DEL DERECHO Y LO SIMBÓLICO

Water masses formed on the continental shelf are controlled by the variability of the air/sea forcing (see Chapter 2) as well as by the glacial meltwater released from the local ice shelves. Different air/sea forcing are used for both simulations (see Section 4.2). Within the Mertz Glacier Polynya (MGP), north of Watt Bay (averaged over 5 × 5 grid points), the surface heat flux is -220 ± 10 W m−2 _{for the PRE}

scenario, and decreased by 36% in the same area (-140 ±30 W m−2_{) for the POST}

scenario. Similarly, the fresh water flux present in the air/sea model forcing shows a corresponding increase of 60%, with a fresh water flux for the same area within the MGP PRE (POST) of -3.7 ±0.3 cm day−1 _(-1.5 ± _{0.6 cm day}−1_{). The cumulative}

sea ice production for the PRE and the POST simulations is shown in Appendix B (Figure B.4) to illustrates the changes in sea ice location and production for the region.

Water mass properties along the continental shelf near the Ad´elie and Mertz de- pressions have changed dramatically between the PRE and POST simulations (see Figures 5.4, 5.5 and Table 5.1). The seasonal (summer and winter) changes in poten- tial temperature and salinity at the bottom layer of the model are seen in Figure 5.4 and 5.5 respectively for the Ad´elie and Mertz depressions. Water mass properties for specific areas in the bottom layer of the model are summarised in Table 5.1 to quantify the regional changes. The potential temperature at the base of the Ad´elie depression, the main reservoir of DSW in the PRE simulation, is essentially unchanged in POST and is close to the surface freezing temperature (Figure 5.4). This is confirmed in Watt Bay, shown in Table 5.1, where the potential temperature increased by only 0.04±0.01◦C between the two simulations. Overall this suggests that wintertime convection due to polynya activity is still strong enough to form DSW after the calving. However, the DSW properties have freshened (-0.1 north of Watt Bay, Table 5.1 and Figure 5.5), most likely in response to the decrease in sea ice formation and associated brine rejection.

The greatest change in bottom potential temperature is found over the Mertz Bank and Mertz Depression, north and northeast of the Ad´elie Depression, respectively. The bottom potential temperature increased by∼1◦C (Figures 5.4 and Table 5.1). The Mertz depression is affected by the inflow from the east described previously (section 5.2.1) allowing warmer water to get closer to the ice shelf. Pre-calving the

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B9B iceberg (grounded at the Ninnis bank) acted as a barrier against warmer water accessing the continental shelf from the east. In addition, in the PRE scenario, the latent heat polynya region in the lee of B9B iceberg modified any warmer water flowing underneath the fast ice near the B9B iceberg, via intense brine rejection. The salinity pattern (Figures 5.5) illustrates the freshening of the water masses on the continental shelf due in part to additional glacial meltwater being released by the increase of ice shelf basal melting, but also as a consequence of the decrease in sea ice production (brine rejection) in the area, reducing the salinity of High Salinity Shelf Water (similar density than DSW but at surface freezing temperature – HSSW) at the base of the depression.

In the PRE simulation, the maximum bottom salinity on the continental shelf is observed within the Ad´elie depression, the Ninnis trough, the Mertz cavity and within Commonwealth Bay in winter. Observations post-calving have noticed a decrease in bottom salinity of 0.12 between observations taken in 2010 and 2012 in an area between the Mertz Glacier and Watt Bay [Lacarra et al. 2014]. In the model POST simulation, the bottom salinity in the Ad´elie depression has decreased by 0.11 ± 0.01, which is in good agreement with the observations. On the other hand, in the model, the salinity on the Mertz bank has increased by 0.08 ± 0.03, with an increase of the potential temperature of 0.9 ± 0.2 ◦C, due to the inflow of mCDW that is less mixed by ambient cold and fresher water masses formed on the continental shelf. However, no measurements were taken near the Mertz bank post-calving to compare with the model as shown on Figure 1 from Lacarra et al. [2014].

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Table 5.1: Time averaged bottom potential temperature (θ: ◦C) and salinity (S), for PRE and POST simulations, with the differences (POST-PRE) for several key areas on the continental shelf. For each area, 5× 5 grid points at the bottom layer of the model are averaged to estimate the bottom water properties. The deepest part of the deep Ad´elie depression is defined for an area north of Watt Bay. The deepest area is used along the Ad´elie and Mertz sill sections and within the Mertz depression. The shallowest area of the Mertz bank is used to quantify the warming of the intruding water masses.

PRE POST POST - PRE

Deep Ad´elie depression

θ -1.915±0.007 -1.87±0.01 +0.04±0.01 S 34.62 ±0.01 34.51± 0.01 -0.11±0.01

Ad´elie sill

θ -1.89±0.04 -1.4 ±0.2 +0.5±0.1 S 34.61 ±0.02 34.56± 0.007 -0.05±0.02

Top of Mertz bank

θ -1.87±0.04 -1.0 ±0.2 +0.9±0.2 S 34.51 ±0.03 34.589 ± 0.003 +0.08±0.03

Deep Mertz depression

θ -1.86±0.04 -0.85±0.03 +1.01±0.04 S 34.61 ±0.01 34.616 ± 0.001 +0.01±0.01

Mertz sill

θ -1.3 ±0.1 -0.87±0.06 +0.4±0.2 S 34.608 ±0.005 34.610 ± 0.004 +0.002 ±0.005

5.2. OCEAN CIR CULA TION AND W A TER MASS PR OPER TIES ON THE AD ´ELIE LAND CONTINENT AL SHELF 121

Figure 5.4: Potential temperature (◦C) at the bottom layer of the model averaged over summer (December, January and February – a-c) and winter (June, July and August – d-f) for PRE (a and d), POST (b and e) and their differences (POST - PRE – c and f).

5.2. OCEAN CIR CULA TION AND W A TER MASS PR OPER TIES ON THE AD ´ELIE LAND CONTINENT AL SHELF 122

Figure 5.5: Salinity at the bottom layer of the model averaged over summer (December, January and February – a-c) and winter (June, July and August – d-f) for PRE (a and d), POST (b and e) and their differences (POST - PRE – c and f).

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5.2.3

Transport through key sections before and after the