Decadal to Centennial Southern Ocean Deep Convection Variability in CMIP5 Models and in the Kiel Climate Model
Decadal to Centennial Southern Ocean Deep Convection Variability in CMIP5 Models and in the Kiel Climate Model
Abstract:
In global climate models, deep convection in the Southern Ocean is subject to large biases. For example, convection on the continental shelves around Antarctica is not represented in many models and Antarctic bottom water—if at all—is often formed in open ocean deep convection cells instead. Among the CMIP5 models, the time scale, region, and intensity of such deep convection variability vary widely affecting both the Southern Ocean mean state and Southern Ocean sector climate variability. For example, the time scale of deep convection variability ranges from multi-decadal to multi-centennial. Further, some models simulate deep-convection only in the Weddell Sea, other models only in the Ross Sea, and again other models in both regions. We systemically analyze the differences in the Southern Ocean deep convection and its variability as simulated by a number of CMIP5 models, but the reasons for the differences remain unclear.
To obtain further insight into the factors controlling Southern Ocean deep convection variability we additionally analyze integrations of different versions of the Kiel Climate Model (KCM) in which Antarctic Bottom Water is formed by open ocean deep convection in the Atlantic sector of the Southern Ocean. We conclude from the KCM simulations that both sea ice and stratification are critical parameters determining the behavior of deep convection. Sea ice acts as a lid preventing heat loss from the ocean to the atmosphere. When sea ice volume is large (small), the default state of the model tends to be non-convective (convective). The underlying period of deep convection variability is controlled by the local ocean stratification. Further, Southern Ocean deep convection variability has the potential to impact the North Atlantic via the Atlantic Meridional Overturning Circulation which is sensitive to the Antarctic Bottom Water formation rate. Conclusions drawn from the KCM are assessed by contrasting them with the results obtained from the CMIP5 models.
To obtain further insight into the factors controlling Southern Ocean deep convection variability we additionally analyze integrations of different versions of the Kiel Climate Model (KCM) in which Antarctic Bottom Water is formed by open ocean deep convection in the Atlantic sector of the Southern Ocean. We conclude from the KCM simulations that both sea ice and stratification are critical parameters determining the behavior of deep convection. Sea ice acts as a lid preventing heat loss from the ocean to the atmosphere. When sea ice volume is large (small), the default state of the model tends to be non-convective (convective). The underlying period of deep convection variability is controlled by the local ocean stratification. Further, Southern Ocean deep convection variability has the potential to impact the North Atlantic via the Atlantic Meridional Overturning Circulation which is sensitive to the Antarctic Bottom Water formation rate. Conclusions drawn from the KCM are assessed by contrasting them with the results obtained from the CMIP5 models.