Analysis of factors influencing tropical lower stratospheric water vapor during 1980–2017

Abstract

Tropical cold point tropopause temperature (CPT) anomalies determine lower stratospheric water vapor (LSWV) variations, leading to a high correlation between variations in tropical average CPT and changes in tropical average LSWV. However, this high correlation is only found in winter and spring. This work revisits the factors controlling LSWV variations using observations and simulations over the past~40 years. It is found that the first and second empirical orthogonal function (EOF) modes of tropical CPT variations together explain the tropical average LSWV changes much better than the tropical average CPT variations. The high correlation between the first and second EOF modes of tropical CPT variations and tropical average LSWV changes holds in all four seasons. A further analysis shows that the first and second EOF modes of tropical CPT variations are related to canonical El Niño-Southern Oscillation (ENSO) activity and sea surface temperature (SST) variations in the central Pacific Ocean, respectively. ENSO Modoki is also an important factor that affects LSWV variations by influencing the vertical velocity at the tropopause. The quasi-biennial oscillation (QBO) affects the CPT, and is the third process modulating the LSWV changes. The simulations also support the results.

Impacts of Ozone Changes in the Tropopause Layer on Stratospheric Water Vapor

“…Weakening of the activity of planetary waves 1 and 2 will lead to weakening of the vertical transport. It is well known that vertical transport may also have important effects on LSWV changes in the tropics, and stronger upwelling transports more water vapor into the stratosphere [10,52,53]. It is necessary here to examine the potential impact of tropopause layer ozone changes on vertical transport.…”

“…It has been recognized that SWV may respond to tropospheric and surface climate changes through a variety of mechanisms. Tropical tropopause temperature is the main factor controlling tropical SWV through dehydration [1,[8][9][10][11], as determined by radiative, chemical, and dynamical processes in the tropical tropopause layer (TTL) [12,13]. These coupling processes are largely related to the exchange of air masses and minor constituents (e.g., water vapor and ozone) across the tropopause layer.…”

“…It is widely agreed that the water vapor entering the stratosphere mainly in the tropics is limited to a few parts per million by volume (ppmv) due to the intense dehydration process that occurs in the TTL due to the extremely low temperatures near the tropical tropopause [16]. The tropopause temperature is driven by radiative processes [17,18], equatorial planetary waves, including tropical Rossby and Kelvin waves [10], variations in the stratospheric Brewer-Dobson (BD) circulation driven by extratropical stratospheric waves [19,20], and so forth.…”

“…During El Niño phases, enhanced tropical upwelling warms the upper troposphere and cools the lower stratosphere in the tropics [24,25], thus influencing large-scale tropical water vapor concentrations [11,26,27]. Due to the different propagation and dissipation patterns of ultralong Rossby waves in the middle-latitude stratosphere, ENSO Modoki can cause changes in the tropopause temperature and induce SWV anomalies, although the influences of ENSO Modoki are smaller than those of canonical ENSO events [10,27]. The tropopause temperature variations associated with the QBO drive a ~2-year cycle of interannual water vapor anomalies near the TTL, and these anomalies propagate vertically and latitudinally with mean stratospheric transport circulation (in a manner similar to the seasonal "tape recorder") [28].…”

Enhanced upward motion through the troposphere over the tropical western Pacific and its implication to transport of trace gases from the troposphere to the stratosphere

“…For example, the Hadley cell has been extended to the subtropics and the Walker circulation over the Pacific has been shifted westward over the past decades (e.g., Lu et al, 2007;Garfinkel et al, 2015;Ma and Zhou, 2016). At the same time, SSTs over most of areas are getting warmer (Cane et al, 1997;Deser et al, 2010), which modulates the deep convection and atmospheric wave activities in the troposphere and then lead to changes of atmospheric circulations from the troposphere and the stratosphere (e.g., Garfinkel et al, 2013;Xie et al, 2012Xie et al, , 2014aWang et al, 2015;Hu et al, 2016;Lu et al, 2020).…”