DAVE BONAN

Current Projects

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Response of ocean's global overturning circulation to orbital forcing

with Andy Thompson, Emily Newsom, and Jess Adkins

Understanding how the ocean's global overturning circulation responds to orbital forcing is important for interpreting paleoclimate records, especially during the glacial-interglacial cycles of the late Pleistocene. Here, we are using a state-of-the-art climate model that was forced with a range of greenhouse-gas concentrations and different combinations of obliquity and precession to study the equilibrium structure of the ocean's global overturning circulation under various climate states.

ocean overturning, ocean circulation

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A conceptual model of the ocean-atmosphere system

with Andy Thompson, Tapio Schneider, and Laure Zanna

Over the past few decades there have been substantial advances in our understanding of how the climate system responds to external forcing through conceptual models of the ocean and atmosphere. However, these models typically treat the atmosphere or ocean as a fixed component that cannot feedback on the other. Here, we are coupling an idealized model of the ocean's global overturning circulation to an energy balance model of the atmosphere to understand how ocean circulation responds to external forcing on long timescales and understand it's role in setting ocean heat uptake.

climate dynamics, feedbacks, ocean overturning, ocean circulation

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Transient and equilibrium responses of the ocean's overturning circulation to climate change

with Andy Thompson, Tapio Schneider, and Laure Zanna

Idealized models have been fundamental in our understanding of the ocean's global overturning circulation. In most conceptual models, there is no explicit representation of temperature and salinity. Here, we are developing an idealized model of the ocean's global overturning circulation that represents both temperature and salinity to better understand the response of the large-scale overturning circulation to climate change, including both changes in temperature and the large-scale hydrological cycle.

climate dynamics, feedbacks, ocean overturning, ocean circulation

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Previous Projects

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Contributions to regional precipitation change under warming

with Nicole Feldl, Mark Zelinka, and Lily Hahn

In response to increased greenhouse-gas concentrations, climate models predict the largest relative change in precipitation will occur in the polar regions, where a substantial absolute increase in precipitation coincides with small precipitation rates in the present-day climate. Here we used an atmospheric energy budget to identify the dominant contributors to the polar-amplified pattern of precipitation change. We also examine contributing factors to the intermodel spread in regional precipitation change.​

precipitation, climate change, feedbacks

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Mechanisms of low-frequency variability in Arctic and Antarctic sea ice

with Jakob Dörr, Marius Årthun, Robb Wills, Andy Thompson, and Lea Svendsen

The sea ice cover of each hemisphere exhibits a large degree of variability on interannual-to-decadal timescales, oftentimes masking the response of sea ice to increased greenhouses-gas concentrations. Here, we used a statistical method to identify mechanisms of Arctic and Antarctic sea ice variability over the observational record. In the Arctic, we quantified the contribution of internal variability to observed Arctic sea ice trends. In the Antarctic, we identified sources of decadal sea ice variability that have contributed to the observed sea ice expansion and abrupt decline events.

sea ice, climate dynamics, ice-ocean-atmosphere interactions, internal variability

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Role of climate feedbacks on precipitation changes

with Nicole Feldl, Nick Siler, Jen Kay, Kyle Armour, Gerard Roe, Ian Eisenman, and Emma Beer

Changes to the pattern of precipitation under warming are set by interactions between ocean heat uptake, radiative forcing, climate feedbacks, and atmospheric energy transport. Here we used an idealized energy balance model with a modified Hadley Cell parameterization to examine the role of individual climate feedbacks on changes to the pattern of precipitation. We also examined how each of these components shape the pattern of precipitation, such as the polar-amplified pattern of relative precipitation change.​

climate dynamics, climate change, feedbacks

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Response of the hydrologic cycle to warming

with Kyle Armour, Gerard Roe, and Nick Siler

Recent studies have shown that an idealized model that makes an assumption about how atmospheric heat transport behaves is remarkably successful at emulating the hydrologic response of climate models to an increase of carbon-dioxide. Through the lens of this simple model we studied how the pattern of changes in precipitation and evaporation depends on the pattern of climate feedbacks through changes in large-scale circulation features like the Hadley Cells. We also used this idealized model to explain the spread in the predicted patterns of evaporation and precipitation made by climate models under global warming (see Bonan et al., 2023).

