Large scale slow-down of the South Indian Ocean circulation with global warming

Annette Stellema^{1, 2}, Alex Sen Gupta^{1, 2}, Andrea Taschetto^{1, 2} 

1. Climate Change Research Centre, University of New South Wales, Sydney, NSW, Australia
2. ARC Centre of Excellence for Climate Extremes, Sydney, NSW, Australia

Indian Ocean circulation affects marine life and global climate through important oceanic teleconnections with the Pacific, South Atlantic and Southern Ocean. Based on 28 Coupled Model Intercomparison Project Phase 5 (CMIP5) models, this study finds that many aspects of the South Indian Ocean circulation slow significantly with increased greenhouse gas emissions. Off the west coast of Australia, the models project a consistent weakening of the poleward flowing Leeuwin Current and equatorward flowing Leeuwin Undercurrent. These changes are linked to a reduction in downwelling between these currents and the zonal geostrophic transport that feeds the Leeuwin Current. We find that projected intermodel differences in the zonal flows is related to the meridional pressure gradient along the west coast of Australia. While both the Indonesian Throughflow and the meridional pressure gradient are projected to weaken, there is no intermodel relationship between these changes. Additionally, the local southerlies are projected to intensify, which might be expected to weaken the Leeuwin Current, however, again we find no intermodel relationship between Leeuwin Current strength and meridional wind stress. In the southwest Indian Ocean, the models project a robust weakening of the poleward flowing East Madagascar Current, Mozambique Channel and Agulhas Current. These changes are compensated by a small reduction of upper-ocean transport, reduced Indonesian Throughflow transport and a substantial reduction of deep ocean transport entering the Indian Ocean from the south.

Stabilised frequency of extreme positive Indian Ocean Dipole under 1.5 °C warming target

Guojian Wang¹, Wenju Cai¹, Agus Santoso^{2, 1} 

1. Centre for Southern Hemisphere Oceans Research (CSHOR), CSIRO Oceans and Atmosphere, Hobart
2. Australian Research Council (ARC) Centre of Excellence for Climate System Science, The University of New South Wales, Sydney

Publish consent withheld.

Downscaling the centennial changes of the Indonesian Throughflow and impacts on the Indian Ocean heat balance

Ming Feng¹, Jie Ma² 

1. CSIRO, Crawley, WA, Australia
2. Ocean University of China, Qingdao, China

In this presentation, we use an eddy-resolving ocean model to assess the centennial changes of the Indonesian throughflow and the poleward temperature transport in South Indian Ocean during the 21st century in response to enhanced greenhouse warming. A significant decrease of poleward temperature transport in the Indian Ocean is captured in the downscaling ocean model, which can be primarily explained by the centennial decrease in the Indonesian Throughflow volume transport, which is associated with the weakening trend of the deep upwelling in the Pacific basin. The reduction of the Indonesian Throughflow drives the long term increase of the zonal thermocline depth differences between west and east boundaries, which causes anomalous northward geostrophic volume transport in subtropical Indian Ocean. The decrease of southward temperature transport at 32°S is mainly associated with the weakening of the Agulhas Current temperature transport, with additional contributions from a weakening Leeuwin Current. Based on a Sverdrup balance calculation, we demonstrate that the spin down of Indian Ocean subtropical gyre circulation associated with cyclonic wind stress curl changes is of secondary importance, compared to the direct influence from the Indonesian Throughflow, to the reduction of the Agulhas Current transport.

References:

Feng, M., Zhang, X., Sloyan, B. and Chamberlain, M., (2017), Contribution of the deep ocean to the centennial changes of the Indonesian Throughflow. Geophysical Research Letters, 44(6), pp.2859-2867.
Ma, J., Feng, M., Sloyan, B. Lan, J. (2018), Pacific influences on the meridional temperature transport of the Indian Ocean. Journal of Climate, under revision.

Comparison of air-sea heat fluxes from concurrent mooring observations in the Southeast Indian and Southeast Pacific

Veronica Tamsitt^{1, 2}, Ivana Cerovečki³, Simon Josey⁴, Sarah Gille³, Eric Schulz⁵ 

1. University of New South Wales, Sydney, NSW, Australia
2. Centre for Southern Hemisphere Oceans Research, Hobart, TAS, Australia
3. Scripps Institution of Oceanography, La Jolla, CA, USA
4. National Oceanography Centre, Southampton, UK
5. Bureau of Meteorology, Melbourne, VIC, Australia

Subantarctic Mode Water (SAMW) formation and subduction north of the Subantarctic Front in the Southern Ocean plays an important role in global budgets of heat, carbon and nutrients. Air-sea heat fluxes are a key process driving the formation of SAMW, but there are few direct observations of fluxes, particularly during the winter. The Ocean Observatories Initiative (OOI) Southern Ocean mooring (in the Southeast Pacific) and the Southern Ocean Flux Site (SOFS, in the Southeast Indian) provide the first concurrent, multi-year, time series of air-sea fluxes in the Southern Ocean, and are well-placed in two key SAMW formation regions. In this work we compare and contrast characteristics and variability of air-sea heat fluxes, mixed-layer depths and SAMW formation from observations at these two mooring location, and combine these with Argo float data and atmospheric reanalyses to provide temporal and spatial context for the mooring observations.

We show that stronger ocean heat loss events measured at the SOFS site than at the OOI site result from larger latent heat losses associated with warmer sea surface temperatures and stronger winds. The interannual variability of wintertime ocean heat loss is larger at OOI than SOFS site. inter mixed-layer depth anomalies tend to be in phase at the two moorings, where anomalously deep mixed layers are associated with anomalous advection of cold air from the south, and conversely shallow mixed layers correspond to warm air from the north. However, both the winter heat flux and mixed- layer depth anomalies show a complex spatial pattern, with a zonal dipole pattern in both the Indian and Pacific basins that we relate to the leading modes of climate variability in the Southern Ocean.