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Centre for Southern Hemisphere Oceans Research

Southern Ocean Dynamics, Circulation and Water Mass Formation

Matthew England, Veronica Tamsitt, Annie Foppert, Andrew Lenton 23 July 2020

Project 4: Research Team

Matthew England (UNSW) Annie Foppert (postdoc) Veronica Tamsitt (postdoc) Steve Rintoul (CSIRO) Andrew Lenton (CSIRO) PhD students:

• David Webb (2020)
• Zhi Li (2021)
• Hannah Dawson (2023)
• Julia Neme (2023)

We aim to advance our understanding of the dynamics, circulation and water-mass formation of the Southern Ocean, its role in the uptake of heat and carbon, and its impact on global climate

Analysis of Subantarctic Mode Water formation and variability at two mooring sites in the southeast Indian and southeast Pacific Analyses of the dynamics of an eddy/upwelling hostpot in the Antarctic Circumpolar Current Analyses of bottom water properties, variability and pathways, from Deep-Argo floats and historical

• bservations

Studying exchange of water masses across and along the Antarctic shelf with modelled Lagrangian particle expts Quantifying the exchange of mass and heat in the Ross and Weddell gyres Quantify subduction rates

• f Subantarctic

Mode Water and Antarctic Intermediate Water using Argo data (Lagrangian, WMT, and volumetric methods) Dynamics and properties of eddy variability in the ACC

We aim to advance our understanding of the dynamics, circulation and water-mass formation of the Southern Ocean, its role in the uptake of heat and carbon, and its impact on global climate

Subantarctic Mode Water formation, subduction, and variability

Subantarctic Mode Water formation and variability using Argo measurements

Zhi Li, England, Groeskamp, Cerovečki and Luo Journal of Physical Oceanography Subantarctic Mode Water formation rates estimated during the Argo era, using a water mass transformation framework alongside Eulerian and Lagrangian approaches. SAMW subduction pathways also calculated using a Lagrangian analysis.

Subantarctic Mode Water formation and variability using Argo measurements

Zhi Li, England, Groeskamp, Cerovečki and Luo Journal of Physical Oceanography

April - June July - Aug Sept - Oct Nov - March

Work planned for 2020/2021:

• 1. Analyze the processes controlling the

transport and variability of the Ross and Weddell gyres (including heat transport toward the shelf) Weddell gyre

Work planned for 2020/2021:

• 1. Analyze the processes controlling the transport

and variability of the Ross and Weddell gyres (including heat transport toward the shelf)

• 2. Quantify subduction rates and pathways of

Antarctic Intermediate Water ventilation using Argo data in combination with both Lagrangian and Eulerian approaches

Work planned for 2020/2021:

• 1. Analyse the processes controlling the transport

and variability of the Ross and Weddell gyres (including heat transport toward the shelf)

• 2. Quantify subduction rates and pathways of

Antarctic Intermediate Water ventilation using Argo data in combination with both Lagrangian and Eulerian approaches 3. Examine the two-time scale response of the Southern Ocean to global warming and wind intensification across ocean model simulations at 1, 0.25 and 0.1 degree resolutions

Kerguelen Plateau

Kerguelen Plateau

SOFS Mooring

Credit: MNF

OOI Southern Ocean flux mooring

• 0.1° horizontal resolution ocean, sea-

ice model, released

• Water mass transformation limits CDW

access to ice shelves in dense shelf regions even when there is strong upslope CDW transport

• CDW has a short residence time in

dense shelf regions (Prydz, Ross/Adelie, Weddell), but there are highly localised spatial patternsparticles every 5 days for 1 year along 1000 m isobath and track on continental shelf for 5 years

Lagrangian pathways and residence time of warm Circumpolar Deep Water on the Antarctic continental shelf

Mean cumulative residence time on shelf (days since release)

• CDW in dense/fresh shelf regions

undergoes a two-timescale transformation: isopycnal (cooling+freshening) on the slope followed by diapycnal (cooling+salinification) on the shelf

• Future: Ongoing work to understand

seasonality and regional patterns, submission of manuscript to GRL (Tamsitt et al. in prep)

Future work

• Extend Antarctic shelf work to look at sensitivity of warm water on shelf to different atmospheric

forcing, and inclusion of tides in the model. Other projects near completion:

• The imprint of the Polar Front on air-sea interaction and heat fluxes - manuscript to be submitted this year

(Bharti, Tamsitt et al)

• Submesoscale upwelling of Circumpolar Deep Water in the Polar Front - manuscript to be submitted this year

(Wang, Tamsitt et al) Temperature anomalies in the Amundsen Sea in response to +/- Southern Annular Mode conditions

Dynamics of eddying hotspot in the ACC

Survey Area

South  Distance from PF core [km]  North

Data from Oct-Nov 2018 voyage

Distribution of T-S profiles Mean T at 300-m depth

Elephant seal data used to estimate cross-slope eddy transport for the first time around East Antarctica (red arrows = eddy transport hot spots) Eddy transport hot spots

Water mass exchange across the continental slope

”Along-slope variability of cross-slope eddy transport in East Antarctica” - Foppert, Rintoul, and England (GRL, 2019)

• Cross-slope eddy transport inferred

from observed density-layer thickness gradient in East Antarctica

• Enhanced transport (1.5 x East Antarctic

mean) of warm, salty Circumpolar Deep Water found in regions of high isopycnal spice variability

• Eddy-induced overturning carries about

0.8 m2/s of CDW poleward, increasing the reservoirs of heat and salt available to cross the shelf break in those eddying regions

Water mass exchange across the continental slope

Future work: participation on R/V Investigator voyage to conduct a hydrographic survey of the Antarctic slope/shelf region near Prydz Bay in summer 2021

• Quantify the structure of the Antarctic Slope Front/Current system
• Characterize the regional hydrography for krill population and distribution studies lead by AAD
• Deployment of BGC-Argo floats

• Synthesis of observations and models
• Very large decadal SO Variability
• SO CO2 sink variability is more

dominated by climate var than CO2 var

Trends in SO uptake(GtC/yr/decade)

• Vertical BGC structure characterized (1st time)
• Controlled by eddy dynamics below MLD &

biological processes in the mixed layer.

