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Amy Solomon

Abstract

Initialized decadal hindcasts are used to assess simulations of 1970–2009 equatorial Pacific SST, zonal wind stress, and surface flux trends. Initialized hindcasts are useful to assess how well the models simulate observed trends, as well as how simulations of observed trends (due primarily to natural variability) differ from ensemble-mean forecasted trends (due to the response to an increase in external forcing). All models forecast a statistically significant warming trend in both the warm-pool and cold-tongue regions. However, while the warm-pool warming trend is within the observed estimates, the cold-tongue warming trend is an order of magnitude larger than an ENSO residual estimated using SST instrumental reconstructions. Multimodel ensemble means formed using forecasts 6–10 years from initialization with 40 ensemble members do not produce an unambiguous zonal SST gradient response to an increase in external forcing. Systematic biases are identified in forecasts of surface fluxes. For example, in the warm-pool region all year-1 forecasts produce SST trends similar to observations but ocean mixed layer and net surface heat flux trends with an opposite sign to air–sea datasets. In addition, year-1 forecasts produce positive shortwave feedbacks on decadal time scales, whereas 6–10-yr forecasts produce negative or statistically insignificant shortwave flux feedbacks on decadal time scales, suggesting sensitivity to circulations forced by the initialized ocean state. In the cold-tongue region initialized ensembles forecast positive net radiative flux trends even though shortwave flux trends are negative (i.e., for increasing cloudiness). This is inconsistent with air–sea datasets, which uniformly show that the net surface radiative flux feedback is a damping of the underlying SSTs.

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Amy Solomon
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Amy B. Solomon

Abstract

The preferred spatial and temporal scales of transient baroclinic waves that are responsible for the transport of sensible heat in midlatitudes are evaluated as a function of pressure, season, and hemisphere. The 7-level initialized ECMWF operational global analyses for the years 1980–88 are used in this study. A clear seasonal cycle in the spatial scales of the baroclinic waves is seen in the Southern Hemisphere lower troposphere. The scale selection and phase relationships of the baroclinic waves are observed to be similar in the Southern and Northern Hemispheres, indicating that the same mechanism may be responsible for the transport of heat in both hemispheres even though there are very large differences in the magnitude of the heat transported by stationary waves and in the structure of baroclinic zones. The secondary heat flux maximum at 200 hPa is dominated by large-scale wavenumbers 2–3 in the winter and spring in both hemispheres. This may be an indication that there is a mechanism that is responsible for the heat transport at 200 hPa that is different from the mechanism at the primary heat flux maximum, at 850 hPa where intermediate-scale waves dominate.

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Amy Solomon and Matthew D. Shupe

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This study investigates cloud formation and transitions in cloud types at Summit, Greenland, during 16–22 September 2010, when a warm, moist air mass was advected to Greenland from lower latitudes. During this period there was a sharp transition between high ice clouds and the formation of a lower stratocumulus deck at Summit. A regional mesoscale model is used to investigate the air masses that form these cloud systems. It is found that the high ice clouds form in originally warm, moist air masses that radiatively cool while being transported to Summit. A sensitivity study removing high ice clouds demonstrates that the primary impact of these clouds at Summit is to reduce cloud liquid water embedded within the ice cloud and water vapor in the boundary layer due to vapor deposition on snow. The mixed-phase stratocumulus clouds form at the base of cold, dry air masses advected from the northwest above 4 km. The net surface radiative fluxes during the stratocumulus period are at least 20 W m−2 larger than during the ice cloud period, indicating that, in seasons other than summer, cold, dry air masses advected to Summit above the boundary layer may radiatively warm the top of the Greenland Ice Sheet more effectively than warm, moist air masses advected from lower latitudes.

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Amy Solomon and Peter H. Stone

Abstract

The role of waves in maintaining the midlatitude tropospheric climate is investigated in a dry high-resolution quasigeostrophic β-plane channel model coupled to both a simplified model of the atmospheric boundary layer and an interactive static stability.

