Seasonal cycles of temperature, salinity and dissolved oxygen observed in the Hawaii Ocean Time-series

Profiles of temperature, salinity and dissolved oxygen observed during the first 5 years of the Hawaii Ocean Time-series (HOT) were fit with annual and semi-annual harmonics to describe the annual variation of water mass properties in the North Pacific subtropical gyre. The fit was made to data on pressure and density surfaces. Generally, there are three areas in the water column where the fit is best, one near the surface, another near 500 m/26.6 σφ and a third at about 800 m/27.2 σφ. For temperature, the fit is best within 120 m of the surface, as the surface layer warms and cools with the local seasonal forcing. Minimum surface temperature is in mid-March, while minimum temperature at 100 m is in mid-May. For salinity, the most significant seasonal cycle is found in the core of the North Pacific Intermediate Water near 500 m. This seasonal cycle is associated with oscillations in the salinity of the main salinity minimum of 50 mpsu, with vertical displacements of the salinity minimum of 40 m. For oxygen, the most significant fits are at the surface and in the Antarctic Intermediate Water oxygen minimum near 800 m, where vertical displacements of the oxygen minimum of 25 m and variations of 2 μM kg−1 were observed. Examination of the residuals left after removing the annual cycle indicates that the remaining variability generally has timescales of less than 6 months with little interannual variability. The exception to this is in the near-surface waters, where larger interannual variability is observed. Residuals show strong vertical coherence as non-seasonal variation tend to extend throughout a large portion of the water column.