Humans’ ability to combat heat stress through sweat-based evaporative cooling is modulated by air temperature and humidity, rendering human health highly sensitive to humid heat extremes. While the field of climate science has studied extreme dry bulb temperatures for decades, exploration of the physical drivers producing extreme humid heat is nascent in comparison. Two major areas of development for the evolving field are: 1) improving understanding of the local drivers of extreme humid heat, and 2) collating a set of universal physical mechanisms which generate humid heat extremes across the planet. The four chapters of this dissertation together advance each of these goals.
Chapter 1 relates the occurrence of extreme humid heat in the Persian Gulf and South Asia to two related modes of intraseasonal climate variability, namely the Madden–Julian oscillation (MJO) and the boreal summer intraseasonal oscillation (BSISO). Wet bulb temperatures (Tw) sufficiently high to impact human health are found to be almost twice as likely during certain oscillation phases than in others. Humid heat anomalies in each region are driven by distinct local circulation patterns and variations in moisture.
Chapter 2 evaluates the influence of monsoon onset and subseasonal precipitation variability on the occurrence of extreme Tw across South Asia. Extreme Tw events often occur on rainy days during the monsoon season. However, the influence of precipitation on Tw varies with the background specific humidity climatology. In climatologically drier areas, positive Tw anomalies tend to occur when precipitation increases due to either early onset or wet spells during the monsoon. In contrast, in climatologically humid areas, positive Tw anomalies occur during periods of suppressed precipitation, including delayed onset and dry spells during the monsoon.
Chapter 3 analyzes the dynamics of the record-breaking extreme heat event in Rio de Janeiro in November 2023. The heatwave was associated with persistent atmospheric blocking, potentially linked to developing El Niño conditions. Temperatures were intensified by declining soil moisture and elevated local sea surface temperatures, and the event was finally terminated due to the onset of precipitation. This chapter also evaluates the influence of climate change on the frequency of heat extremes, finding a significant increase in the frequency of high heat days throughout Brazil over the past four decades. Further, the frequency of spring heat extremes is expected to increase in the future, though highly dependent upon our future emissions pathway.
Chapter 4 explores the combinations of temperature and humidity contributing to humid heat experienced across the globe. In addition to using traditional metrics, this chapter derives a novel variable named “stickiness,” which quantifies the relative contributions of temperature and specific humidity to a given Tw. Consistent across metrics, high magnitudes of Tw tend to occur in the presence of anomalously high moisture, with temperature anomalies of secondary importance. Nonetheless, there is a broad range of stickiness observed for a given Tw across moderate-to-high Tw thresholds relevant to socioeconomic impacts.
| Identifer | oai:union.ndltd.org:columbia.edu/oai:academiccommons.columbia.edu:10.7916/217m-ck95 |
| Date | January 2024 |
| Creators | Ivanovich, Catherine Christine |
| Source Sets | Columbia University |
| Language | English |
| Detected Language | English |
| Type | Theses |
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