Matthew H. Hitchman

A Climatology of Rossby Wave Breaking along the Subtropical Tropopause

Ten years (1986‐95) of global analyses from the European Centre for Medium-Range Weather Forecasts are used to investigate the temporal and spatial distributions of Rossby wave breaking (RWB) at 350 K along the tropopause, herein defined by the 61.5 potential vorticity (PV) unit (10 26 Km 2 s21 kg21) contours. Though many studies acknowledge RWB as an important contributor to the complex of mixing processes in the atmosphere, there exists no prior climatological study of its distribution near the tropopause. As in previous studies, RWB is identified in the global analyses by southward directed PV gradients. At 350 K, RWB along the tropopause occurs preferentially during summer over the midoceans, in relative proximity to the planetary-scale high pressure systems in the subtropics. Isentropic trajectories at 350 K show that outflow from the tops of these subtropical highs directly participates in RWB over the adjacent, downstream oceanic regions. Two regions are highlighted in this study: the North Pacific during boreal summer and the South Atlantic during austral summer. Synoptic maps of breaking Rossby waves in these regions are provided to reveal the acute tropopause folding in the meridional plane, which characteristically accompanies RWB. The rich interaction between the tropical flow and the extratropical westerly current exhibited by these cases suggests that the subtropical highs serve as important agents in the coupling between the tropical troposphere and the extratropical stratosphere. As expected from theoretical considerations, the locations where RWB occurs most frequently, known as ‘‘surf zones,’’ are shown to coexist with regionally weak time-mean wind speeds and horizontal gradients of PV at 350 K.

A climatology of stratospheric aerosol

A global climatology of stratospheric aerosol is created by combining nearly a decade (1979–1981 and 1984–1990) of contemporaneous observations from the Stratospheric Aerosol and Gas Experiment (SAGE I and II) and Stratospheric Aerosol Measurement (SAM II) instruments. One goal of this work is to provide a representative distribution of the aerosol layer for use in radiative and chemical modeling. A table of decadal average 1μm extinction values is included, extending from the tropopause to 35 km and 80°S to 85°N, which allows estimation of surface area density. We find that the aerosol layer is distinctly volcanic in nature and suggest that the decadal average is a more useful estimate of future aerosol loading than a “background” loading, which is never clearly achieved during the data record. This climatology lends insight into the general circulation of the stratosphere. Latitude - altitude sections of extinction ratio at 1 μm are shown, averaged by decade, season, and phase of the quasi-biennial oscillation (QBO). A tropical reservoir region is diagnosed, with an “upper” and a “lower” transport regime. In the tropics above 22 km (upper regime), enhanced lofting occurs in the summer, with suppressed lofting or eddy dilution in the winter. In the extratropics within two scale heights of the tropopause (lower regime), poleward and downward transport is most robust during winter, especially in the northern hemisphere. The transport patterns persist into the subsequent equinoctial season. Ascent associated with QBO easterly shear favors detrainment in the upper regime, while relative descent and poleward spreading during QBO westerly shear favors detrainment in the lower regime. Extinction ratio differences between the winter-spring and summer-fall hemispheres, and differences between the two phases of the QBO, are typically 20–50%. Dynamical implications of the aerosol distributions are explored, with focus on interhemispheric differences, strong subtropical gradients, and the pronounced annual cycle.

Transport of Ozone in the Middle Stratosphere: Evidence for Planetary Wave Breaking

Abstract Data from the Nimbus 7 Limb Infrared Monitor of the Stratosphere (LIMS) for the period 25 October 1978–28 May 1979 are used in a descriptive study of ozone variations in the middle stratosphere. It is shown that the ozone distribution is strongly influenced by irreversible deformation associated with large amplitude planetary-scale waves. This process, which has been described by McIntyre and Palmer as planetary wave breaking, takes place throughout the 3–30 mb layer, and poleward transport of ozone within this layer occurs in narrow tongues drawn out of the tropics and subtropics in association with major and minor warming events. Thew events complement the zonal mean diabatic circulation in producing significant changes in the total column amount of ozone.

