Hydrographic sections

Figure 3.4 shows a similar pattern of large scale circulation to Figure 3.1 although the Polar Front is not well resolved, possibly due to it mostly lying outside the area of study for Figure 3.4. The splitting of the SAF is clear, with the southern branch found to be considerably weaker than the northern branch in terms of volume flux. The area of long residence time also shows as a large area with no streamlines, although Figure 3.4 suggests that this area extends further east than the float trajectories show it. Smaller scale features, such as the closed circulation around the plateau and detrainment into the area of long residence time are not captured, due to the large time and space scales between CTD casts and the lack of casts close to the edge of the plateau, although steep topography causes considerable problems in the interpretation of hydrographic data in such areas.

Figure 3.4 Transport streamfunction taken from Pollard et al. (2007). Data used is from the Crozet cruises Discovery 285 and 286, Discovery cruises 201 and 213 and Suzil (Park

et al. 1993). Streamlines show the transport relative to 4000 m or the bottom if shallower, measured in Sverdrups (1 Sv = 106m3s-1). Solid lines are at 20 Sv intervals between 0 and 100 Sv, then at 40 Sv intervals to 180 Sv. A few intermediate 10 and 5 Sv contours are shown long and short dashed respectively. All station positions on which the contouring is based are shown, apart from those outside the limits of the map.

3.1.3 Absolute Dynamic Topography

Figure 3.5 shows a sample image of absolute dynamic height contours (Rio and Hernandez 2004) from February 2005. The contours, through the geostrophic relationship, are streamlines of the surface geostrophic flow. The image has been deliberately chosen to illustrate all of the features present in the area – whereas each feature is seen frequently in other images, it is rare to have them all present in one image. A full set of weekly images from the 2004/5 season is included as Appendix B.

Figure 3.5 Absolute dynamic height (dyn cm) image from February 2005, selected to illustrate features regularly present.

There is an overall similarity between Figure 3.5, the Argo float trajectories (Figure 3.1) and the map derived from hydrographic sections (Figure 3.4). The comparison confirms that the surface flows shown in the surface height gradients extend to depth, giving confidence in using the surface geostrophic currents to study the overall circulation. Conversely it also gives confidence in using the deep flows shown by float trajectories to interpret the surface currents and so the advection of iron.

The repeated features, relative to those introduced in section 3.1.1 from study of Argo float trajectories, and discussed in section 3.2, include: the strong east-south-east flow to the north of the study area; bifurcation of the SAF south-west of the Crozet Plateau; an indication of detrainment from the northern branch (other images show detrainment from the southern branch north of the islands); closed circulation over the plateau; and the weak flow and closed circulations north of the western part of the plateau.

There is also a closed circulation south of the southern branch of the SAF. This is a regular feature in dynamic height images (see Appendix B). It agrees with the fact that no Argo float has yet entered the area, suggesting that this feature extends to at least 2000m. This area, together with the southern branch of the SAF, separates the area to the south from flow that has passed close to the plateau and/or islands.

A feature seen in Figure 3.5 that is also seen in several ocean colour images is the cyclonic eddy that is shed occasionally to the west (43.5°S, 44°E) as the northern branch of the SAF retroflects eastwards again north of the Del Cano Rise.

Figure 3.6 Location of the cuts through the absolute dynamic height data used to study the variation in strength and location of the surface geostrophic flows.

To investigate some of the flows identified, cuts were made through weekly absolute dynamic height data: an east/west cut 45.8-46.6°S, 40-60°E and two north/south cuts 40- 60°S, 50.3-51°E and 40-60°S, 40.3-41°E (Figure 3.6). Along the first two of these cuts regions were specified and consecutive minima and maxima found within these regions. The gradient between these gives a measure of the surface geostrophic velocities and so

the difference across a flow is a comparative measure of the volume flux close to the surface, on the assumption that there is a consistent depth structure across the time or area of the comparison. These surface geostrophic flows are a significant contribution to the surface flows that advect iron and plankton. In addition, by the overall agreement found above between surface and deep flows, these surface data give an indication of the relative strengths, persistence and location of the deep extending fronts.

To study the variability of the paths of the fronts, the latitudes of specified dynamic heights in the cuts were found. If the dynamic height occurred at more than one latitude the one closest to the mean (specified for first image, then calculated from preceding images) was chosen.

The time series for these latitudes and differences in dynamic height runs from October 1992 until March 2007 (754 weeks). However, data from before April 2002 are either omitted or used with caution due to there being fewer altimetric satellites meaning that there is reduced spatial and temporal information used to create the mapped absolute dynamic topography data.