Finding Answers in Ancient Leaf Waxes<\/strong><\/p>\n To understand how the monsoon changed in the middle Pliocene, Bhattacharya and graduate student Claire Rubbelke, a Ph.D. candidate in Earth and environmental sciences, analyzed Pliocene-era leaf waxes preserved in ocean sediment cores from Baja California and southern California. The hydrogen isotopic composition of these waxes reveals past changes in the monsoon.\u00a0 Since rain is the source of the hydrogen used to produce leaf waxes, measuring the concentration of hydrogen reveals precipitation totals from a particular moment in the past. Researchers extract the leaf waxes by running solvents through sediments at high temperature and pressure and make isotopic measurements using a device called a gas chromatograph-isotope ratio mass spectrometer, which separates out waxes by their molecular mass.<\/p>\n \u201cWith information encoded in leaf waxes we found that the Pliocene featured a stronger summer monsoon in western Mexico stretching all the way to where southern California now is, contrasting with previous work which stated that Pliocene hydroclimate changes were only the result of winter, not summer, rainfall,\u201d notes Bhattacharya.\u00a0\u201cOur paper presents the first direct evidence that monsoon changes caused wet conditions in the middle Pliocene.\u201d<\/p>\n Driven by Temperature Changes<\/strong><\/p>\n Climate modeling expert Ran Feng<\/a>, professor in the University of Connecticut\u2019s Department of Geosciences and second author on the study, carried out simulations to determine how sea surface temperatures may have factored into the stronger North American Monsoon during the mid-Pliocene.\u00a0Her team found that ocean temperatures in the Pacific were arranged in a way to transport more moisture from the tropics to the subtropics. Specifically, there was a reduced subtropical-tropical temperature gradient that drives the strengthening of the North American monsoon.<\/p>\n Temperatures are one of the driving factors behind monsoon intensity. Warmer conditions in the eastern equatorial Pacific cause descending motion over many monsoon regions of the southwestern North America, reducing the favorability of the atmosphere to rainfall. But when the California margin is warmer than the eastern equatorial Pacific \u2013 as can happen today during marine heat wave events – greater amounts of tropical moisture enter into the subtropics, bringing enhanced North American monsoon precipitation.<\/p>\n \u201cBy studying the mid-Pliocene climate, we can determine how our planet operates under warm conditions,\u201d says Feng. \u201cThe mechanism we identified here is already at play during present-day marine heat wave events and we anticipate that it will become more prevalent in the future with a warmer climate and perhaps more frequent marine heat wave events.\u201d<\/p>\n Their results offer confirmation that higher temperatures on the California margin help increase the favorability of the atmosphere for monsoon rainfall \u2013 essentially providing more energy to fuel monsoon storms.<\/p>\n \u201cSummer rainfall and flooding will likely increase in the future in southwestern North America,\u201d says Bhattacharya. \u201cWe believe our work is a nice illustration of how the past can be used to predict future climate hazards.\u201d<\/p>\n![\"\"](\"https:\/\/today.uconn.edu\/wp-content\/uploads\/2022\/11\/IMG_5595_summerstorm-233x300.jpg\")