Do Unprecedented Weather Extremes Since Early 2022 Have an Association with the Hunga Tonga-Hunga Ha'apai Eruption?

In 2023, global mean annual temperature unexpectedly increased 0.2°C above the previous record warmth observed in 2016, prompting concern for the trajectory of future warming. The most immediate culprit of this temperature rise was the development of a strong El Niño, but the particularly large magnitude warming that occurred remains poorly understood. Other potential factors include incremental background warming from greenhouse gas emissions, sharp declines in ship aerosol emissions over the North Pacific this past decade and subsequent radiative impact, and possible lag response to the January 2022 eruption of the submarine volcano Hunga Tonga-Hunga Ha’apai (Tonga). The latter event produced an “unprecedented” injection of water vapor into the stratosphere for the satellite era, leading some to postulate a link between the eruption and the unusual warmth in 2023. However, initial estimates of the direct radiative impact from the Tonga-sourced excess water vapor suggest small, if not negligible warming effect. Other studies emphasize possible dynamical responses that are not yet understood. This thesis utilizes global climate reanalysis and satellite water vapor observations to explore possible teleconnections between the January, 2022 Tonga eruption and observed climate and weather extremes in the ensuing years. Results from this analysis show a series of unusually large meteorological events with standardized anomalies ranging 3–6 standard deviations (σ) above or below a 30-year, 1991–2020 climate reference period. These pronounced extremes appear to be unique "fingerprints” of the post-eruption climate system response. Of particular note among many identified extremes: an Antarctic heat wave in March, 2022 that set a world record for highest temperature above historical climatology; development of a rare “triple-dip” La Niña throughout 2022 that was unusually strong; a record cold anomaly that spread across the Southern Hemisphere lower stratosphere from April through October of 2022 in association with the development of La Niña; a record warm wave and melt event across Greenland in September, 2022; and a record strong atmospheric blocking pattern over Western Canada in May, 2023 that preceded unusually weak winds over the North Atlantic in June, which in turn facilitated record high sea-surface temperatures across the basin in July. From these results, and considering the spread of Tonga-sourced stratospheric water vapor across both polar hemispheres by fall 2022, an hypothesis is developed to explain how the eruption may have facilitated the climate and weather extremes observed in both 2022 and 2023. First, the lower-stratosphere thermal anomaly associated with Tonga water vapor intensified atmospheric circulation over the Pacific and nudged sea surface temperatures toward La Niña, thereby producing the observed “triple-dip” event. The heat-trapping effect of the excess stratospheric water vapor was masked in 2022 because of the perpetuation of La Niña, but, as the Niña dissipated in early 2023, the additional stratospheric heating provided feedback to what became one of the strongest El Niños on record. These two factors – excess stratospheric water vapor and the strong El Niño – led to unusually weak worldwide atmospheric circulation, which in turn produced a greater likelihood of atmospheric blocking from which extreme weather was observed in 2023 and into 2024. This research was undertaken as part of the Systems Approach to Understanding and Navigating the New Arctic (SAUNNA) National Research Traineeship (NRT) program, and as such is relevant to the Arctic. Recent modeling studies have shown that vapor from the Tonga plume could have produced heatwaves in the Arctic in early 2022, and satellite-based estimates indicate that excess stratospheric water vapor reached the Arctic by mid 2022. This thesis also draws a potential teleconnection between a Tonga climate impact and record atmospheric circulation and heat anomalies across the North Atlantic Ocean and Greenland in fall 2022. A description of how this research interfaces with the SAUNNA NRT program is provided in Appendix B of this thesis.

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