Estimation of the Kelvin Wave Contribution to the Semiannual oscillation

Abstract The behavior of Kelvin waves in the equatorial middle atmosphere is investigated with the use of daily mapped temperature derived from the Limb Infrared Monitor of the Stratosphere (LIMS) experiment. Diagrams of wave activity per unit mass and wave activity flux density concisely illustrate bulk properties of Kelvin waves and facilitate tracing of packets to source times near the tropopause. Kelvin wave packets of different zonal wavenumbers propagate separately and appear to be forced separately. During the LIMS data period (25 October 1978–28 May 1979) two Kelvin wave regimes are found. Packets of wave one, wave two, or wave three Kelvin waves occur at irregular intervals prior to April. During April and May a nearly continuous upward flux of wave one activity dominates. For very tall Kelvin waves the observed dependence of vertical wavelength on zonal wind is weaker than predicted by the slowly-varying theory for internal gravity waves. However, most properties of the observed waves are consis...

The separated polar winter stratopause: A gravity wave driven climatological feature

Abstract An examination of satellite-derived temperatures reveals that the winter polar stratopause is usually elevated and warmer than the adjacent midlatitude stratopause. This “separated stratopause” occurs in both hemispheres, but is more pronounced and persistent in the southern winter. It descends with time towards spring and exhibits week to week variability. Observational diagnostics and results from a two dimensional (2-D) model suggest that gravity wave driving can account for this separated polar stratopause by driving a meridional circulation with downwelling over the winter pole. In the model, the solar heating pattern induces stronger winter westerlies than summer easterlies, which leads to a stronger gravity wave driven circulation in the winter hemisphere. Spherical geometry and the high latitude location of the winter westerly jet combine to yield a concentrated region of downwelling. Model results suggest that descent of the temperature maximum with time is probably caused by wave–mean f...

Evolution of the Zonal Mean State in the Equatorial Middle Atmosphere during October 1978-May 1979

Abstract The evolution of the zonal mean state in the equatorial middle atmosphere is investigated with the use of daily mapped temperatures derived from the Limb Infrared Monitor of the Stratosphere (LIMS) experiment. These quasi-global, high vertical resolution data cover the pressure range 100-05 mb and the period 25 October 1978-28 May 1979. The equatorial semiannual oscillation (SAO) in zonal mean temperature, derived zonal wind and meridional shear of the zonal wind is described in detail. Rocket profiles are used to validate features seen in LIMS data. These include ranges in temperature and zonal wind of 20 K and 100 m s−1, and cross-equatorial shears of at 3 day−1 Consistent with the theory that the wave-mean flow interaction is essential to the SAO, flow acclerations over the equator exhibit strong week-to-week variations. While easterly accelerations are moderate and occur in deep cool layers, westerly accelerations are generally stronger and occur in shallow warm layers which descend with time...

Stratospheric response to trace gas perturbations: changes in ozone and temperature distributions.

The stratospheric concentration of trace gases released in the atmosphere as a result of human activities is increasing at a rate of 5 to 8 percent per year in the case of the chlorofluorocarbons (CFCs), 1 percent per year in the case of methane (CH4), and 0.25 percent per year in the case of nitrous oxide (N2O). The amount of carbon dioxide (CO2) is expected to double before the end of the 21st century. Even if the production of the CFCs remains limited according to the protocol for the protection of the ozone layer signed in September 1987 in Montreal, the abundance of active chlorine (2 parts per billion by volume in the early 1980s) is expected to reach 6 to7 parts per billion by volume by 2050. The impact of these increases on stratospheric temperature and ozone was investigated with a two-dimensional numerical model. The model includes interactive radiation, wave and mean flow dynamics, and 40 trace species. An increase in CFCs caused ozone depletion in the model, with the largest losses near the stratopause and, in the vertical mean, at high latitudes. Increased CO2 caused ozone amounts to increase through cooling, with the largest increases again near 45 kilometers and at high latitudes. This CO2-induced poleward increase reduced the CFC-induced poleward decrease. Poleward and downward ozone transport played a major role in determining the latitudinal variation in column ozone changes.

