Date of Award

Spring 2026

Language

English

Embargo Period

4-30-2026

Document Type

Master's Thesis

Degree Name

Master of Science (MS)

College/School/Department

Department of Atmospheric and Environmental Sciences

Program

Atmospheric Science

First Advisor

Aiguo Dai

Committee Members

Brian Rose

Keywords

CMIP6, ERA5, Surface air temperature, climate variability

Subject Categories

Atmospheric Sciences | Climate

Abstract

This thesis investigates historical changes in the seasonality and daily variability of near-surface air temperature (Tas) during 1950–2024 using ERA5 reanalysis and simulations from 21 CMIP6 models. ERA5 and CMIP6 show broadly similar climatological structures of the Tas seasonal cycle and variability, with considerable inter-model discrepancies. Both ERA5 and CMIP6 show decreasing Tas variability over northern mid-high latitudes, while Tas variability increases in the low latitudes in ERA5 but changes little in CMIP6 multi-model mean (MMM) due to different change patterns among the models. To investigate the mechanisms of historical Tas variability changes, I analyzed the relationships between Tas variability and a set of physical variables at both high and low latitudes. In the polar regions, reduction in Tas variability is found to be strongly correlated with decreases in sea ice concentration (SIC), both spatially and temporally. Temporal correlations between Tas variability and mean SIC reach 0.8-0.9 in the Barents-Kara Seas, near Greenland in the Arctic, and near the Ross and Amundsen Seas in the Antarctic. A similar SIC-Tas variability relationship is also identified in CMIP6, though with weaker magnitudes in both Tas variability and SIC changes. The strong relationship with SIC loss is consistent with the amplifying effect of SIC variations on Tas variability found in previous numerical experiments. In low-latitude regions, the drivers of Tas variability changes are more complex. Using ERA5 data, I examined the relationships between Tas variability and five atmospheric stability-related variables, including boundary layer height (BLH), convective inhibition (CIN), convective precipitation frequency (fPRC), relative humidity (RH), and static stability (Sp), using both spatial pattern correlations and temporal analyses at five selected low-latitude locations. BLH and CIN changes exhibit positive correlations with Tas variability in convective regions, suggesting that both variables respond to a common large-scale forcing rather than directly driving Tas variability changes. In contrast, fPRC shows strong negative correlations with Tas variability, indicating that changes in convective precipitation frequency directly modulate Tas variability in equatorial regions. RH also displays strong negative correlations, consistent with a mechanism whereby rising mean Tas reduces RH, suppresses convective precipitation, and thereby amplifies Tas variability. Sp similarly exhibits strong negative correlations with Tas variability, further supporting the role of lower-tropospheric static stability in modulating low-latitude Tas variability. While the spatial correlations between the long-term changes in these variables and Tas variability across 30°S-30°N are generally modest, regionally coherent relationships are identified, particularly over the western Pacific, Amazon, and Congo Basin, where the correspondence is strongest during local wet seasons. In other regions such as the eastern Pacific and Indian Ocean, this seasonal dependence is even more pronounced, reflecting the important role of large-scale ocean-atmosphere interactions in modulating the stability-Tas variability relationship.

License

Creative Commons Attribution 4.0 International License
This work is licensed under a Creative Commons Attribution 4.0 International License.

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