Research

Tropospheric Transport

One of the clear features of global warming is the decreased temperature gradient between the equator and the poles. This change in gradient is associated with changes to the jet location and variability. With Professor Gang Chen, I am exploring this phenomenon in a simple model in order to understand how transport of tracers, such as moisture and pollutants, will change. We are also developing a metric for diagnosing mixing in the troposphere.

Stratospheric Circulation

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The development of the ozone hole in the Southern Hemisphere in September 2015 as seen from satellites. From http://ozonewatch.gsfc.nasa.gov/

The focus of my dissertation was the circulation of the stratosphere. The stratosphere starts around the altitude where planes usually fly (~10 km) goes up to 50 km. The lower stratosphere is where the ozone layer absorbs harmful solar radiation which is important for human health and climate. As its name suggests, the stratosphere is highly stratified, and so the physics that govern its behavior are much like those that govern the ocean. I am studying how air is transported within the stratosphere, which is important for how trace gases (like ozone) are spatially distributed. The strength of the stratospheric circulation could be important for the recovery of the ozone hole in the Southern Hemisphere, for example. Together with my advisor Alan Plumb, we‘ve developed theories for how to get quantitative estimates of the stratospheric circulation from satellite observations of certain trace gases. I’ve tested the theories in a simple atmospheric model and a more comprehensive climate model. [related paper] I’ve also applied this theory to the satellite data that is available to calculate the strength of the global overturning circulation of the stratosphere. [related paper]

Tropical Ocean Eddies

ECCO_EKE
Eddy kinetic energy from the ECCO state estimate. Notice the high values in the western boundary current regions, the Antarctic Circumpolar Current and along the equator.

 

Before leaping up into the sky, I studied heat transport and eddy diffusivity in the tropical Pacific ocean in an ocean model, the ECCO state estimate working with Carl Wunsch and Raffaele Ferrari at MIT. Models of the ocean frequently use a very simplified treatment of how small scale features transport heat, and by using a high resolution state estimate, we tried to understand how significant of an effect that simplification has. The equatorial region has a large number of eddies, as shown in the plot of eddy kinetic energy on the left. We found that these eddies transport heat in a much more complicated way than is often accounted for in ocean models. However, apart from some important regional details near the inter-tropical convergence zone, this discrepancy is small compared to the total model errors.

 

El Niño in Climate Models

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Comparison of sea surface height anomalies in 1997 (previously the largest El Niño on record) and 2015. From https://svs.gsfc.nasa.gov/30629.

 

Before graduate school, I worked with Eli Tziperman at Harvard to look at El Niño in climate models. El Niño refers to unusually warm water in the eastern equatorial Pacific ocean which in turn is connected to a host of more distant phenomena, like rain on the west coast of the US and changes to the Indian monsoon. This pattern of warm water recurs every 2-7 years and is the largest signal of interannual climate variability. The dynamics of El Niño are related to both the atmosphere and the ocean and how they are coupled, so getting the El Niño behavior right is a stringent test of climate models. Using a technique borrowed from controls engineering, I compared the physical processes that lead to El Niño in a number of state of the art climate models and found some intermodel consistency and some large differences. This type of testing is important for understanding the behavior and limitations of these global climate models. [related paper]