Multi-decadal Ocean Change

The (a) surface temperature and (b) salinity time series from the Maria Island station, and c) the decadal patterns of T and S at Maria Island (blue, cyan), satellite composite (red) and the trend of the temperature (black) (Ridgway, 2007a)
Surface temperature and salinity time series from the Maria Island station

Sea surface temperatures around Australia have increased by about 0.6-0.74°C over the past century (Lough and Hobday, 2011, Lough et al., 2012). However, south eastern Australian waters experienced warming three to four times the global average, becoming a global hot spot for ocean temperature change. Holbrook and Bindoff (1997) calculated a depth-averaged (0 to 100 m) warming of 1.5 oC century-1 off Tasmania based on objectively mapped historical vertical temperature profiles over 34 years (1955-1988). More recently, using the Maria Island long term quasi-monthly monitoring station (1944-2002), Ridgway (2007a) reported a sea surface temperature (SST) warming rate of 2.3 °C century-1 and increasing salinity of 0.34 century-1, consistent with the southward penetration of the EAC over the past 60 years.

The intensification of the EAC flow past Tasmania is also seen in recent model studies describing both a spin-up and southward shift of the southern hemisphere subtropical ocean circulation. The oceanic changes are forced by an intensification of the wind stress curl arising from a pole-ward shift in the circumpolar westerly winds due to the trend in the Southern Annular Mode (SAM). A decline in overall rainfall in the region with very low winter/spring rainfall linked to rising temperatures and associated changes in the large-scale atmospheric circulation such as SAM, ENSO and the Indian Ocean Dipole (IOD).

The following high-level science questions will guide the Southeast Australia IMOS observing strategy in this area:

Ocean Heat Content:

  • How are the global energy balance and the broadscale ocean temperature patterns changing and its impact on sea level rise in the South East Australia region? 
  • How are open ocean temperature changes related to temperature changes on the South East Australia shelf?

Ocean Circulation:

  • How and why are the East Australian Current, Leeuwin and Antarctic Circumpolar Current changing?
  • How is the shelf – open exchange changing as a result of, among other processes, Bass Strait winter discharge into the Tasman Sea.

Global hydrological cycle:

  • How is ocean salinity in the SEA region changing and what do these changes in salinity tell us about the response of the regional hydrological cycle to climate change? 

Global carbon budget:

  • What is the seasonal through interannual variability in air-sea CO2 fluxes for SEA shelves and regional seas?
  • What are the key biological and physical processes driving air-sea CO2 exchange in the Southern Ocean and South East Australian region and how sensitive are they to climate change?
  • What is the role of microbes in stabilising ‘blue C’ stocks via production of recalcitrant C. in coastal oceans?