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Kevin Hamilton

Abstract

A 48-yr integration was performed using the Geophysical Fluid Dynamics Laboratory SKYHI troposphere–stratosphere–mesosphere GCM with an imposed zonal momentum forcing designed to produce a quasi-biennial oscillation (QBO) in the tropical stratosphere. In response to this forcing, the model generates a QBO in the tropical circulation that includes some very realistic features, notably the asymmetry between the strength of the descending easterly and westerly shear zones, and the tendency for the initial westerly accelerations to appear quite narrowly confined to the equator. The extratropical circulation in the Northern Hemisphere (NH) winter stratosphere is affected by the tropical QBO in a manner similar to that observed. In particular, the polar vortex is generally weaker in winters in which there are easterlies in the tropical middle stratosphere. Roughly two-thirds of the largest midwinter polar warmings occur when the equatorial 30-mb winds are easterly, again in rough agreement with observations. Despite this effect, however, the total interannual variance in the zonal-mean extratropical circulation in the model apparently is slightly decreased by the inclusion of the tropical QBO. The observed QBO dependence of the winter-mean stratospheric extratropical stationary wave patterns is also quite well reproduced in the model.

The QBO was also found to have a profound influence on stratospheric stationary waves at low latitudes. Near and above 10 mb the NH stationary waves were found to penetrate across the equator during the westerly QBO phase, but to be restricted to latitudes poleward of ∼10°N during the easterly phase. This means that the equatorial QBO in prevailing wind near and above 10 mb has a significant zonally asymmetric component. If this is also true in the real atmosphere, there are important implications for the adequacy of the current observational rawindsonde network near the equator.

Analysis of the zonal-mean zonal momentum budget in the tropical stratosphere reveals that the resolved waves in the model are strong enough to force the realized accelerations through much of the QBO cycle. The exception appears to be the easterly acceleration phase below about 20 mb. The implications of this for the generation of a self-consistent QBO by GCMs will be considered.

The effects of the imposed QBO on the troposphere were found to be very modest. There does appear to be a statistically significant weakening (by ∼1 m s−1) of the high-latitude winter vortex in the middle and upper troposphere. Given the very high predictability of the stratospheric QBO itself, this effect could possibly be used to enhance the skill of seasonal weather forecasts. No significant QBO influence was found in the model precipitation field.

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Kevin Hamilton

Abstract

A zonal-mean climatology of the temperature, geostrophic zonal winds and the beat and momentum transports associated with the geostrophic winds has been constructed on the basis of almost four years of routine weekly analyses from the National Meteorological Center (NMC). The region considered extends from 10 to 85°N and from 100 to 0.4 mb. The complete set of tables describing this climatology is available in an NCAR technical report (Hamilton, 1982). In the present paper the rationale for using the NMC analyses is given and a few of the highlights of the results are displayed.

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Kevin Hamilton

Abstract

A 21-year record of monthly mean determinations of the solar semidiurnal surface pressure oscillation [S 2(p) at Batavia (6.2°S) was analyzed to detect long-period variability. When the S 2(p) determination were resolved into components which peak at local midnight (and noon) and 0900 (and 2100) local solar time, considerable evidence was found for a quasi-biennial variation in the 0900 component (but not in the midnight component). It is shown that this is consistent with the expected response of S 2(p) to the familiar quasi-biennial oscillation of the tropical stratosphere.

Also apparent in the record is a very long term trend in S 2(p). It is suggested that this way be an indication of a similar trend in stratospheric ozone, and the possibility of using the surface pressure oscillation in monitoring long-term changes in atmospheric ozone is discussed.

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Kevin Hamilton

Abstract

This paper reports on interannual variability of the Northern Hemisphere winter stratospheric circulation as simulated by the 40-level GFDL “SKYHI” general circulation model. A 31-year control simulation was performed using a climatological annual cycle of sea surface temperatures. The interannual variability of the stratospheric circulation in this model has some realistic features. In particular, the simulated variance of monthly mean, zonal-man temperature and wind in the. extratropical Northern Hemisphere agrees fairly well with observations. The day-to-day variability of the circulation also appears to be rather well simulated, with midwinter warmings of realistic intensity and suddenness appearing in the polar regions. The major deficiency is the absence of a realistic quasi-biennial oscillation (QBO) in the simulated winds in the tropical lower stratosphere. There is also an indication of long period (∼10 year) variability in the winter polar vortex. This appears not to be related to any obvious source of long-term memory in the atmosphere such as surface boundary conditions or the flow in the tropical stratosphere.