The climate of the model’s equilibrated state is found to be separated into two dynamical regimes, one within the boundary layer and the other within the free troposphere. Thermal diffusion in the atmospheric boundary layer prevents the eddies from modifying the mean temperature structure there by damping temperature fluctuations. The potential vorticity gradients are essentially eliminated in the lower troposphere above the boundary layer, in agreement with observations. The homogenization of potential vorticity occurs in the region where the baroclinic waves have a critical layer, and is accomplished mainly by an increase in the static stability in the lower troposphere due to the vertical eddy heat fluxes.

Even though the model has kinetic energy and enstrophy spectra characteristic of a fully turbulent flow, the equilibrated state of the model is essentially maintained by wave–mean flow interaction, primarily by the interaction between wave 5 and the zonal mean state. The zonal mean of the equilibrated state is found to be linearly stable to all waves. The largest-scale wave in the fully nonlinear state, wave 4, is found to be maintained by an energy cascade from the higher wavenumbers. However when wave 4 is large, stability analysis indicates that it is unstable, with the growing mode dominated by wave 6. This instability appears to saturate quickly and hand its energy over to wave 5. The result is that the amplitude of waves 4 and 5 in the equilibrated state are strongly anticorrelated, but the fluctuations in total eddy kinetic energy are strongly correlated with the fluctuations in the sum of the energy in waves 4 and 5.

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Amy Solomon and Richard S. Lindzen

Abstract

The barotropic point jet problem is used to study the impact of resolution on a numerical simulation of barotropic instability. This particular problem is studied because of the close relation of the linearized version of the problem to baroclinic instability.

This study finds that the channel-averaged wave enstrophy and the fluxes at the jet in the wave–mean flow equilibrated state are underestimated when the resolution of the model is inadequate to resolve the analytic linear growth rates. Moreover, the resolution of the model is found to impact the wave–mean flow equilibrated state even when the analytic linear growth rates are resolved. This is due to potential vorticity gradients and fluxes in the equilibrated state being largely independent of resolution, as long as the linear growth rates are adequately resolved. In a coarse-resolution model this results in momentum fluxes and shear at the vertex of the jet that are dependent upon the resolution of the model.

The results of this study suggest that resolution will also impact the numerical simulation of baroclinic instability.

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Amy Solomon and Peter H. Stone

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The sensitivity of the equilibrated state of a dry high-resolution quasigeostrophic β-plane channel model, coupled to both a simplified model of the atmospheric boundary layer and an interactive static stability, to changes in forcings is investigated. An earlier study with the same model found that with standard parameter values, the potential vorticity in the center of the channel just above the atmospheric boundary layer was homogenized. The new experiments show that this result is robust does not vary strongly with variations in forcing over a wide range of forcing parameters. This is so even though the meridional temperature gradients and static stability are generally sensitive to the forcing; that is, the changes in these cooperate to keep the meridional potential vorticity gradient zero. The potential vorticity gradients at higher levels are also robust although nonzero. The homogenization in the lower troposphere does disappear if the differential diabatic heating is decreased sufficiently or if the tropopause level is lowered sufficiently.

The model results are also used to assess proposed parameterizations of eddy effects. Stone's parameterization of the meridional eddy heat flux is most successful at reproducing the model's results for most of the experiments. However, no parameterizations of the eddy heat flux captured the results of the experiments in which the diabatic heating timescale was varied. In these experiments, changes in the eddy heat fluxes kept the tropospheric temperature structure essentially unchanged even though the timescale changed from 5 to 80 days.

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Amy Solomon and Fei-Fei Jin

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Concurrent with most large El Niño events, cold sea surface temperature (SST) anomalies are observed over the western Pacific warm pool region (WPWP). Observational evidence that SST anomalies that form in the off-equatorial western Pacific during El Niño–Southern Oscillation (ENSO) cycles are forced by subsurface ocean processes equatorward of 12°N and air–sea fluxes poleward of 12°N is presented. It is demonstrated that diurnal mixing in the ocean equatorward of 12°N plays a significant role in bringing subsurface temperature anomalies to the sea surface during an El Niño event.

