Search

The summer monsoon anticyclones are the dominant climatological features of the Northern Hemispheric (NH) summertime circulation in the upper troposphere and lower stratosphere (UTLS). However, the response of these anticyclones to the increased levels of CO2 remains highly uncertain, as does the impact on the distribution of UTLS ozone and other tracers. This study examines the response of the NH summertime monsoon anticyclones and UTLS ozone to the abrupt increase in CO2 forcing using output from a suite of coupled ocean–atmosphere general circulation model simulations. These models show an equatorward shift of the Asian summer monsoon anticyclone, a weakening of the North American summer monsoon anticyclone, and a stronger westerly flow penetrating deep into the tropics above the Pacific Ocean and North America. We use additional idealized experiments from atmosphere‐only general circulation models with prescribed SSTs and sea ice concentration to isolate the direct atmospheric radiative effects from the indirect effect of SST warming on the UTLS monsoon anticyclones. Comparison between atmosphere‐only and coupled ocean–atmosphere experiments shows that SST warming is the principal mechanism producing UTLS monsoonal circulation changes. The 4×CO2 experiments result in a significant reduction up to 40%–50% of the UTLS ozone in the northern tropics, which could have an impact on radiative balance near the surface. Key Points: Models simulate an equatorward shift of the Asian and disappearance of the North American monsoon anticyclones in 4 × CO2 simulationsChanges in the upper troposphere and lower stratosphere anticyclones are dominated by the effect of indirect SST warming by CO2Circulation changes due to 4 × CO2 result in a significant reduction of ozone in the northern tropics at 100 hPa [ABSTRACT FROM AUTHOR]

Seasonal differences in the impact of the Madden–Julian oscillation (MJO) on tropical and extratropical upper troposphere–lower stratosphere (UTLS) temperature, circulation, and trace gases are examined using trace gases (ozone, carbon monoxide, and water vapor) and temperature from measurements from the Microwave Limb Sounder (MLS) and meteorological fields from the Modern-Era Retrospective Analysis for Research and Applications, version 2 (MERRA-2). During boreal winter months (November–February), atmospheric fields exhibit a well-known planetary-scale perturbation consistent with the upper-level flow modeled by Gill, with twin high and low pressure extratropical systems associated with a Rossby wave response. However, the circulation anomalies in the UTLS differ during boreal summer months (June–September), when background UTLS circulation north of the equator is dominated by the Asian summer monsoon anticyclone. The twin high and low pressure extratropical systems are much weaker but with a stronger equatorial Kelvin wave front that encircles the globe as the MJO propagates eastward. These differences are explained in terms of seasonal variations in vertically propagating Kelvin waves that strongly depend on the zonal structure of the climatological background winds. The trace gas response to the MJO is strongly coherent with circulation anomalies showing strong seasonal differences. The stronger equatorial Kelvin wave front during the summer produces enhanced upwelling in the tropical tropopause layer, resulting in significant cooling of this region, reduced ozone and water vapor, and enhanced carbon monoxide. [ABSTRACT FROM AUTHOR]

Abstract: The interannual variability of tropical lower stratosphere ozone and its connections to sea surface temperatures in the equatorial Pacific are examined using a combination of chemistry climate model simulations, satellite observations, and reanalyses. The model simulations and observations show large differences in the magnitude of interannual variability in ozone between northern tropic (NT; EQ‐18° N) and southern tropic (EQ‐18° S) during boreal summer but small differences in winter. The interannual variability during boreal summer is highly correlated with summer sea surface temperatures in the eastern and central Pacific Ocean and El Niño–Southern Oscillation (ENSO) events. Larger variability in NT ozone is primarily due to meridional advection, connected to the changes in the onset date and strength of the Asian summer monsoon anticyclone. The Asian summer monsoon anticyclone forms earlier in a season and tends to be stronger during cold (La Niña) events leading to more isentropic transport of ozone from the extratropics into the NT, with the reverse for warm (El Niño) events. [ABSTRACT FROM AUTHOR]

