Info

Fig. 5.89 Annual mean salinity difference between 200 and 500 m depth. See color plate section.

At low temperatures, salinity can make a crucial contribution to the stratification in the ocean; this can be illustrated through the following diagnosis based on the World Ocean Atlas 2001 (WOA-01; Conkright et al., 2002) climatological mean T, S properties in the world's oceans. We first examine how much salinity contributes to the vertical stratification in the water column. As an example, we show the stratification ratio, which is defined as follows.

The commonly used stratification can be extended as

Po dz

where NT is defined as the equivalent stratification due to the vertical distribution of temperature only, with salinity set to a constant value of S0 — 35. The difference between N2 and NT2 is defined as NS2, which indicates the contribution of salinity to the stratification. For example, the places where |N|/N21 ^ 1 indicate that the contribution of the vertical salinity gradient to the local stratification is quite small. As shown in Figure 5.90a, in the upper ocean stratification is primarily due to the density gradient associated with temperature profile, while the salinity gradient tends to weaken the stratification. At nearly 1.5 km below sea level in the Southern Hemisphere, the stratification is primarily controlled by the relatively fresh water of Antarctic Intermediate Water (Fig. 5.90b, c).

Fig. 5.90 Contribution of temperature and salinity to stratification along 30.5° W, inferred from climatology: a N2(heavy line), N2 due to temperature only (thin line), and N2 due to salinity only (dashed line), in 10-4/s2; b stratification contribution due to temperature; c stratification contribution due to salinity.

Fig. 5.90 Contribution of temperature and salinity to stratification along 30.5° W, inferred from climatology: a N2(heavy line), N2 due to temperature only (thin line), and N2 due to salinity only (dashed line), in 10-4/s2; b stratification contribution due to temperature; c stratification contribution due to salinity.

In the Atlantic Ocean, salinity's control of the stratification can be clearly identified from depth below the upper kilometer (Fig. 5.90b). In particular, the salinity gradient associated with both Antarctic Intermediate Water and North Atlantic Deep Water clearly plays a major role in shaping the stratification at mid depth.

On the other hand, although the temperature contribution dominates the stratification for the upper ocean in the Pacific Ocean, the contribution of salinity to the stratification below the top 1 km is very important (Fig. 5.91). In particular, the contribution of salinity is very large for latitudes higher than 20° off the equator.

Another way to illustrate the role of salinity is through its contribution to the horizontal pressure gradient. Since the sea-surface elevation is unknown, we will omit the potential contribution due to difference in sea-surface elevation and focus on the baroclinic pressure gradient associated with the T, S distribution only. Furthermore, we will also subtract the horizontal mean pressure, and present the deviation from the horizontal mean pressure at each level only. Similar to the discussion above, we separate the pressure into two components as follows:

pdz, Pt (T, S0) = J p (T, S0) dz, ps = p - Pt (5.127)

Was this article helpful?

0 0

Post a comment