On the climate response to spatial patterns of absorbing aerosol and sea-surface temperatures

Earth’s climate demonstrates rich regional variations; no two places on our planet are quite the same. From rain forests to deserts and doldrums to storm tracks, the character of the atmospheric circulation and its links to rainfall and cloudiness vary markedly across the globe. These regional features present a problem to the climate science community when faced with the problem of understanding how human activities (which also have a distinct spatial pattern) impact on climate. This thesis aims to bolster our understanding of how the geographical location and pattern of perturbations impact on their climate response. To do this, we focus on two problems; the climate response to localized absorbing aerosol and to sea- surface temperature anomalies. In both of these cases, we use an idealized modeling approach to isolate the key features of the problem and provide mechanistic, physical insights into the ‘spatial sensitivities’ we uncover.
This thesis begins by investigating the relationship between global-mean effective radiative forcing (ERF) and global-mean aerosol optical depth (AOD) for the case of absorbing aerosol, in Chapter 2. We use a large ensemble of atmosphere-only simulations with identical aerosol absorption perturbations in different geographical locations to show that the interactions between deep convection and diabatic heating resulting from aerosol absorption (such as occurs during large wildfire events) leads to an “aerosol pattern effect”, where the ERF from an identical burden of absorbing aerosol can vary by an order-of-magnitude and even change sign depending on where the aerosol heating is located relative to the region of tropical ascent.
Building on this work, we then investigate the implications of this “aerosol pattern effect” for understanding the global- and local- precipitation response to absorbing aerosol in Chapter 3. Contrary to the assumptions in recent work, we find that the relationship between global-mean AOD and global-mean precipitation changes depends strongly on the geographical location of the aerosol plume. We use an energetic decomposition of the global-mean precipitation changes into contributions from shortwave absorption, longwave cooling and sensible heat fluxes and link changes in these quantities to the dynamical and thermodynamical responses to localized aerosol heating. Further to this, we also explore the local precipitation response to absorbing aerosol, confirming the ‘tropical-extratropical’ contrast found by previous studies. We then provide a robust physical mechanism for this contrast in terms of simple atmospheric dynamics.
In Chapter 4 we then turn our attention to the related question of how sea- surface temperature anomalies in different regions impact on the global-mean top-of-atmosphere (TOA) radiation budget, a phenomenon often referred to as the “SST pattern effect”. Previous work has attempted to quantify the SST pattern effect using a Green’s function approach, which assumes that TOA fluxes respond linearly to SST anomalies in a given region and that they combine linearly. However, using idealized model simulations, I demonstrate that the response of global-mean TOA fluxes to localized tropical SST perturbations is both non-linear and non- additive. I then explain that this behavior arises as a robust consequence of convective quasi-equilibrium and weak (but non-zero) temperature gradients in the tropical free-troposphere, which I encapsulate in a ‘circus tent’ model of the tropical atmosphere. These results demonstrate that the climate response to SST perturbations is fundamentally non-linear, due to the physics of deep convection.
All-in-all, this thesis highlights the crucial role that the spatial pattern of perturbations plays in determining their end climate response, which is an aspect which is often ignored in simple, ‘top-down’ models of the climate system. Furthermore, we have used an idealized modeling approach to find simple, physical mechanisms which explain how localized perturbations (in the form of absorbing aerosol and sea-surface temperature anomalies) generate different responses depending on their location. These ‘spatial sensitivities’ are often largest in the tropics, where the interactions between local perturbations and deep convection can lead to large, non-local responses; this suggests a deeper understanding of the links between deep convection and the large-scale atmosphere is needed to fully understand the climate response to spatial patterns of forcing.

Advisor: Stier, P