Seasonal variation of ozone in the tropical lower stratosphere: Southern tropics are different from northern tropics

Abstract

We examine the seasonal behavior of ozone by using measurements from various instruments including ozonesondes, Aura Microwave Limb Sounder, and Stratospheric Aerosol and Gas Experiment II. We find that the magnitude of the annual variation in ozone, as a percentage of the mean ozone, exhibits a maximum at or slightly above the tropical tropopause. The maximum is larger in the northern tropics than in the southern tropics, and the annual maximum of ozone in the southern tropics occurs 2 months later than that in the northern tropics, in contrast to usual assumption that the tropics can be treated as a horizontally homogeneous region. The seasonal cycles of ozone and other species in this part of the lower stratosphere result from a combination of the seasonal variation of the Brewer-Dobson circulation and the seasonal variation of tropical and midlatitude mixing. In the Northern Hemisphere, the impacts of upwelling and mixing between the tropics and midlatitudes on ozone are in phase and additive. In the Southern Hemisphere, they are not in phase. We apply a tropical leaky pipe model independently to each hemisphere to examine the relative roles of upwelling and mixing in the northern and southern tropical regions. Reasonable assumptions of the seasonal variation of upwelling and mixing yield a good description of the seasonal magnitude and phase in both the southern and northern tropics. The differences in the tracers and transport between the northern and southern tropical stratospheres suggest that the paradigm of well-mixed tropics needs to be revised to consider latitudinal variations within the tropics.

Radiative Effects of Stratospheric Seasonal Cycles in the Tropical Upper Troposphere and Lower Stratosphere

“…The maximum radiative warming is found in October, ranging from 2.3 K at 208S to 1.2 K at 108S. This result is consistent with Stolarski et al (2014), who emphasized that the tropical lower-stratosphere ozone seasonal cycle is smaller in the SH than the NH, and SH seasonal cycle lags the NH cycle by 1-2 months (cf. Figs.…”

“…A recent study by Stolarski et al (2014) found that asymmetrical upwelling and mixing between the Northern and Southern Hemispheres (NH and SH, respectively) leads to a larger amplitude ozone seasonal cycle in the NH (08-208N) than the SH (208S-08). In particular, lowerstratospheric upwelling and horizontal mixing impacts on ozone are in phase and additive in the NH tropics, whereas they are out of phase in the SH tropics.…”

“…6). This feature arises because of the migration of the tropical region on the 85-hPa pressure surface, moving northward in the boreal summer and southward in the austral summer (Stolarski et al 2014). Strong upwelling and weak horizontal in mixing across the subtropical jet promotes reductions in ozone at these latitudes from January to March (Chen 1995;Stolarski et al 2014).…”

“…The distinct horizontal patterns in NH and SH ozone, related to the increased mixing in the NH surf-zone region, the migration of the tropical lower stratosphere, and the phasing of upwelling and horizontal mixing in the two hemispheres (Stolarski et al 2014), result in meridionally asymmetric radiative effects in the tropics. When considering the impacts of radiative effects on lower-stratospheric or near-tropopause phenomena in detail (such as the outflow of tropical cyclones or the amount of water vapor entering the stratosphere), the latitudinal variability examined here should be taken into account.…”

“…1c). Horizontal in-mixing from the extratropics during each hemisphere's respective summer also contributes to ozone's tropical seasonal cycle, amplifying it and contributing to hemispheric asymmetries (Konopka et al 2010;Ploeger et al 2011Ploeger et al , 2012Stolarski et al 2014; and discussed further in sections 2 and 3). The tropical ozone seasonal cycle transitions from an annual to a semiannual cycle between about 55 and 30 hPa.…”