Robert Jnglin Wills

Climate Scientist, ETH Zürich

Research Interests

In my research I use physical principles, climate models, and statistical analysis of large data sets to understand large-scale atmospheric and oceanic circulation changes and their impacts at the Earth’s surface. This includes studying the atmosphere-ocean dynamics of decadal climate variability and the response of the large-scale atmospheric circulation to climate change. I focus on aspects of the atmospheric and oceanic circulation that are most relevant for regional climate impacts, such as for heat waves, drought, extreme rainfall, snowpack, and sea ice.

The study of climate variability and change is inherently interdisciplinary, as coupling between the atmosphere, ocean, cyrosphere, land surface, biosphere, and human sector plays a fundamental role in shaping climate on time scales from seasons to millennia. I enjoy reaching out from my expertise in atmosphere, ocean, and climate dynamics to collaborate on a broad range of questions on past and future climates, Earth system dynamics, and climate impacts.

Forced and Unforced Contributions to the Pattern of Climate Change

Separating the forced and unforced components of climate change is critical challenge in climate science, as it is a necessary step in understanding the causes of observed changes and in improving projections of future changes in regional climate. We use pattern recognition methods to distinguish between forced and unforced components of global climate changes. This helps to detect changes that are small relative to the noise from internal climate variability. For example, after removing patterns of temperature change associated with El Niño, we find that observed temperatures in the eastern tropical Pacific have increased less than in other parts of the tropics, a change that is not well represented in climate models. Pattern recognition methods also help us identify hard to detect climate responses within climate models, such as changes in the jet stream over the North Atlantic and a reduction in rainfall over the U.S. Southwest in the years following tropical volcanic eruptions. We show that these methods help to separate the forced and unforced components of climate change with up to ten times fewer ensemble members than traditional methods, even allowing for an approximation of the forced and unforced components of observed changes.

Relevant publications:

Wills, R.C.J., Y. Dong, C. Proistosescu, K.C. Armour, and D.S. Battisti, 2022: Systematic climate model biases in the large-scale patterns of recent sea-surface temperature and sea-level pressure change. Geophysical Research Letters, 49. [PDF] [SI] [Official Version] [UW News] [Presentation in Smile Webinar, July 2022]

Rugenstein, M., S. Dhame, D. Olonscheck, R. Jnglin Wills, M. Watanabe, and R. Seager, 2023: Connecting the SST pattern problem and the hot model problem. Geophysical Research Letters, 50. [PDF] [SI] [Official Version]

Armour, K.C., C. Proistosescu, Y. Dong, L.C. Hahn, E. Blanchard-Wrigglesworth, A.G. Pauling, R.C.J. Wills, T. Andrews, M.F. Stuecker, S. Po-Chedley, I. Mitevski, P.M. Forster, and J.M. Gregory, 2024: Sea-surface temperature pattern effects have slowed global warming and biased warming-based constraints on climate sensitivity. Proceedings of the National Academy of Sciences, 121 (12) e231209312. [Official Version]

Bonan, D.B., J.S. Dörr, R.C.J. Wills, A.F. Thompson, and M. Årthun, 2024: Sources of low-frequency variability in observed Antarctic sea ice. The Cryosphere, 18, 2141–2159. [PDF] [Official Version]

Dörr, J.S., D.B. Bonan, M. Årthun, L. Svendsen, and R.C.J. Wills, 2023: Forced and internal components of observed Arctic sea-ice changes. The Cryosphere, 17, 4133–4153. [PDF] [Official Version]

Maher, N., R.C.J. Wills, P. DiNezio, J. Klavans, S. Milinski, S.C. Sanchez, S. Stevenson, M.F. Stuecker, and X. Wu, 2023: The future of the El Niño-Southern Oscillation: Using large ensembles to illuminate time-varying responses and inter-model differences. Earth System Dynamics, 14, 413–431. [PDF] [SI] [Official Version]

