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Gerd Breßer and Steven Pawson

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A narrowband (5 cm−1) radiation transfer scheme has been used to calculate scale-dependent radiative dissipation rates for finite-amplitude temperature disturbances. Eight bands of five atmospheric trace gases have been examined. As previously reported, the CO2 15-µm bands are dominant, and the O3 9.6-µm bands can play a significant role, particularly in the lower stratosphere. The minor bands of CO2 (4.3 and 10.7 µm) are unimportant. Of the other gases considered, H2O (integrated across the longwave spectrum) makes a significant contribution to the dissipation rates in the lower stratosphere. Likewise, the O3 14.3-µm bands contribute to the cooling to space in the lower stratosphere. The 7.66-µm CH4 as well as the 7.78-µm NO2 bands make almost no contribution above the lower stratosphere.

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Lawrence Coy and Steven Pawson

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The major stratospheric sudden warming (SSW) of 6 January 2013 is examined using output from the NASA Global Modeling and Assimilation Office (GMAO) Goddard Earth Observing System version 5 (GEOS-5) near-real-time data assimilation system (DAS). GEOS-5 analyses showed that the SSW of January 2013 was a major warming by 1200 UTC 6 January, with a wave-2 vortex-splitting pattern. Upward wave activity flux from the upper troposphere (~23 December 2012) displaced the ~10-hPa polar vortex off the pole in a wave-1 pattern, enabling the poleward advection of subtropical values of Ertel potential vorticity (EPV) into a developing anticyclonic circulation region. While the polar vortex subsequently split (wave-2 pattern) the wave-2 forcing [upward Eliassen–Palm (EP) flux] was smaller than what was found in recent wave-2, SSW events, with most of the forcing located in the Pacific hemisphere. Investigation of a rapidly developing tropospheric weather system over the North Atlantic on 28–29 December 2012 showed strong transient upward wave activity flux from the storm with influences up to 10 hPa; however, the Pacific hemisphere wave forcing remained dominate at this time. Results from the GEOS-5 five-day forecasts showed that the forecasts accurately predicted the major SSW of January 2013. The overall success of the 5-day forecasts provides motivation to produce regular 10-day forecasts with GEOS-5, to better support studies of stratosphere–troposphere interaction.

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Jonathan S. Kinnersley and Steven Pawson

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The influence of vertical advection on the descent rate of the zero-wind line in both phases of the equatorial quasi-biennial oscillation (QBO) is investigated with the help of the “THIN AIR” stratosphere two-and-a-half-dimensional model. The model QBO is forced by two symmetric easterly and westerly waves, and yet the model reproduces qualitatively the observed asymmetry in the descent rates of the two shear zones due to the enhanced heating during easterly descent combined with the equatorial heating induced by the extratropical planetary waves. Observations show that the maximum easterly accelerations occur predominantly from May until July, which is when the modeled equatorial planetary-wave-induced heating rates are weakest. Hence, model results are consistent with the theory that vertical advection induced by extratropical planetary waves slows significantly the descent of the easterly shear zone. The model also shows the observed increase in vertical wind shear during stalling of the easterly descent (which increases the impact of vertical advection). In the model, the effect of cross-equatorial advection of momentum by the mean flow is negligible compared to the vertical advection. Changes in the propagation of planetary waves depending on the sign of the equatorial zonal wind have a small effect on the modeled equatorial heating rates and therefore do not play a large part in producing the modeled asymmetry in descent rates.

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Katrin M. Müller, Ulrike Langematz, and Steven Pawson

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The tropical stratopause semiannual oscillation (SAO) in the Berlin troposphere–stratosphere–mesosphere general circulation model (TSM GCM) is investigated. The model is able to produce a semiannual oscillation that is properly located in time and space in comparison with observations; however, the westerly phase is 10–15 m s−1 too weak while the second easterly phase of the year is about the same amount too strong.

A case study is performed to examine the westerly forcing of the SAO in detail. At 1 hPa in one particular March, when the strongest westerly acceleration of the year takes place, only 18% of the forcing is due to the dissipation of Kelvin waves, whereas 72% of the forcing is due to the dissipation of other slow eastward propagating planetary-scale waves and 10% is due to the dissipation of eastward propagating inertio–gravity waves. Throughout the year the Kelvin wave contribution to the total body force is below 50%. The modeled Kelvin waves are dissipated at higher altitudes than observed Kelvin waves with the same phase speeds.

