Ozone and HCl distributions in the equatorial stratosphere in relation to the QBO and the SAO observed by the Superconducting Submillimeter-Wave Limb-Emission Sounder (SMILES) and the Microwave Limb Sounder (MLS)

In association with the phase of the quasi-biennial oscillation (QBO), it is sometimes seen that meridional distribution of ozone around the equatorial middle stratosphere shows a double-peak structure with a slight minimum at the equator, which is called “rabbit-ears” by Randel and Wu (1996). In this study it is investigated by using data from the Superconducting Submillimeter-Wave Limb-Emission Sounder (SMILES) for the period of October 2009 to April 2010 when the SMILES could clearly capture the double-peak structure. The SMILES is a sounder to measure atmospheric minor constituents in the middle atmosphere globally with very low system noise temperature suppressed by a 4-K mechanical cooler and superconducting mixers (Kikuchi et al., 2010). We also used ozone data from the Microwave Limb Sounder (MLS) to extend our analysis period for roughly three QBO cycles, and found that there exist the double-peak structures in 2005 and 2007 in addition to 2009-2010. The structure captured during the SMILES observation period was most pronounced within the three cases. To further investigate the effect of vertical advection, we additionally use the data for HCl both from SMILES and MLS.

The ozone distribution observed by the SMILES clearly showed the double-peak structure around 10 hPa in October 2009 and April 2010, while the structure almost disappeared in January 2010. In accordance with the ozone structure the HCl distribution showed double-minima with a slight maximum over the equator around 10 hPa. The ozone minimum over the equator with a negative vertical gradient of ozone mixing ratio and the HCl maximum with a positive HCl gradient can be understood as an effect of descending motion which should be associated with the QBO and possibly with the semiannual oscillation (SAO). Then we investigated equatorial zonal wind and temperature fields by using the ERA interim dataset. During the SMILES observation period the phase of the QBO was in easterlies at 20 hPa, and we see warm anomalies around 10 hPa, roughly corresponding to the westerly shear zone at 10 hPa. The latitudinal gradient of temperature being warmer at the equator and the westerly shear is consistent with the thermal-wind relationship. As to the SAO in the upper stratosphere during the SMILES observation period, there were westerlies in the equinoctial seasons when the SAO extended down to enhance the westerly shear above the QBO easterlies.

Similar analyses have been made with use of the MLS data. The double-peak structures of ozone were found in 2005 and 2007 in addition to 2009 and 2010. All these cases are in the westerly shear between the QBO and the SAO, and it is enhanced by the SAO westerly in the equinoctial seasons. Thus the negative anomalies from the climatological mean of ozone and the positive anomalies of HCl are found in such cases over the equator. These results clearly show the importance of the phase matching between the QBO and the SAO to induce stronger downward motion in the westerly shear zone, resulting in the “rabbit-ears” of the meridional ozone distribution.