Wills, R.C.J., D.S. Battisti, K.C. Armour, T. Schneider, and C. Deser, 2020: Pattern recognition methods to separate forced responses from internal variability in climate model ensembles and observations. Journal of Climate, 33, 8693–8719. [PDF] [SI] [Official Version] [PCC Research Highlight]

Wills, R.C.J., S. Sippel, and E. A. Barnes, 2020: Separating forced and unforced components of climate change: The utility of pattern recognition methods in large ensembles and observations. US CLIVAR Variations, 18, 1–10, Not Peer Reviewed. [PDF] [Official version] [Webinar Presentation, September 2020]

Parsons, L.A., M.K. Brennan, R.C.J. Wills, and C. Proistosescu, 2020: Magnitudes and spatial patterns of interdecadal temperature variability in CMIP6. Geophysical Research Letters, 47. [PDF] [SI] [Official Version]

Wills, R.C., T. Schneider, J.M. Wallace, D.S. Battisti, and D.L. Hartmann, 2018: Disentangling global warming, multidecadal variability, and El Niño in Pacific temperatures. Geophysical Research Letters, 45, 2487–2496. [PDF] [SI] [Official version] [Code] [Science Editor's Note] [PCC Research Highlight]

Mechanisms of Climate Variability

Unforced climate variability on decadal and longer time scales has garnered much attention, both because of its potential predictability and because it often masks the influence of externally forced climate change. We focus on identifying patterns of sea-surface temperature (SST) variability that are associated with low-frequency climate variability in the Pacific and the Atlantic. Our work provides insight into the mechanisms governing this variability, particularly the role of ocean circulation in maintaining persistent SST anomalies, the relative role of air-sea fluxes of heat and momentum in ocean circulation dynamics, and the feedback of persistent SST anomalies onto the atmospheric circulation.

This work has shown that the decadal variability of the Pacific is more independent of El Niño than previously thought and has improved our ability to separate the signals of global warming, Pacific Decadal Oscillation (PDO), and El Niño in observations. It has also established the role of ocean circulations in the PDO and Atlantic Multidecadal Oscillation (AMO) in climate models, including developing simple models for how the associated ocean circulation features respond to forcing. We are also working to quantify the impacts of different modes of variability on Earth's energy budget and global-mean surface temperature.

Relevant publications:

Wills, R.C.J., A. R. Herrington, I.R. Simpson, D.S. Battisti: Resolving weather fronts increases the large-scale circulation response to Gulf Stream SST anomalies in variable resolution CESM2 simulations. Journal of Advances in Modeling Earth Systems, submitted. [Official Preprint] [PDF] [SI]

Oldenburg, D., R.C.J. Wills, K.C. Armour, L. Thompson, 2022: Resolution dependence of atmosphere-ocean interactions and water-mass transformation in the North Atlantic. Journal of Geophysical Research: Oceans, 127. [PDF] [SI] [Official Version]

Shi, H., F.-F. Jin, R.C.J. Wills, M.G. Jacox, B.A. Black, D.J. Amaya, R. R. Rykaczewski, S.J. Bograd, M. García-Reyes, and W.J. Sydeman, 2022: Global decline in ocean memory over the 21st century. Science Advances, 8. [PDF] [SI] [Official Version] [Press Release] [EurekAlert] [Nature Research Highlight] [EOS]

Wills, R.C.J., K.C. Armour, D.S. Battisti, C. Proistosescu, and L.A. Parsons, 2021: Slow modes of global temperature variability and their impact on climate sensitivity estimates. Journal of Climate, 34, 8717–8738. [PDF] [Official Version] [Corrigendum] [Presentation in ECS and Cloud Feedback symposium, April 2022]

Oldenburg D., R.C.J. Wills, K.C. Armour, L. Thompson, and L.C. Jackson, 2021: Mechanisms of low-frequency variability in Atlantic northward ocean heat transport and AMOC. Journal of Climate, 34, 4733–4755. [PDF] [Official Version]

