2021 Texas cold snap: Manifestation of natural variability and a recent warming trend

• Published in: Weather and Climate Extremes
• Main Authors: Pei-Chun Hsu, Huang-Hsiung Hsu, Hao-Jhe Hong, Ying-Ting Chen, Yu-Luen Chen, Wan-Ling Tseng
• Format: Article in Journal/Newspaper
• Language: English
• Published: Elsevier 2022
• Subjects: Extreme cold event; North America; Natural variability; Warming trend; Pacific–north American pattern; Sea surface temperature; Meteorology. Climatology; QC851-999; Pacific; North Atlantic
• Online Access: https://doi.org/10.1016/j.wace.2022.100476; https://doaj.org/article/804cd4a2750548b28a3ed2c391598e26

In this paper, we demonstrate that the extreme cold snap in Texas in February 2021 resulted from a large-scale background circulation anomaly over the North Pacific and North America that was a manifestation of both natural variability (namely, a negative Pacific–North American pattern, PNA) and a warming trend that began in the late 1990s. Numerical experiments revealed that the negative PNA pattern could be driven by negative (La Niña-like) sea surface temperature (SST) anomalies in the equatorial eastern Pacific on an interannual–decadal timescale and the SST warming trend in the subtropical North Atlantic on a longer timescale. We propose that the excitation of the most unstable mode of the Northern Hemisphere winter, which exhibits a combined negative PNA and positive East Atlantic pattern resulting from SST anomalies and trends, leads to the dominance of the large-scale circulation anomaly observed in February 2021 and over the last few decades.Our study demonstrated the key role of extratropical wave activity and the intrinsic mode in causing the extreme cold in North America. The arguments favoring polar or tropical influences may partially explain the source of variability, considering that extratropical weather and climate variability could be modulated by not only polar or tropical perturbations but also the enhanced tropical–extratropical interaction and mid-latitude instability. A systematic approach involving both empirical diagnostics and a suite of carefully designed numerical experiments is required to determine the relative influence of various sources of variability.Regarding future changes, models of the Coupled Model Intercomparison Project Phase 6 project a weakening, rather than an enhancement, of the negative PNA pattern that favors the occurrence of cold snaps despite failing to simulate the observed enhanced trend since the late 1990s.