J. Eric Nielsen

A New Look at Stratospheric Sudden Warmings. Part II: Evaluation of Numerical Model Simulations

The simulation of major midwinter stratospheric sudden warmings (SSWs) in six stratosphere-resolving general circulation models (GCMs) is examined. The GCMs are compared to a new climatology of SSWs, based on the dynamical characteristics of the events. First, the number, type, and temporal distribution of SSW events are evaluated. Most of the models show a lower frequency of SSW events than the climatology, which has a mean frequency of 6.0 SSWs per decade. Statistical tests show that three of the six models produce significantly fewer SSWs than the climatology, between 1.0 and 2.6 SSWs per decade. Second, four process-based diagnostics are calculated for all of the SSW events in each model. It is found that SSWs in the GCMs compare favorably with dynamical benchmarks for SSW established in the first part of the study. These results indicate that GCMs are capable of quite accurately simulating the dynamics required to produce SSWs, but with lower frequency than the climatology. Further dynamical diagnostics hint that, in at least one case, this is due to a lack of meridional heat flux in the lower stratosphere. Even though the SSWs simulated by most GCMs are dynamically realistic when compared to the NCEP-NCAR reanalysis, the reasons for the relative paucity of SSWs in GCMs remains an important and open question.

Stratosphere-troposphere coupling and annular mode variability in chemistry-climate models

The internal variability and coupling between the stratosphere and troposphere in CCMValA¢Â�Â�2 chemistryA¢Â�Â�climate models are evaluated through analysis of the annular mode patterns of variability. Computation of the annular modes in long data sets with secular trends requires refinement of the standard definition of the annular mode, and a more robust procedure that allows for slowly varying trends is established and verified. The spatial and temporal structure of the modelsA¢Â�Â� annular modes is then compared with that of reanalyses. As a whole, the models capture the key features of observed intraseasonal variability, including the sharp vertical gradients in structure between stratosphere and troposphere, the asymmetries in the seasonal cycle between the Northern and Southern hemispheres, and the coupling between the polar stratospheric vortices and tropospheric midlatitude jets. It is also found that the annular mode variability changes little in time throughout simulations of the 21st century. There are, however, both common biases and significant differences in performance in the models. In the troposphere, the annular mode in models is generally too persistent, particularly in the Southern Hemisphere summer, a bias similar to that found in CMIP3 coupled climate models. In the stratosphere, the periods of peak variance and coupling with the troposphere are delayed by about a month in both hemispheres. The relationship between increased variability of the stratosphere and increased persistence in the troposphere suggests that some tropospheric biases may be related to stratospheric biases and that a wellA¢Â�Â�simulated stratosphere can improve simulation of tropospheric intraseasonal variability.

Two‐dimensional and three‐dimensional model simulations, measurements, and interpretation of the influence of the October 1989 solar proton events on the middle atmosphere

The very large solar proton events (SPEs) which occurred from October 19 to 27, 1989, earned substantial middle-atmospheric HOx and NOx constituent increases. Although no measurements of HOx increases were made during these SPEs, increases in NO were observed by rocket instruments which are in good agreement with calculated NO increases from our proton energy degradation code. Both the HOx and the NOx increases can cause ozone decreases; however, the HOx-induced ozone changes are relatively short-lived because HOx species have lifetimes of only hours in the middle atmosphere. Our two-dimensional model, when used to simulate effects of the longer-lived NOx, predicted lower-stratospheric polar ozone decreases of greater than 2% persisting for one and a half years past these SPEs. Previous three-dimensional model simulations of these SPEs (Jackman et al., 1993) indicated the importance of properly representing the polar vortices and warming events when accounting for the ozone decreases observed by the solar backscattered ultraviolet 2 instrument two months past these atmospheric perturbations. In an expansion of that study, we found that it was necessary to simulate the November 1, 1989, to April 2, 1990, time period and the November 1, 1986, to April 2, 1987, time period with our three-dimensional model in order to more directly compare to the stratospheric aerosol and gas experiment (SAGE) II observations of lower stratospheric NO2 and ozone changes between the end of March 1987 and 1990 at 70°N. Both the NOx increases from the October 1989 SPEs and the larger downward transport in the 1989–1990 northern winter compared to the 1986–1987 northern winter contributed to the large enhancements in NO2 in the lower stratosphere observed in the SAGE II measurements at the end of March 1990. Our three-dimensional model simulations predict smaller ozone decreases than those observed by SAGE II in the lower stratosphere near the end of March 1990, indicating that other factors, such as heterogeneous chemistry, might also be influencing the constituents of this region.

