salinity, and dissolved oxygen emerge without any sophisticated data processing. Here, we update and discuss these hydrographic variations in the near-surface, intermediate, deep, and bottom water over the past three decades. The dominant interannual signal in the near-surface and near-bottom temperatures is related to the El Niño-Southern Oscillation phenomenon with near-surface variations related to local forcing and bottom variations related to remote forcing by baroclinic waves. The bottom water variability is influenced by cold overflow events, produced when near-bottom relatively cold and salty water from the Maui Basin “overflows” into the Kauai Deep, where Station ALOHA is located. Recent results show a warmer-than-normal sea surface temperature from 2019 through the end of 2020. Additionally, during the same period, the upper 200 m of the water column displayed the strongest and deepest reaching freshening observed since 1989.