The model has also been run through a large number of boreal winter simulations with imposed perturbations. In one set of experiments the Pacific sea surface temperatures have been changed to these appropriate for strong El Niño or La Niña conditions. The model is found to reproduce the observed extratropical stratospheric response to El Niño conditions quite well. Interestingly, the results suggest that including the interannual variations in SST would not greatly enhance the simulated interannual variance of the extratropical stratosphere circulation.

Another set of integrations involved arbitrarily altering the mean flow in the tropical lower stratosphere to be appropriate for different extremes of the QBO. The effect of these modifications on the simulated zonal-mean circulation in the extratropical winter stratosphere is found to be quite modest relative to that seen in comparable observations. The model results do display a clear effect of the imposed tropical lower-stratospheric wind perturbations on the extratropical summer mesospheric circulation. This could reflect the influence of the mean flow variations on the gravity waves forced in the Tropics, propagating upward and poleward and ultimately breaking in the extratropical mesosphere. The model behavior in this regard may be related to reported observations of an extratropical mesospheric QBO.

The equilibration of the stratospheric water vapor field in the long SKYHI control integration is examined. The results suggest that the mean residence time for upper-stratospheric air in the model is about 4 years.

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Kevin Hamilton

Abstract

Surface meteorological data at several stations over the period 1875–1936 are examined in relation to solar activity. In particular an attempt is made to we if these historical data can be reconciled with the sun-QBO-weather relationship recently found in modern (post-1950) data by van Loon and Labitzke (vLL). The basic problem in extending vLL's analysis to earlier periods is ignorance of the phase of the QBO. In the present study, vLL's computations are repeated for the historical data using several million possible sequences for the phase of the QBO. The results reveal problems in reproducing vLL's results in the earlier data. This indicates either that the QBO behaved differently in the past, or that vLL's results for a solar-weather relationship are not stable over the long term.

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Kevin Hamilton

Abstract

The consequences of the hypothesis of Lindzen (1978) that latent heat release may be a significant excitation mechanism for the semidiurnal atmospheric tide are examined in some detail. Harmonic analysis of hourly rainfall data from 79 tropical stations shows that the semidiurnal variation of rainfall in the tropics is ∼1 mm day−1 and has a phase near 0300 LST, just as Lindzen's theory requires. Analysis of data at 85 midlatitude stations shows that the sermidiurnal rainfall oscillation there has its phase rather later (about 0600). The results of simple classical tidal theory calculations which indicate that the geographical distribution of the surface pressure response to latent heat forcing largely follows that of the forcing itself are presented. This result is then used to suggest a plausible explanation for the observed seasonal cycle of the semidiurnal pressure oscillation in midlatitudes. Further calculations show that the magnitude of the non-migrating components of the semidiurnal barometric oscillation produced by latent heat excitation is not likely to be unrealistically large. These calculations also suggest that Lindzen's hypothesis might be verified by observing the phase of the semidiurnal pressure oscillation in particularly and regions.

The rainfall observations also show a strong diurnal (24 h) component in the rainfall both in the tropics and in midiatitudes. The effects of latent heat release on the 24 h tide are briefly discussed.

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Kevin Hamilton

The unusual spring and summer weather of the year 1816 in central Canada is discussed using Canadian newspaper accounts together with manuscript records from early amateur weather observers. Many of the spectacular meteorological events of this year that are known to have affected the northeast United States have close parallels in central Canada. The available instrumental records suggest that the summer of 1816 in central Canada was colder than any that has been observed in more-recent times.

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Kevin Hamilton and Li Yuan

Abstract

A 30-level version of the rhomboidal-15 GFDL spectral climate model was constructed with roughly 2-km vertical resolution. In common with other comprehensive general circulation models, this model fails to produce a realistic quasi-biennial oscillation (QBO) in the tropical stratosphere.

