Variability of Water Vapor in the Tropical Middle Atmosphere Observed From Satellites and Interpreted Using SD‐WACCM Simulations

Abstract

Water vapor in the middle atmosphere plays an essential role in global warming, ozone depletion, and the formation of polar stratospheric and mesospheric clouds. We show that tropical middle atmospheric water vapor simulated with the specified‐dynamics version of the Whole Atmosphere Community Climate Model (SD‐WACCM) is consistent with changes observed in a merged satellite data set, which encompasses the period 1993–2020. Consistent with previous work, we find no significant trend in the stratosphere in either the observations or the simulation; in the mesosphere, we find a long‐term trend of 0.1 ppmv per decade, but only in the observations. We also analyze an SD‐WACCM simulation for the longer period 1980–2019 to quantify the contribution of various factors to the decadal variation of middle atmospheric water vapor. Over 1980–1995, the simulated water vapor in the upper stratosphere and mesosphere, averaged zonally and over ±30° latitude, increases by 0.30 ppmv per decade due to increasing methane emissions. After 1995, a significant abrupt decrease of water vapor of 0.37 ppmv per decade and then a gradual increase of 0.33 ppmv per decade result from changes in stratospheric cold point temperature. The cold‐point temperature is strongly influenced by the strength of the Brewer‐Dobson circulation. The acceleration of the Brewer‐Dobson circulation before about 2003 leads to a cooler tropical tropopause and a decrease of water vapor, and the deceleration thereafter leads to corresponding warming of the tropopause and an increase in water vapor.

WACCM6 Projections of Polar Mesospheric Cloud Abundance Over the 21st Century

“…The water vapor in SSP3‐7.0 keeps increasing monotonically and shows a higher value at the end of the 21st century than in SSP5‐8.5, where the water vapor begins to drop after the 2070s. Water vapor in the middle atmosphere is mainly determined by two factors: oxidation of methane in the middle atmosphere and water vapor entering from the troposphere constrained by the tropical cold‐point tropopause temperature (Oman et al., 2008; Yu et al., 2022). More discussion regarding the water vapor budget will be presented in Section 5.1.…”

“…We separate the water vapor into water vapor derived from methane (H 2 O_fromCH 4 ) and water vapor entering the stratosphere directly from the troposphere (H 2 O_entry) according to the method described in Yu et al. (2022). We regard the tropical tropopause (10°N–10°S, 87 hPa) as the entry point and calculate the age of air from the entry point to each model grid point.…”

“…The conclusions about the PMC trend drawn from the polar mean column-integrated IWC in each scenario are also valid at mid-latitudes and on different vertical layers. We estimate the trend of the IMD using the multi-variate regression method, with the signals of the QBO, ENSO, and the solar cycle removed as described in Yu et al (2022); we show the spatial distribution of the trend in Figure 3 for the different SSP scenarios. The vertical distribution of the trend shows consistent increasing or decreasing values except for SSP2-4.5, where the IMD shows a slight and not significant positive trend above the mesopause and in mid-latitudes, and a negative trend elsewhere.…”

“…Previous studies on the long-term changes of the PMC emphasize the importance of increasing methane to the mesospheric water vapor and thus the PMC (Lübken et al, 2018). The water vapor entering the middle atmosphere from the troposphere also plays a role in the middle atmospheric water vapor budget (Gettelman et al, 2009(Gettelman et al, , 2010Oman et al, 2008;Yu et al, 2022). In this section, we divide the polar mesospheric water vapor from the CMIP6-WACCM6 model output into different sources and calculate the contribution of each.…”

Opposite Impacts of Interannual and Decadal Pacific Variability in the Extratropics

“…Randel, Garcia, et al, 2009;W. J. Randel, Shine, et al, 2009;Solomon et al, 2010;Yu et al, 2022). El Niño warms the cold point (Calvo et al, 2010;Free & Seidel, 2009;García-Herrera et al, 2006; W. J.…”