climate dynamics, climate change, feedbacks

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Transient and equilibrium responses of the Atlantic overturning circulation to warming

with Andy Thompson, Emily Newsom, Shantong Sun, and Maria Rugenstein

Understanding how the ocean's global overturning circulation responds to increasing greenhouse-gas concentrations is important for predicting future climate. Here, we studied the transient and equilibrium responses of the Atlantic overturning circulation to warming in a suite of millennial-length simulations from state-of-the-art climate models. We showed that a simple expression — which is widely used in idealized ocean models — can be used to explain both the initial weakening and various levels of recovery of the Atlantic overturning circulation in climate models. This study improves our understanding of the short- and long- term changes of the Atlantic overturning circulation to external forcing and highlights the unique role of salinity and temperature dynamics (see Bonan et al., 2022).

ocean overturning, ocean circulation, climate change

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Constraints on the loss of Arctic sea ice

with Tapio Schneider, Ian Eisenman, and Robb Wills

Projections of Arctic sea ice area are marred by largest uncertainties, which arise primarily because of structural differences between climate models and how they respond to rising greenhouse-gas concentrations. To constrain this uncertainty, we used a simple model that relates past sea ice to future sea ice through a metric known as the local sea ice sensitivity. This simple model enabled us to explain what processes cause the inter-model spread in model projections of Arctic sea ice and how these processes influence estimates of when the Arctic will be seasonally free of sea ice (see Bonan et al., 2021b).

sea ice, climate change

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Uncertainty in projections of Arctic sea ice

with Flavio Lehner and Marika Holland

Arctic sea ice has declined rapidly over the past few decades and is projected to continue declining throughout the 21st century. Internal variability of the climate system can mask human-induced sea-ice loss on decadal timescales, meaning it must be properly accounted for when interpreting observations and characterizing projections. Using a suite of fully-coupled global climate model ensembles that represent different realizations of internal variability, we studied how internal variability confounds estimates of regional and total Arctic sea ice loss in the coming decades (see Bonan et al., 2021a).

sea ice, internal variability, climate change, predictability

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Influence of the Pacific Ocean on Arctic sea ice 

with Ed Blanchard-Wrigglesworth

Our understanding of atmospheric teleconnections is derived from the temporally-limited observational record, which means we are only seeing a glimpse of these patterns. Using an ensemble of fully-coupled global climate models, we studied how a relationship between the Pacific Ocean and Arctic sea ice evolves in time and showed that this relationship is non-stationary in time. In the context of observed Arctic sea ice, when this mode shifts in the future, we can expect significant changes to the magnitude of sea ice loss (see Bonan and Blanchard-Wrigglesworth, 2020).

sea ice, ice-ocean-atmosphere interactions, internal variability, predictability

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Glacier trends and natural variability in the climate system

with Knut Christianson and John Christian

Glacier retreat is an iconic symbol of anthropogenic climate change. Mass loss from any particular glacier, however, is the result of both anthropogenic and natural changes. Using a statistical method known as dynamical adjustment, we studied how circulation anomalies affect seasonal glacier mass-balance trends to quantify the influence of natural variability on glacier mass loss in the North Atlantic (see Bonan et al., 2019b).

climate variability, internal variability, ice-ocean-atmosphere interactions

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Regional predictions of Arctic sea ice

with Mitch Bushuk and Mike Winton

Accurately predicting Arctic sea ice is of interest to many stakeholders, including indigenous communities, fisheries, and the shipping industry. Using an ensemble of fully-coupled global climate models and a simple linear regression model, we demonstrated that there is a robust springtime predictability barrier across dynamical models, which suggests there is a fundamental limit on accurate forecasts of regional Arctic sea ice and a physical mechanism universal to all global climate models (see Bonan et al., 2019a and Bushuk et al., 2020).

predictability, sea ice 

 

Uncertainty in the spatial pattern of warming

with Kyle Armour, Gerard Roe, Nick Siler, and Nicole Feldl

As global climate models have increased in complexity, the number of physical processes representing the climate system has also increased. A central goal of climate science is to understand how uncertainty in these physical processes translates into uncertainty in the forced response. Within climate models, though, it is a challenge to disaggregate the individual factors contributing to uncertainty and explore each in a systematic way. To circumvent this problem, we used a simple model —  which describes how energy is transported from the tropics to the poles — to study uncertainty in the spatial pattern of warming (see Bonan et al., 2018 and this EOS spotlight).

climate dynamics, climate change, feedbacks

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Effects of orography on large-scale atmospheric and oceanic circulation 

with Dargan Frierson and Rachel White 

Mountains play an important role in shaping the Earth's climate. Not only do these features modulate the circulation of the atmosphere, but they also control the strength of circulation in the ocean through changes in precipitation and wind patterns. By running idealized experiments in which mountains were removed from specific geographic regions in global climate models, we studied the influence of orography on large-scale oceanic and the atmospheric circulation.

ocean circulation, ocean overturning, atmospheric circulation