A Freshening of the Upper Pycnocline in the North Pacific Subtropical Gyre
Roger Lukas (Dept. of Oceanography, U. Hawaii, 1000 Pope Rd.,Honolulu,Hawaii 96822, USA)
e-mail: rlukas@hawaii.edu
ABSTRACT
The Hawaii Ocean Time-series (HOT) project has observed full-depth water mass variability at 22°45'N, 158°W since October 1988. A pronounced freshening of the upper pycnocline started in 1991, continuing into 1998 when it began to reverse in the shallowest layers. The time scale of the signal is decadal. The freshening is most pronounced (〜0.15psu) near 25 σθ 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 24.1-27.3 σθ 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 24-26.5 σθ. Compared to a long-term climatology, the upper 100m at the HOT site is fresher, and the 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 decadal time-scale 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.
INTRODUCTION
Much progress has occurred in understanding, monitoring and predicting the ENSO phenomenon, and in recent years, attention has turned towards decadal variability of the coupled ocean-atmosphere system in the Pacific. This natural variability is also important to monitor, understand and predict because it has significant effects on climate in many places around the Pacific, and because these effects may combine with those of ENSO to produce climate extremes. Also, we must take these natural decadal variations into account in attempting to identify anthropogenic changes to the Earth's climate.
Unfortunately, there are very few long-term observations of Pacific Ocean variability which can be used to quantify and improve our understanding of such decadal modes of climate variability. Most of the long time series of subsurface properties are along the coasts, rather than in the deep ocean interior. In addition to the direct thermal variations of the coupled ocean-atmosphere climate system, the oceanic branch of the hydrological cycle may be an important component of decadal variability, but we have very few accurate long-term observations of salinity in the ocean. For the evaluation of climate models, collocated, multivariate observations are most powerful (IOC, 1998). The Hawaii Ocean Time-series (HOT) program (Karl and Lukas, 1996) was established, in part, to address these deficiencies.
