Figure 5. Difference between climatological mean salinity near Station ALOHA and the average salinityduring the HOT program, as a function of potential density.
In the potential density framework, the salinity anomalies (relative to the HOT climatology) appear to propagate downwards with time (Fig. 2d). This is similar in nature to the penetration of thermal anomalies in the central North Pacific Ocean(Deser et al., 1996; hereafter DAT96), who suggested that this was a manifestation of thermocline ventilation. Subducation of mixed layer anomalies, or anomalous subduction rates, should appear first along the shallowest isopycnals at Station ALOHA, because the strongest flows are near the surface. Deeper isopycnals will experience the ventilation anomaly at later times because advection is slower (Bingham, 1998), and also possibly because the ventilation location may be further away than for shallower isopycnals (Huang and Russell, 1994).
DAT96 raised the issue of the degree to which the thermal anomalies that they observed were density compensated (i.e., by salinty anomalies of opposite sign and appropriate magnitude). In potential density coordinates (Fig. 2d) the salinity anomalies are exactly compensated by temperature anomalies. The freshening trend corresponds to a 0.5℃ cooling on some isopycnals (Fig. 4), which is comparable to the thermal anomalies of DAT96.
These anomalies of salinity are relative to the HOT climatology, but how representative of long term conditions is that climatology? Fig. 5 shows the difference between the mean HOT salinity profile and the Boyd and Levitus (1997) climatological profile for the region around the HOT site. Above 24σθthe HOT mean conditions are fresher than climatology. Between 24 and 26.25 σθ, the ocean near Station ALOHA is markedly saltier during the 1988-1997 time period.
