"Extreme tropical variability under greenhouse warming"

October 14, 2021

Pedro DiNezio

Hosted by Charlotte DeMott

Abstract

Whether tropical climate variability will increase in response to greenhouse warming is highly uncertain. This question remains unanswered because externally-forced changes in modes of tropical variability cannot be ascertained using historical records or model predictions. We overcame these issues by studying changes in modes of variability under a range of past external forcings, validating them against proxy-inferred changes in variability during Earth’s geological history. For the Indian Ocean, we discovered a mode active during the last glacial period driving much stronger variability than currently observed. This mode – inactive today – could remerge by mid-century as this ocean develops oceanographic conditions favoring large-scale air-sea interactions. For the Pacific Ocean, simulations of glacial intervals show that a deeper mixed-layer in the central Pacific, disrupts the generation of extreme El Niño, as seen in paleoclimate records. A deeper mixed layer makes ocean currents less sensitive to wind variations during the onset of El Niño – limiting their ability to reach extreme amplitude. The opposite occurs under greenhouse warming, when a shallower mixed layer makes currents highly responsive to wind fluctuations favoring the onset of extreme El Niño. These results reveal a heightened risk of increasing climate extremes throughout a large part of the world if greenhouse gas emissions continue to increase.