Årthun, M., R.C.J. Wills, H. Johnson, L. Chafik, and H.R. Langehaug, 2021: Mechanisms of decadal North Atlantic climate variability and implications for the recent cold anomaly. Journal of Climate, 34, 3421–3439. [PDF] [SI] [Official Version]

Wills, R.C.J., K.C. Armour, D.S. Battisti, and D.L. Hartmann, 2019: Ocean-atmosphere dynamical coupling fundamental to the Atlantic Multidecadal Oscillation. Journal of Climate, 32, 251–272. [PDF] [Official version]

Wills, R.C.J., D.S. Battisti, C. Proistosescu, L. Thompson, D.L. Hartmann, and K.C. Armour, 2019: Ocean circulation signatures of North Pacific decadal variability, Geophysical Research Letters, 46, 1690–1701. [PDF] [SI] [Official version]

Large-scale Atmospheric Circulation in a Changing Climate

Planetary-scale variations in the atmospheric circulation, known as stationary waves, are the principal cause of longitudinal variations in Earth’s surface climate (e.g., the warmth and wetness of winters in the Pacific Northwest relative to similar latitudes in eastern Asia). Uncertainties in the stationary wave response to climate change are thus a key source of uncertainty in future projections of regional climate. I work to understand the response of stationary waves to climate change, using a hierarchy of models to better understand which processes are important. During my Ph.D., I developed mechanistic theories that give a better understanding of how tropical and midlatitude circulations are forced by large-scale topography and by warm ocean regions, and identified the importance of moist processes and the vertical structure of the atmosphere in the response of these circulations to climate change. My ongoing research focuses on predicting and understanding changes in atmospheric circulations that are relevant for regional climate impacts, such as stationary waves, monsoonal circulations, and storm tracks.

Relevant publications:

Wills, R.C.J., R.H. White, and X.J. Levine, 2019: Northern Hemisphere stationary waves in a changing climate, Current Climate Change Reports. [PDF] [SI] [Official version]

Wills, R.C.J. and T. Schneider, 2018: Mechanisms setting the strength of orographic Rossby waves across a wide range of climates in a moist idealized GCM. Journal of Climate, 31, 7679–7700. [PDF] [Official version]

Wills, R.C., X.J. Levine, and T. Schneider, 2017: Local energetic constraints on Walker circulation strength. Journal of the Atmospheric Sciences, 74, 1907–1922. [PDF] [Official version]
Corrigendum. Journal of the Atmospheric Sciences, 76, 3965. [Corrigendum]

Wills, R.C., 2016: Stationary Eddies and Zonal Variations of the Global Hydrological Cycle in a Changing Climate. Ph.D. Thesis, California Institute of Technology. (chapters 5-6) [PDF]

Ocean-Atmosphere Dynamics in Past Climates

The atmosphere exerts a fundamental influence on the ocean circulation through surface fluxes of freshwater, heat, and momentum. For example, the absence of deep water formation and meridional overturning in the modern North Pacific results in part from an excess of precipitation over evaporation, the result of atmospheric moisture flux convergence. We are interested in how the ocean circulation differed in past climates, in understanding how these changes are related to differences in atmospheric forcing, and in developing a theoretical understanding of large-scale asymmetries in the global ocean circulation.

We have focused in particular on changes in the North Pacific ocean circulation over the last 21,000 years. We have found evidence that at the last glacial maximum, 21,000 years ago, the jet stream was further south and the Aleutian Low was stronger than in the present climate, due in part to the stationary waves forced by the Laurentide Ice Sheet over North America. Model evidence suggests that this led to a stronger and larger North Pacific subpolar gyre circulation. These atmospheric and oceanic changes made the North Pacific less stratified and allowed for stronger ventilation down to intermediate depths, potentially even allowing North Pacific deep water formation during stadial events.

