Hobart

Seminar Abstract

Friday 7 November 2008, 11.30am (Tas time)
CSIRO Auditorium, Hobart

Stephanie Downes
UTAS/CSIRO/ACE-CRC Oceanography PhD student
IASOS University of Tasmania

Multi-model analysis of the Southern Ocean upper limb water masses at the end of the 21st century

Sub-Antarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) ventilate the subtropical gyres via the process of subduction, carrying temperature and salinity properties from deep winter mixed layers into the ocean interior and redistributing them throughout the Southern Hemisphere and beyond. Warming and freshening trends at the ocean surface increase stratification in the mixed layer, and this could ultimately lead to reduced vertical transport of carbon in the Southern Ocean in the future. I use the model output from the Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report to estimate the projected changes in the subduction of SAMW and AAIW, the associated atmospheric surface fluxes, as well as changes in the water mass properties and circulation in density coordinates.

The models are first assessed by how well they represent the observed changes over the second half of the 20^th century. The ability to simulate changes in the wind stress curl, as well as heat and freshwater at the ocean surface, is important for the subduction calculation. Further, the variability of the depth, temperature and salinity properties of density surfaces is compared with observations from the Hydrobase2 and Southern Ocean Database climatologies.

Following this, the decadal mean 1950s 20^th century climate is compared with the 2090s IPCC A2 projection, where atmospheric CO₂ reaches 860 ppm by 2100. The projected model mean decreases in the SAMW and AAIW subduction rates are associated with an increase in the air-sea fluxes and upper ocean stratification, and a decrease in gradient of the winter mixed layer depth. The model SAMW and AAIW move to lighter density classes by about 0.3 kg m^-3, and show warming and freshening. Cooling and freshening patterns observed during the late 20^th century along density surfaces are predicted to continue into the 21^st century, and correspond to the warming and freshening trends at the ocean surface. The projected depth of isopycnals deepens due to a poleward shift in the density layer outcrop, and a vertical displacement is the cause of shoaling of density surfaces north of the subtropics. The changes in the northward volume transport of SAMW and AAIW, and the variability of the ACC transport across the Southern Ocean chokepoints, are also presented and discussed.

Seminar Recording

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Location:
CSIRO = Marine Laboratories Auditorium, Castray Esplanade, Hobart

For further information, or to schedule a seminar, contact:
To schedule a seminar, contact:
Clothilde Langlais, (Oceanographic seminars)
CSIRO Marine and Atmospheric Research (03) 6232 5399
Natalie Kelly, (Biology/Modelling seminars)
CSIRO Marine and Atmospheric Research 0438 452 483
Jillian Enraght-Moony, (seminar administrator)
CSIRO Marine and Atmospheric Research (03) 6232 5320
Communications Manager, Antarctic Climate and Ecosystems CRC (03) 6226 2265
Margaret Hazelwood, Institute of Antarctic and Southern Ocean Studies (IASOS) University of Tasmania (03) 6226 2971

Last updated 21/07/09