Water Mass Variations in the Subtropical North Pacific

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Fredrick M. Bingham*
Roger Lukas

March 24, 1994

HOT Results: Water Mass Variations in the Subtropical North Pacific Since October, 1988, investigators at the University of Hawaii have been conducting a deep-ocean observational project, the Hawaii Ocean time-series (HOT), funded by NSF as part of the U.S. contribution to the WOCE and JGOFS programs. The site for our deep-ocean observations is known as station ALOHA at 22� 45'N, 158�W (Figure 1), 100 km north of Oahu, in about 4750 m of water. It is located to be as representative of the interior of the subtropical gyre as possible, while still affording reasonable logistics for a sustained long-term effort. The interior of the North Pacific subtropical gyre is supposed to be a region of rather small horizontal gradients of water mass properties such as salinity, dissolved oxygen and inorganic nutrients, whose temporal variations are thus expected to be slow. However, we have observed energetic water mass variations even within the intermediate and deep waters.

Cruises to the deep water station are made approximately monthly. The cruises are 5 days in duration during which we make one or two deep hydrographic casts when possible and do a series of casts to 1000 m over a 36 hour period in order to resolve energetic internal tidal motion. We collect a full suite of water column measurements, temperature, salinity, dissolved oxygen, nutrients as well as important biological and geochemical properties. We have done 53 such cruises to date.

In Figure 2, we show a contoured time series of salinity for the first five years of the program, using potential density (sigma theta) as a vertical coordinate. The potential density at the sea surface, which is shown by the solid line at the top of the colored area of the figure, undergoes an annual cycle. The near-surface salinity changes in response to changes in evaporation and rainfall, with a low salinity in mid- to late 1989, a high in late 1990 and lows in mid-1991 and late 1992. There is a salinity maximum found near the surface at potential density of 24.5 kg/m^3 or an average depth of about 120 m. This is a ubiquitous feature of the subtropical ocean associated with a large area of high salinity found at the surface in the central subtropical gyre. In the HOT data we see significant variability of this feature, especially in 1989. In early 1991, the surface water gets dense enough so that the water of the salinity maximum is almost ventilated, or almost comes to the surface. The variability in this water mass is also associated with variations observed in the waters of the main thermocline below at 25-26 kg/m^3.

Moving down in the water column, at a density of approximately 26.7 kg/m^3 we find the water mass known as North Pacific Intermediate Water, which is identified by a vertical salinity minimum. It is thought to be formed off of the coast of northern Japan by a confluence of subtropical and subpolar waters. Again, significant variability of this feature is observed with time scales of a couple of months which does not appear to be related to the variations mentioned above in the upper waters. What is not shown in this picture is significant small vertical scale intrusions of higher salinity water observed in some cruises and not in others. The variability shown here could be a result of changes in the conditions where this water was last at the surface. They could be a result of variations in the large scale flow patterns of the subtropical gyre. They could also result from local changes in the large-scale flow caused by the presence of the Hawaiian Islands.

* Current Address
Department of Physics
University of North Carolina at Wilmington
601 S. College Rd.
Wilmington, NC 28403-3297

Reference:

U.S. WOCE Implementation Report #6

Reference:

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