Home > Results > Research Abstracts > Abstract
Time-series Hydrography
The hydrographic data collected during the first eleven years of HOT are presented in a series of contour plots. The CTD data used in these plots are obtained by averaging the data collected during the 36-hour period of burst sampling (Santiago-Mandujano et al., 1999). Therefore, much of the variability which would otherwise be introduced by internal tides in the upper ocean has been removed. Figures 1 and 2 show the contoured time-series record for potential temperature and density in the upper 1000 dbar for the HOT cruises. seasonal variation in temperature for the upper ocean is apparent in the maximum of near-surface temperature of about 26 �C and the minimum of approximately 23 �C. Oscillations in the depth of the 5 deg �C isotherm below 500 dbar appear to be relatively large with displacements up to 100 dbar. The main pycnocline is observed between 100 and 600 dbar, with a seasonal pycnocline develpoing between June and December in the 50-100 dbar range (Figure 2). The cruise-to-cruise changes between February and July 1989 in the upper pycnocline illustrate that variability in density is not always resolved by our quasi-monthly sampling.Figures 3 and 4 show the contoured time-series record for salinity in the upper 1000 dbar plotted against pressure and potential density. Surface salinity is variable from cruise-to-cruise, with no obvious seasonal cycle and some substantial interannual variability. Relatively low surface salinities occurred during 1989, the early part of 1995, and during 1996. A relative increase in surface salinity that started in the late months of 1997 has continued throughout 1998 and 1999, intensifying in the first half of 1999. This increase is also present in deeper layers reaching 200 dbar.
The salinity maximum is generally found between 50 and 150 dbar, and within the range 24-25 sigma-theta. A salinity maximum region extends to the sea surface in the later part of 1990, 1993 and during 1998 and 1999, as indicated by the 35.2 contour reaching the surface. The maximum shows salinities lower than normal in early 1995 and 1996, and throughout these two years the values are below 35.2 During 1997 the salinities decrease even further, with values below 35.1, to recover rapidly after February 1998 to values prior to 1995. The increase continues throughout 1999 reaching record values of up to 35.45 in the first half of 1999.
The maximum value of salinity in the salinity maximum region is subject to short-term variations of about 0.1 which is probably due to the proximity of Station ALOHA to the region where this water is formed at the sea surface (Tsuchiya, 1968). The variability of this feature is itself variable. Throughout 1989 there were extreme variations of a couple of months duration with 0.2 amplitude. The variability was much smaller and slower thereafter, except for a few months of rapid variation in earlier 1992.
The salinity minimum is found between 400 and 600 dbar (26.35-26.85 sigma-theta). There is no obvious seasonal variation in this feature, but there are distinct periods of higher than normal minimum salinity in early 1989, in the fall of 1990, in early 1992 and in the summer of 1996. These variations are related to the episodic appearance at Station ALOHA of energetic fine structure and submesoscale water mass anomalies (Lukas and Chiswell, 1991; Kennan and Lukas, 1995).
Figures 4 and 5 show contoured time-series data for oxygen in the upper 1000 dbar at Station ALOHA. The oxygen data show a strong oxycline between 400 and 625 dbar (26.25-27.0 sigma-theta), and an oxygen minimum centered near 800 dbar (27.2 sigma-theta). A recurrent decrease in the oxygen concentration can be seen throughout the time-series between 25 and 26.25 sigma-theta. This feature is accompanied by a decrease in salinity and an increase in the nutrient concentration (see discussion below).
The oxygen minimum exhibits some interannual variability, with values less than 30 umol kg-1 appearing frequently during the time-series. This variability can be seen in a plot of the mean oxygen in the intermediate waters spanning the oxygen minimum (27-27.8 sigma-theta, ). Superimposed to this variability is a general trend towards lower oxygen values from 1989 throughout 1998, that started increasing in 1999.
The surface layer shows a seasonality in oxygen concentrations, with highest values in the winter. This roughly corresponds to the minimum in surface layer temperature. (Figure 1).
Figures 6-11 show [nitrate+nitrite] (Figures 6 and 7), soluble reactive phosporous (SRP) (Figures 8 and 9) and silica (Figures 10 and 11 ) at Station ALOHA plotted against both pressure and potential density. The nitricline is located between about 200 and 600 dbar (25.75-27 sigma-theta). Most of the variations seen in these data are associated with vertical displacements of the density structure, and when [nitrate+nitrite] is plotted versus potential density, most of the contours are level. A recurrent increase in [nitrate+nitrite] can be seen throughout the series between 25-26.25 sigma-theta. These events are accompanied by a decrease in the oxygen concentration mentioned above. The most obvious events occurred in March-April 1990, January 1992, May 1992, February-March 1995, early 1996, mid- to late 1997, and July-September 1999. These events can likely be attributed to mesoscale features such as eddies. It is possible for eddies to transport water with different biogeochemical characteristics from distant sources into the region of Station ALOHA (Santiago-Mandujano and Lukas, 2000). The SRP variability is similar to the [nitrate+nitrite] in the upper water column.
References:
Kennan, S. C. and R. Lukas. 1995. Saline intrusions in the intermediate waters north of Oahu, Hawaii. Deep-Sea Research, 43, 215-241.Lukas R. and S. Chiswell. 1991. Submesoscale water mass variations in the salinity minimum of the north Pacific near Hawaii. WOCE Notes, 3(2), 6-8.
Santiago-Mandujano, F. and R. Lukas. 2000. Eddy Transport of Anomalous Waters in the Hawaii Ocean Time-series. J. Geophys. Res., in preparation.
Santiago-Mandujano, F., L. Tupas, C. Nosse, D. Hebel, L. Fujieki, R. Lukas, and D. Karl. 1999. Hawaii Ocean Time-series Program Data Report 10, 1998. SOEST Tech. Report 99-5, School of Ocean and Earth Science and Technology, Univ. of Hawaii, Honolulu, HI, 246 pp.
Tsuchiya, M. 1968. Upper Waters of the Intertropical Pacific Ocean. Johns Hopkins Oceanographic Studies, 4, 49 pp.