Wookap Choi

Transport Circulation Deduced from SAMS Trace Species Data

Abstract Three years of zonally averaged N2O and CH4 data from the SAMS instrument on Nimbus 7 are utilized to investigate the annual and semiannual cycles in long-lived tracer mixing ratios. The annual and semiannual variations are shown to be approximately antisymmetric and symmetric about the equator, respectively. Using the first three components of the annual cycle to estimate the time tendency, the tracer continuity equation is solved diagnostically to obtain the effective transport velocity (i.e., the meridional circulation that can produce the observed seasonal variations in the tracer fields). The resulting circulation is qualitatively in agreement with the diabatic circulations computed by other workers. The present calculations, however, exhibit a stronger equinoctial subsidence in the equatorial upper stratosphere than deduced in other studies as required to produce a “double peak” tracer structure that has the amplitude and vertical extent that is observed.

Role of the quasi-biennial oscillation in the transport of aerosols from the tropical stratospheric reservoir to midlatitudes

The temporal evolution of the stratospheric aerosol distribution in the tropical stratospheric reservoir after the eruption of Mount Pinatubo was observed from 1992 to 1995 by the HALOE instrument on the UARS satellite. Since the spatial gradient of aerosol loading is large at the boundaries of the tropical stratospheric reservoir due to the volcanic aerosols, the effect of the meridional circulation on the distribution is seen clearly. The mechanism for dispersal of aerosol in the lower stratosphere from the tropics into midlatitudes strongly depends on the phase of the equatorial zonal wind. The time-latitude crosssections of the normalized distribution of aerosol on isentropic surfaces are used to observe the equatorial variation as well as change in meridional dispersal during the quasibiennial period. Observed tropical stratospheric winds are used with a simple analytical dynamical model to examine transport processes of tracers from the tropics during several phases of the quasibiennial oscillation (QBO) from 1992 to 1995. The Lagrangian meridional circulation in the tropics is consistent with the vertical and meridional velocities correlated with the QBO in the zonal wind. We find that vertical motion plays a crucial role in vertical and subsequent meridional transport. The pattern of meridional divergence derived from the vertical velocity is closely related to the observed HALOE aerosol distributions and their temporal development in the equatorial region. The westerly (easterly) shear phase of the QBO is associated with sinking (rising) motions at the equator and subsequent poleward (equatorward) transport in the lower stratosphere.

On the secondary meridional circulation associated with the quasi-biennial oscillation

Abstract Concentrations and distributions of stratospheric aerosol, hydrogen fluoride and ozone from the Halogen Occultation Experiment (HALOE) on the Upper Atmosphere Research Satellite (UARS) are used to investigate features associated with transport by the secondary meridional circulation induced by the quasi-biennial oscillation (QBO). The points of maxima in the divergence and convergence of the QBO-induced meridional velocity at the equator are identified from the meridional gradients of the tracers. Such points can be identified from the tracer fields in the westerly shear zones but not in the easterly shear zones. The temporal variation of tracer concentration at the equator is determined mainly by vertical advection, which is significantly larger during the westerly shear phase of the QBO than during the easterly shear phase, since the QBO-induced equatorial sinking motion amplifies the vertical gradient. Thus, the vertical advection associated with the secondary circulation has a stronger influence on the equatorial tracer variation during the westerly shear phase than during the easterly shear phase.

Intercomparison of ILAS and HALOE ozone at high latitudes

Atmospheric O 3 mixing ratios measured by ILAS and HALOE are compared for periods from March 25-31, 1997 in the NH and from November 20-24, 1996 and December 13-14, 1996 in the SH. Both instruments observe consistent vertical and horizontal structure in the highly variable polar vortex regions. In the ozone peak region, ILAS ozone values are smaller compared to HALOE values in both hemispheres, but the differences are less than 13% and 20% in 40-20 km altitude range, respectively, for the NH and the SH, which are both close to the sum of uncertainties of the two instruments. When validation is complete, ILAS data will be useful for the study of the chemistry and dynamics at high latitudes.

Extinction coefficients and properties of Pinatubo aerosol determined from Halogen Occultation Experiment (HALOE) data

Relative extinction coefficients as a function of wavelength are determined for stratospheric aerosols from the Mount Pinatubo eruptions in June of 1991, using the Halogen Occultation Experiment (HALOE) data at latitudes 20°–55° in the northern hemisphere from November 1991 to February 1992. Extinction coefficients at each of the eight ELALOE channels are obtained from the ratio of two transmittance profiles of consecutive occultation measurements separated by 25° longitude, one loaded with aerosols larger than the other during the early stage of aerosol dispersion after the eruptions. These coefficients are compared to theoretical Mie calculation values. Composition and a single mode particle size distribution are derived as a function of altitude. The retrievals indicate that the weight percentage of H2SO4 for 45 occultation cases is larger than the equilibrium value by about 5 wt %, while the size distribution parameters are within the range of those measured in situ at Laramie, Wyoming.

