by Qiuxian Li, UW CICOES Postdoctoral Scholar

When I arrived in Seattle in the fall of 2023, the city’s gray skies felt both foreign and exciting. It was my first time living abroad, and everything felt new: the people, the language, and the steady rain and cloudy weather of the temperate oceanic climate. As I would soon realize, those clouds above Seattle were more than just scenery; they were central to my research.

Over two years as a postdoctoral scholar at CICOES, I explored how clouds, ocean heat transport, and atmospheric feedback interact to shape Earth’s climate. Working with mentors Wei Cheng, Kyle Armour, and LuAnne Thompson, I completed four interconnected projects that advanced understanding of global climate sensitivity, which refers to how much the Earth will warm if carbon dioxide levels double.

Project 1: What truly controls ocean heat transport

The first project began with the very first meeting with my mentors. I still remember that afternoon in LuAnne’s office, my first real scientific conversation after arriving in Seattle. We talked about my PhD work, where I had used a new set of climate experiments to separate the contribution of ocean circulation changes to the ocean temperature response under global warming. After listening carefully, Kyle made a comment that led me in a new direction. He said, “This framework might also help us decompose ocean heat transport changes toward the poles.”

There has been a long-standing debate in the literature about whether changes in ocean circulation or temperature dominate the changes in poleward ocean heat transport—the process by which seawater carries heat from warmer regions (like the tropics) to colder regions (like the poles). Much of this disagreement comes from differences in how researchers separate these two effects. In many earlier studies, the estimated contribution from temperature changes actually included part of the circulation effect, making it difficult to distinguish between the two.

Through a set of new analyses, I was able to isolate the portion of temperature change not influenced by circulation changes, more accurately quantifying its contribution to ocean heat transport. The results showed that previous methods tended to overestimate the role of circulation changes, suggesting that temperature-driven effects are more important than previously thought.

Project 2: How tropical clouds shape Arctic warming

At the same time, I began participating in a larger project led by Wei Cheng that explores how cloud feedback influences climate sensitivity. Clouds can act like a mirror or a blanket, reflecting sunlight or trapping heat. Cloud feedback refers to how clouds change in response to warming, either amplifying the warming (positive feedback) or dampening it (negative feedback). This project involved cloud-locking experiments, where we allowed everything in the climate model to change except the radiative properties of clouds in order to isolate their influence. These experiments had been completed by scientists from NOAA’s Pacific Marine Environmental Laboratory and the Department of Energy’s Pacific Northwest National Laboratory.

As I started analyzing the results, I was surprised by how strongly clouds outside the polar regions, particularly in the tropics, affected polar temperatures. Changes in tropical clouds altered how much heat was transported through the atmosphere toward the poles, which in turn impacted Arctic warming. This was when I truly began to appreciate the interconnected nature of Earth’s climate system.

Project 3: A colder start, a warmer future

After finishing the second project, I went through a short period of uncertainty. I wasn’t sure what to do next. Then another CICOES scientist, Jiaxu Zhang, invited me to analyze a set of experiments she had completed. These experiments examined how different strengths of ocean heat transport influence global climate. That conversation opened the door to my third project.

During our discussions, Kyle noticed that the difference between two experiments closely matched the difference between observations and model simulations. This meant we could use the experiments to understand not only how ocean heat transport shapes climate, but also how model biases can affect climate sensitivity.

We discovered something interesting: when ocean heat transport was weaker, the climate became cooler, especially in polar regions. But under greenhouse gas warming, these cooler climates warmed more strongly. The reason is simple yet powerful: weaker poleward ocean heat transport allows more sea ice to exist, which amplifies warming through albedo feedback—when more sea ice exists, more of it melts during warming, exposing darker ocean surfaces that absorb more sunlight and further amplify warming. This project deepened my understanding of how the mean-state ocean, meaning the background ocean temperature and circulation patterns that exist before any external forcing is applied, can shape not only today’s climate but also its sensitivity to future warming.

Project 4: The trouble with two rain belts

The idea for my final project came up during one of our weekly discussions. Since we had seen how mean-state ocean conditions influence the climate’s response to external forcing, we began to wonder whether other persistent model biases could also affect climate sensitivity. That was when LuAnne brought up the double ITCZ bias, one of the most common and stubborn climate model errors. In the real world, most tropical rainfall happens in a single band just north of the equator, known as the Intertropical Convergence Zone (ITCZ). But many climate models make a common mistake: they produce two such rain bands, one on each side of the equator.

To test how this bias might influence the climate, we designed a set of experiments that artificially forced the ITCZ to move southward, mimicking the double ITCZ pattern commonly seen in models. We found that when an ITCZ forms in the Southern Hemisphere, subtropical low clouds in the southeast Pacific decrease significantly. This region’s cloud feedback turned out to be crucial; it controls how the Southern Ocean and the tropics interact and also influences the global surface temperature response.

Moving forward

Through these projects, I began to see a common thread running through my work, a single question that ties everything together: what sets Earth’s temperature response to carbon dioxide increases? Different climate models give varying answers. Through the four projects I completed during my postdoc, we gained some important clues. One key source of uncertainty lies in cloud feedback, which varies widely among models. Another comes from mean-state biases, including how models underestimate ocean heat transport and long-standing issues such as the double ITCZ in the tropics. Each project has helped me see a different piece of this puzzle.

As I move forward, I hope to keep exploring this question, to better understand the physical processes behind climate sensitivity, and to bring us one step closer to answering how our planet will respond to human influence.

Looking back, I realize that all four of my projects were born and completed through collaboration. Each one began with a conversation. Working closely with my mentors and collaborators taught me not only how to design experiments and interpret results, but also how science grows through teamwork and curiosity.