The decrease in mid-stratospheric tropical ozone since 1991

While global stratospheric O3 has begun to recover, there are localized regions where O3 has decreased since 1991. Specifically, we use measurements from the Halogen Occultation Experiment (HALOE) for the period 1991–2005 and the NASA Aura Microwave Limb Sounder (MLS) for the period 2004–2013 to demonstrate a significant decrease in O3 near ~ 10 hPa in the tropics. O3 in this region is very sensitive to variations in NOy, and the observed decrease can be understood as a spatially localized, yet long-term increase in NOy. In turn, using data from MLS and from the Atmospheric Chemistry Experiment (ACE), we show that the NOy variations are caused by decreases in N2O which are likely linked to long-term variations in dynamics. To illustrate how variations in dynamics can affect N2O and O3, we show that by decreasing the upwelling in the tropics, more of the N2O can photodissociate with a concomitant increase in NOy production (via N2O + O(1D) → 2NO) at 10 hPa. Ultimately, this can cause an O3 decrease of the observed magnitude.

The decrease in mid-stratospheric tropical ozone since 1991

While global stratospheric O3 has begun to recover, there are localized regions where O3 has decreased since 1991. Specifically, we use measurements from the Halogen Occultation Experiment (HALOE) for the period 1991–2005 and the NASA/Aura Microwave Limb Sounder (MLS) for the period 2004–2013 to demonstrate a significant decrease in O3 near ~10 hPa in the tropics. O3 in this region is very sensitive to variations in NOy, and the observed decrease can be understood as a spatially localized, yet long term increase in NOy. In turn, using data from MLS and from the Atmospheric Chemistry Experiment (ACE), we show that the NOy variations are caused by decreases in N2O which are likely linked to long term variations in dynamics. To illustrate how variations in dynamics can affect N2O and O3, we show that by decreasing the upwelling in the tropics, more of the N2O can photodissociate with a concomitant increase in NOy production (via N2O+O(1D → 2 NO) at 10 hPa. Ultimately, this can cause an O3 decrease of the observed magnitude.

The effects of convection on the summertime mid-latitude overworld

Halogen Occultation Experiment measurements of H2O are used to investigate the influence of mid-latitude convection on the summertime overworld between 30° N and 40° N. We find that most of the convective influence over this latitude range occurs over the Asian monsoon and over North America. Over North America, the effects of convection extend to ~410 K (17.5 km). Over Asia, the effects of convection extend to ~460 K (19 km), about 50 K (1.5 km) higher than over North America.

Technical note: A stratospheric climatology for O₃, H₂O, CH₄, NO_x, HCl and HF derived from HALOE measurements

The Halogen Occultation Experiment (HALOE) on board the Upper Atmosphere Research Satellite (UARS) has observed mixing ratios of important trace species in the stratosphere for more than a decade since 1991. Here we present a climatology for the stratosphere compiled from HALOE O3, H2O, CH4, NOx, HCl, and HF data for the period from 1991 to 2002. In this approach, the data are averaged over equivalent latitude instead of latitude in order to correctly reproduce the gradients at the transport barriers like the polar vortex edge. The climatology is compiled for 5 degree equivalent latitude bins. Seasonal dependence is taken into account by choosing intervals of one month. The climatology is available as an electronic supplement.

Technical note: A stratospheric climatology for O₃, H₂O and CH₄ derived from HALOE measurements

The Halogen Occultation Experiment (HALOE) on board the Upper Atmosphere research satellite (UARS) has observed mixing ratios of important trace species in the stratosphere over more than a decade since 1991. Here we present a climatology for the stratosphere compiled from the HALOE data for ozone, H2O and CH4 for the period from 1991 to 2002. In this approach, the data are averaged over equivalent latitude instead of latitude in order to correctly reproduce the gradients at the transport barriers like the polar vortex edge. The climatology is compiled for 5 degree equivalent latitude bins. The seasonal dependence is taken into account by choosing intervals of one month. The climatology is available as an electronic supplement.