Stratospheric Ozone in the Last Glacial Maximum

Abstract

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.

Quasi-Biennial Oscillation and Sudden Stratospheric Warmings during the Last Glacial Maximum

“…The latter is supported by satellite observations [15,16]. Fu et al [5] and Wang et al [17] used the Whole Atmosphere Community Climate Model version 6 (WACCM6) [18] to examine the BDC and stratospheric O 3 during the LGM. They showed that the stratospheric O 3 changes in the LGM as compared to the preindustrial climate were caused by chemical processes in the upper stratosphere associated with a temperature increase and OH and NO x decreases, a slow-down of the BDC in the lower stratosphere, and a lowering of the tropopause in the extratropics [17].…”

“…The WACCM6 has a realistic, internally generated QBO in the stratosphere and simulates a realistic frequency of SSWs [18,39]. We analyzed five simulations from Fu et al [5] and Wang et al [17], who used the WACCM6 with a horizontal resolution of 0.9 • latitude and 1.25 • longitude and 70 vertical levels. They consist of (1) a LGM simulation with prescribed SSTs based on models (LGM PMIP3 ), (2) a LGM simulation with SSTs based on proxy data (LGM PROXY ), (3) a preindustrial climate simulation (PI), (4) a modern climate simulation (MC), and (5) a modern climate simulation without ozone-depleting substances (ODS) (MC no_ODS ).…”

“…In the LGM simulations, the WACCM6 was coupled to the CLM4.0 LGM land boundary conditions [43] with the ice sheet topography from Abe-Ouchi et al [44]. The greenhouse gas concentrations and orbital parameters corresponding to the LGM, preindustrial, and modern climate were used [5,17]. The ozone depletion substances (ODSs) in MC were taken from the year 2000.…”

“…Atmosphere 2020, 11, x FOR PEER REVIEW 3 of 12 modern climate were used [5,17]. The ozone depletion substances (ODSs) in MC were taken from the year 2000.…”

“…Fu et al [5] and Wang et al [17] used the Whole Atmosphere Community Climate Model version 6 (WACCM6) [18] to examine the BDC and stratospheric O 3 during the LGM. They showed that the stratospheric O 3 changes in the LGM as compared to the preindustrial climate were caused by chemical processes in the upper stratosphere associated with a temperature increase and OH and NO x decreases, a slow-down of the BDC in the lower stratosphere, and a lowering of the tropopause in the extratropics [17]. In this study, we examine two aspects of stratospheric variability in the LGM-the quasi-biennial oscillation (QBO) and sudden stratospheric warmings (SSWs)-both with known links to the troposphere [19,20], by analyzing the same simulations described in Fu et al [5] and Wang et al [17].…”