Sea ice is a fundamental component of the climate system and it is rapidly changing. Arctic sea ice, in particular, has undergone striking changes over the past few decades and is projected to continue to change throughout the next century. One of the goals of my research is to identify processes that control the short- and long- term evolution of Arctic sea ice and explain how sea ice cover in both hemispheres interacts with the atmosphere and ocean. I am also interested in the seasonal-to-decadal predictability of Arctic sea ice. My work uses state-of-the-art climate models, observations, and idealized models. I am also broadly interested in improving the representation of sea ice in climate models.
The ocean's global overturning circulation regulates Earth's climate by transporting heat and freshwater between hemispheres, influencing the rate of ocean heat uptake, and ventilating the interior of the ocean. Sparse observations necessitates the use of idealized, conceptual models that help to reveal primary controls on the ocean's overturning circulation. Such models have been fundamental in our understanding of the ocean’s deep stratification and meridional overturning circulation. Recent and ongoing work examines: how the ocean's global overturning circulation responds to warming on centennial-to-millennial timescales; the role of ocean heat transport in polar climate change; the influence of ocean-atmosphere coupling on the ocean's global overturning circulation; and the role of low-latitude surface forcing in transitions of the oceans's global overturning circulation in past climates.
The atmosphere exerts a fundamental influence on the ocean through exchange of freshwater, heat, and momentum, and in turn the ocean influences the atmosphere through its control on sea surface temperature and heat transport. How the ocean responds to changes in atmospheric conditions depends crucially on the processes that set surface water mass transformation. I am interested in how these relationships set the current climate and how they have differed in past climates. To disentangle these processes, I use simple expressions and climate model hierarchies that reveal fundamental aspects of the coupled ocean-atmosphere system.
The spatial pattern of climate change is set by interactions between atmosphere and ocean heat transport, radiative forcing, radiative feedbacks, and ocean heat uptake. I am interested in the relative importance of each and how idealized energy balance models can be used to infer processes that set features such as polar amplified warming and the pattern of precipitation and evaporation. This research uses simple methods and idealized models to identify how each component contributes to the pattern of climate change. I am also interested in estimating the forced and un-forced patterns of climate change using statistical methods.
Response of global ocean overturning to orbital forcing
Understanding how the general ocean 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 ocean circulation after experiencing large changes in external forcing.
ocean overturning, ocean circulation
Polar clouds and sea ice
Clouds play an important role in the Arctic environment and depend crucially on coupled ice-ocean-atmosphere interactions. Yet, precisely how the retreat of sea ice impacts the development of clouds in the Arctic and feedbacks on the climate system is unclear. Here, we are coupling a model of the atmospheric column to a simplified sea ice model to understand primary controls on cloud behavior and its radiative effects in the Arctic.
climate dynamics, feedbacks, ice-ocean-atmosphere interactions
A conceptual model of the ocean's global overturning circulation
with Andy Thompson
Idealized models have been fundamental in our understanding of the ocean's overturning circulation. In most conceptual models, the representation of deep-water formation in the North Atlantic has been prescribed or not allowed to feedback on changes in surface conditions. Here, we are developing a way to represent the formation of North Atlantic Deep Water (NADW) and couple it to existing theories of the ocean's global overturning circulation to better understand the response of the large-scale overturning circulation to warming and its role in ocean heat uptake.
ocean overturning, ocean circulation, ice-ocean-atmosphere interactions
A toy model of the ocean-atmosphere system
Over the past few decades there have been substantial advances in our understanding of how the climate system responds to external forcing through toy models of the ocean and atmosphere. However, these toy 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 temperature changes on long timescales and understand it's role in setting ocean heat uptake.
climate dynamics, feedbacks, ocean overturning, ocean circulation
Mechanisms of low-frequency variability in Arctic and Antarctic sea ice
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 anthropogenic greenhouses gases. Here, we are using a statistical method to identify mechanisms of Arctic and Antarctic sea ice variability over the observational record and in in coupled climate models.
sea ice, climate dynamics, ice-ocean-atmosphere interactions, internal variability
Response of the hydrologic cycle to global warming
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 warming and circulation changes. We also used this model to explain the spread in the predicted patterns of evaporation and precipitation made by climate models under global warming.
climate dynamics, climate change, feedbacks
Transient and equilibrium responses of the Atlantic overturning circulation to warming
Understanding how the ocean's 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 eventual recovery of the Atlantic overturning circulation. 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.
ocean overturning, ocean circulation, climate change
Constraints on the loss of Arctic sea ice
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
Uncertainty in projections of Arctic sea ice
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
Influence of the Pacific Ocean on Arctic sea ice
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
Regional predictions of Arctic sea ice
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
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
Glacier trends and natural variability in the climate system
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
Effects of orography on large-scale atmospheric and oceanic circulation
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