Research

Atmospheric transport

Gases and particulates are transported far from their origins by the atmosphere and its strongest currents (the jet streams), affecting atmospheric chemistry, air quality, and climate. However, transport processes are subject to large errors in climate models, in part because numerical models are overly diffusive. In parallel, changes in the behavior of the jets in response to human activity remain uncertain. This has important implications for the validity of climate projections, especially since highly nonlinear chemical and dynamical processes can lead to very large errors. My research in this area is primarily driven by the following questions:

• What is a jet, anyway? Dr. Jezabel Curbelo and I set to develop a robust and objective framework to understand how jets may behave in the future. We started by revisiting the very definition of the jets, showing that a Lagrangian view has many advantages over the traditional Eulerian view (see JetLag). Our work is currently under review with Nat. Commun. Earth & Env.
• How badly do climate models overestimate the dispersion of airborne material? I am working with Dr. Arlene Fiore and Dr. Sebastian Eastham to develop a new definition of coherent ‘air mass’ and quantify the rate at which wildfire smoke and urban emissions are dispersed by the real atmosphere. We hope to quantify model errors, so that they can eventually be corrected.

Emergence of climate trends

The planet is warming, but some long-term changes remain difficult to confirm observationally (e.g., shifts in the jets, trends in midlatitude lower stratospheric ozone, changes in the Brewer-Dobson Circulation and in the Atlantic Meridional Overturning Circulation). After all, the climate system is noisy, making it difficult to distinguish trends from noise, and our historical records are limited in time and space, making it difficult to see the full picture. In order to determine whether a trend is ‘real’ or an artifact, I seek to answer the following questions:

• Have we collected enough data?
• Are observed changes significantly larger than random fluctuations of the climate system?

I developed a new method with collaborators at SCRIPPS and Harvard to answer these questions and properly assess the degree of confidence we should place in long-term trends calculated from historical records. The method has already been applied to long-term trends in the stratospheric circulation and the ozone layer, and it also informed the budget and logistics for a satellite mission proposal led by JPL.

Atmospheric composition

Without the stratospheric ozone layer, life on Earth would likely be severely limited by ultraviolet (UV) radiation exposure. The ozone hole has understandably garnered tremendous scientific concern. Its recovery since the 2000s is the object of ongoing research and monitoring, in particular, the interaction between the ozone layer and the stratospheric circulation. Ozone also plays a role lower down, for instance in the upper tropical troposphere where it acts as a potent greenhouse gas. Multiple factors contribute to changes in ozone abundances there, making it challenging to attribute observed trends to specific processes. Some questions I am working to address include:

• Is the expected acceleration of the stratospheric circulation detectable using observations? Some of my work appears in a review paper and uses the time of emergence framework described above to answer this question. I have also contributed to the development of a new satellite mission at Caltech’s Jet Propulsion Laboratory to monitor the circulation of the stratosphere and its connection with its composition.
• What degree of confidence should we place in long-term ozone trends seen by satellites? A study I led with collaborators from JPL, Harvard, and Princeton showed that some satellites, contrary to expectations, provide a distorded view of long-term changes in stratospheric ozone. This may help to explain discrepancies in recent literature.
• Can models help us understand ozone change and variability in the historical record? Human activity has already been shown to have a major impact, and I am working to understand the role of another major influence: biomass burning.

Tropical cyclones

Tropical cyclones have historically been studied from the surface up, mostly because of the extensive damage they produce at the surface. However, tropical cyclones also affect the upper atmosphere around them, with climate impacts that are still uncertain. During my PhD, I asked:

• What is cooling the tropopause above tropical cyclones?
• What effects, if any, does this cooling have on tropical cyclones and on the climate system at large?

I started by providing a new detailed view of the fine-scale structure of cold anomalies found above tropical cyclones, and ruled out cloud radiative effects as the main mechanism. The cooling appears to be of dynamical nature (a response to the release of latent heat by convective activity in the core of the cyclones), but the effects of the cooling on the cyclones themselves remains an open question.