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Elisa Manzini and Kevin Hamilton

Abstract

The excitation and propagation of equatorial planetary waves and inertia-gravity waves were studied by comparing simulations from the comprehensive GFDL troposphere-stratosphere-mesosphere SKYHI general circulation model (GCM) and from a linear primitive equation model with the same domain and numerical resolution. The basic state of the linear model is time dependent and is derived from the mean zonal wind and temperature obtained from a simulation with the full SKYHI model. The latent and convective heating fields of this SKYHI integration are used as the forcing for the linear model in a parallel simulation.

The wavelength and frequency characteristics of the prominent vertically propagating equatorial Kelvin and Rossby-gravity waves are remarkably similar in the linear model and in SKYHI. Amplitudes are also similar in the lower stratosphere, indicating that the latent and convective heating is the dominant mechanism producing equatorial wave activity in the GCM. The amplitude of these waves in the upper stratosphere and mesosphere is larger in the linear model than in SKYHI. Given that the linear and SKYHI models have comparable radiative damping and horizontal subgrid scale diffusion, it appears that the wave amplitudes in SKYHI are limited by some nonlinear saturation, possibly involving the subgrid-scale vertical mixing.

At low latitudes the linear model reproduces the flux of upward-propagating inertia-gravity waves seen in the full model. The results also show that a significant fraction of the inertia-gravity wave activity found in the midlatitude mesosphere of the SKYHI model can be accounted for by tropical convective heating.

The global-scale Rossby normal modes seen in observations were also identified in the analyses of westward-propagating planetary waves in both models. They are of realistic amplitude in the SKYHI simulation but are much weaker in the linear model. Thus, it appears that latent and convective heating is not the main source of excitation for the Rossby normal modes.

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Axel Lauer and Kevin Hamilton

Abstract

Clouds are a key component of the climate system affecting radiative balances and the hydrological cycle. Previous studies from the Coupled Model Intercomparison Project phase 3 (CMIP3) showed quite large biases in the simulated cloud climatology affecting all GCMs as well as a remarkable degree of variation among the models that represented the state of the art circa 2005. Here the progress that has been made in recent years is measured by comparing mean cloud properties, interannual variability, and the climatological seasonal cycle from the CMIP5 models with satellite observations and with results from comparable CMIP3 experiments. The focus is on three climate-relevant cloud parameters: cloud amount, liquid water path, and cloud radiative forcing. The comparison shows that intermodel differences are still large in the Coupled Model Intercomparison Project phase 5 (CMIP5) simulations, and reveals some small improvements of particular cloud properties in some regions in the CMIP5 ensemble over CMIP3. In CMIP5 there is an improved agreement of the modeled interannual variability of liquid water path and of the modeled longwave cloud forcing over mid- and high-latitude oceans with observations. However, the differences in the simulated cloud climatology from CMIP3 and CMIP5 are generally small, and there is very little to no improvement apparent in the tropical and subtropical regions in CMIP5.

Comparisons of the results from the coupled CMIP5 models with their atmosphere-only versions run with observed SSTs show remarkably similar biases in the simulated cloud climatologies. This suggests the treatments of subgrid-scale cloud and boundary layer processes are directly implicated in the poor performance of current GCMs in simulating realistic cloud fields.

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Kevin Hamilton and J. D. Mahlman

Abstract

A study has been made of the evolution of the zonal-mean zonal wind and temperature in a multiyear integration of the 40-level, 3° × 3.6° resolution “SKYHI” general circulation model (GCM) that has been developed at GFDL. In the tropical upper stratosphere the mean wind variation is dominated by a strong semiannual oscillation (SAO). The peak SAO amplitude in the model is almost 25 m s−1 and occurs near the 1 mb level. The phase of the SAO near the stratopause is such that maximum westerlies occur shortly after the equinoxes. These features are in good agreement with the available observations. In addition the meridional width of the stratopause SAO in the GCM compares well with observations.

A diagnostic analysis of the zonal-mean momentum balance near the tropical stratopause was performed using the detailed fields archived during the GCM integration. It appears that the easterly accelerations in the model SAO are provided by a combination of (i) divergence of the meridional component of the Eliassen-Palm flux associated with quasi-stationary planetary waves and (ii) mean angular momentum advection by the residual meridional circulation. The effects of the residual circulation dominate in the summer hemisphere, while the eddy contributions are more important in the winter hemisphere. The westerly accelerations in the model SAO result from the convergence of the vertical momentum transport associated with gravity waves that have a broad distribution of space and time scales. Thus, in contrast to some simple theoretical models, large-scale equatorial Kelvin waves appear to play only a very minor role in the dynamics of the SAO in the SKYHI GCM.

A second equatorial SAO amplitude maximum was found in the tropical upper mesosphere of the GCM. This apparently corresponds to the mesopause SAO that has been identified in earlier observational studies. While the observed phase of this oscillation is reproduced in the model, the simulated amplitude is unrealistically small.

The model integration included the computation of the concentration of N2O. The results show a fairly realistic simulation of the semiannual variation of tropical stratospheric N2O mixing ratio seen in satellite observations.

