J. P. McCormack

Origin of extreme ozone minima at middle to high northern latitudes

Extreme ozone minima represent localized and temporally brief (several days) reductions in column ozone amounts below some chosen absolute level. Although such minima at middle to high northern latitudes are known to be primarily dynamical in origin, a remaining issue is whether heterogeneous chemical loss processes may also contribute significantly to their formation. A case in point is the record low 165 Dobson units (DU) minimum occurring on November 30, 1999, when temperatures near 30 hPa at the location of the minimum were lower than the threshold for the formation of type I polar stratospheric clouds (PSC). An examination of Polar Ozone and Aerosol Measurement III data for times surrounding the event shows that PSCs were indeed present in the vicinity where the ozone minimum was observed. However, archived data show that a similar extreme minimum of 167 DU with characteristics comparable to those of the November 30, 1999, minimum occurred on October 30, 1985, when no PSCs were present. An ensemble of 71 extreme ozone minima with amplitudes under 215 DU exhibit a nearly linear relationship between ozone minimum deviations from the zonal mean and corresponding 30-hPa temperature deviations. Such a relationship is predicted by analytic transport models which assume that vertical motions (i.e., upwelling) are responsible for the ozone minima. Temperature deviations near 30-hPa were unusually large for both the November 30, 1999, and the October 30, 1985, events, implying unusually rapid upward transport for these events. All 71 minima occur in regions where deviations from the zonal mean of 330 K potential vorticity are negative, implying an additional contribution to their formation by quasi-horizontal transport. The timescale for column ozone reductions during extreme ozone minima events is also determined and found to be at least 20 times more rapid than expected from known chemical loss processes. The data are therefore most consistent with a purely dynamical origin for extreme ozone minima in general and the November 30, 1999, event in particular. As was shown by earlier work, the basic dynamical process involves a combination of isentropic transport of ozone-poor air from the tropical upper troposphere and rapid upwelling over upper tropospheric anticyclonic disturbances resulting from poleward Rossby wave breaking events.

The influence of the 11‐year solar cycle on the quasi‐biennial oscillation

[1] The zonally averaged CHEM2D photochemical-dynamical middle atmosphere model is used to investigate the effect of the 11-year cycle in solar ultraviolet (UV) irradiance on the quasi-biennial oscillation (QBO) in equatorial lower stratospheric zonal wind. Model calculations show the duration of the westerly (easterly) phase of the modeled QBO is ∼1 month shorter (longer) at solar maximum than at solar minimum. This effect is most apparent when the modeled QBO period is 28 months, and it is dependent on the magnitude of the imposed solar UV variations. The model results also show that a realistic simulation of the semi-annual oscillation in equatorial zonal wind is necessary to produce solar cycle changes in QBO behavior. This is the first fully interactive modeling study to show that changes in solar UV can influence the behavior of the QBO, and lends support to the current working theory of sun-climate connections.

Solar‐QBO interaction and its impact on stratospheric ozone in a zonally averaged photochemical transport model of the middle atmosphere

This work was supported in part by the NASA Living with a Star TR&T program and by the Office of Naval Research.

Decadal variability of the tropical stratosphere: Secondary influence of the El Niño–Southern Oscillation

This work was supported in part by grants from the Office of Naval Research and from the National Aeronautics and Space Administration under grants NNX06AC06G (L. Hood, P.I.) and NNH08AI67 (J. McCormack, P.I.) issued through the Living With a Star research TR&T program. NOGAPS‐ALPHA simulations were made possible by a grant of computer time from the DoD High Performance Computing Modernization Program at the U.S. Air Force Research Laboratory.

Simulations of the effects of vertical transport on the thermosphere and ionosphere using two coupled models

We have explored the sensitivity of the thermosphere and ionosphere to dynamical forcing from altitudes near the mesopause (~95 km). We performed five simulations, all for the year 2009, with the National Center for Atmospheric Research (NCAR)/Thermosphere Ionosphere Electrodynamics General Circulation Model (TIEGCM). Two simulations were driven with the NCAR Global Scale Wind Model, and three used output from the Advanced Level Physics High Altitude (ALPHA) version of the Navys Operational Global Atmospheric Prediction System (NOGAPS). Use of NOGAPS-ALPHA allows for realistic meteorological variability from the lower atmosphere to propagate up into the TIEGCM, including a rich spectrum of nonmigrating tides. We find that the additional vertical transport from these tides causes a significant reduction in the calculated peak electron density of the ionospheric F2 layer (NmF2). The mechanism for this effect is the enhanced downward transport of atomic oxygen to the base of the thermosphere. In turn, this yields a greater relative abundance of N2 and hence enhanced recombination of ions and electrons. To get improved agreement with observed electron densities, we must reduce (Kzz) by a factor of 5. However, even with lower Kzz, our calculation still underestimates the NmF2 compared with radio occultation observations by the Constellation Observing System for Meteorology, Ionosphere and Climate satellite system. This underestimate of NmF2 may be linked to an overestimate of the nonmigrating tides in the coupled TIEGCM-NOGAPS calculations or to uncertainties in the bottom boundary for atomic oxygen in the TIEGCM.

