Diagnosing Trends in Tropical Pacific Sea Surface Temperatures Using a Heuristic Atmospheric Longwave Emissivity Model
Open Access
- Author:
- Milcarek, Colton
- Graduate Program:
- Meteorology and Atmospheric Science
- Degree:
- Master of Science
- Document Type:
- Master Thesis
- Date of Defense:
- March 24, 2021
- Committee Members:
- Sukyoung Lee, Thesis Advisor/Co-Advisor
Eugene Edmund Clothiaux, Committee Member
Jerry Y Harrington, Committee Member
David J. Stensrud, Program Head/Chair - Keywords:
- emissivity
radiative transfer
atmospheric dynamics
climate change
walker circulation
atmospheric thermodynamics - Abstract:
- Atmospheric emissivity is crucial for determining how and where energy is stored and transmitted through the atmosphere. Over the past several decades, west tropical Pacific water has been warming more rapidly than the eastern part of the ocean. These sea surface temperature (SST) trends have widespread implications on weather and climate around the globe through teleconnections. In this thesis research, physical processes behind the SST trend over the period of 1979-2019 are explored. For this study, the European Centre for Medium-Range Weather Forecasts Reanalysis 5th Generation (ERA5) is used with 0.25°x0.25° monthly grid spacing. A surface energy balance analysis reveals striking resemblances between downward longwave radiation (DLR) trend pattern and skin temperature trend patterns. To investigate the cause of the longwave radiation trend pattern, a ‘gross’ emissivity model is developed constrained by ERA5 top-of-atmosphere outgoing longwave radiation (OLR) and DLR. Atmospheric emissivity for the entire column is determined by running the Rapid Radiative Transfer Model (RRTM) when the surface emissivity and reflectivity are zero, and then comparing to ERA5 OLR. The difference of these two determines the amount of upward longwave radiation absorbed by the atmosphere, and hence, gross atmospheric emissivity. Emission temperatures are determined for the RRTM-derived atmospheric OLR and ERA5 DLR by setting each respective quantity equal to the Stefan-Boltzmann Law incorporating gross emissivity. The zonal gradient in atmospheric emissivity is found to increase across the Pacific Ocean. DLR trends are mostly explained by emission temperature trends. Emissivity and emission temperature trends are consistent with a strengthening Walker Circulation. Monthly indices are created for the tropical domain by projecting emissivity, land skin temperature, ocean skin temperature, emission temperature, precipitation, and vertically integrated moisture divergence, onto their respective trend patterns. Lead-lag correlations of these indices indicate that there is a statistically significant basis for the land skin temperature trend patterns to proceed precipitation and vertically integrated moisture divergence trend patterns representative of a strengthening Walker Circulation. Moisture distribution changes lead emission temperature and emissivity trend patterns, as well as the SST trend pattern. These lead-lag relationships suggest that the trend towards La-Niña-like SST conditions, at least in part, are realized through the tropical land skin temperature trend acting to strengthen the Walker Circulation. This land surface temperature trend component may have a greater prevalence in boreal winter months, leading to an ocean SST trend response in boreal spring months. The increasing land-sea thermal contrast of the tropical regions of the Americas and Africa may be of interest in inciting strengthening trends of the Walker Circulation.