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R. John Wilson and Kevin Hamilton

Abstract

This paper discusses the thermotidal oscillations in simulations performed with a newly developed comprehensive general circulation model of the Martian atmosphere. With reasonable assumptions about the effective thermal inertia of the planetary surface and about the distribution of radiatively active atmospheric aerosol, the model produces both realistic zonal-mean temperature distributions and a diurnal surface pressure oscillation of at least roughly realistic amplitude. With any reasonable aerosol distribution, the simulated diurnal pressure oscillation has a very strong zonal variation, in particular a very pronounced zonal wavenumber-2 modulation. This results from a combination of the prominent wave-2 component in the important boundary forcings (topography and surface thermal inertia) and from the fact that the eastward-propagating zonal wave-1 Kelvin normal mode has a period near 1 sol (a Martian mean solar day of 88 775 s). The importance of global resonance is explicitly demonstrated with a series of calculations in which the global mean temperature is arbitrarily altered. The resonant enhancement of the diurnal wave-1 Kelvin mode is predicted to be strongest in the northern summer season. In the model simulations there is also a strong contribution to the semidiurnal tide from a near-resonant eastward-propagating wave-2 Kelvin mode. It is shown that this is significantly forced by a nonlinear steepening of the diurnal Kelvin wave. The daily variations of near-surface winds in the model are also examined. The results show that the daily march of wind at any location depends strongly on the topography, even on the smallest horizontal scales resolved in the model (∼ few hundred km). The global tides also play an important role in determining the near-surface winds, especially so in very dusty atmospheric conditions.

The results for the diurnal and semidiurnal surface pressure oscillations in seasonal integrations of the model are compared in detail with the observations at the two Viking Lander sites (22°N and 48°N). The observations over much of the year can be reasonably reproduced in simulations with a globally uniform aerosol mixing ratio (and assuming more total aerosol in the northern winter season, when the largest dust storms are generally observed). There are features of the Viking observations that do not seem to be explainable in this way, however. In particular, in early northern summer, the model predicts amplitudes for the diurnal pressure oscillation at both lander sites that are at least a factor of 2 larger than observed. Results are presented showing that the low amplitudes observed could be explained if the dust distribution tended to be concentrated over the highlands, rather than being uniformly mixed. Annual cycle simulators with a version of the model with an interactive dust transport do in fact reveal the tendency of the circulation to organize so that larger dust mixing ratios occur over highlands, particularly near subsolar latitudes. When the model includes globally uniform surface dust injection and parameterized dust sedimentation, the annual cycle of the diurnal and semidiurnal tides at both lander sites can be rather well reproduced, except for the periods of global dust storms. The attempts to simulate the observed rapid evolution of the tidal pressure oscillations during the onset of a global dust atom also demonstrate the importance of a nonuniform dust concentration. Simulations with the version of the model incorporating interactive dust are able to roughly reproduce the Viking observations when a strong zonally uniform dust injection is prescribed in the Southern Hemisphere Tropics and subtropics.

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John N. Koshyk and Kevin Hamilton

Abstract

Horizontal kinetic energy spectra simulated by high-resolution versions of the Geophysical Fluid Dynamics Laboratory SKYHI middle-atmosphere general circulation model are examined. The model versions considered resolve heights between the ground and ∼80 km, and the horizontal grid spacing of the highest-resolution version is about 35 km. Tropospheric kinetic energy spectra show the familiar ∼−3 power-law dependence on horizontal wavenumber for wavelengths between about 5000 and 500 km and have a slope of ∼−5/3 at smaller wavelengths. Qualitatively similar behavior is seen in the stratosphere and mesosphere, but the wavelength marking the transition to the shallow regime increases with height, taking a value of ∼2000 km in the stratosphere and ∼4000 km in the mesosphere.

The global spectral kinetic energy budget for various height ranges is computed as a function of total horizontal wavenumber. Contributions to the kinetic energy tendency from nonlinear advective processes, from conversion of available potential energy, from mechanical fluxes through the horizontal boundaries of the region, and from parameterized subgrid-scale dissipation are all examined. In the troposphere, advective contributions are negative at large scales and positive over the rest of the spectrum. This is consistent with a predominantly downscale nonlinear cascade of kinetic energy into the mesoscale. The global kinetic energy budget in the middle atmosphere differs significantly from that in the troposphere, with the positive contributions at most scales coming predominantly from vertical energy fluxes.

The kinetic energy spectra calculated from two model versions with different horizontal resolution are compared. Differences between the spectra over the resolved range of the lower-resolution version are smallest in the troposphere and increase with height, owing mainly to large differences in the divergent components. The result suggests that the parameterization of dynamical subgrid-scale processes in middle-atmosphere general circulation models, as well as in high-resolution tropospheric general circulation models, may need to be critically reevaluated.