Abstract: Observations from satellites and a ground‐based station are combined to construct a global data set for investigating the tertiary ozone maximum in the winter mesosphere for the period August 2004 to June 2017. These give a comprehensive picture of this ozone maximum in latitude, pressure, and time. The location of the tertiary ozone maximum shifts in latitude and pressure with the evolving season; the ozone peak occurs at lower latitude and higher pressure around the winter solstice. Highest average nighttime ozone concentrations and greatest degree of interannual variability are seen in late winter in the Northern Hemisphere (NH). The hemispheric differences and interannual variability in nighttime ozone are related to variations of temperature, H2O, and OH associated with dynamical activity. Elevated stratopause events in the NH winter are associated with transport of air that is depleted in H2O and enhanced in OH; photochemistry then leads to downward displacement of the altitude of maximum ozone and enhancement in the ozone amount. Transport by planetary waves in the NH extends the region of high ozone further from the pole and leads to longitudinal variations. The analysis shows that while the tertiary ozone maximum responds to a particular radiative situation as shown in previous studies, it is also the result of very dry air found in the winter polar mesosphere. [ABSTRACT FROM AUTHOR]

The quasi-biennial oscillation (QBO) is a quasiperiodic alternation between easterly and westerly zonal winds in the tropical stratosphere, propagating downward from the middle stratosphere to the tropopause with a period that varies from 24 to 32 months (~28 months on average). The QBO wind oscillations affect the distribution of chemical constituents, such as ozone (O3), water vapor (H2O), nitrous oxide (N2O), and hydrochloric acid (HCl), through the QBO-induced meridional circulation. In the 2015-2016 winter, radiosonde observations revealed an anomaly in the downward propagation of the westerly phase, which was disrupted by the upward displacement of the westerly phase from ~30 hPa up to 15 hPa and the sudden appearance of easterlies at 40 hPa. Such a disruption is unprecedented in the observational record from 1953 to the present. In this study we show the response of trace gases to this QBO disruption using O3, HCl, H2O, and temperature from the Aura Microwave Limb Sounder (MLS) and total ozone measurements from the Solar Backscatter Ultraviolet (SBUV) Merged Ozone Data Set (MOD). Results reveal the development of positive anomalies in stratospheric equatorial O3 and HCl over ~50-30 hPa in May-September of 2016 and a substantial decrease in O3 in the subtropics of both hemispheres. The SBUV observations show near-record low levels of column ozone in the subtropics in 2016, resulting in an increase in the surface UV index during northern summer. Furthermore, cold temperature anomalies near the tropical tropopause result in a global decrease in stratospheric water vapor. [ABSTRACT FROM AUTHOR]

The quasi-biennial oscillation (QBO) is a quasi-periodic alternation between easterly and westerly zonal winds in the tropical stratosphere, propagating downward from the middle stratosphere to the tropopause with a period that varies from 24 to 32 months (∼28 months on average). The QBO wind oscillations affect the distribution of chemical constituents, such as ozone (O3), water vapor (H2O), nitrous oxide (N2O) and hydrochloric acid (HCl), through the QBO induced meridional circulation. In the 2015-2016 winter, radiosonde observations revealed an anomaly in the downward propagation of the westerly phase, which was disrupted by the upward displacement of the westerly phase from ∼30 hPa up to 15 hPa, and the sudden appearance of easterlies at 40 hPa. Such a disruption is unprecedented in the observational record from 1953-present. In this study we show the response of trace gases to this QBO disruption using O3, HCl, H2O and temperature from the Aura Microwave Limb Sounder (MLS) and total ozone measurements from the Solar Backscatter Ultraviolet (SBUV) Merged Ozone Data Set (MOD). Results reveal development of positive anomalies in stratospheric equatorial O3 and HCl over ∼50-30 hPa in May-September of 2016 and a substantial decrease in O3 in the subtropics of both hemispheres. The SBUV observations show near record low levels of column ozone in the subtropics in 2016, resulting in an increase of surface UV index during northern summer. Furthermore, cold temperature anomalies near the tropical tropopause result in a global decrease in stratospheric water vapor. [ABSTRACT FROM AUTHOR]