Annual Sections of Dynamic Height

Sections of dynamic height for each year in the various layers are found in Figure 3.26. The layers used were:

1. 1500 dbar relative to 1900 dbar: this is the approximate depth range of the upper Labrador Sea Water (Pickart, 1992).

2. 1000 dbar relative to 1500 dbar: this broadly covers the lower thermocline and Mediter-ranean Water.

3. 500 dbar relative to 1000 dbar: this broadly covers the middle and upper thermocline water.

4. 300 dbar relative to 500 dbar: this includes the 18-degree water.

5. 100 dbar relative to 300 dbar.

6. Surface relative to 100 dbar: the seasonal mixed layer.

For completeness, plots of dynamic height at the surface relative to 1000 dbar and 1500 dbar are included in Figure 3.25. The gaps in several plots near the eastern boundary (e.g.

Figures 3.25a, 3.25b, 3.26a and 3.26b), are due to a lack of Argo floats profiling deeper than 1000 dbar in this region.

Figures 3.25a and 3.25b show the same general pattern in each year, with a sharp increase in dynamic height moving west-to-east across the Gulf Stream, followed by a much shallower decline across the subtropical gyre to the eastern boundary. In general, there is very little year-to-year variability in the dynamic height of the eastern basin. Much greater variability is seen in the western basin, both in the magnitude of the dynamic height maximum and in its position. The location of the maximum is generally immediately offshore of the Gulf Stream (600 km along the baseline), but in both 2002 and 2005, the peak was significantly further east. In terms of the interannual variability, there is good agreement between Figures 3.25a and 3.25b with both displaying the largest dynamic height peak in 2003 and the lowest in 2005.

Examining the dynamic height in layers (Figure 3.26), several important comments can be made. Firstly, the gentle east to west slope of dynamic height across the recirculation region

0 1000 2000 3000 4000 5000 6000

(a) Relative to 1500 dbar

0 1000 2000 3000 4000 5000 6000

0.4

(b) Relative to 1000 dbar

Figure 3.25: Dynamic height (in dyn. m) at the surface by year relative to (a) 1500 dbar and (b) 1000 dbar.

is largely confined to the upper 1000 dbar, with layers below 1000 dbar having a relatively flat dynamic height profile away from the western boundary (Figures 3.26a and 3.26b).

This observation implies that the baroclinic part of the recirculation is largely confined to the top 1000 dbar with relatively weak flows beneath this level. However, all plots show a strong gradient in dynamic height across the Gulf Stream itself suggesting this feature extends to at least 2000 dbar. Secondly, in all layers above 1000 dbar, there is much stronger interannual variability in the region west of 3000 km along the baseline than in the eastern basin. This conclusion does not hold in the 1000 dbar to 1500 dbar layer as the influence of the Mediterranean Water east of 4500 km increases the variability of this region. Finally, there is a strong increase in interannual variability in all regions in the surface to 100 dbar layer due to the influence of the annual cycle in solar insolation on this part of the water column.

To investigate the geographical and depth range of the variability more carefully, the stan-dard deviation in dynamic height per unit pressure is plotted in Figure 3.27. The overall pattern is that the interannual variability in dynamic height increases towards the surface, but this masks a more complicated result. In the deepest two layers (the Upper Labrador Sea Water and lower thermocline), the variability in dynamic height is low across the entire basin, suggesting these layers are relatively unimportant in controlling the variability in the baroclinic component of the interior transport. Whilst the Mediterranean Water increases the relative importance of the eastern basin compared to the west, it is not important in controlling the overall dynamic height gradient from year to year. In contrast, the 1000 dbar to 500 dbar layer, whilst continuing to have a small standard deviation east of 3000 km, has a much larger standard deviation in the western basin, suggesting that year-to-year changes in the middle and lower thermocline of this region are strongly affecting the dynamic height field. A similar pattern is observed in both the 500 dbar to 300 dbar and 300 dbar to 100 dbar layers, with strong variability west of the Mid-Atlantic Ridge and weak variability close to the eastern boundary. The surface layer has the strongest variability in all regions,

0 1000 2000 3000 4000 5000 6000

(a) 1500 dbar relative to 1900 dbar (depth range of Upper Labrador Sea Water)

0 1000 2000 3000 4000 5000 6000

0.2

(b) 1000 dbar relative to 1500 dbar (depth range of lower thermocline and Mediterranean Water)

0 1000 2000 3000 4000 5000 6000

0

(c) 500 dbar relative to 1000 dbar (depth range of middle and upper thermocline water)

0 1000 2000 3000 4000 5000 6000

0.2

(d) 300 dbar relative to 500 dbar (depth range of EDW)

0 1000 2000 3000 4000 5000 6000

0.2

(e) 100 dbar relative to 300 dbar

0 1000 2000 3000 4000 5000 6000

0.1

(f) Surface relative to 100 dbar (seasonal mixed layer)

Figure 3.26: Dynamic height (in dyn. m) by year in various depth layers. Note the different vertical scales on each plot.

0 1000 2000 3000 4000 5000 6000 0

5e!05 0.0001 0.00015 0.0002 0.00025 0.0003 0.00035 0.0004 0.00045 0.0005 0.00055 0.0006

Distance (km)

Standard deviation in dynamic height per unit depth (m/dbar)

1900 dbar to 1500 dbar 1500 dbar to 1000 dbar 1000 dbar to 500 dbar 500 dbar to 300 dbar 300 dbar to 100 dbar 100 dbar to surface

Figure 3.27: Standard deviation in dynamic height per unit pressure for six pressure layers from the annual estimates of 2002 to 2007.

controlling the majority of the variability in the eastern basin. Therefore, two different mechanisms of variability emerge from this analysis. Year-to-year variations in dynamic height in the western basin are controlled in part by density changes within the thermocline and in part by density variability in the surface layer caused by differential solar heating from year-to-year. In contrast, almost all of the variability at the eastern end of the section occurs in the surface layer. Our results concur with the findings of Sato and Rossby (1995), who found that the main source of variability on the Sargasso Sea side of their Gulf Stream transects was the vertical displacement of the main thermocline (their Figure 4).