1. Liping Zhang, Thomas L. Delworth, Will Cooke, and Xiaosong Yang
Decadal variability and predictability in the Southern Ocean - implications for interpreting recent observed trends
Liping Zhang, Thomas L. Delworth, Will Cooke, and Xiaosong Yang
GFDL/NOAA
17 Sep, 2018
Motivations:
Observed decadal scale cooling and sea ice increase in the Southern Ocean
Simulated multidecadal variability in the Southern Ocean, related to deep ocean convection, with impacts on temperature and sea ice
Goals:
Better understand the mechanism of the simulated variability
Assess the predictability of the simulated variability
Assess whether this type of variability could play a role in the recent observed trends

2. Simulated low frequency climate variability over the Southern Ocean (SO)
Physical controls
of convection
Period: ~80yr
Subsurface
heat build up
leads to
strong
convection
Weddell Sea deep convection
MLD and SST in Peak Positive phase
Power spectrum of mixed layer depth in Weddell Sea simulated by climate model
Surface
freshening
weakens
convection
Zhang and Delworth 2016 JGR-ocean; Zhang et al. 2017a, J.Clim

3.  First, diagnose predictability from Control simulation
Is that low frequency climate variability predictable?
 First, diagnose predictability from Control simulation
Prognostic potential Predictability (PPP): Large PPP value means small ensemble spread and high predictability
SST Spatial
Pattern of PPP averaged
in lead year 1-10 (top)
and year (bottom)
Zhang et al. 2017b, J.Clim

4. Could such low frequency climate variability play a role in explaining observed trends?
Recent ( ) observed SST and sea ice trends over the Southern Ocean (SO)
Observed SO surface cooling and sea ice expansion over the last several decades
Observed SST trend
Observed Sea Ice trend
Modeled SST trend
Modeled Sea Ice trend
The panels on the right show ensemble mean model simulated changes over the last several decades in response to radiative forcing changes, including greenhouses gases, aerosols, and stratospheric ozone. The simulated trends do not reproduce the observed trends around the Antarctic. Instead, the model simulates a steady warming and Antarctic sea ice loss.

5. Southern Ocean(SO) internal variability in coupled model
MLD response in active convection resembles
observed 1976/1977 Weddell Polynya
Southern Ocean deep convection variability
This natural variability can produce ~30-yr trends in SST and SIC that resemble the observations, with cooling trends in the Ross and Weddell Seas and warming trends over the Amundsen-Bellingshausen Seas

6. Reproduces observed trends!
If we initialize a coupled model from a strong phase of the convective cycle, does the model reproduce the observed trends over the period ?
Ensemble simulations
for :
A: Initialize from active phase
of convective cycle
B: Initialize from inactive phase
C: Initialize from neutral phase
(The images that went on the right side of this slide are not posted – sorry for the inconvenience).
Reproduces observed trends!
Does NOT reproduce observed trends!
Does NOT reproduce observed trends!

7. Summary and Discussion
Pronounced multidecadal model variability in the Southern Ocean is related to a buildup of heat in the deep ocean that is periodically vented to the atmosphere via deep convection.
This variability is highly predictable on decadal scales.
These multidecadal variations drive decadal scale trends in model ocean temperature and sea ice that look like observed trends.
Simulations of the last 30+ years that are initialized from a phase of this variability (with active convection) can reproduce observed trends in the Southern Ocean.
These results suggest a possible role for internal variability in explaining observed trends … and with good observations and models they may be predictable.
Caveats:
Sparse observations in Southern Ocean make it difficult to evaluate simulated variability
Model physics, especially ocean model (e.g., bias, mean state) and resolution
Internally generated SO deep convection likely not the only driver – important roles for radiative forcing & atmospheric circulation changes, including changes driven from Tropics