Paleo and present oceanic modelling of the Ross Sea (Antarctica): evolution of water masses and ice shelf – ocean interactions during the last glacial cycle (21-0 ka)

In this work, we want to investigate the influences of water masses on the basal melting under the RIS at present and in the past. In particular, the research aimed at understanding the influences of Ross Sea water masses variability on the RIS basal melting both at present and in the past. A regional adaptation of the Massachusetts Institute of Technology general circulation model (MITgcm) was implemented on the Ross Sea to simulate ocean circulation on the continental shelf and under the RIS. A present-day transient run, forced by ocean (GLORYS12V1) and atmospheric (ERA5) reanalysis over the period 1993-2018, shows that: [1] simulated water masses present different timescales of variability in their properties: Circumpolar Deep Water and Antarctic Surface Waters show a strong seasonal cycle, modulated by strong interannual variability. High Salinity Shelf Water and Low Salinity Shelf Water, on the other hand, show a weaker seasonal cycle and a decadal oscillation in their salinity. Variability of CDW and AASW is probably related to wind variability associated with the Southern Annular Mode, the Amundsen Sea Low, and El-Niño Southern Oscillation, mediated by sea ice. Variability of HSSW and LSSW is probably related to variability of the sea ice and meltwater input, and katabatic wind strength, in turn associated with the Polar Cell. The same variability is observed for the water masses beneath the RIS. [2] Basal melting presents a distinct pattern, related to the current at draft level, and variability related to the changing water masses properties. A new method based on mixing of water masses was developed to disentangle the effect of mixing, and highlight the melting variability associated to each water mass. Results show basal melting of ∼78 Gt/yr, in line with the observations, and presenting variability at the seasonal, interannual and decadal scale indicative of changing water masses properties or volume expansion inside the cavity. Then, we run 21 snapshots at intervals of 1000 years, over the Last Deglaciation (∼21-0 kyears BP): each snapshot was 26 years long and branched on a separate 120 years-long spinup. Simulations are forced by the outputs from an existent transient global paleoclimate experiment TraCE-21ka. The purpose of the paleo experiment was: 1) to analyse the evolution of the water masses with varying deglacial climatic conditions, and 2) how circulation resumed on the continental shelf, starting from a condition restricted by a grounded ice sheet at LGM (∼21 ka), and retreating during the deglaciation. Results show that: [1] initially, circulation was limited to three sub-ice shelf cavities in the Western Ross Sea. In Pennel trough warm CDW water reached the cavity, whereas in the Drygaslki and Joides troughs, HSSW filled the bottom level. [2] During the millenium following the Meltwater Pulse 1-A (14.6-14.3 ka), deep ocean warming and sub-surface ocean freshening caused a weakening of the Antarctic Slope Front, fostered CDW flow in Pennel and the Whales Deep cavity, which experienced high rates of basal melting. HSSW production in the Drygaslki and Joides stopped during this event. [3] In the Early Holocene (∼11.8 ka) grounding line retreat uncovered growingly portions of the continental shelf, allowing stronger atmospheric cooling and resumption of HSSW production. At ∼10ka the RIS cavity began to form, and was melted on the Westward side by HSSW, and on the Eastward side by advected mCDW; therefore, the stronger melting role shifted to the HSSW at that time.

In this work, we want to investigate the influences of water masses on the basal melting under the RIS at present and in the past. In particular, the research aimed at understanding the influences of Ross Sea water masses variability on the RIS basal melting both at present and in the past. A regional adaptation of the Massachusetts Institute of Technology general circulation model (MITgcm) was implemented on the Ross Sea to simulate ocean circulation on the continental shelf and under the RIS. A present-day transient run, forced by ocean (GLORYS12V1) and atmospheric (ERA5) reanalysis over the period 1993-2018, shows that: [1] simulated water masses present different timescales of variability in their properties: Circumpolar Deep Water and Antarctic Surface Waters show a strong seasonal cycle, modulated by strong interannual variability. High Salinity Shelf Water and Low Salinity Shelf Water, on the other hand, show a weaker seasonal cycle and a decadal oscillation in their salinity. Variability of CDW and AASW is probably related to wind variability associated with the Southern Annular Mode, the Amundsen Sea Low, and El-Niño Southern Oscillation, mediated by sea ice. Variability of HSSW and LSSW is probably related to variability of the sea ice and meltwater input, and katabatic wind strength, in turn associated with the Polar Cell. The same variability is observed for the water masses beneath the RIS. [2] Basal melting presents a distinct pattern, related to the current at draft level, and variability related to the changing water masses properties. A new method based on mixing of water masses was developed to disentangle the effect of mixing, and highlight the melting variability associated to each water mass. Results show basal melting of ∼78 Gt/yr, in line with the observations, and presenting variability at the seasonal, interannual and decadal scale indicative of changing water masses properties or volume expansion inside the cavity. Then, we run 21 snapshots at intervals of 1000 years, over the Last Deglaciation (∼21-0 kyears BP): each snapshot was 26 years long and branched on a separate 120 years-long spinup. Simulations are forced by the outputs from an existent transient global paleoclimate experiment TraCE-21ka. The purpose of the paleo experiment was: 1) to analyse the evolution of the water masses with varying deglacial climatic conditions, and 2) how circulation resumed on the continental shelf, starting from a condition restricted by a grounded ice sheet at LGM (∼21 ka), and retreating during the deglaciation. Results show that: [1] initially, circulation was limited to three sub-ice shelf cavities in the Western Ross Sea. In Pennel trough warm CDW water reached the cavity, whereas in the Drygaslki and Joides troughs, HSSW filled the bottom level. [2] During the millenium following the Meltwater Pulse 1-A (14.6-14.3 ka), deep ocean warming and sub-surface ocean freshening caused a weakening of the Antarctic Slope Front, fostered CDW flow in Pennel and the Whales Deep cavity, which experienced high rates of basal melting. HSSW production in the Drygaslki and Joides stopped during this event. [3] In the Early Holocene (∼11.8 ka) grounding line retreat uncovered growingly portions of the continental shelf, allowing stronger atmospheric cooling and resumption of HSSW production. At ∼10ka the RIS cavity began to form, and was melted on the Westward side by HSSW, and on the Eastward side by advected mCDW; therefore, the stronger melting role shifted to the HSSW at that time.