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

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

 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 11-20 (bottom) Zhang et al. 2017b, J.Clim

Could such low frequency climate variability play a role in explaining observed trends? Recent (1979-2012) 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.

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

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 1979-2012? Ensemble simulations for 1979-2012: 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!

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