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 role of mountain ranges and ocean heat transport in setting the structure of planetary-scale atmospheric circulations. 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.
Mechanisms of decadal climate variability
Collaborators: David Battisti, Kyle Armour, LuAnne Thompson, Dennis Hartmann, Mike Wallace (University of Washington)
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. My work focuses on identifying patterns of sea-surface temperature (SST) variability that are associated with low-frequency climate variability in the Pacific and the Atlantic. My 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. My work also aims to quantify the impacts of different modes of variability on Earth's energy budget and global-mean surface temperature.
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]
Wills, R.C., D.S. Battisti, D.L. Hartmann, and T. Schneider, 2017: Extracting modes of variability and change from climate model ensembles. Proceedings of the 7th International Workshop on Climate Informatics: CI 2017, V. Lyubchich, N.C. Oza, A. Rhines, and E. Szekely, Eds., NCAR Technical Note NCAR/TN-536+PROC, 25–28. [PDF] [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]
Regional variation in the hydrological cycle
Collaborators: Tapio Schneider (Caltech), Michael Byrne (Imperial College London)
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.
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. Corrigendum. Journal of Climate, 30, 8841–8842. [PDF] [Official version] [Corrigendum]
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., 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., 2016: Stationary Eddies and Zonal Variations of the Global Hydrological Cycle in a
Changing Climate. Ph.D. Thesis, California Institute of Technology. (see chapters 2-4) [PDF]
Stationary eddy circulation changes with warming
Collaborators: Tapio Schneider (Caltech), Xavier Levine, Rachel White (Barcelona Supercomputing Center)
Partly motivated by their importance for the zonal variation of P - E, I would like to know how the amplitude of stationary eddies changes with climate change. To begin to answer this question, I have sets of experiments in an idealized GCM, where stationary eddies are forced by a large-scale Gaussian mountain or an equatorial ocean heat flux convergence. The stationary eddies from these two sources respond differently to climate changes in this idealized model.
In the Gaussian mountain case, the response depends on the zonal wind hitting the mountain, the latent heating in the orographic flow, and the top-of-mountain surface pressure and thus can behave in a number of different ways depending on the latitude of the mountain.
In the equatorial heating case, stationary waves in midlatitudes are driven by a Walker circulation along the equator. The strength of this Walker circulation can be constrained with energetic arguments focused on the net column energy input and the gross moist stability, an effective static stability for tropical circulations.
Wills, R.C.J., R.H. White, and X.J. Levine: Midlatitude stationary waves in a changing climate, submitted to Current Climate Change Reports (invited contribution).
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]
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. (see chapters 5-6) [PDF]
Atmospheric influences on the ocean at the Last Glacial Maximum
Collaborators: James Rae (St Andrews), William Gray (St. Andrews)
Stationary eddy circulations were substatially altered during the Last Glacial Maximum, when large continental-scale ice sheets covered North America and Europe, providing additional stationary-wave forcing. The surface winds, surface heat fluxes, and net precipitation associated with the stationary waves provide forcing to the Northern Hemisphere oceans. We would like to understand how stationary wave and ocean circulation differences simulated by models fit with sediment core based proxies of temperature, ocean convection, and surface nutrient availability. Changes in the ocean circulation associated with this stationary wave forcing could have implications for the release of CO2 from the deep ocean, an important driver of glacial cycles.
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]
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: Expansion and strengthening of the North Pacific subpolar gyre during the Last Glacial Maximum, submitted to Geophysical Research Letters.
Rae, J.W.B., R.C.J. Wills, A. Burke, I. Eisenman, B. Fitzhugh, W.R Gray, R. Rees-Owen, and B. Taylor: Overturning circulation, nutrient limitation, and warming in the Glacial North Pacific, in revision.