Browsing by Author "Hurrell, James, advisor"

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Open Access
Investigating the impact of forced and internal climate variability on future convective storm environments in subtropical South America: a large ensemble approach
(Colorado State University. Libraries, 2023) Chakraborty, Anindita, author; Rasmussen, Kristen, advisor; Hurrell, James, advisor; Anderson, Brooke, committee member
Subtropical South America (SSA) has some of the most intense deep convection in the world. Large hail and frequent lightning are just two of the hazards that profoundly affect people, agriculture, and infrastructure in this region. Therefore, it is important to understand the future convective storm environments over SSA associated with climate change and how these large-scale environmental changes are likely to change high-impact weather events in the future. Previous studies have used convection-permitting regional models and radar data to examine convective storm environments in the current climate across different regions of South America. Here, we use a large ensemble of Earth system model simulations to quantify anthropogenically-driven future changes in large-scale convective environments, as well as how those forced changes might be modified by unforced, internal climate variability. Specifically, we examine changes in different thermodynamic parameters of relevance to severe weather events over SSA in austral spring and summer (September-February). We use daily data from a 50-member ensemble from 1870-2100 performed with version two of the Community Earth System Model (CESM2). Results indicate that no forced changes in convective environments are evident until very late in the 20th century. However, increases in convective available potential energy and atmospheric stability, as well as an increase in lower tropospheric vertical wind shear, became apparent around 1990, and these trends are projected to continue throughout the rest of this century. The implication is that future large-scale environments may be favorable for less frequent, but perhaps more intense and severe convective modes and their associated hazards. Results also demonstrate that anthropogenic changes are likely to be significantly modified, regionally, by internal climate variability.
Open Access
The signature of the western boundary currents on tropospheric climate variability
(Colorado State University. Libraries, 2024) Larson, James, author; Hurrell, James, advisor; Thompson, David, advisor; Willis, Megan D., committee member
Oceanic western boundary currents play a crucial role in transporting heat poleward, thereby influencing the midlatitude climatological-mean climate and serving as an important role for midlatitude storm tracks that provide rainfall to land regions. It is not yet firmly established what role these oceanic currents play in influencing atmospheric variability. Characterized by the presence of mesoscale features such as oceanic eddies and sharp sea surface temperature (SST) gradients, the western boundary currents define a uniquely separate regime for air-sea interactions on climatic timescales relative to the rest of the ocean basins. In this study, simple but robust observational and modeling evidence reveals that anomalous precipitation and vertical motion co-vary with local SST anomalies in the western boundary currents, with a measurable influence extending into the upper troposphere. Periods of anomalously warm SSTs are associated with anomalous, co-located upward motion of > 0.02 Pa/s and precipitation anomalies of ~0.6 mm/day when averaged over a month. Yet, the standard resolution of most climate models, with grid cells on the order of 100 kilometers, fail to capture this co-variability. It is demonstrated that sharpening the horizontal resolution in both a climate model and in atmospheric reanalyses alters the spatial patterns both of sea surface temperature and of regional atmospheric processes. Given the significant influence of these western boundary currents on the broader regions surrounding them, climate projections conducted with grid cells coarser than 50 kilometers may overlook crucial processes.