Caroline C. Ummenhofer

USGS2020

Increased typhoon activity in the Pacific deep tropics driven by Little Ice Age circulation changes

The instrumental record reveals that tropical cyclone activity is sensitive to oceanic and atmospheric variability on inter-annual and decadal scales. However, our understanding of the influence of climate on tropical cyclone behaviour is restricted by the short historical record and the sparseness of prehistorical reconstructions, particularly in the western North Pacific, where coastal communities suffer loss of life and livelihood from typhoons annually. Here, to explore past regional typhoon dynamics, we reconstruct three millennia of deep tropical North Pacific cyclogenesis. Combined with existing records, our reconstruction demonstrates that low-baseline typhoon activity prior to 1350 CE was followed by an interval of frequent storms during the Little Ice Age. This pattern, concurrent with hydroclimate proxy variability, suggests a centennial-scale link between Pacific hydroclimate and tropical cyclone climatology. An ensemble of global climate models demonstrates a migration of the Pacific Walker circulation and variability in two Pacific climate modes during the Little Ice Age, which probably contributed to enhanced tropical cyclone activity in the tropical western North Pacific. In the next century, projected changes to the Pacific Walker circulation and expansion of the tropics will invert these Little Ice Age hydroclimate trends, potentially reducing typhoon activity in the deep tropical Pacific.

Nature Geoscience↗

USGS2020

Coupling of Indo-Pacific climate variability over the last millennium

The Indian Ocean Dipole (IOD) impacts climate and rainfall across the world, and most severely in nations surrounding the Indian Ocean1-4. The frequency and intensity of positive IOD events increased during the 20th Century5 and may continue to intensify in a warming world6; however, confidence in future IOD changes is limited by known biases in model representations of the IOD7 and the lack of information on natural IOD variability prior to anthropogenic climate change. Here we use precisely dated and highly resolved coral records from the eastern equatorial Indian Ocean, where the signature of IOD variability is optimised, to produce a semi-continuous reconstruction of IOD variability that covers five centuries of the last millennium. Our reconstruction demonstrates that extreme positive IOD events were rare prior to 1960. However, the strongest event on record (1997) is not unprecedented as at least one event that was approximately 27% to 42% larger occurred naturally during the 17th Century. We further show that a persistent, tight coupling existed between variability of the IOD and the El Niño-Southern Oscillation during the last millennium. Indo-Pacific coupling was characterised by weak interannual variability prior to ~1590 CE which likely altered teleconnection patterns, and anomalously strong variability during the 17th Century that was associated with societal upheaval in tropical Asia. A tendency for clustering of positive IOD events is evident in our reconstruction, which together with the identification of extreme IOD variability and persistent tropical Indo-Pacific climate coupling may have implications for improving seasonal and decadal prediction schemes and managing the climate risks of future IOD variability.

Nature↗