Atmospheric effects of the total solar eclipse of 4 December 2002 simulated with a high-altitude global model

Abstract : The atmosphere s response to the total solar eclipse of 4 December 2002 is studied using a prototype high-altitude global numerical weather prediction model (NOGAPS-ALPHA). Local reductions in solar ultraviolet (UV) radiation during the eclipse are estimated using astronomical calculations of umbral and penumbral surface trajectories and observed solar limb darkening at ~ 200-300 nm. In NOGAPS-ALPHA these UV eclipse shadows yield stratospheric radiative cooling rate footprints peaking near 27 K day 1, a value 2 3 times larger than assumed in previous modeling. Difference fields between NOGAPS-ALPHA runs with and without this eclipse forcing reveal vertically deep middle atmospheric responses, with three-dimensional horizontal structures very similar to the large-scale bow-wave response first proposed by Chimonas (1970). Such structure appears clearly only at later times when total eclipses have abated and gravity waves generated in the stratosphere have had time to propagate vertically. Bow-wave amplitudes and direct thermal cooling responses are both small (]1 K for temperature and ]2 3 m s 1 for horizontal winds), contradicting some rocketsonde measurements that suggest much larger responses near 50 60 km altitude. We also find clear evidence of a bow-wave-like response in the model s surface pressure fields, with an amplitude 0.1 0.5 hPa, while surface air temperatures in NOGAPS-ALPHA show 4 K cooling over Africa during the eclipse. Both findings are consistent with surface atmospheric data acquired during previous eclipse passages.

Summer mesospheric warmings and the quasi 2 day wave

High-altitude meteorological analyses are used to study the interannual variability of mesospheric weather in the Southern extratropics over five recent Januarys (2005, 2006, 2008, 2009, and 2010). Two features are apparent. First, there is significant variability in the quasi 2 day wave (Q2DW) with the largest amplitudes in January 2006 and also the last half of January 2005. Second, these periods coincide with high-latitude temperature enhancements of 8–12 K. Previous studies have linked summer mesospheric warmings to interhemispheric coupling (IHC); however, the observed temperature and zonal wind anomalies do not agree with the predictions of IHC. Rather it appears that the westward momentum forcing from these Q2DW enhancements counteracts the gravity wave drag forcing which normally produces the cold summer mesopause. Since these temperature increases have been linked to Polar Mesospheric Cloud (PMC) disappearance, the present study supports the suggestion that the Q2DW may be an important factor governing PMC variability.

Intraseasonal and interannual variability of the quasi 2 day wave in the Northern Hemisphere summer mesosphere

This study uses global synoptic meteorological fields from a high-altitude data assimilation system to investigate the quasi 2 day wave (Q2DW) and migrating diurnal tide during the Northern Hemisphere (NH) summers of 2007–2009. By applying a two-dimensional fast Fourier transform to meridional wind and temperature fields, we identify Q2DW source regions and diagnose propagation of Q2DW activity into the upper mesosphere and lower thermosphere. We find that the Q2DW in NH summer is composed primarily of westward propagating zonal wave number 3 and wave number 4 components that originate within baroclinically unstable regions along the equatorward flank of the summer midlatitude easterly jet. The amplitude of the wave number 3 Q2DW tends to peak in July while the amplitude of the wave number 4 Q2DW tends to peak in late June and again in early August. The seasonal mean Q2DW amplitudes are largest in 2009, when the amplitude of the migrating diurnal tide in the upper mesosphere near 30°N was relatively weak. However, there is no evidence of rapid amplification of the Q2DW via nonlinear interaction with the diurnal tide. Instead, variations of Q2DW amplitudes during NH summer appear to be linked to variations in the strength and location of the mesospheric easterly jet from one summer to the next, with a stronger jet producing larger Q2DW amplitudes. Linear instability model calculations based on the assimilated wind fields indicate that the fastest-growing modes are zonal wave numbers 3 and 4 with periods near 2 days that originate in the vicinity of the easterly jet.

Stratospheric Analysis and Forecast Errors Using Hybrid and Sigma Coordinates

AbstractPast investigations have documented large divergent wind anomalies in stratospheric reanalyses over steep terrain, which were attributed to discretization errors produced by the terrain-following (sigma) vertical coordinate in the forecast model. However, forecasting experiments have reported negligible differences in skill between sigma- and hybrid-coordinate models. This leads to the paradoxical conclusion that discretization errors in the forecast model yield significant stratospheric analysis errors, but insignificant stratospheric forecast errors. The authors reexamine this issue by performing two forecast-assimilation experiments that are identical except for the vertical coordinate: one uses a sigma coordinate and the other uses a hybrid coordinate. The sigma-coordinate analyses exhibit large divergent wind anomalies over terrain that extend from the surface to the model top and distort explicitly resolved orographic gravity waves. Above the tropopause, divergent wind errors are suppressed ...

Origin of the 2016 QBO Disruption and Its Relationship to Extreme El Niño Events

The descent of the westerly phase of the quasi-biennial oscillation (QBO) in equatorial stratospheric zonal wind was interrupted by the development of easterlies near 40 hPa (~23 km altitude) in early 2016. We use tropical meteorological analyses of wind and temperature to describe in detail the special circumstances by which equatorward-propagating planetary waves produced this unprecedented disruption in the QBO. Our findings show that the subtropical easterly jet in the winter lower stratosphere during the 2015-2016 winter was anomalously weak owing to (1) the timing of the QBO relative to the annual cycle and (2) an extreme El Nino event. The weak jet allowed an unusually large flux of westward momentum to propagate from the extratropical Northern Hemisphere to the equator near the 40 hPa level. Consequently, the QBO westerlies at that level experienced sustained easterly acceleration from extratropical wave breaking, leading to the observed wind reversal.