Abstract
Abstract. Temporal variability of the upwelling near the tropical tropopause on daily to annual timescales is investigated using three different estimates computed from the ERA-Interim reanalysis. These include upwelling archived by the reanalysis, plus estimates derived from thermodynamic and momentum balance calculations. Substantial variability in upwelling is observed on both seasonal and sub-seasonal timescales, and the three estimates show reasonably good agreement. Tropical upwelling should exert strong influence on temperatures and on tracers with large vertical gradients in the lower stratosphere. We test this behavior by comparing the calculated upwelling estimates with observed temperatures in the tropical lower stratosphere, and with measurements of ozone and carbon monoxide (CO) from the Aura Microwave Limb Sounder (MLS) satellite instrument. Time series of temperature, ozone and CO are well correlated in the tropical lower stratosphere, and we quantify the influence of tropical upwelling on this joint variability. Strong coherent annual cycles observed in each quantity are found to reflect the seasonal cycle in upwelling. Statistically significant correlations between upwelling, temperatures and tracers are also found for sub-seasonal timescales, demonstrating the importance of upwelling in forcing transient variability in the lower tropical stratosphere.
Highlights
The mean circulation in the tropical lower stratosphere is characterized by upwelling, which transports air masses across the tropical tropopause into the lower stratosphere
Tropical upwelling is a key aspect of the global stratospheric circulation, but fundamental aspects such as forcing mechanisms and temporal variability are poorly understood
In this study we evaluated the variability and quality of zonal average tropical upwelling estimates derived from different techniques (w∗, w∗m and w∗Q)
Summary
The mean circulation in the tropical lower stratosphere is characterized by upwelling, which transports air masses across the tropical tropopause into the lower stratosphere. Konopka et al (2010) and Ploeger et al (2012) suggest that in-mixing of air from the extra-tropics into the TTL makes a major contribution to the seasonal cycle in ozone concentration above the tropical tropopause They propose that horizontal transport associated with the upper-level circulation of the Asian monsoon plays a dominant role in building the ozone maximum observed in boreal summer. In addition to the seasonal cycle, high-temporal resolution satellite measurements of ozone and CO reveal variability on subseasonal timescales in this region The analysis of these faster variations can provide complementary information on the relative roles of the different forcing mechanisms. We combine the meteorological data with satellite observations of tracer concentrations and examine correlated variability among temperature, ozone and CO in terms of coherence with upwelling, focusing separately on seasonal and sub-seasonal timescales
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