Dynamics of the Stratospheric Semiannual Oscillation (original) (raw)
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The Dynamics of Intraseasonal Atmospheric Angular Momentum Oscillations
Journal of the Atmospheric Sciences, 1997
The global and zonal atmospheric angular momentum (AAM) budget is computed from seven years of National Centers for Environmental Prediction data and a composite budget of intraseasonal (30-70 day) variations during northern winter is constructed. Regressions on the global AAM tendency are used to produce maps of outgoing longwave radiation, 200-hPa wind, surface stress, and sea level pressure during the composite AAM cycle. The primary synoptic features and surface torques that contribute to the AAM changes are described. In the global budget, the friction and mountain torques contribute about equally to the AAM tendency. The friction torque peaks in phase with subtropical surface easterly wind anomalies in both hemispheres. The mountain torque peaks when anomalies in the midlatitude Northern Hemisphere and subtropical Southern Hemisphere are weak but of the same sign. The picture is different for the zonal mean budget, in which the meridional convergence of the northward relative angular momentum transport and the friction torque are the dominant terms. During the global AAM cycle, zonal AAM anomalies move poleward from the equator to the subtropics primarily in response to momentum transports. These transports are associated with the spatial covariance of the filtered (30-70 day) perturbations with the climatological upper-tropospheric flow. The zonally asymmetric portion of these perturbations develop when convection begins over the Indian Ocean and maximize when convection weakens over the western Pacific Ocean. The 30-70-day zonal mean friction torque results from 1) the surface winds induced by the upper-tropospheric momentum sources and sinks and 2) the direct surface wind response to warm pool convection anomalies. The signal in relative AAM is complemented by one in ''Earth'' AAM associated with meridional redistributions of atmospheric mass. This meridional redistribution occurs preferentially over the Asian land mass and is linked with the 30-70-day eastward moving convective signal. It is preceded by a surface Kelvin-like wave in the equatorial Pacific atmosphere that propagates eastward from the western Pacific region to the South American topography and then moves poleward as an edge wave along the Andes. This produces a mountain torque on the Andes, which also causes the regional and global AAM to change.
Longitudinal Variations of the Stratospheric Quasi-biennial Oscillation
Journal of the Atmospheric Sciences, 2004
The longitudinal dependence of interannual variations of tropical stratospheric wind is examined in a detailed general circulation model simulation and in the limited observations available. A version of the SKYHI model is run with an imposed zonally symmetric zonal momentum source that forces the zonal-mean zonal wind evolution in the tropical stratosphere to be close to an estimate of the observed zonal wind based on radiosonde observations at Singapore during the period 1978-99. This amounts to a kind of simple assimilation model in which only the zonal-mean wind field in the tropical stratosphere is assimilated, and other quantities are allowed to vary freely. A total of five experiments were run, one covering the full 1978-99 period and four for 1989-99.
Longitudinal Variation of the Stratospheric Quasi-Biennial Oscillation
Journal of the Atmospheric Sciences, 2004
The longitudinal dependence of interannual variations of tropical stratospheric wind is examined in a detailed general circulation model simulation and in the limited observations available. A version of the SKYHI model is run with an imposed zonally symmetric zonal momentum source that forces the zonal-mean zonal wind evolution in the tropical stratosphere to be close to an estimate of the observed zonal wind based on radiosonde observations at Singapore during the period 1978-99. This amounts to a kind of simple assimilation model in which only the zonal-mean wind field in the tropical stratosphere is assimilated, and other quantities are allowed to vary freely. A total of five experiments were run, one covering the full 1978-99 period and four for 1989-99. The results at and above about 30 hPa are fairly simple to characterize. When the zonal-mean wind near the equator at a particular level is easterly, the monthly mean wind has only very small zonal contrasts. When mean westerlies are present near the equator, significant zonal asymmetries occur at low latitudes, most notably easterly anomalies over South America and westerly anomalies in the eastern Pacific region. These anomalies generally display a continuous meridional phase propagation with the extratropical quasi-stationary eddy field in the winter hemisphere. The net result is a significantly weaker peak-to-peak amplitude of the quasi-biennial oscillation (QBO) in zonal wind over the South American sector than over the rest of the equatorial band. The zonal contrast in QBO amplitude near 10 hPa exceeds 10%. In the lower stratosphere the zonal asymmetries in the prevailing wind are fairly small. Asymmetries seem to reflect the upward extension of the tropospheric Walker circulation, and are less strongly modulated by the quasi-biennial oscillation in zonal-mean circulation. The model results were checked against limited station observations at Nairobi (1.3ЊS, 36.7ЊE), Singapore (1.4ЊN, 103.9ЊE), Rochambeau (4.8ЊN, 52.4ЊW), and Bogota (4.7ЊN, 74.1ЊW). Overall reasonable agreement was found between the monthly mean zonal winds in the model simulation and these station data. The low-latitude wind field in monthly mean NCEP gridded analyses was also examined. These analyses have some obviously unrealistic features in the tropical stratosphere, but some of the behavior seen in the SKYHI model simulations can be identified as well in the NCEP analyses.