Episodic total ozone minima and associated effects on heterogeneous chemistry and lower stratospheric transport

A description of the January 31, 1989, ozone minihole over Stavanger, Norway, is given on the basis of three-dimensional model simulations. This minihole is typical (though of large magnitude) of many transient events in the lower stratosphere that arise because of cyclonic-scale disturbances in the troposphere. The ozone reduction is a short-lived reversible dynamical event. However, through heterogeneous chemical processes there can be a significant transfer of chlorine from reservoir molecules to active radicals. This chemically perturbed air is defined as processed air, and it is found that a single event can produce enough processed air to reduce the HCl in the entire polar vortex. Chemical processing on clouds associated with transient events is shown to be a major source of processed air in the polar vortex in December before background temperatures are cold enough for more uniform heterogeneous conversion. In the model, intense cyclonic scales propagating close to the vortex edge and large planetary wave events (especially stratospheric warmings) are the major mechanisms of extra-vortex transport. Only a small amount of processed air is found outside of the polar vortex. The processed air is a strong function of longitude, and it is virtually excluded from the Pacific Basin.

The ozone response to ENSO in Aura satellite measurements and a chemistry-climate simulation

[1] The El Nino–Southern Oscillation (ENSO) is the dominant mode of inter-annual variability in the tropical ocean and troposphere. Its impact on tropospheric circulation causes significant changes to the distribution of ozone. Here we derive the lower tropospheric to lower stratospheric ozone response to ENSO from observations by the Tropospheric Emission Spectrometer (TES) and the Microwave Limb Sounder (MLS) instruments, both on the Aura satellite, and compare to the simulated response from the Goddard Earth Observing System Chemistry-Climate Model (GEOSCCM). Measurement ozone sensitivity is derived using multiple linear regression to include variations from ENSO as well as from the first two empirical orthogonal functions of the quasi-biennial oscillation. Both measurements and simulation show features such as the negative ozone sensitivity to ENSO over the tropospheric tropical Pacific and positive ozone sensitivity over Indonesia and the Indian Ocean region. Ozone sensitivity to ENSO is generally positive over the midlatitude lower stratosphere, with greater sensitivity in the Northern Hemisphere. GEOSCCM reproduces both the overall pattern and magnitude of the ozone response to ENSO obtained from observations. We demonstrate the combined use of ozone measurements from MLS and TES to quantify the lower atmospheric ozone response to ENSO and suggest its possible usefulness in evaluating chemistry-climate models.

Three‐dimensional simulations of wintertime ozone variability in the lower stratosphere

The evolution of ozone has been calculated for the winters of 1979 and 1989 using winds derived from our stratospheric data assimilation system (STRATAN). The ozone fields calculated using this technique are found to compare well with satellite-measured fields for simulations of 2–3 months. Here we present comparisons of model fields with both satellite and sonde measurements to verify that stratospheric transport processes are properly represented by this modeling technique. Attention is focussed on the northern hemisphere middle and high latitudes at the 10-hPa level and below, where transport processes are most important to the ozone distribution. First-order quantities and derived budgets from both the model and satellite data are presented. By sampling the model with a limb-viewing satellite and then Kalman filtering the “observations” of the model, it is shown that transient subplanetary-scale features that are essential to the ozone budget are missed by the satellite system.

The effects of the October 1989 solar proton events on the stratosphere as computed using a three‐dimensional model

Very large solar proton events (SPEs) occurred from October 19–27, 1989. These SPEs are predicted to produce short-lived increases in HOx and long-lived increases in NOx species, which both can lead to ozone destruction. December 1989 SBUV/2 measurements of upper stratospheric ozone show substantially more ozone depletion in the Northern than in the Southern Hemisphere even though the amount of HOx and NOx produced in both hemispheres should be similar from these SPEs. Our two-dimensional (2D) model simulations predict only a modest interhemispheric difference in the ozone depletion in December caused by the October 1989 SPEs. In an attempt to better understand the interhemispheric difference in the observed ozone depletion, we have used the GSFC three-dimensional (3D) chemistry and transport model to simulate the distribution of NOx and ozone after the SPEs. Our 3D model computations of ozone and NOx behavior for two months after the October 1989 SPEs indicate differences in the constituent behavior in the two hemispheres during the October–November–December 1989 time period which are qualitatively consistent with SBUV/2 ozone observations. These differences are caused by: 1) Substantial mixing of perturbed air in the Southern Hemisphere from the polar region with unperturbed lower latitude air during the November final warming; and 2) Significant confinement of the photochemically perturbed air in the Northern Hemisphere in the winter-time polar vortex.

