Overview
I am broadly interested in the understanding and model simulation of climate change and variability. In the past, I have studied changes in hydroclimate and atmospheric circulation from anthropogenic forcing. I have also studied the uncertainties in climate projections on both global and regional scales. One of my current research projects focuses on tropical air-sea coupling. The goal is to quantify various coupling feedback processes and to build a simple and practical framework for modeling air-sea interactions. I am also working on the connection between transient climate sensitivity and regional ocean heat uptake.
Subtropical Precipitation Decline
Time evolution of global mean surface warming (left) and zonal mean precipitation changes (right) from abrupt 4xCO2 simulations. Note the slow global warming and the rapid precipitation decline at approximately 30o latitude. |
The subtropics encompass many of the world's driest human habitats, and climate models ubiquitously project a future decline in subtropical precipitation. This precipitation decline has been widely attributed to either the increase in atmospheric moisture or the poleward expansion of the Hadley cell - both mechanisms are associated with the slow warming of the global surface temperature. However, by abruptly increasing CO2, we found that the subtropical precipitation decline forms much faster than the global surface warming and is therefore unrelated to the previously proposed mechanisms. Then what actually causes the subtropical precipitation decline?
Find out more in He and Soden [2016, Nature Climate Change].
Climate Sensitivity and Ocean Circulation
Time evolution of global mean surface warming from 1%CO2 simulations branching off from a pre-industrial control simulation (1860-start, blue) and a present-day control simulation (1990-start, red) and the difference between the two (black). |
Climate models show large uncertainty in the simulation of transient climate sensitivity. Much of this uncertainty could result from biases in the simulation of the base climate. However, because models differ in multiple ways, it is often difficult to narrow the uncertainty to specific dynamical processes. In this study, we compared two simulations with the same model but in which CO2 is increased from either a pre-industrial or a present-day control simulation. This allowed different base climate ocean circulations that are representative of those in current climate models to be imposed upon a single model. As a result, the model projects different transient climate sensitivities that are comparable to the multi-model spread. What can we learn from these simulations?
Find out more in He et al. [2016, Journal of Climate].
Weakening of the Tropical Circulation
Percentage changes in tropical mean precipitation (P), surface moisture (q) and convection (M) from fully coupled (top), uncoupled CO2 only (bottom left) and uncoupled warming only (bottom right) simulations. Note the negative changes from M in both CO2 only and warming only simulations. |
As the climate warms, the tropical overturning circulation slows down. It is commonly believed that the circulation weakening is largely associated with the warming. However, by increasing CO2 in an uncoupled simulation with fixed surface temperature, we found that substantial weakening also occurs. So what causes the weakening of tropical circulation?
Find out more in He and Soden [2015, Journal of Climate] & Ma et al. [2016, Environmental Research Letters].
Climate Change and Sea Surface Warming Pattern
Land precipitation change from a fully coupled simulation (left) and an uncoupled simulation with only a spatially uniform warming (right). Note the similarity between the two. |
Studies found that the pattern of future changes in sea surface temperature (SST) plays a dominant role in circulation and precipitation changes over tropical oceans. It is expected that the pattern of SST changes should also have a large influence on the climate change over land. This expectation is based on the fact that natural variations in tropical SST (e.g., ENSO) often have substantial remote climatic impact. However, we found that land climate change is actually quite similar in a fully coupled simulation and an uncoupled simulation with only a uniform SST warming. So why is climate change over land insensitive to the pattern of future SST changes?
Find out more in He et al. [2014, Geophysical Research Letters].