Freshening of the Upper Pycnocline in the North Pacific Subtropical Gyre
Roger Lukas
Department of Oceanography, University of Hawaii
The Hawaii Ocean Time-series (HOT) project has observed full-depth water mass variability at 22.75N, 158 W since October 1988. A pronounced freshening of the upper pycnocline started between 1991 and 1994, continuing into 1998. The freshening is most pronounced (〜0.15 psu) near 25σt just below the salinity maximum; its phase appears progressively later on deeper isopycnals.
The first multivariate EOF of salinity, dissolved oxygen and potential vorticity over the potential density range 24.1-27.3σt explains 36% of the non-seasonal variance, and it includes the trend-like freshening since 1991. The freshening was accompanied by a decrease in dissolved oxygen and an increase in potential vorticity over 24-26.5σt. The time scale of the signal is at least decadal, but we cannot know the actual time scale until the trend reverses. Compared to climatology, the upper 100m at the HOT site is fresher, and the upper pycnocline is saltier, so the freshening trend is apparently a recovery from some previous anomalous event or cycle.
One hypothesis is that ventilation of the upper pycnocline has carried the signature of atmospheric anomalies into the pycnocline. This is consistent with observed atmospheric signals and with the delay in the arrival of the signal at the HOT site with depth, because advective pathways are slower with depth. Another hypothesis is that gyre "wobble" associated with long, low-frequency Rossby waves results in meridional displacement of the mean salinity, oxygen and potential vorticity fields. This hypothesis is consistent with the multivariate EOF structure of the freshening signal, but it cannot account for the delay of the signal with depth. Because the two hypotheses are not mutually exclusive, it is necessary to combine skillful ocean models with high quality surface forcing information to estimate the relative contributions of the hypothesized processes to the observed signal.