• Eddy transport supports productivity outside the

Southern Ocean (i.e. at lower latitudes)

Climate+CO2 CO2 Climate

Andrew to add a slide here on FUTURE WORK Decoupling of projected oceanic uptake of carbon and heat in the 21st century- 1/10o simulations) High resolution projections of the ocean carbon cycle changes under different emissions pathways (1/10o simulations)

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ACCESS-ESM ACCESS-ESM

Belkin, I., A. Foppert, H. T. Rossby, T., S. Fontana, and C. Kincaid (2020) A Double-Thermostad Warm-Core Ring of the Gulf Stream. Journal of Physical Oceanography, 50, 2. doi:10.1175/JPO-D-18-0275.1 Bharti, V., V. Tamsitt, H. E. Phillips, and N. L. Bindoff. The imprint of the Southern Ocean Polar Front on air-sea fluxes. In preparation for submission to a special collection of Frontiers in Marine Science. Foppert, A., S. R. Rintoul, and M. H. England (2019) Along-slope variability of cross-slope eddy transport in East Antarctica. Geophysical Research Letters, 46. doi: 10.1029/2019GL082999 Foppert, A., et al. 2020: Variability of bottom water properties in the Australian Antarctic Basin: A perspective from Deep Argo. In preparation for submission to JGR Oceans, Holmes, R. M., J. D. Zika, R. Ferrari, A. F. Thompson, E. R. Newsom and

• M. H. England, 2019: Atlantic Ocean heat transport enabled by Indo-

Pacific heat uptake and mixing, Geophys. Res. Lett., 46, 13,939-13,949. Lago, V., and M. H. England (2019). Projected slowdown of Antarctic Bottom Water formation in response to amplified meltwater contributions, Journal of Climate, 32(19), 6319-6335. Li, Q., S. Lee, M. H. England, and J. L. McClean (2019). Seasonal-to- interannual response of Southern Ocean mixed layer depth to the Southern Annular Mode from a global 1/10° ocean model. Journal of Climate, 32(18), 6177-6195. Li, Q., and M. H. England, 2020: Tropical Indo-Pacific teleconnections to Southern Ocean mixed layer variability. Geophys. Res. Lett., in press. Li, Z., M. H. England, S. Groeskamp, I. Cerovečki, and Y. Luo, 2020: The Origin and fate of Subantarctic Mode Water in the Southern Ocean, Journal of Physical Oceanography, in prep.

2019/2020 publications

Meijer, J. J., H. E. Phillips, N. L. Bindoff, S. R. Rintoul, and A. Foppert. Changing water masses and their mechanisms in a standing meander of the Antarctic Circumpolar Current. In prep. for to JGR Oceans. Meijers, A., I. Cerovečki, B. A. King, and V. Tamsitt (2019). A see-saw in Pacific Subantarctic Mode Water formation driven by atmospheric modes, Geophysical Research Letters, 46(22), 13152-13160. Morrison, A. K. , A. McC. Hogg, M. H. England, and P. Spence (2020), Warm Circumpolar Deep Water transport towards Antarctica driven by local dense water export in canyons, Sci. Adv., in press. Patel, R, Llort, J , Strutton, P. G., Moreau S., Pardo P. C. , Phillips. H. E. , Lenton A, The biogeochemical structure of Southern Ocean mesoscale eddies, JGR-Oceans, in press Silvano, A., A. Foppert, S. R. Rintoul, et al. (2020) Recovery of Antarctic Bottom Water formation driven by anomalous climate forcing. In review at Nature Geoscience. Tamsitt, V., I. Cerovečki, S. Josey, S. Gille, and E. Schulz (2020). A comparison of mooring air-sea heat flux and mixed layers in the Southeast Indian and Southeast Pacific Subantarctic Mode Water formation regions, Journal of Climate, 33(7), 2757-2777. Tamsitt, V. et al. Lagrangian pathways and residence time of warm Circumpolar Deep Water on the Antarctic continental shelf, In prep GRL. Thomas, G., Purkey, S. G., Roemmich, D., Foppert, A, and Rintoul S., Spatial variability of Antarctic Bottom Water in the Australian Antarctic Basin from 2018-2020 captured by Deep Argo. In review at GRL. Wei, Y., Gille, S. T., Mazloff, M. R., Tamsitt, V., Swart, S., Chen, D., & Newman, L. (2020). Optimizing mooring placement to constrain Southern Ocean air-sea fluxes. Journal Atmos. Ocean. Tech.. Wang, K., V. Tamsitt, H. E. Phillips, and N. L. Bindoff. A detailed submesoscale survey of the Polar Front. In prep. for JGR:Oceans.

Centre for Southern Hemisphere Oceans Research

Southern Ocean Dynamics, Circulation and Water Mass Formation

Matthew England, Veronica Tamsitt, Annie Foppert, Andrew Lenton 23 July 2020