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Marvin A. Geller, Tiehan Zhou, and Kevin Hamilton

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Sensitivity tests of a mechanistic model of the mean meridional circulation driven by specified eddy forcing are conducted to investigate how the morphology of tropical upwelling in the lower stratosphere is related to the structure of the forcing expected to be associated with the stratospheric surf zone. The basic morphology of tropical upwelling is found to be similar among the mechanistic model forced with reasonable eddy fluxes, the Geophysical Fluid Dynamics Laboratory (GFDL) SKYHI GCM, U.K. Met Office (UKMO) analyses, and other climate models, indicating the robustness of the upwelling features. Atmospheric data are analyzed to characterize the interannual variability of wave drag. The influence of such variations on the interannual variability of tropical upwelling in the lower stratosphere is explored, which may help explain the observed interannual variability of stratospheric water vapor.

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Bei-Wei Lu, Lionel Pandolfo, and Kevin Hamilton

Abstract

A nonlinear principal component analysis (NLPCA) is applied to monthly mean zonal wind observations from January 1956 through December 2007 taken at seven pressure levels between 10 and 70 hPa in the stratosphere near the equator to represent the well-known quasi-biennial oscillation (QBO) and investigate its variability and structure. The NLPCA is conducted using a simplified two–hidden layer feed-forward neural network that alleviates the problems of nonuniqueness of solutions and data overfitting that plague nonlinear techniques of principal component analysis. The QBO is used as a test bed for the new compact model of NLPCA.

The two nonlinear principal components of the dataset of the equatorial stratospheric zonal winds, determined by the compact NLPCA, offer a clear picture of the QBO. In particular, their structure shows that the QBO phase consists of a predominant 28.3-month cycle that is modulated by an 11-yr cycle as well as by longer cycles. The differences in wind variability between westerly and easterly regimes and between Northern Hemisphere winter and summer seasons and the tendency for a seasonal synchronization of the QBO phases are well captured.

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Yoshio Kawatani, Jae N. Lee, and Kevin Hamilton

Abstract

By analyzing the almost-decade-long record of water vapor measurements from the Microwave Limb Sounder (MLS) instrument on the NASA Aura satellite and by detailed diagnostic analysis of the results from state-of-the art climate model simulations, this study confirmed the conceptual picture of the interannual variation in equatorial stratospheric water vapor discussed in earlier papers (e.g., Geller et al.). The interannual anomalies in water vapor are strongly related to the dynamical quasi-biennial oscillation (QBO), and this study presents the first QBO composite of the time–height structure of the equatorial water vapor anomalies. The anomalies display upward propagation below about 10 hPa in a manner analogous to the annual “tape recorder” effect, but at higher levels they show clear downward propagation. This study examined these variations in the Model for Interdisciplinary Research on Climate (MIROC)-AGCM and in four models in phase 5 of the Coupled Model Intercomparison Project (CMIP5) that simulate realistic QBOs. Diagnostic budget analysis of the MIROC-AGCM data and comparisons among the CMIP5 model results demonstrate (i) the importance of temperature anomalies at the tropopause induced by the QBO for lower-stratospheric water vapor variations and (ii) that upper-stratospheric water vapor anomalies are largely driven by advection of the mean vertical gradient of water content by the QBO interannual fluctuations in the vertical wind.

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Yoshio Kawatani, Kevin Hamilton, and Akira Noda

Abstract

The effects of sea surface temperature (SST) and CO2 on future changes in the quasi-biennial oscillation (QBO) are investigated using a climate model that simulates the QBO without parameterized nonstationary gravity wave forcing. Idealized model experiments using the future SST with the present CO2 (FS run) and the present SST with the future CO2 (FC run) are conducted, as are experiments using the present SST with the present CO2 (present run) and the future SST with the future CO2 (future run). When compared with the present run, precipitation increases around the equatorial region in the FS run and decreases in the FC run, resulting in increased and decreased wave momentum fluxes, respectively. In the midlatitude lower stratosphere, westward (eastward) wave-forcing anomalies form in the FS (FC) run. In the middle stratosphere off the equator, westward wave-forcing anomalies form in both the FS and FC runs. Corresponding to these wave-forcing anomalies, the residual vertical velocity significantly increases in the lower stratosphere in the FS run but decreases to below 70 hPa in the FC run, whereas residual upward circulation anomalies form in both the FS and FC runs in the middle equatorial stratosphere. Consequently, the amplitude of the QBO becomes smaller in the lower stratosphere, and the period of the QBO becomes longer by about 1–3 months in the FS run. On the other hand, in the FC run, the QBO extends farther downward into the lowermost stratosphere, and the period becomes longer by 1 month.

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Kevin Hamilton and Rolando R. Garcia

This paper reports on an investigation into the chronology of El Niño/Southern Oscillation (ENSO) events during the period from the arrival of Europeans in Peru in 1531 until the year 1841 when conventional barometric data became available in the tropical regions. A number of probable ENSO events can be dated from anecdotal reports of significant rainfall in the coastal desert of northern Peru. In many of the years with anomalous Peruvian rainfall it is also possible to use various types of proxy data to identify aspects of the global teleconnection patterns usually associated with tropical ENSO events.

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