Hawaii Ocean Time-series (HOT)
in the School of Ocean and Earth Science and Technology at the University of Hawai'i
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Salinity samples were collected, stored and analyzed as described in Tupas et al. (1993). IAPSO samples were measured to standardize the salinometer, and samples from a large batch of "secondary standard" (substandard) seawater were measured after every 24-48 bottle samples of each cruise to detect drift in the salinometer. Standard deviations of the secondary standard measurements were less than ± 0.0010 for all the 2015 cruises (Table below).
The secondary standard seawater batches are made from 60 liters of seawater taken from a depth of 1000 m from Station ALOHA. Two batches of secondary standard seawater were used during 2015. Batch #58 was created on June 4th, 2014. Batch #59 was created on December 11th, 2014.
Prior to making each substandard batch, all Substandard making materials and supplies were cleaned thoroughly. The plastic carboy used to collect the Substandard seawater on the cruise was bleached before collection. The glass carboys were washed with liquinox, rinsed with water, hexane, rinsed with water again, and then rinsed with 99% alcohol before drying. The glass rods were rinsed with hexane and 99% alcohol. The carboy (and glass rod) was rinsed with the collected Substandard water before being filled and capped with a layer of white oil to prevent oxygenation and evaporation. The filled carboy was then wrapped in black bags to prevent light from reaching the stored Substandard water.
Figure 10 and Figure 11 show the contoured time-series record for salinity in the upper 1000 dbar for all HOT cruises through 2015. Some of the bottles in Figure 11 are plotted at density values lower than the indicated sea surface density. This is due to surface density changing from cast to cast within each cruise, and even between the downcast and the upcast during a single cast.
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, during 1996, and 2004. A relative increase in surface salinity that started in the late months of 1997 continued throughout 2003, intensifying in the first half of 1999 and remaining with high values during the major part of 2000, 2001, and early 2002, showing a decrease in mid-2002, mid-2003, during the second half of 2004, in early 2005, during 2007, and mid-2008; and increasing again by the end of 2002, early 2003, late 2003, early 2004, early 2009, early to mid 2010, and early and late 2011, 2012, 2013, 2014 and 2015. 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 σθ. A salinity maximum region extends to the sea surface in the later part of 1990, 1993 and during 1998 throughout the early months of 2002, during late 2002 and early 2003, and again in the late part of 2003, early 2004, late 2004, early 2006, late 2008, early and late 2009, early to mid 2010, and early 2011, 2012, 2013, 2014 and 2015, 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 2004, reaching record values of up to 35.45 in the first half of 1999. During 2005 and 2006, the salinities decreased to values comparable to those in 1998, and even further during 2007, to increase again in 2008, and to continue increasing to values above 35.3 thoughout 2015. These salinity anomalies seem to be related to rainfall anomalies in the central North Pacific dominated by the El Niņo/Southern Oscillation phenomenon and by the Pacific Decadal Oscillation (Lukas, 2001). During the 1998 through 2004 period of high salinities in the salinity maximum, brief periods of relatively lower salinities are observed during the second half of 1998, 1999, and 2003.
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.
In the thermocline region below the salinity maximum (between 150 and 300 dbar), the salinities present a decreasing trend starting around 1995 until mid-2008, when it started increasing until mid 2010, and decreasing again until 2012.
The salinity minimum is found between 400 and 600 dbar (26.35-26.85 σθ). 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, in the summer of 1996, in the fall of 2006, late in 2007, fall 2008 and 2009, the second half of 2010, in the summer of 2011 and 2012 and during 2013. 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). The anomalous high salinity centered at 400 dbar in early 2001 was apparently caused by the passing of an eddy during HOT-122 (Lukas and Santiago-Mandujano, 2001). This caused anomalous values in all the hydrographic variables observed at the ALOHA station. A similar feature centered at 350 dbar was observed in mid-2012 during HOT-241, however its anomalous values were not as extreme as during HOT-122.