  Stratocumulus clouds cover about a fifth of Earths surface, and due to their
albedo and low-latitude location, they have a strong effect on Earths radiation
budget. Previous studies using Large Eddy Simulations have shown that multiple
equilibria (both cloud-covered and cloud-free states) exist as a function of
fixed-SST, with relevance to equatorward advected air masses. Multiple
equilibria have also been found as a function of atmospheric CO2, with a
subtropical SST nearly 10 K higher in the cloud-free state, and with suggested
relevance to warm-climate dynamics. In this study, we use a mixed-layer model
with an added surface energy balance and the ability to simulate both the
cloud-covered and cloud-free states to study both types of multiple equilibria
and the corresponding hysteresis. The model's simplicity and computational
efficiency allow us to explore the mechanisms critical to the stratocumulus
cloud instability and hysteresis as well as isolate key processes that allow
for multiple equilibria via mechanism denial experiments not possible with a
full-complexity model. For the hysteresis in fixed-SST, we find that decoupling
can occur due to either enhanced entrainment warming or a reduction in
cloud-top longwave cooling. The critical SST at which decoupling occurs is
highly sensitive to precipitation and entrainment parameterizations. In the CO2
hysteresis, decoupling occurs in the simple model used even without the
inclusion of an active SST and SST-cloud cover feedbacks, and the width of the
hysteresis displays the same sensitivities as the fixed-SST case. Overall, the
simple model analysis and results motivate further studies using higher
complexity models.

Atmospheric and Oceanic Physics