Question

lai Whj do the cross-sections in Figure fi,17lal and lb) indicate the presence ol a large volume of Labrador Sea Water?

thi Wh\ are the contour patterns in Figure ft. 17 consistent with cyclonic winds'.1 And why would the effect of cyclonic inds render the uppermost waters of the Labrador Sea more susceptible to being destabilized'.1

Figure 6.17 Section across the southern Labrador Sea at about 60° N, showing vertical distributions of (a) temperature (°C), (b) salinity and (c) oxygen (ml H). As will be discussed in Section 6.5, atmospheric oxygen dissolves readily in cold surface water, so high concentrations of dissolved oxygen indicate water recently in contact with the atmosphere (but see the answer to Question 6.7(b)).

Figure 6.17 Section across the southern Labrador Sea at about 60° N, showing vertical distributions of (a) temperature (°C), (b) salinity and (c) oxygen (ml H). As will be discussed in Section 6.5, atmospheric oxygen dissolves readily in cold surface water, so high concentrations of dissolved oxygen indicate water recently in contact with the atmosphere (but see the answer to Question 6.7(b)).

In the Mediterranean, too. the triggering of deep convection is aided by the fact that the cold, dry air of the Mistral blows over water circulating in a cyclonic gyre.

After sinking. Labrador Sea Water spreads out at mid-depths both eastward across the northern Atlantic, and southward in a current known as the deep western boundary current. As Figure 6.17 shows. Labrador Sea Water is very distinctive, with temperatures of 3-4 °C and low salinities (< 34.92), and often occupies a well-developed pycnostad. It is therefore considered to be a subpolar mode water.

The subpolar mode water that forms in the Antarctic Polar Frontal Zone (cf. Question 6.6) is known as Antarctic Intermediate Water (AAIW) and is the most widespread intermediate water mass in the oceans (Figure 6.16). It is characterized by temperatures of 2-4 °C and salinities of about 34.2. and is therefore significantly fresher than Western Atlantic Sub-Arctic/ Labrador Sea Water.

Can Antarctic Intermediate Water he identified on Figure 5.31 ?

Yes. indeed it can. In Figure 5.31(a). a "tongue' of cold water can be seen extending down from the Antarctic Front to a depth of about 800 m: in the corresponding region in Figure 5.31(b). the near-surface isohalines dip downwards, indicating the sinking of relatively low-salinity water. The downward flow of Antarctic Intermediate Water may be seen in the slope of the isotherms and isohalines from this surface region to a depth of about 1500 m at 56° S (the northerly limit of the section).

After sinking at convergences in the Frontal Zone. Antarctic Intermediate Water spreads northwards throughout the Southern Hemisphere: in the Atlantic Ocean, its low salinity enables it to be traced to at least 20° N (Figure 6.18). The relationship between the flow of Antarctic Intermediate Water and the major deep and bottom water masses of the Atlantic Ocean is shown in Figure 6.19.

Figure 6.18 The spread of the least-mixed layer of Antarctic Intermediate Water in the Atlantic, as shown by the position of the salinity minimum at about 500-900 m depth. The numbers on the contours are salinity values; the numbers in blue give the approximate depth of the salinity minimum (in m). The thin dashed line is the 600 m isobath. The salinity contours in the north-western Atlantic show the southward spread of Labrador Sea Water (Western Atlantic Sub-Arctic Water).

Figure 6.18 The spread of the least-mixed layer of Antarctic Intermediate Water in the Atlantic, as shown by the position of the salinity minimum at about 500-900 m depth. The numbers on the contours are salinity values; the numbers in blue give the approximate depth of the salinity minimum (in m). The thin dashed line is the 600 m isobath. The salinity contours in the north-western Atlantic show the southward spread of Labrador Sea Water (Western Atlantic Sub-Arctic Water).

Antarctic Polar Frontal Zone

Figure 6.19 Meridional cross-section of the Atlantic Ocean, showing movement of the major water masses; NADW = North Atlantic Deep Water; AAIW = Antarctic Intermediate Water; AABW = Antarctic Bottom Water. Water with salinity greater than 34.8 is shown yellow; note how the low-salinity tongue of AAIW extends northwards from the Antarctic Polar Frontal Zone, to overlie the more saline NADW. The M at about 35° N indicates the Inflow of water from the Mediterranean. Water warmer than 10°C is shown pink/orange, and that cooler than 0 °C (corresponding approximately to the distribution of AABW) is shown blue. The 'Subtropical Convergences' correspond to the centres of the subtropical gyres. The oxygen maxima and minima will be explained in Section 6.5.

Subtropical Convergence

Subtropical Convergence

Antarctic Polar Frontal Zone

Antarctic

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