A number of simulations were conducted in which the zonal-mean winds and temperatures in the equatorial lower and middle stratosphere were instantaneously perturbed and the model was integrated while the mean state relaxed toward its equilibrium. The time scale for the mean wind relaxation varied from somewhat over one month at 40 km to a few months in the lower stratosphere. This is similar to the time scales of observed QBO wind reversals. The wind relaxations in the model also displayed the downward phase propagation characteristic of QBO wind reversals, and mean wind anomalies of opposite sign to the imposed perturbation appear at higher levels. In the GCM, however, the downward propagation is clear only above about 20 mb.

Detailed investigations were made of the zonal-mean zonal momentum budget in the equatorial stratosphere in these experiments. The mean flow relaxations above 20 mb were mostly driven by the vertical Eliassen-Palm flux convergence. The anomalies in the horizontal Eliassen-Palm fluxes from extratropical planetary waves, however, were found to be the dominant effect forcing the mean flow back to its equilibrium at altitudes below 20 mb. The vertical eddy momentum fluxes near the equator in the model were decomposed using space-time Fourier analysis. While total fluxes associated with easterly and westerly waves are comparable to those used in simple mechanistic models of the QBO, the GCM has its flux spread over a very broad range of wavenumbers and phase speeds.

The effects of vertical resolution were studied directly by repeating part of the control integration with a 69-level version of the model with greatly enhanced vertical resolution in the lower and middle stratosphere. The results showed that there is almost no sensitivity of the simulation in the tropical stratosphere to the increased vertical resolution.

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Takatoshi Sakazaki and Kevin Hamilton

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We used newly available ERA5 hourly global data to examine the variations of atmospheric circulation on global scales and high frequencies. The space–time spectrum of surface pressure displays a typical red background spectrum but also a striking number of isolated peaks. Some peaks represent astronomically forced tides, but we show that most peaks are manifestations of the ringing of randomly excited global-scale resonant modes, reminiscent of the tones in a spectrum of a vibrating musical instrument. A few such modes have been tentatively identified in earlier observational investigations, but we demonstrate the existence of a large array of normal mode oscillations with periods as short as 2 h. This is a powerful and uniquely detailed confirmation of the predictions of the theory of global oscillations that has its roots in the work of Laplace two centuries ago. The delineation of the properties of the modes provides valuable diagnostic information about the atmospheric circulation. Notably the amplitudes and widths of the normal mode spectral peaks contain information on the forcing mechanisms and energy dissipation for the modes, and the simulation of these properties for each of the many modes we have identified can serve as tests for global climate and weather prediction models.

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Markus Stowasser and Kevin Hamilton

Abstract

The relations between local monthly mean shortwave cloud radiative forcing and aspects of the resolved-scale meteorological fields are investigated in hindcast simulations performed with 12 of the global coupled models included in the model intercomparison conducted as part of the preparation for Intergovernmental Panel on Climate Change (IPCC) Fourth Assessment Report (AR4). In particular, the connection of the cloud forcing over tropical and subtropical ocean areas with resolved midtropospheric vertical velocity and with lower-level relative humidity are investigated and compared among the models. The model results are also compared with observational determinations of the same relationships using satellite data for the cloud forcing and global reanalysis products for the vertical velocity and humidity fields. In the analysis the geographical variability in the long-term mean among all grid points and the interannual variability of the monthly mean at each grid point are considered separately. The shortwave cloud radiative feedback (SWCRF) plays a crucial role in determining the predicted response to large-scale climate forcing (such as from increased greenhouse gas concentrations), and it is thus important to test how the cloud representations in current climate models respond to unforced variability.

Overall there is considerable variation among the results for the various models, and all models show some substantial differences from the comparable observed results. The most notable deficiency is a weak representation of the cloud radiative response to variations in vertical velocity in cases of strong ascending or strong descending motions. While the models generally perform better in regimes with only modest upward or downward motions, even in these regimes there is considerable variation among the models in the dependence of SWCRF on vertical velocity. The largest differences between models and observations when SWCRF values are stratified by relative humidity are found in either very moist or very dry regimes. Thus, the largest errors in the model simulations of cloud forcing are prone to be in the western Pacific warm pool area, which is characterized by very moist strong upward currents, and in the rather dry regions where the flow is dominated by descending mean motions.

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