Representation of the Tropical Stratospheric Zonal Wind in Global Atmospheric Reanalyses
2016
This paper reports on a project to compare the representation of the monthly-mean zonal wind in the equatorial stratosphere among major global atmospheric reanalysis datasets. The degree of disagreement among the reanalyses is characterized by the standard deviation (SD) of the monthly-mean zonal wind and this depends on latitude, longitude, height and the phase of the quasi-biennial oscillation (QBO). At each height the SD displays a prominent equatorial maximum, indicating the particularly challenging nature of the reanalysis problem in the low-latitude stratosphere. At 50-70 hPa the geographical distributions of SD are closely related to the density of radiosonde observations. The largest SD values are over the eastern Pacific, where few in situ observations are available. At 10-20 hPa the spread among the reanalyses and differences with in situ observations both depend significantly on the QBO phase. Notably the easterly-to-westerly phase transitions in all the reanalyses except MERRA are delayed relative to those directly observed at Singapore. In addition, the timing of the easterly-to-westerly phase transitions displays considerable variability among the different reanalyses and this spread is much larger than for the timing of the westerly-to-easterly phase changes. The eddy component in the monthly mean zonal wind near the equator is dominated by zonal wavenumber 1 and 2 quasi-stationary planetary waves propagating from mid-latitudes in the westerly phase of the QBO. There generally is considerable disagreement among the reanalyses in the details of the quasi-stationary waves near the equator. At each level, there is a tendency for the agreement to be best near the longitude of Singapore, suggesting that the Singapore observations act as a strong constraint on all the reanalyses. Our measures of the quality of the reanalysis clearly show systematic improvement over the period considered (1979-2012). The SD among the reanalysis declines significantly over the record, although the geographical pattern of SD remains nearly constant. the troposphere. What makes the low latitude stratosphere so remarkable is that the forcing of the zonal-mean flow by these waves leads to the very large-amplitude, low-frequency quasi-periodic cycle known as the quasi-biennial oscillation (QBO). In the tropical stratosphere, the QBO clearly dominates other aspects of interannual variability and even swamps the annual and semiannual variations in the zonal-mean circulation, at least up to ~3 hPa. Although rooted in the tropics, the QBO has global impacts. The QBO strongly influences interannual variations in circulation and composition throughout the stratosphere. The QBO also affects the circulation at the Earth's surface and is an important consideration in extended-range weather forecasts (e.g. Baldwin et al., 2001). The state of the QBO up to the middle stratosphere can be characterized by the time series of monthly mean, near-equatorial zonal winds at levels between 10 and 70 hPa maintained by the Free University of Berlin (FUB, e.g. Naujokat, 1986) since 1953. The monthly values in the FUB series are based on operational balloon soundings, and the FUB record has been stitched together from such observations at Canton Island (2.8°S, 172°W from January 1953 to March 1967), Gan (0.7°S, 73°E from September 1967 to December 1975), and Singapore (1.4°N, 104°E since 1976). The high quality of these balloon data, and the close proximity of the stations to the equator, has led FUB series to be widely used, despite being based on only a single station each month and the possible inhomogeneity introduced by the changes in the station employed. Global atmospheric analyses that assimilate all available satellite remote sensing and in situ observations are another potential source of information about the QBO and other aspects of the circulation in the tropical stratosphere. However, a number of factors combine to make the global meteorological analysis process particularly challenging in the tropical middle atmosphere. (i) One challenge is the relative paucity of in situ data, even if attention is restricted to levels at and below 10 hPa (i.e. the usual upper bound for most operational balloon soundings). There are large ocean areas in the tropics with no balloon observations. Even over land areas, the observations in the stratosphere at many stations are sparse. At stratospheric levels, the zonal wind measurements at many stations near the equator tend to have short overall records, or records with many months that have no observations (or not enough to compute a stable monthly mean). This leads to gaps in time series of monthly-mean winds (e.g. Hamilton, 1984; Kawatani and Hamilton, 2013). (ii) Near the equator the Coriolis parameter is small; so observations of the temperature