The role of SST anomalies equatorward of 12°N in ENSO cycles is tested in the Zebiak–Cane coupled model, modified to allow for the impact of subsurface temperatures on SSTs. This coupled model successfully simulates cold SST anomalies in the off-equatorial northwestern Pacific that are observed to occur during the warm phase of ENSO and the atmospheric response to these anomalies, which is composed of both westerlies in the central Pacific and easterlies in the far western equatorial Pacific. It is found that there is little net change in the zonal mean wind stress at the equator, suggesting that the westerlies cancel the impact of the easterlies on the basin-scale tilt of the equatorial zonal mean thermocline depth. The anomalous westerly winds in the central equatorial Pacific are found to increase the amplitude of an El Niño event directly by increasing anomalous warm zonal advection and reducing upwelling. Moreover, the off-equatorial anticyclonic wind stress associated with the cold SST anomalies during the warm phase of ENSO tends to reduce the discharge of the equatorial heat content. Thus, the coupled processes over the western Pacific warm pool can serve as a positive feedback to amplify ENSO cycles.

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Kevin E. Trenberth and Amy Solomon

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The information available on different scales in the atmosphere for a number of different variables is explored using the global ECMWF analyses by examining the spatial spectra at T106 resolution. In most atmospheric spectra, a low wavenumber regime can be identified that does not follow a power law and is dominated by the stationary forced part of the flow. A higher wavenumber regime, where an approximate power law does appear to hold, can also usually be found. For the rotational part of the flow in the upper troposphere, the observed spectra follow quite closely that expected for quasi-two-dimensional geostrophic turbulence between about wavenumbers 12 and 70, with a kinetic energy spectrum falling off as n −3, where n is the total spherical harmonic wavenumber. In the lower troposphere, there is more power at high wavenumbers than would be expected from geostrophic turbulence, most likely due to the influence and close proximity of the lower boundary. Changes in the global analyses since 1979 have mainly influenced the spectra in the lower troposphere and the more recent analyses for 1988 have more power at higher wavenumbers. In the stratosphere, the spectra at high wavenumbers do not follow a power law behavior very well.

The widespread practice of using a coarse grid without the appropriate truncation or smoothing first can result in unresolved scales being aliased and excessively noisy fields; an example is the 2.5° gridded fields made available by ECMWF. Appropriate procedures are described for truncating the T106 ECMWF spectral archive for scalar and vector fields. T42 resolution is an adequate representation for diagnostic calculations depicting most quantities within a few percent accuracy, although some spatial structure, which may partly be noise, is lost for the w, divergence, and moisture fields. In contrast, errors greater than 10% can occur at T21 or R15 resolution, although these truncations can be useful for displaying results.

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Yang Zhang, Peter H. Stone, and Amy Solomon

Abstract

A β-plane multilevel quasigeostrophic channel model with interactive static stability and a simplified parameterization of atmospheric boundary layer physics is used to study the role of different boundary layer processes in eddy equilibration and their relative effect in maintaining the strong boundary layer potential vorticity (PV) gradient.

The model results show that vertical thermal diffusion, along with the surface heat exchange, is primarily responsible for limiting PV homogenization by baroclinic eddies in the boundary layer. Under fixed SST boundary conditions, these two processes act as the source of the mean flow baroclinicity in the lower levels and result in stronger eddy heat fluxes.

Reducing surface friction alone does not result in efficient elimination of the boundary layer PV gradient, but the equilibrium state temperature gradient is still largely influenced by surface friction and its response to changes in surface friction is not monotonic. In the regime of strong surface friction, with reduced poleward eddy heat flux, a strong temperature gradient is still retained. When the surface friction is sufficiently weak along with the stronger zonal wind, the critical level at the center of the jet drops below the surface. As a result, in the lower levels, the eddy heat flux forcing on the mean flow moves away from the center of the jet and the equilibrium state varies only slightly with the strength of the vertical momentum diffusion in the boundary layer.

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