Temperature and salinity evolution

To examine the temporal variability of sea surface properties within eddies, Hovm¨oller diagrams of SST (Figure 3.9), Sea Surface Salinity (SSS; Figure 3.10), and their anoma- lies (SSTA and SSSA) in the ocean model along the idealised pathway are produced.

A SST front between warm tropical waters from the Coral Sea (> 22o_{C) and cold}

temperate waters from the Tasman Sea (<10o_{C) exists in the region of interest (Figure}

3.9a). The mean location of this front is at∼40o_{S, extending further south off the eastern}

Australia coast, due to advection of warmer waters by the EAC and its eddies. Therefore, eddies propagating along the pathway form in warmer regions and propagate towards colder regions. The SSTA standard deviation is higher close to the EAC separation region and southeast of Tasmania (Figure 3.9b). Here, the seasonal signal is removed, therefore this variability is mainly attributed to local mesoscale variability.

The evolution of SST along the pathway has a strong seasonal signal (Figure 3.9c). This signal hinders the observation of eddy propagation in the SST Hovm¨oller diagram. Despite this, it is possible to see the SST signature of the eddies previously seen in the ocean model SLA Hovm¨oller diagram (Figure 3.2b) propagating following the pathway. With the seasonal signal removed, the SSTA associated with the eddies is clearer (Figure 3.9d). Due to their warm-core nature, EAC anticyclonic eddies are seen as a positive SSTA. Although less persistent than the positive SLA associated with eddies (Figure 3.2c), the positive SSTA can be tracked as far as “F”, off western Tasmania, in most cases (e.g. eddies 2, 3, 6, 8, 10, 12, 13).

A SSS front between saltier tropical waters from the Coral Sea (>35.6 psu) and fresh temperate waters from the Tasman Sea (<35 psu) is found in this region (Figure 3.10a). The mean location of this SSS front is also at∼40o_{S, extending further south off eastern}

Australia coast towards Tasmania. The eddy pathway transits through different SSS values, encountering saltier waters of the EAC and fresher waters south of Australia. The standard deviation of SSSA is higher close to the EAC separation region, south of Tasmania and south of Western Australia (Figure 3.10b). This variability is not attributed to a seasonal signal. At the EAC separation region, this variability can be explained by the changes in the location of this current. South of Tasmania, this variability can be explained by movements of the subtropical convergence (Wyrtki, 1960, Ridgway and Dunn, 2003).

The evolution of SSS along the pathway has no strong seasonal signal (Figure 3.10c). In this case, the salinity differences between the Coral and the Tasman Seas (between “C”

a)

b)

c)

d)

Figure 3.9: a) Idealised pathway of anticyclonic eddies shed at the EAC separation

region in the ocean model, overlaid on mean SST in the model between 1993 and 2012. The letters (“A” to “G”) indicate key locations along the pathway; b) map of SST Anomaly (SSTA) standard deviation in the model between 1993 and 2012; c) Hovm¨oller diagram of SST in the model on the “A–G” pathway; d) as for (c), but for

a)

b)

c)

d)

Figure 3.10: a) as for Figure 3.9a, but for Sea Surface Salinity (SSS); b) as for Figure

3.9b, but for SSS Anomaly (SSSA); c) as for Figure 3.9c, but for SSS; d) as for Figure 3.9d, but for SSSA.

and “D” in the SSS Hovm¨oller diagram) are very clear. Even with this strong signal, the signature of the eddy propagation along the pathway is evident. Consideration of seasonal anomalies of SSS yields a clearer picture of the eddy propagation following the pathway (Figure 3.10d). However, after the “D” location, there is a misleading increase in SSSA. This increase does not represent an increase in the SSS associated with the eddies, but a higher SSSA due to the eddy propagating along a highly variable region (Figure 3.10b).

To explore the evolution of the potential temperature (θ) and salinity (S) at depth, θS diagrams are produced (Figure 3.11). The spread of these properties within all eddies following the pathway ranges from 0 to 25o_{C and from 34.4 to 35.9 psu, and includes}

several water masses (Figure 3.11b).

Two regimes are evident when considering θS within eddies at “A-G” locations along the pathway (Figure 3.11c). These regimes are seen in the scattered data at the surface and sub-surface of eddies located between “A” and “D” (circle 1 in Figure 3.11c), and at the scattered data at the surface and sub-surface of eddies located between “E” and “G” (circle 2 in Figure 3.11c). This scattered data represents the effect of seasonal variability at the surface of the eddy.

The evolution ofθS properties shown in Figure 3.11c is consistent with cooling and fresh- ening of the surface waters of the eddies (Figures 3.9 and 3.10). At the EAC separation (i.e., “A”), eddies are warm and salty, retaining characteristics of their formation region. These characteristics are maintained until the eddies reach Tasmania (i.e., between “C” and “D”). South of Tasmania (i.e., “D”), the surface and sub-surface properties of eddies are cooler and fresher. As eddies advect westwards in the Eastern Indian Ocean (i.e., “E–G”), they no longer carry waters from their formation region at the surface. This is expected, considering that by the time the eddies reach south of Tasmania they are no longer non-linear, and have lost the capacity of trapping fluid. The denser waters within the eddies also change - becoming saltier as eddies propagate (see salinity minimum in theθS curves).