To examine the effect of the QBO on the wave spectrum a relaxation of the zonal-mean wind to an idealized QBO has been implemented into the model. During the westerly phase of the QBO, slow eastward propagating waves with wavenumbers greater than 1 can no longer reach the upper stratosphere. This leads to a reduction of the westerly momentum available to generate the westerly phase of the SAO, and the westerly winds do not descend below 1 hPa as in the control experiment. This reduction in the descent is also present in observations, but there it is less pronounced. As the upper-stratospheric Kelvin waves in the model are partly too slow, they are too strongly affected by the changes in the propagation properties due to the QBO.

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Klaus Fraedrich, Steven Pawson, and Risheng Wang

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Empirical orthogonal function analyses of the time-height series of monthly mean zonal wind in the stratosphere have been performed. Conventional EOF analysis on the time series reveals that the quasi-biennial oscillation (QBO) of the zonal wind is a quasi-regular oscillation with a period near 28 months, but the noisy structure of the first two EOFs, representing 57.18% and 36.24% of the variance of the time series, leads to little new insight concerning the dynamics of the QBO. A second type of analysis is performed by applying windows to the data and calculating the EOFs of the time development of the spatial structure. By using a window of around one-half of the period of the dominant oscillation, the EOF analysis reveals that the QBO is essentially a linear feature, with successive wind regimes propagating smoothly downwards with a spectral peak near 28 months; this oscillation has a very smooth phase portrait and cross-correlation analysis of the first two EOFs reveals that it is a quasi-linear feature with good predictability. Delays in the downward propagation of the easterly phase of the QBO are shown to be nonlinear, unpredictable features, represented by higher-order EOFS.

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Risheng Wang, Klaus Fraedrich, and Steven Pawson

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Extended empirical orthogonal functions (EOFs) are used to define a phase space for the analysis of tropical stratospheric wind data, extending our previous study of the quasi-biennial oscillation (QBO) in several manners. First, the sensitivity of the analysis to the length of the window (w) is discussed in some detail. As w increases, the leading pair of EOFs become more concentrated on the period near 28 months; simultaneously, the signals contained in higher-order EOFs become more significant, with more clearly defined periodicities; however, for large w more EOFs are required to represent the same variance. There appear to be two stable regimes: when w is less than 20 months the first two EOFs describe a QBO with some irregularities in the onset of easterly wind regimes, whereas when w exceeds 30 months such irregularities are represented by the third and fourth EOFS. Second, the first pair of EOFs with w = 40 are regarded as representing a pure QBO signal, subject to variations in cycle length (ranging from 22 to 33 months) and amplitude but propagating smoothly. Its phase–space characteristics are examined in some detail; this oscillation is regarded as a limit cycle, subject to low-frequency variability, presumably due to fluctuations in the forcing mechanisms at work. No annual cycle is evident in its propagation in phase space. Third, departures from this pure QBO are examined. These are represented by higher-order signals with w = 40. EOFs 3 and 4 describe much of the irregularity in downward propagation of the wind regimes, with dominant periods in a broad band centered on 28 months; EOF 5 does not represent a propagating signal but some low-frequency variability (probably externally forced) in the vertical wind shear; E0Fs 6 and 7 are the subharmonics of the QBO; EOFs 8 and 9 represent the annual cycle.

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Andrew Tangborn, Robert Cooper, Steven Pawson, and Zhibin Sun

Abstract

A source inversion technique for chemical constituents is presented that uses assimilated constituent observations rather than directly using the observations. The method is tested with a simple model problem, which is a two-dimensional Fourier–Galerkin transport model combined with a Kalman filter for data assimilation. Inversion is carried out using a Green’s function method and observations are simulated from a true state with added Gaussian noise. The forecast state uses the same spectral model but differs by an unbiased Gaussian model error and emissions models with constant errors. The numerical experiments employ both simulated in situ and satellite observation networks. Source inversion was carried out either by directly using synthetically generated observations with added noise or by first assimilating the observations and using the analyses to extract observations. Twenty identical twin experiments were conducted for each set of source and observation configurations, and it was found that in the limiting cases of a very few localized observations or an extremely large observation network there is little advantage to carrying out assimilation first. For intermediate observation densities, the source inversion error standard deviation is decreased by 50% to 90% when the observations are assimilated with the Kalman filter before carrying out the Green’s function inversion.

