Stratosphere-troposphere exchange (STE) of ozone represents a significant source for the tropospheric ozone budget (e.g.,

Using the Whole Atmosphere Community Climate Model version 6, stratospheric ozone in the Last Glacial Maximum (LGM) is investigated. It is shown that, compared with preindustrial (PI) times, LGM modeled stratospheric temperatures are increased by up to 8 K, leading to faster ozone destruction rates for gas phase reactions, especially via the Chapman mechanism. On the other hand, stratospheric hydroxyl radical (OH) and nitrogen oxides (NO x) concentrations are decreased by 10-20%, which decreases catalytic ozone destruction, thereby decreasing ozone loss rates. The net effect of these two compensating mechanisms in the upper stratosphere (above 15 hPa) is a vertically integrated 1-3 Dobson unit (DU) decrease during the LGM. In the lower stratosphere (tropopause to 15 hPa), changes in the stratospheric overturning circulation and resulting transport dominate changes in ozone. Consistent with a weakening of the residual circulation in the LGM, lower stratospheric ozone is increased by 2-5 DU in the tropics and decreased by 5-10 DU in the extratropics, but the latter is partly compensated by ozone increases due to a lower tropopause. It is found that tropospheric ozone is decreased by about 5 DU in the LGM versus PI. Combined changes in stratospheric and tropospheric ozone lead to a decrease in total ozone column everywhere except over the northeast North America, equatorial Indian and West Pacific Oceans. Surface ultraviolet radiation in the LGM versus PI is increased over the Northern Hemisphere middle and high latitudes, especially over the ice caps, and over the Southern Hemisphere near 60°S.

The Brewer‐Dobson circulation during the Last Glacial Maximum (LGM) is investigated in simulations using the Whole Atmosphere Community Climate Model version 6. We examine vertical mass fluxes, age of stratospheric air, and the transformed Eulerian mean stream function and find that the modeled annual‐mean Brewer‐Dobson circulation during the LGM is almost everywhere slower than that in the modern climate (with or without anthropogenic ozone depleting substances). Compared to the modern climate, the annual‐mean tropical upwelling in the LGM is 11.3–16.9%, 11.2–15.8%, and 4.4–10.2% weaker, respectively, at 100, 70, and 30 hPa. Simulated decreases in annual‐mean mass fluxes at 70 and 100 hPa are caused by a weaker parameterized orographic gravity wave drag and resolved wave drag, respectively.

As an important component of the tropospheric ozone budget, the stratosphere-troposphere exchange (STE) of ozone can substantially impact tropospheric ozone concentrations (e.g.,

The quasi-biennial oscillation (QBO) and sudden stratospheric warmings (SSWs) during the Last Glacial Maximum (LGM) are investigated in simulations using the Whole Atmosphere Community Climate Model version 6 (WACCM6). We find that the period of QBO, which is 27 months in the preindustrial and modern climate simulations, was 33 months in the LGM simulation using the proxy sea surface temperatures (SSTs) and 41 months using the model-based LGM SSTs. We show that the longer QBO period in the LGM is due to weaker wave forcing. The WACCM6 simulations of the LGM, preindustrial, and modern climates do not support previous modeling work that suggests that the QBO amplitude is smaller (larger) in a warmer (colder) climate. We find that SSWs in the LGM occurred later in the year, as compared to the preindustrial and modern climate, but that time of the final warming was similar. The difference in SSW frequency is inconclusive.