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!
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.
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.
Coupled impacts of sea ice variability and North Pacific atmospheric circulation on Holocene hydroclimate in Arctic Alaska (published in PNAS December 2020)
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.
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.
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.
An Arctic watershed observatory at Lake Peters, Alaska: weather–glacier–river–lake system data for 2015–2018 (published in ESSD December 2019).