Research

Reconstructing changes in the jet stream using tree rings and documentary data

My postdoctoral research is focused on reconstructing past changes in the jet stream. As the Arctic warms, there is increasing interest in possible relationships between this high latitude warming and lower latitude ocean and atmospheric circulation, including possible connections to the position, speed, and waviness of the jet stream, or the "polar vortex." Paleoclimate records can help clarify warming-caused changes in the jet stream by providing long-term context with which to compare current jet stream behavior.

To this end, I am currently involved in multiple projects that seek to reconstruct various aspects of the jet stream's behavior using existing tree ring records. In the Northern Hemisphere, part of each annual tree ring (the "latewood") is sensitive to climate conditions during the summer months. As such, we can use changes in the characteristics of this latewood over time to reconstruct summer season jet stream behavior, using carefully chosen sites that correspond to weather patterns associated with different jet positions. I am currently analyzing these data in order to understand changes in jet stream wave numbers, and (with many collaborators) am combining a tree-ring-based North Atlantic jet stream reconstruction with documentary climate records to understand the influence of summer jet extremes on both weather and societies in Europe in the past. Results coming soon!

Superposed epoch analysis showing the relationship between reconstructed summer North Atlantic jet stream extremes and poor harvests in the British Isles and the Balkans (above) and fires in the Balkans (below).

Above: the prevailing patterns of storm tracks (orange arrows) in years with a strong (A) versus weak (B) Aleutian Low. Orange star shows location of the Kenai Peninsula.
Below: purified diatoms from Sunken Island Lake in the Kenai lowlands, which were analyzed for oxygen isotopes, under a scanning electron microscope. Both images from Broadman et al. (2020).

Reconstructing Holocene hydroclimatic change in southern Alaska

A primary goal of my PhD research was to reconstruct past climate conditions for the Kenai Peninsula lowlands in southern Alaska. This region is very sensitive to ocean and atmospheric circulation patterns in the North Pacific Ocean. Depending on these large-scale circulation patterns, storms arriving in the Kenai Peninsula travel different paths over the land and ocean, which can make them stronger (more precipitation) or weaker (less precipitation).

I reconstruct changes in these past hydroclimate conditions using various environmental indicators found in lake sediment cores, especially the oxygen isotope composition of diatoms. Main findings include:

the last ~4,000 years has generally been the wettest time in the Holocene (last ~11,000 years), likely in part due to strengthening of the Aleutian Low atmospheric pressure cell.
groundwater sourced from melting glaciers is likely a key component of hydrologic and isotope mass balance in lakes near retreating glaciers, both now and in the past.

Relevant publications:

Multi-proxy evidence for millennial-scale changes in North Pacific Holocene hydroclimate from the Kenai Peninsula lowlands (published in QSR July 2020)

Reconstructing postglacial hydrologic and environmental change in the eastern Kenai Peninsula lowlands using proxy data and mass balance modeling (published in QR March 2022)

Media:

Masters in Mud, by Emily Stone

Drivers of Holocene precipitation in Arctic Alaska

Rising temperatures in recent decades have resulted in rapid reductions in the annual extent and duration of Arctic sea ice. Holocene Arctic sea ice extent has been reconstructed from marine sediment cores, but little is known about the impacts of these past sea ice conditions on terrestrial climate in Arctic Alaska. Using sedimentary oxygen isotope data, modern water isotope data, and isotope enabled model output, I am investigating the role of past sea ice dynamics on Arctic Alaska's climate, and found that reduced sea ice has been associated with wetter conditions, in line with projections for the future. These Arctic sea ice and precipitation dynamics may also be linked past changes in North Pacific ocean-atmosphere circulation.

Relevant publications:

Coupled impacts of sea ice variability and North Pacific atmospheric circulation on Holocene hydroclimate in Arctic Alaska (published in PNAS December 2020)

Anomalies for the oxygen isotope composition of precipitation (A, B, C) and precipitation amount (D, E, F) in three sea ice reduction scenarios (the smallest reduction on the left progressing to the largest reduction on the right). (Broadman et al., in revision).

The bright white line is a volcanic ash (tephra) deposit in sediment collected from Sunken Island Lake, in the Kenai Peninsula lowlands.

Quaternary geochronology

Our reconstructions of past environmental conditions are only meaningful because we are able to set them to age scales, and figure out the timing of past events. Therefore, improving age control in geologic records is very important. I am interested in many Quaternary geochronologic methods. Lake sediment records most commonly rely heavily on radiocarbon (14C) chronologies, but I am currently working to improve some new and existing such chronologies from the Kenai Peninsula by using layers of volcanic ash (tephras). I have also developed soil chronosequences to characterize the relative ages of soil profiles that formed atop lava flows in northern Arizona.

Relevant abstracts:

A soil chronosequence from loess deposits on late Pleistocene lava flows, northern Arizona, USA (GSA 2013)

Human influence on paleoenvironments

Humans have an immense impact on the natural environment. Sediments from lakes, marshes, and soils can contain evidence of these impacts, often in the form of dramatic changes in sedimentation rate or other sediment characteristics. To this end, I have reconstructed the impact of European settlement and population growth in mid-coastal California during the 18th and 19th centuries, using non-native pollen and basic sediment properties to discern changes in the environment. I have also studied the impact of volcanic eruptions on soil fertility in northern Arizona in the last millennium.

Relevant abstracts:

Late Holocene Environmental History of the Los Osos Watershed, Morro Bay, CA (AGU 2014).

Prehistoric Agriculture and Soil Fertility on Lava Flows in Northern Arizona, USA (AGU 2013).

After the establishment of Europeans during the Mission period (late 18th century), sedimentation rate accelerated drastically in the Morro Bay area on the central California coast, likely due to a change in land use practices. Shown here are several indicators that demonstrate this change; the amount of organic matter, carbonate, and other mineral matter are all deposited much more quickly following the establishment of the local Mission (Broadman et al., in prep).

These panels show several consecutive days of a high flow event where Carnivore Creek (entering the lake to the left on these images) deposited sediment into Lake Peters. The colors indicate the level of turbidity, or cloudiness, in the lake, with red being the most turbid, or cloudly, due to suspended sediments (Broadman et al., 2019).

Environmental monitoring and data curation

Collecting high resolution, high quality meteorological data is important for understanding current climate conditions, and for providing a baseline to compare with future changes. Such data also can be useful for validating climate and environmental models. In remote locations, these data can be difficult to collect, but are very valuable. I was involved in a 4-year environmental monitoring effort in the Arctic National Wildlife Refuge, where we collected (sub)hourly weather, river, lake, glacier, and sediment based datasets. I then curated these data at the Arctic Data Center, where they can be freely and reliably accessed and used by scientists worldwide.

Relevant publications:

An Arctic watershed observatory at Lake Peters, Alaska: weather–glacier–river–lake system data for 2015–2018 (published in ESSD December 2019).

Associated datasets:

Collaborative research: Developing a System Model of Arctic Glacial Lake Sedimentation for Investigating Past and Future Climate Change (NSF Arctic Data Center).