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Future climate change:

Often curiosity-driven research in climate dynamics or the study of past climates lead us to insights possibly relevant to future climate change. Here are a few examples.

  • "Convective cloud feedback" warming the winter Arctic in a high CO2 world; with Dorian Abbot. The study of the climate of 50 Myr ago (Eocene) led us to a suggestion that tropical-like atmospheric convection may have occurred over the Arctic, and that the strong greenhouse effect of the resulting clouds helped keep the Arctic warm. We then found this feedback to be active in essentially all climate models projecting future climate at high CO2 (Abbot and Tziperman 2008a, 2008b, 2009, Abbot Walker & Tziperman 2009, Arnold et al 2014).

  • Enhancement of tropical "Madden-Julian Oscillation" in a warmer climate; with Nathan Arnold and collaborators. While studying evidence for what seems to be a "permanent El Nino" during 2–5 Myr ago (Pliocene), we suggested that a superrotation (westerly winds over the equator) due to stronger tropical storminess may have played a role. We then found that the tropical Madden-Julian Oscillation is indeed expected to be stronger in warmer climate, suggested a specific mechanism, and found that that this leads to superrotation tendencies in a state-of-the-art "super-parameterized" atmospheric model. (Tziperman & Farrell 2009; Arnold, Tziperman & Farrell 2012; Arnold et al 2014).

  • Suppression of Arctic air formation; with Tim Cronin. The high latitude continental northern US sees occasional extreme cold events during winter due to the penetration of "Arctic air" from Canada. It has been observed that warming over the past couple of decades is especially enhanced over such high-latitude continents during winter time. The two main feedbacks responsible for this enhanced warming are the albedo feedback due to melting snow and sea ice, and a "lapse-rate feedback" which is simply another way of saying that the warming is surface-intensified. We suggested a mechanism that explains this enhanced surface warming as being due to low level clouds developing because of the condensation of warmer and moister air arriving from the ocean. We also showed that as the oceans become even warmer (say 50 Myr ago or perhaps even in a high CO2 future scenario), the process of Arctic air formation that occurs over high latitude continents may be suppressed, leading to a very significant warming there (Cronin & Tziperman 2015; Cronin, Lee & Tziperman 2017; Hu, Cronin & Tziperman 2018).

  • More frequent Sudden Stratospheric Warming (SSW) events due to strengthening of the Madden-Julian Oscillation (MJO) in a warmer climate; with Wanying Kang. Following the above work in which we found a strengthening of the MJO in a warmer climate and explained the mechanism, we looked into some of the consequences. SSW events involve an abrupt warming of tens of degrees of the Arctic stratosphere, at a height of ~30 km above the surface within days. These events occur about six times per decade on average, and are followed by extreme weather events near the surface. We find that a stronger MJO leads to a significant increase in the frequency of SSW events. The mechanism involves two parts. First, a direct propagation of MJO-forced atmospheric planetary waves to the Arctic stratosphere. Second and more significantly, the MJO-forced waves interact nonlinearly with the mid-latitude tropospheric jet, modify it and lead to stronger stationary waves emitted from this jet toward the Arctic stratosphere, leading in turn to more frequent SSWs. (Kang and Tziperman 2017, 2018a, 2018b)

  • A wet California in a warmer future climate; with Minmin Fu. The Pliocene (5.3–3 Myr), characterized by warmer temperatures and similar CO2 concentrations to present day, is considered a useful analog for future warming scenarios. Geological evidence suggests that at that time, many modern-day desert regions such as the South-West United States, including Death Valley in California, received higher levels of rainfall and supported large lakes and wetter vegetation types. These wetter conditions have been difficult to reconcile with model predictions of 21st century drying over the same areas. We show that this discrepancy between past evidence and future projections may be due to the models missing an important feedback: Increasing sea surface temperature (SST) due to a weakening of the California coastal upwelling leads to wetter conditions over nearby land, and wetter land leads to a weakening of the wind that forces the upwelling. (Fu, Cane, Molnar & Tziperman 2021a, 2022)

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