from satellite remote sensing do not constrain the wind field as strongly as at higher latitudes. Even if we assume the near-equatorial flow really is close to thermal wind-balance (Reed, 1962; Randel et al., 1999) the computed geostrophic wind shears are extremely sensitive to small errors in the observed temperatures. (iii) The flow in the tropical stratosphere exhibits variations on very small vertical scales. This limits the usefulness of the relatively coarse-resolution satellite remote-sensing temperature retrievals. Even the monthly-mean zonal wind in this region displays thin layers where the wind can change by ~30 m s-1 over ~3 km. Satellite radiances used in global assimilations have an effective vertical resolution of several kilometers (e.g. Huesmann and Hitchman, 2003). Huesmann and Hitchman (2003) note that in such shear regions the assimilation scheme will have to reconcile the strong wind-shears measured
Realistic semiannual oscillation simulated in a middle atmosphere general circulation model
Geophysical Research Letters, 2001
Although several general circulation modelling studies have presented simulations containing a tropical middle atmosphere semiannual oscillation (6 month period), none so far have been able to reproduce the observed amplitude and vertical structure extending into the mesosphere. Here we present simulations with the Canadian Middle Atmosphere Model which utilize a nonlinear broad-spectrum gravity-wave parameterization [Medvedev and Klaassen, 1995, 2000] and exhibit a semiannual oscillation in good agreement with observations. 1. Introduction Oscillations of the mean zonal wind and temperature in the tropical strato-mesosphere with a periodicity of 6 months constitute a phenomenon known as the semiannual oscillation (SAG). The observed climatological structure of the oscillation has been widely documented, e.g. [Hamilton, 1998; Garcia et al., 1997; Burrage et al, 1996; Ortland et al., 1996]. It is well-known that the middle atmosphere undergoes two distinct out-of-phase semiannual oscillations: one has maximum amplitude at the stratopause (SSAG), while the second is observed near the mesopause (MSAG). Although the SAG is clearly associated with the annual cycle of radiative forcing (the sun crosses the equator twice a year), the detailed physical mechanism driving the SAG is not completely understood. Dunkerton [1979] showed that the eastward phase of the stratospheric SAG can be maintained by large-scale Kelvin waves propagating from below. Holton and Wehrbein [1980] demonstrated that the crossequatorial transport of westward momentum by the mean meridional circulation could explain the westward phase of the SAO. Planetary Rossby waves propagating into the tropics from the winter hemisphere also supply westward momentum in the tropical stratosphere at the solstices [Hamilton, 1986; Ray et al, 1998]. Hitchman and Leovy [1988] have estimated that only 30% to 70% of the required eastward acceleration in the stratosphere is produced by large-scale Kelvin waves, and that the rest of eastward torque can be attributed to gravity waves. Dunkerton [1982] suggested that vertically propagating gravity waves (GW) are filtered by the stratopause SAG, thereby providing both the eastward and westward accelerations required to drive the mesospheric SAO. This theory also accounts for the observed 3-month phase lag between the MSAG and SSAG. Sassi and Garcia [1997] argued that Corresponding author
Icarus, 2019
Planets with high obliquity receive more radiation in the polar regions than at low latitudes, and thus, assuming an ocean-covered surface with sufficiently high heat capacity, their meridional temperature gradient was shown to be reversed for the entire year. The objective of this work is to investigate the drastically different general circulation of such planets, with an emphasis on the tropical Hadley circulation and the mid-latitude baroclinic eddy structure. We use a 3D dry dynamic core model, accompanied by an eddy-free configuration and a generalized 2D Eady model. When the meridional temperature gradient is reversed, the Hadley cell becomes much weaker, shallower and thermally indirect, as seen by other studies, though not in the context of high obliquity planets. For both the normal and reverse temperature gradient cases, we show that the surface friction and eddy momentum transport are the primary drivers of the Hadley cell. Because mid-latitude baroclinic eddies concentrate westerly momentum toward the mid-latitude baroclinic zone, the surface wind pattern is easterly-westerly-easterly from the equator to the pole in both cases. As a result, low-latitude air parcels near the surface gain westerly momentum through friction, and are redirected equatorward by the Coriolis force, forming thermally indirect (direct) circulation in the reverse (normal) case. The Hadley cell under a reverse temperature gradient configuration is shallow and weak, even when the magnitude of the gradient is the same as in the normal case. This shallow structure is a result of the bottom-heavy structure of the baroclinic eddies in the reverse case, and the relatively weak wave activity. We propose a new mechanism to explain the mid-latitude eddy structure for both cases, and verify it using the generalized Eady model. With seasonal variations included, the annual mean circulation resembles that under perpetual annual