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Richard S. Stolarski, Anne R. Douglass, Stephen Steenrod, and Steven Pawson

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Stratospheric ozone is affected by external factors such as chlorofluorcarbons (CFCs), volcanoes, and the 11-yr solar cycle variation of ultraviolet radiation. Dynamical variability due to the quasi-biennial oscillation and other factors also contribute to stratospheric ozone variability. A research focus during the past two decades has been to quantify the downward trend in ozone due to the increase in industrially produced CFCs. During the coming decades research will focus on detection and attribution of the expected recovery of ozone as the CFCs are slowly removed from the atmosphere. A chemical transport model (CTM) has been used to simulate stratospheric composition for the past 30 yr and the next 20 yr using 50 yr of winds and temperatures from a general circulation model (GCM). The simulation includes the solar cycle in ultraviolet radiation, a representation of aerosol surface areas based on observations including volcanic perturbations from El Chichon in 1982 and Pinatubo in 1991, and time-dependent mixing ratio boundary conditions for CFCs, halons, and other source gases such as N2O and CH4. A second CTM simulation was carried out for identical solar flux and boundary conditions but with constant “background” aerosol conditions. The GCM integration included an online ozonelike tracer with specified production and loss that was used to evaluate the effects of interannual variability in dynamics. Statistical time series analysis was applied to both observed and simulated ozone to examine the capability of the analyses for the determination of trends in ozone due to CFCs and to separate these trends from the solar cycle and volcanic effects in the atmosphere. The results point out several difficulties associated with the interpretation of time series analyses of atmospheric ozone data. In particular, it is shown that lengthening the dataset reduces the uncertainty in derived trend due to interannual dynamic variability. It is further shown that interannual variability can make it difficult to accurately assess the impact of a volcanic eruption, such as Pinatubo, on ozone. Such uncertainties make it difficult to obtain an early proof of ozone recovery in response to decreasing chlorine.

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Young-Kwon Lim, Robin M. Kovach, Steven Pawson, and Guillaume Vernieres

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The 2015/16 El Niño is analyzed using atmospheric and oceanic analysis produced using the Goddard Earth Observing System (GEOS) data assimilation systems. As well as describing the structure of the event, a theme of this work is to compare and contrast it with two other strong El Niños, in 1982/83 and 1997/98. These three El Niño events are included in the Modern-Era Retrospective Analysis for Research and Applications (MERRA) and in the more recent MERRA-2 reanalyses. MERRA-2 allows a comparison of fields derived from the underlying GEOS model, facilitating a more detailed comparison of physical forcing mechanisms in the El Niño events. Various atmospheric and oceanic structures indicate that the 2015/16 El Niño maximized in the Niño-3.4 region, with a large region of warming over most of the Pacific and Indian Oceans. The eastern tropical Indian Ocean, Maritime Continent, and western tropical Pacific are found to be less dry in boreal winter, compared to the earlier two strong events. Whereas the 2015/16 El Niño had an earlier occurrence of the equatorial Pacific warming and was the strongest event on record in the central Pacific, the 1997/98 event exhibited a more rapid growth due to stronger westerly wind bursts and the Madden–Julian oscillation during spring, making it the strongest El Niño in the eastern Pacific. Compared to 1982/83 and 1997/98, the 2015/16 event had a shallower thermocline over the eastern Pacific with a weaker zonal contrast of subsurface water temperatures along the equatorial Pacific. While the three major ENSO events have similarities, each is unique when looking at the atmosphere and ocean surface and subsurface.

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Lawrence Coy, Paul A. Newman, Steven Pawson, and Leslie R. Lait

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A significant disruption of the quasi-biennial oscillation (QBO) occurred during the Northern Hemisphere (NH) winter of 2015/16. Since the QBO is the major wind variability source in the tropical lower stratosphere and influences the rate of ascent of air entering the stratosphere, understanding the cause of this singular disruption may provide new insights into the variability and sensitivity of the global climate system. Here this disruptive event is examined using global reanalysis winds and temperatures from 1980 to 2016. Results reveal record maxima in tropical horizontal momentum fluxes and wave forcing of the tropical zonal mean zonal wind over the NH 2015/16 winter. The Rossby waves responsible for these record tropical values appear to originate in the NH and were focused strongly into the tropics at the 40-hPa level. Two additional NH winters, 1987/88 and 2010/11, were also found to have large tropical lower-stratospheric momentum flux divergences; however, the QBO westerlies did not change to easterlies in those cases.

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