Internal ocean-atmosphere variability in kilometer-scale
radiative-convective equilibrium
Abstract
We describe internal, low-frequency variability in a 21-year simulation
with a cloud-resolving model. The model domain is the length of the
equatorial Pacific and includes a mixed-layer ocean, which permits
coherent cycles of sea surface temperature (SST), atmospheric
convection, and the convectively coupled circulation. The warming phase
of the cycle is associated with near-uniform SST, less organized
convection, and sparse low cloud cover, while the cooling phase exhibits
strong SST gradients, highly organized convection, and enhanced low
cloudiness. Both phases are quasi-stable but, on long timescales, are
ultimately susceptible to instabilities resulting in rapid phase
transitions.
The internal cycle is leveraged to understand the factors controlling
the strength and structure of the tropical overturning circulation and
the stratification of the tropical troposphere. The overturning
circulation is strongly modulated by convective organization, with SST
playing a lesser role. When convection is highly organized, the
circulation is weaker and more bottom-heavy. Alternatively, tropospheric
stratification depends on both convective organization and SST,
depending on the vertical level. SST-driven variability dominates aloft
while organization-driven variability dominates at lower levels. A
similar pattern is found in ERA5 reanalysis of the equatorial Pacific.
The relationship between convective organization and stratification is
explicated using a simple entraining plume model. The results highlight
the importance of convective organization for tropical variability and
lay a foundation for future work using coupled, idealized models that
explicitly resolve convection.