Understanding the Changes of Stratospheric Water Vapor in Coupled Chemistry-Climate Model Simulations

Abstract Past and future climate simulations from the Goddard Earth Observing System Chemistry–Climate Model (GEOS CCM), with specified boundary conditions for sea surface temperature, sea ice, and trace gas emissions, have been analyzed to assess trends and possible causes of changes in stratospheric water vapor. The simulated distribution of stratospheric water vapor in the 1990s compares well with observations. Changes in the cold point temperatures near the tropical tropopause can explain differences in entry stratospheric water vapor. The average saturation mixing ratio of a 20° latitude by 15° longitude region surrounding the minimum tropical saturation mixing ratio is shown to be a useful diagnostic for entry stratospheric water vapor and does an excellent job reconstructing the annual average entry stratospheric water vapor over the period 1950–2100. The simulated stratospheric water vapor increases over the 50 yr between 1950 and 2000, primarily because of changes in methane concentrations, offse...

The global structure of upper troposphere‐lower stratosphere ozone in GEOS‐5: A multiyear assimilation of EOS Aura data

Eight years of ozone measurements retrieved from the Ozone Monitoring Instrument and the Microwave Limb Sounder, both on the EOS Aura satellite, have been assimilated into the Goddard Earth Observing System Version 5 (GEOS-5) data assimilation system. This study evaluates this assimilated product, highlighting its potential for science. The impact of observations on the GEOS-5 system is explored by examining the spatial distribution of the observation-minus-forecast statistics. Independent data are used for product validation. The correlation of the lower stratospheric (the tropopause to 50 hPa) ozone column with ozonesondes is 0.99 and the (high) bias is 0.5%, indicating the success of the assimilation in reproducing the ozone variability in that layer. The upper tropospheric (500 hPa to the tropopause) assimilated ozone column is about 10% lower than the ozonesonde column, but the correlation is still high (0.87). The assimilation is shown to realistically capture the sharp cross-tropopause gradient in ozone mixing ratio. Occurrence of transport-driven low ozone laminae in the assimilation system is similar to that obtained from the High Resolution Dynamics Limb Sounder (HIRDLS) above the 400 K potential temperature surface, but the assimilation produces fewer laminae than seen by HIRDLS below that surface. Although the assimilation produces about 25% fewer occurrences per day during the 3 years of HIRDLS data, the interannual variability is captured correctly. This data-driven assimilated product is complementary to ozone fields generated from chemistry and transport models. Applications include study of the radiative forcing by ozone and tracer transport near the tropopause.

Interannual Variability and Trends of Extratropical Ozone. Part I: Northern Hemisphere

The authors apply principal component analysis (PCA) to the extratropical total column ozone from the combined merged ozone data product and the European Centre for Medium-Range Weather Forecasts assimilated ozone from January 1979 to August 2002. The interannual variability (IAV) of extratropical O3 in the Northern Hemisphere (NH) is characterized by four main modes. Attributable to dominant dynamical effects, these four modes account for nearly 60% of the total ozone variance in the NH. The patterns of variability are distinctly different from those derived for total O3 in the tropics. To relate the derived patterns of O3 to atmospheric dynamics, similar decompositions are performed for the 30–100-hPa geopotential thickness. The results reveal intimate connections between the IAV of total ozone and the atmospheric circulation. The first two leading modes are nearly zonally symmetric and represent the connections to the annular modes and the quasi-biennial oscillation. The other two modes exhibit in-quadrature, wavenumber-1 structures that, when combined, describe the displacement of the polar vortices in response to planetary waves. In the NH, the extrema of these combined modes have preferred locations that suggest fixed topographical and land–sea thermal forcing of the involved planetary waves. Similar spatial patterns and trends in extratropical column ozone are simulated by the Goddard Earth Observation System chemistry–climate model (GEOS-CCM). The decreasing O3 trend is captured in the first mode. The largest trend occurs at the North Pole, with values 1 Dobson Unit (DU) yr 1 . There is almost no trend in tropical O3. The trends derived from PCA are confirmed using a completely independent method, empirical mode decomposition, for zonally averaged O3 data. The O3 trend is also captured by mode 1 in the GEOS-CCM, but the decrease is substantially larger than that in the real atmosphere.