Eddy–Zonal Flow Feedback in the Southern Hemisphere
Journal of the Atmospheric Sciences, 2001
The eddy-zonal flow feedback in the Southern Hemisphere (SH) winter and summer is investigated in this study. The persistence time scale of the leading principal components (PCs) of the zonal-mean zonal flow shows substantial seasonal variation. In the SH summer, the persistence time scale of PC1 is significantly longer than that of PC2, while the persistence time scales of the two PCs are quite similar in the SH winter. A storm-track modeling approach is applied to demonstrate that seasonal variations of eddy-zonal flow feedback for PC1 and PC2 account for the seasonal variations of the persistence time scale. The eddy feedback time scale estimated from a storm-track model simulation and a wave-response model diagnostic shows that PC1 in June-August (JJA) and December-February (DJF), and PC2 in JJA, have significant positive eddy-mean flow feedback, while PC2 in DJF has no positive feedback. The consistency between the persistence and eddy feedback time scales for each PC suggests that the positive feedback increases the persistence of the corresponding PC, with stronger (weaker) positive feedback giving rise to a longer (shorter) persistence time scale.
Climatology and trends in the forcing of the stratospheric zonal-mean flow
The momentum budget of the Transformed Eulerian-Mean (TEM) equation is calculated using the European Centre for Medium-Range Weather Forecasts (ECMWF) reanalysis (ERA-40) and the National Centers for Environmental Prediction (NCEP) Reanalysis 2 (R-2). This study outlines the considerable contribution of unresolved waves, deduced to be gravity waves, to the forcing of the zonal-mean flow. A trend analysis, from 1980 to 2001, shows that the onset and break down of the Northern Hemisphere (NH) stratospheric polar night jet has a tendency to occur later in the season in the more recent years. This temporal shift follows long-term changes in planetary wave activity that are mainly due to synoptic waves, with a lag of one month. In the Southern Hemisphere (SH), the polar vortex shows a tendency to persist further into the SH summertime. This also follows a statistically significant decrease in the intensity of the stationary EP flux divergence over the 1980-2001 period. Ozone depletion is well known for strengthening the polar vortex through the thermal wind balance. However, the results of this work show that the SH polar vortex does not experience any significant long-term changes until the month of December, even though the intensification of the ozone hole occurs mainly between September and November. This study suggests that the decrease in planetary wave activity in November provides an important feedback to the zonal wind as it delays the breakdown of the polar vortex. In addition, the absence of strong eddy feedback before November explains the lack of significant trends in the polar vortex in the SH early spring. A long-term weakening in the Brewer-Dobson (B-D) circulation in the polar region is identified in the NH winter and early spring and during the Correspondence to: E. Monier (emonier@mit.edu) SH late spring and is likely driven by the decrease in planetary wave activity previously mentioned. During the rest of the year, there are large discrepancies in the representation of the B-D circulation and the unresolved waves between the two reanalyses, making trend analyses unreliable.
The Atmospheric General Circulation and Its Variability
Journal of the Meteorological Society of Japan. Ser. II, 2007
Progress in understanding the general circulation of the atmosphere during the past 25 years is reviewed. The relationships of eddy generation, propagation and dissipation to eddy momentum fluxes and mean zonal winds are now sufficiently understood that intuitive reasoning about momentum based on firm theoretical foundations is possible. Variability in the zonal-flow can now be understood as a process of eddy, zonal-flow interaction. The interaction of tropical overturning circulations driven by latent heating with extratropical wave-driven jets is becoming a fruitful and interesting area of study. Gravity waves have emerged as an important factor in the momentum budget of the general circulation and are now included in weather and climate models in parameterized form. Stationary planetary waves can largely be explained with linear theory.