Long-term variation in seawater composition at the base of the thermocline

Abstract

Measurements of the chemical composition of seawater over a time scale of years at a fixed point in the ocean below the main thermocline have not been previously reported because of navigational limitations, ship and hydrowire instability, and, to a large extent, cost. The deep-water pipeline at the Natural Energy Laboratory of Hawaii at Keahole Point (Fig. 1a) provides a unique opportunity to address the question of time-varying water composition at a fixed site in the ocean below the mixed layer. In this study we examine the composition of sub-thermocline seawater through weekly sampling (surface-water data are also available, but are not discussed here). The mean deep-water composition obtained over about five years is representative of regional mean-water composition, and significant variation in water composition is observed on time scales exceeding one year. This unexpected long-term variation probably represents cyclic displacement of deep water masses in response to variation in the regional ocean circulation.