Relevant publications:

Gray, W.R., C. deLavergne, R.C.J. Wills, L. Menviel, P. Spence, M. Holzer, M. Kageyama, and E. Michel, 2023: Poleward shift in the Southern Hemisphere westerly winds synchronous with the deglacial rise in CO₂. Paleoceanography and Paleoclimatology, 38. [PDF] [SI] [Official Version]

Rae, J.W.B., W.R Gray, R.C.J. Wills, I. Eisenman, B. Fitzhugh, E.F.M. Littley, P. Rafter, R. Rees-Owen, A. Ridgwell, B. Taylor, and A. Burke, 2020: Overturning circulation, nutrient limitation, and warming in the glacial North Pacific. Science Advances, 6. [PDF] [SI] [Official Version] [UW News]

Gray, W.R., R.C.J. Wills, J.W.B. Rae, A. Burke, R. Ivanovic, W.H.G. Roberts, D. Ferreira, and P.J. Valdes, 2020: Wind-driven evolution of the North Pacific subpolar gyre over the last deglaciation, Geophysical Research Letters, 47. [PDF] [SI] [Official Version]

Gray, W.R., J.W.B. Rae, R.C.J. Wills, A.E. Shevenell, G.L. Foster, C.H. Lear, and B. Taylor, 2018: Deglacial upwelling, productivity and CO2 outgassing in the North Pacific Ocean. Nature Geoscience, 11, 340–344. [Official version] [News and Views]

Ferreira, D., P. Cessi, H. Coxall, A. de Boer, H.A. Dijkstra, S.S. Drijfhout, T. Eldevik, N. Harnik, J.F. McManus, D.P. Marshall, J. Nilsson, F. Roquet, T. Schneider, R.C. Wills, 2018: Atlantic-Pacific asymmetry in deep water formation. Annual Reviews of Earth and Planetary Sciences, 46, 327–352. [Official version]

Nilsson, J., D. Ferreira, T. Schneider, and R.C.J. Wills, 2021: Is the surface salinity difference between the Atlantic and Indo-Pacific a signature of the Atlantic Meridional Overturning Circulation? Journal of Physical Oceanography, 51, 769–787. [PDF] [Official Version]

Regional Variation in the Hydrological Cycle

Atmospheric circulations transport water that is evaporated in one region and deposit it as precipitation in another. In the zonal mean, the Hadley cell and synoptic eddies transport water from the dry evaporative regions of the subtropics to regions of high precipitation in the Intertropical Convergence Zone and extratropical storm track. Variations about this zonal mean are responsible for, for example, the relative wetness of Southeast Asia and the eastern US and the relative dryness of central Asia. I am interested in how stationary eddy circulations lead to these spatial patterns in net precipitation through there influence on vertical motion in the lower troposphere and how changes in stationary-eddy circulations influence the response of the hydrological cycle to global warming.

Relevant publications:

Wills, R.C., M.P. Byrne, and T. Schneider, 2016: Thermodynamic and dynamic controls on changes in the zonally anomalous hydrological cycle. Geophysical Research Letters, 43, 4640–4649. [PDF] [SI] [Official version] [EOS Spotlight]

Wills, R.C. and T. Schneider, 2016: How stationary eddies shape changes in the hydrological cycle: Zonally asymmetric experiments in an idealized GCM. Journal of Climate, 29, 3161–3179. [PDF] [Official version]

Wills, R.C. and T. Schneider, 2015: Stationary eddies and the zonal asymmetry of net precipitation and ocean freshwater forcing. Journal of Climate, 28, 5115-5133. [PDF] [Official version]
Corrigendum. Journal of Climate, 30, 8841–8842. [Corrigendum]

Wills, R., 2016: Stationary Eddies and Zonal Variations of the Global Hydrological Cycle in a Changing Climate. Ph.D. Thesis, California Institute of Technology. (chapters 2-4) [PDF]