Amplitude-dependent relationship between the Southern Annular Mode and the El Niño Southern Oscillation in austral summer

Co-variability between the Southern Annular Mode (SAM) and the El Niño Southern Oscillation (ENSO) during the austral summer is examined, and it is found that there exists an apparent co-variability of a negative (positive) SAM during the mature period of El Niño (La Niña). However, this co-variability is largely controlled by the small number of strong ENSO cases. When strong ENSO cases are excluded, the correlation becomes non-significant. This behavior in the relationship between SAM and ENSO is supported by a series of general circulation model experiments with prescribed sea surface temperature boundary conditions that represent the incremental strengthening of El Niño (La Niña) conditions. The modeled Antarctic sub-polar jet exhibits similar behavior to that identified through observational analysis. Marked changes in both the magnitude and position of the sub-polar jet are largely controlled by particularly strong transient eddy forcing. Planetary wave forcing plays only a minor role in the co-variability, but it can explain in part the asymmetric response of the sub-polar jet between El Niño and La Niña.

Variations of the stratospheric chemical species in the Antartic spring observed by ILAS

Temporal variation of chemical species in the stratosphere was investigated based on observations by the Improved Limb Atmospheric Spectrometer onboard the Advanced Earth Observing Satellite. The zonal mean mixing ratio of nitrous oxide at high latitudes in the Southern Hemisphere increased rapidly during November 1996. During the period the circulation pattern changed from the winter polar vortex to the summer pattern and was influenced by the periodic fluctuation of the planetary wave activities. The altitude on which the polar vortex broke down gradually changed from the upper stratosphere to the lower stratosphere. The vertical gradient of the zonal mean mixing ratio of nitrous oxide showed positive value at the level of the rapid temporal increase. The level of the positive vertical gradient is closely associated with that of the polar vortex breakdown. The variation of the tracer mixing ratio was investigated with respect to the boundary of the polar vortex. The mixing ratios inside and outside of the polar vortex evolved with distinctive patterns with time. The evolution of the polar vortex and the shape of it have significant influences on the zonal mean mixing ratio of tracers.

Properties of Stratospheric Aerosol Estimated from HALOE Data

Extinction coefficients for stratospheric aerosols at 8 HALOE wavelengths are determined by comparing transmittances data for two adjacent solar occultation measurements, where one limb path is loaded with aerosols but the other path is free of aerosols. These extinction coefficients are used to infer the aerosol properties such as composition and size distribution parameters. Mie theory has been used to calculate the extinction coefficients, and a nonlinear least square method is applied to determine the aerosol properties. Sixteen cases are selected for the retrieval in southern hemisphere at latitudes from 21 to 48 degrees S for the period of 29 Mar - 31 May 1992. Retrieved size width ranges from 1.1 to 1.5 and radius ranges from 0.25 to 0.45 micrometer. These size parameters are within the ranges of in situ measurements at Laramie, Wyoming. Retrieved weight % of H2SO4 is larger than the equilibrium value by about 5 to approximately 10 weight %, similar to the results for northern hemisphere at latitudes 20 to 55 degree N for the period from Nov 1991 to Feb 1992.

Eddy heat flux convergence in the troposphere and its effect on the meridional circulation and ozone distribution

In this study the vertical convergence of the eddy heat flux, found as a forcing term in the thermodynamic energy equation of the transformed Eulerian mean formulation, is estimated in the troposphere and in the lower stratosphere from climatalogical data. Results show that while the heating rates caused by these eddy effects are small in the stratosphere they may play an important role in tropospheric circulation. The eddy-caused additions to the forcing field are seen as a region of significant cooling in the midladitudes at the midtroposphere level and of weak heating throughout the tropical region. This net global cooling is important in balancing net global heating. In addition, the heating due to meridional heat flux is found to dominate compared to heating due to the vertical heat flux. To study circulation changes, the residual mean circulation is calculated with and without the estimated eddy heating effects. The added forcing causes additional circulation in each hemisphere that coincides with the primary circulation due to zonal-mean diabatic heating. Therefore, the eddy heat flux convergence has a significant role in enhancing the zonal-mean residual circulation in the troposphere. 26 refs., 9 fig.