Atmosphere Ocean Science Colloquium

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Non-linearities in the tropical SST pattern effect explained by the 'circus tent' model

Speaker: Dr. Andrew Williams, NOAA Geophysical Fluid Dynamics Laboratory

Location: Warren Weaver Hall 1302

Date: Wednesday, October 4, 2023, 3:30 p.m.

Synopsis:

In recent decades it has become evident that the spatial pattern of sea-surface temperatures (SSTs) exerts a strong control over Earth’s radiation balance by altering tropical low clouds. To quantify this so-called ‘pattern effect’, previous studies have used atmosphere-only simulations forced with localized SST perturbations to construct mappings (‘Green’s Functions’) between global-mean top-of-atmosphere (TOA) radiative fluxes and regional SSTs. However, such approaches assume that the sensitivity of Earth’s radiation balance to regional SSTs is linear with respect to the sign of the perturbation and its magnitude.

Contrary to this, we use model simulations to demonstrate that the TOA response to localized tropical SST perturbations exhibits substantial non-linearities. These non-linearities manifest both through an "asymmetry" in the response to positive and negative SST changes and a "magnitude dependence" with respect to the size of the SST perturbation. We then explain how these non-linearities arise as a simple consequence of convective quasi-equilibrium and weak (but non-zero) temperature gradients in the tropical free-troposphere, which impose a threshold on the sub-cloud moist static energy below which deep convection does not occur. We encapsulate this explanation in an intuitive, “circus tent”, model of the tropical atmosphere and present some preliminary observational evidence in support of it.

These results demonstrate that the climate response to SST perturbations is fundamentally non-linear and have important implications for past and future studies which seek to understand the response of Earth’s radiation balance to patterns of SST.