A climatology of stratospheric polar vortices and anticyclones

[1] United Kingdom Meteorological Office global analyses from 1991 to 2001 are used to create a global climatology of stratospheric polar vortices and anticyclones. New methodologies are developed that identify vortices in terms of evolving three-dimensional (3-D) air masses. A case study illustrates the performance of the identification schemes during February and March of 1999 when a merger of anticyclones led to a stratospheric warming that split the Arctic polar vortex. The 3-D structure and temporal evolution of the Arctic vortex and identified anticyclones demonstrates the algorithm’s ability to capture complicated phenomena. The mean geographical distribution of polar vortex and anticyclone frequency is shown for each season. The frequency distributions illustrate the climatological location and persistence of polar vortices and anticyclones. A counterpart to the Aleutian High is documented in the Southern Hemisphere: the ‘‘Australian High.’’ The temporal evolution of the area occupied by polar vortices and anticyclones in each hemisphere is shown as a function of potential temperature. Large polar vortex area leads to an increase in anticyclone area, which in turn results in a decrease in the size of the polar vortex. During Northern winter and Southern spring, 9 years of daily anticyclone movement are shown on the 1200 K (36 km, 4 hPa) isentropic surface. Preferred locations of anticyclogenesis are related to cross-equatorial flow and weak inertial stability. Regimes of traveling and stationary anticyclones are discussed. INDEX TERMS: 3309 Meteorology and Atmospheric Dynamics: Climatology (1620); 3319 Meteorology and Atmospheric Dynamics: General circulation; 3334 Meteorology and Atmospheric Dynamics: Middle atmosphere dynamics (0341, 0342); KEYWORDS: polar vortex, stratospheric anticyclones

On The Relationship between the QBO and Tropical Deep Convection

Abstract The height and amount of tropical deep convection are examined for a correlation with the stratospheric quasi-biennial oscillation (QBO). A new 23-yr record of outgoing longwave radiation (OLR) and a corrected 17-yr record of the highly reflective cloud (HRC) index are used as measures of convection. When binned by phase of the QBO, zonal means and maps of OLR and HRC carry a QBO signal. The spatial patterns of the maps highlight the QBO signal of OLR and HRC in typically convective regions. Spectral analysis of zonal mean OLR and HRC near the equator reveals significant peaks at QBO frequencies. Rotated empirical orthogonal function (REOF) analysis is used to determine if ENSO variations of convection are aliased into the observed QBO signals. Some analyses are repeated using the OLR record after ENSO REOF modes have been removed, yielding very similar results compared to the original analyses. It appears that the QBO signal is distinct from the ENSO signal, although the relative brevity of the ...

The stratospheric quasi‐biennial oscillation in the NCEP reanalyses: Climatological structures

Global quasi-biennial variation in the lower stratosphere and tropopause region is studied using 41 years (1958–1998) of reanalyses from the National Centers for Environmental Prediction (NCEP). Horizontal wind, temperature, geopotential height, tropopause temperature and pressure fields are used. A new quasi-biennial oscillation (QBO) indexing method is presented, which is based on the zonal mean zonal wind shear anomaly at the equator and is compared to the Singapore index. A phase difference compositing technique provides “snapshots” of the QBO meridional-vertical structure as it descends, and “composite phases” provide a look at its time progression. Via binning large amounts of data, the first observation-based estimate of the QBO meridional circulation is obtained. High-latitude QBO variability supports previous studies that invoke planetary wave-mean flow interaction as an explanation. The meridional distribution of the range in QBO zonal wind is compared with the stratospheric annual cycle, with the annual cycle dominating poleward of ∼12° latitude but still being significant in the deep tropics. The issues of temporal shear zone asymmetries and phase locking with the annual cycle are critically examined. Subtracting the time mean and annual cycle removes ∼2/3 of the asymmetry in wind (and wind shear) zone descent rate. The NCEP data validate previous findings that both the easterly and westerly QBO anomalous wind regimes in the lower stratosphere change sign preferentially during northern summer. It is noteworthy that the NCEP QBO amplitude and the relationships among the reanalyzed zonal wind, temperature, and meridional circulation undergo a substantial change around 1978.