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Long-term hydrographic Variations Observed
in the Hawaii Ocean Time-Series

Roger Lukas, Fernando Santiago-Mandujano and Matt Cochran

The Hawaii Ocean Time-Series is now 12 years long, allowing us to detect interannual variations in water properties from the near-surface layer to the bottom (4800 m). Despite the energetic cruise-to-cruise variations associated with mesoscale eddies, low frequency variability of temperature, salinity and dissolved oxygen emerge without any sophisticated data processing.
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. Near-surface salinity reflects more clearly the interannual variations of surface forcing.	Near surface variability
	Near Bottom variability
Currents measured with a shipboard acoustic Doppler current profiler along the cruise tracks to and from the ALOHA station have revealed the existence of a mean flow to the west-northwest along the Hawaiian Ridge at 158°W (Firing, 1996). This North Hawaiian Ridge Current is highly variable from cruise to cruise and from year to year.	Near Surface Current Variablility
Within the water column, there are 3 hydrographic features which dominate the mean profiles: These features are the shallow salinity maximum (S_max) in the upper thermocline, the salinity minimum (S_min) associated with the North Pacific Intermediate Water mass, and the dissolved oxygen (dO_2 min) associated with the Antarctic Intermediate Water mass. When these features are analyzed on potential density surfaces, variations in conservative properties such as T and S can only be attributed to horizontal advection and diapycnal mixing. Variability associated with advection may be due to current anomalies, or may be due to changes in the remote surface forcing of these features in regions of water mass formation. Our observations cannot distinguish between these two modes of variability.	Interior Variability
So far, the biggest surprises are the detection of ENSO time-scale variability in the near-bottom temperatures, and the trend towards lower values of dO₂ in the Antarctic Intermediate Water. That trend may be associated with trends in other properties and other portions of the water column which are masked by other, more energetic, variability. A longer Hawaii Ocean Time-Series will tell.

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