Two types of multi-centennial variability of the Southern Ocean deep convection: How sea ice tips the scale 1
Torge Martin1, Wonsun Park1, and Mojib Latif1
Introduction
The Mechanism: Convection Controls
• Large ocean heat capacity is important for decadal to centennial climate variability • Deep convection is one driver of the global ocean thermohaline circulation • OGCMs can produce stable climate states with convection in one (asymmetric) or both hemispheres (symmetric) [Bryan, 1986] • OGCMs also show self-sustaining oscillations between states of active and inactive convection [e.g. Winton and Sarachik,
Conclusions
heat accumulation at mid depth vs. freshwater lid and sea ice cover
3
Oscillations of open ocean deep convection with multi-
temperature maximum, immediate convection
centennial time-scale are found in a complex CGCM. These are driven by temperature and freshwater
surface
accumulation at mid depth and at the surface, respectively. Southern Ocean sea ice cover extent and thickness are effective
-1000
moderators of convection occurrence frequency. The convection is seen in the strength of the Antarctic Circumpolar
event CTRL-2
Current and the Atlantic Meridional Overturning Circulation, and causes fluctuations in global mean surface air temperature.
1993; Drijfhout et al., 1996]
• Oscillations caused by competing role of
surface
heat and freshwater fluxes [Pierce at al., 1995]
4
-1000
Kiel Climate Model Experiments The KCM is a global coupled general circulation model [Park and Latif, 2008] with • ECHAM5 atmosphere, T31L19 (~3.75°)
0.1 m
4200 years
ICE
0.6 m
1300 years
*constant thickness of newly formed, laterally growing ice
September sea ice thickness difference ICE-CTRL
2
bold = 5 m, dashed = 200 m
4500
Ice growth(+)/melt(-) [1012 kg/month]
P-E [ 10 12 kg/month]
convection area
year
3232 3310
year
1. Surface freshening leads 3. Diffusive rising of heat shutdown event, caused from mid depth re-starts by doubled precipitation convection
September sea ice edge (15%):
Corresponding Author Torge Martin
[email protected]
CTRL
ICE
The convection starts where the density gradient is weakest between the surface and the depth of maximum temperature.
SST [°C]
CTRL
ice area fraction
Length Salinity
Experiment
Ice thickness*
temperature maximum, delayed convection Shutdown and re-start of convection event CTRL-2 (see figure above)
Depth of max. Temp. [m] Mixed layer depth [m]
• NEMO (OPA9-LIM2) ice-ocean model, 2° grid (0.5° in the tropics), 31 levels
2. Only stable with formation of extensive sea ice coverage in the same winter
4. Only if the energy is sufficient to melt all ice and surface cooling destabilizes water column
Institute of Marine Sciences (IFM-GEOMAR) Düsternbrooker Weg 20, 24105 Kiel, Germany
The Effect: Southern Ocean Heat Balance
• The convection extracts heat from the deep Atlantic and Indian Oceans. • The heat depletion triggers enhanced heat inflow from the North Atlantic accelerating the Meriodional Overturning Circulation by 2-3 Sv. • The strength of the Antarctic Circumpolar Current is ~25 Sv stronger during active convection.
Southern Ocean heat content in the Atlantic …
Ocean heat content depletion due to convection event in ICE
… and Indian sectors
The transition between CTRL and ICE includes extensive freshening of the sea surface due to increased sea ice melt.
In ICE density of water column is lower and sea ice forms a much more stable lid that needs to be melted by rising heat from below before convection can start.
Western boundary current heat transport in the South Atlantic below 800 m
surface salinity difference CE-CTRL
References 1Leibniz
5
Bryan, F., 1986, High-latitude salinity effects and interhemispheric thermohaline circulation, Nature, 323, 301-304. Drijfhout, S., C. Heinze, M. Latif, E. Maier-Reimer, 1996, Mean Circulation and Internal Variability in an primitive Equation Model, J. Phys. Ocean., 26, 559-580.
no convection or
convection in the Weddell Sea region
Park, W., M. Latif, 2008, Multidecadal and multicentennial variability of the meridional overturning circulation, Geophys. Res. Lett., 35, L22703, doi:10.1029/2008GL035779. Pierce, D.W., T.P. Barnett, U. Mikolajewicz, 1995, Competing Roles of Heat and Freshwater Flux in Forcing Thermohaline Oscillations, J. Phys. Ocean., 25, 2046-2064.
