Multi-decadal Dynamics of Southern Ocean Open-Ocean Polynyas and Deep Convection: Insights from Climate Models
Open-ocean deep convection and polynyas in the Southern Ocean play a fundamental role in regulating global climate through its influence on water mass formation, sea ice, and the large-scale overturning circulation. However, its variability, underlying mechanisms, and global impacts remain incompletely understood, particularly on multi-decadal to centennial timescales. This thesis investigates the dynamics and climate significance of Southern Ocean deep convection in preindustrial (non-anthropogenic) conditions, using diagnostic outputs from multiple climate models. First, the relationship between deep convection and open-ocean polynyas (OOPs) is examined across 49 CMIP6 models. While both phenomena are commonly simulated, their co-occurrence is highly variable. The results demonstrate that polynyas are not a reliable indicator of the deep convection, motivating a distinction between "deep OOPs" associated with convective overturning and "shallow OOPs" driven by atmospheric and/or sea ice processes. Second, the global impacts of Southern Ocean convection are explored using long preindustrial simulations from three generations of GFDL climate models. The analysis reveals a self-sustained oscillatory mode linking convection in the Weddell and Ross Seas to variability in the Atlantic Meridional Overturning Circulation (AMOC). Convective events enhance Antarctic Bottom Water formation and strengthen the lower overturning cell, while weakening the upper cell, producing a multi-decadal to centennial "seesaw" in AMOC structure that propagates globally through wave and advective processes. Finally, a detailed investigation of a high-resolution eddy-permitting model (GFDL-CM4) identifies a dominant centennial mode of Southern Ocean variability involving coupled fluctuations of subpolar gyres, the Antarctic Circumpolar Current (ACC), and deep convection. Convection in the Ross and Weddell Seas show distinct regional characteristics but is dynamically linked through both rapid coastal propagation and slower circumpolar advection. These interactions give rise to a self-sustained oscillatory system in which convection, gyre dynamics, and ACC transport are tightly coupled. Together, these results demonstrate that Southern Ocean deep convection is a key source of internally generated, low-frequency climate variability, with essential impacts on the global ocean circulation. This work provides a unified mechanistic framework for understanding Southern Ocean variability and highlights the importance of basin-scale connectivity and mesoscale processes in shaping climate variability on multi-decadal to centennial timescales.
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