III. OBSTÁCULOS ONTOGENÉTICOS
6. EVALUACIÓN DEL OVA
The topography affects both the time-mean location of the fronts and the variability around this. The influence on the time-mean can be seen in Figure 3.9 as the surface geostrophic streamlines follow paths of approximately constant depth for long periods.
d) b)
c)
Figure 3.1 and Figure 3.4 also show this pattern, and Figure 3.10 shows that the variation of latitude of the fronts with time is small compared to the effects of topography on the latitude of the fronts. The physical reasons (potential vorticity) for this effect on flows on a rotating sphere have been investigated in section 1.2.3.
The dynamic heights 110dyn cm, 85dyn cm and 45dyn cm were chosen to represent the Northern SAF branch (NSAF), Southern SAF branch (SSAF) and Polar Front
respectively. The criteria for selecting these heights were as follows: NSAF, a height that always passed south of the Del Cano Rise and then turned northward to the west of the Crozet Plateau; SSAF, a height that always passed close to the south of the Crozet Plateau; PF, a height that always passed between the Del Cano Rise and Ob-Lena (Conrad) Rise close to the Ob-Lena (Conrad) Rise. Figure 3.9 shows how the flows approximately follow lines of constant depth, as expected. The divergence of bathymetry contours south of the Crozet Plateau east of 45°E causes the 45 and 85 dyn cm contours to diverge as they follow the two sides. This will cause an area of weak flow, as is observed.
Figure 3.9 Example image of the locations of the (from north) 110, 85 and 45 dyn cm contours, over bathymetry (Sandwell and Smith 1997). Contour intervals are 1000m.
The latitudes of these dynamic heights were found at 40°E and 50°E and the time series of these latitudes are shown in Figure 3.10.
Figure 3.10 Latitude of dynamic heights 110 dyn cm, 85 dyn cm and 45 dyn cm, used to show the locations of the Northern SAF branch, Southern SAF branch and Polar Front respectively.
The standard deviation of the latitude for each height is shown in Table 3.1. The paths of the SSAF at 50°E and PF at 40°E are tightly controlled by the topography of the Crozet Plateau and the Ob-Lena (Conrad) Rise respectively. The NSAF at 40°E (or at least the dynamic height that picks it out at 50°E, there being no overall need to distinguish the SSAF and NSAF at this longitude) is also influenced, to a lesser extent, by the Del Cano Rise. The SSAF at 40°E and NSAF and PF at 50°E are not strongly controlled by bathymetry and this is reflected in the greater variation seen in their location. Argo float trajectories, Figure 3.3, also show greater variability in location away from topographic control, this being especially evident south of the Crozet Plateau where there is
consistency in the location and direction of the trajectories close to the plateau but greater variability further south.
The fact that the locations of some of the flows in the area are controlled by topography decreases the interannual variability in the location of different biogeochemical zones. This consistency allows comparisons between the same geographical area between years with reduced problems of observed differences being driven by changes in the overall biogeochemical zone being sampled.
Height\Longitude 40°E 50°E NSAF 110 dyn cm 0.33 0.44 SSAF 85 dyn cm 0.58 0.23 PF 45 dyn cm 0.25 0.59
Table 3.1 Standard deviations of latitude of three dynamic heights representing the three main flows in the region.
3.2.3.1 Relative strengths of the SAF branches
Time series of the absolute dynamic height differences across the SSAF and NSAF are shown in Figure 3.11. The SSAF here is defined as the difference between a maximum in 45.6-46.8°S and a minimum in 47.4-49.3°S at 50°E. The NSAF is defined as the
difference between a maximum in 44.7-48.0°E and a minimum in 47.0-50.7°E.
Figure 3.11 Time series of the strength of the two branches of the SAF, as measured by the absolute dynamic height difference across the front in dyn cm.
The mean dynamic height difference across the SSAF is 33.8 dyn cm. This is very similar to the difference across the NSAF of 34.4 dyn cm. There are however issues with a direct comparison. Firstly the ‘SSAF’ in this location and quantified in this way includes the closed circulation around the plateau because both currents flow in the same direction south of the plateau whereas the closed circulation flows against the NSAF and is
therefore excluded. There is an average height difference of 7.5 dyn cm across the closed circulation (section 3.2.6) so it would be expected that a similar contribution from the closed circulation has been included in the SSAF. Accounting for the closed circulation,
the relative strengths are 34 and 26 dyn cm for the NSAF and SSAF respectively, approximately a 4:3 ratio.
This ratio contrasts with evidence from hydrography (Figure 3.4) that shows transport estimates of 30 and 5 Sv for the NSAF and SSAF. Floats drifting at 2000m also seem to be more likely to follow the path of the NSAF (4/6 follow NSAF) compared to shallower floats that are more likely to follow the path of the SSAF (3/9 follow NSAF). Due to the depth of the Del Cano Rise (approximately 1500m) it is to be expected that deep flows will be influenced by it more than surface flows. The SSAF therefore can be seen to be an important surface flow, with implications for the advection of iron, but much less
significant when considering full depth volume transport.