Latitude Circle Research Articles

Successive bifurcations in a simple model of atmospheric zonal-flow vacillation.

Low-frequency variability of the atmospheric flow in the Southern Hemisphere is dominated by irregular changes in the latitude and intensity of the mid-latitude eastward jet about its climatological mean state. This phenomenon, known as atmospheric zonal-flow vacillation, is characterized by the existence of two persistent states of the zonal (i.e., east-west oriented) jet and irregular transitions between them. Nonlinear interactions between the mean flow and the waves play a key role in the dynamics of this vacillation. In the present study, we develop a low-order, deterministic model for the nonlinear dynamics of atmospheric zonal-flow vacillation. Multiple equilibria arise in this model's zonal-mean flow, that is, in the longitudinal flow averaged along a given latitude circle. These equilibria bear a strong resemblance to the two persistent flow regimes found in Southern Hemisphere observations. The two equilibrium states are maintained by wave forcing against surface drag, as in the observations. Successive bifurcations to periodic and chaotic zonal-mean flow regimes occur as the model's dissipation parameter is reduced. (c) 2002 American Institute of Physics.

The non-zonal effect in the dynamical structure of the midlatitude MLT-region

Abstract Ionospheric drift measurements in the LF range, which refer to altitudes between 80 and 100 km, make it possible to permanently monitor the circulation in the upper mesosphere/lower thermosphere region (MLT-region). A long-term (1975–1996) study of the MLT region dynamical structure at stations, well-spaced along 52°N latitude circle has revealed the nonzonality in the MLT dynamical structure. This non-zonality in the nighttime prevailing wind, tidal variability and planetary wave activity is considered including results obtained during the MLTCS, DYANA and CRISTA/MAHRSI Campaigns. The longitudinal variations could be interpreted as sign of “coupling from below” and may be important when empirical and physical upper atmosphere models are being formed.

On the insensitivity of the wind-driven circulation to bottom topography

An analytical model is developed for the wind-driven circulation in a continuously stratified ocean overlying variable bottom topography. In the ocean interior we adopt a linear relation between potential vorticity, density and Montgomery potential, resulting in Welander's solution for an adiabatic internal thermocline. The horizontal structure of the circulation is described by a characteristic equation, obtained by imposing a boundary condition of no-normal flow at the sea floor and a prescribed vertical velocity through the base of the surface Ekman layer. The characteristics, which determine the extent to which bottom topography steers the circulation within the upper ocean, are dominated by latitude circles at low latitudes, but are increasingly influenced by the bottom topography at higher latitudes as the thermocline widens and intersects the sea floor. A solution is evaluated for the full three-dimensional circulation in the North Pacific. We find classical Sverdrup gyres within the thermocline, increasingly zonal flows at mid-depths, and weak topographically-bounded gyres within the abyssal ocean.

Large-scale flow and transport of magnetic flux in the solar convection zone

Horizontal large-scale velocity field describes horizontal displacement of the photospheric magnetic flux in zonal and meridian directions. The flow systems of solar plasma, constructed according to the velocity field, create the large-scale cellular-like patterns with up-flow in the center and the down-flow on the boundaries. Distribution of the large- scale horizontal eddies (with characteristic scale length from 350 to 490 Mm) was found in the broad equatorial zone, limited by 60° latitude circles on both hemispheres. The zonal averages of the zonal and meridian velocities, and the total horizontal velocity for each Carrington rotation during the activity cycles no. 21 and 22 varies during the 11-yr activity cycle. Plot of RMS values of total horizontal velocity is shifted about 1·6 years before the similarly sha ped variation of the magnetic flux.

Does the Potential Vorticity Distribution Constrain the Spreading of Floats in the North Atlantic?

Float trajectories are compared with the distribution of climatological potential vorticity, Q, on approximate isentropic surfaces for intermediate waters in the North Atlantic. The time-mean displacement and eddy dispersion are calculated for clusters of floats in terms of their movement along and across Q contours. For float clusters with significant mean velocities, the mean flow crosses Q contours at an angle of typically less than 208‐308 in magnitude in the ocean interior. The implied Peclet number in the ocean interior ranges from 1 to 19 with a weighted-mean value of 4.4. This mean Peclet number suggests that there is significant eddy mixing in the ocean interior: tracers should only be quasi-conserved along mean streamlines over a subbasin scale, rather than over an entire basin. The mean flow also strongly crosses Q contours near the western boundary in the Tropics, where the implied Peclet number is 0.7; this value may be a lower bound as Q contours are assumed to be zonal and relative vorticity is ignored. Float clusters with a lifetime greater than 200 days show anisotropic dispersion with greater dispersion along Q contours, than across them; float clusters with shorter lifetimes are ambiguous. This anisotropic dispersion along Q contours cannot generally be distinguished from enhanced dispersion along latitude circles since Q contours are generally zonal for these cases. However, for the null case of uniform Q for the Gulf Stream at 2000 m, there is strong isotropic dispersion, rather than enhanced zonal dispersion. In summary, diagnostics suggest that floats preferentially spread along Q contours over a subbasin scale and imply that passive tracers should likewise preferentially spread along Q contours in the ocean interior.

Total‐ozone imaging over north america with GOES‐8 infrared measurements

AbstractInfrared observations from the Geostationary Operational Environmental Satellite (GOES‐8) are used to provide high‐resolution synoptic maps of total ozone over North America for 23/24 February 1996. The infrared retrievals are validated with total ozone measured by the Global Ozone Monitoring Experiment instrument. In the GOES‐derived total‐ozone maps, one can identify fine‐scale structures. A prominent one is a slow‐moving, slender filament with a typical contrast of 30 Dobson units, stretching across North America along the 40°N latitude circle. A high‐resolution ozone simulation using European Centre for Medium‐range Weather Forecasts winds for this period also displays total‐ozone filaments characterized by high contrasts. There are more filamentary structures in the model.

Large-scale Flow and Transport of Magnetic Flux in the Solar Convection Zone

AbstractHorizontal large-scale velocity field describes horizontal displacement of the photospheric magnetic flux in zonal and meridian directions. The flow systems of solar plasma, constructed according to the velocity field, create the large-scale cellular-like patterns with up-flow in the center and the down-flow on the boundaries. Distribution of the largescale horizontal eddies (with characteristic scale length from 350 to 490 Mm) was found in the broad equatorial zone, limited by 60° latitude circles on both hemispheres. The zonal averages of the zonal and meridian velocities, and the total horizontal velocity for each Carrington rotation during the activity cycles no. 21 and 22 varies during the 11-yr activity cycle. Plot of RMS values of total horizontal velocity is shifted about 1.6 years before the similarly shaped variation of the magnetic flux.

Wave Packets and Life Cycles of Troughs in the Upper Troposphere: Examples from the Southern Hemisphere Summer Season of 1984/85

Abstract In this paper, reanalysis data for the Southern Hemisphere summer season of 1984/85, produced by the European Centre for Medium-Range Weather Forecasts, have been analyzed to examine wave packets and life cycles of baroclinic waves. A semiobjective algorithm has been devised to track wave packets. It is found that the perturbations in the upper troposphere are dominated by wave packets. Some of these wave packets can be tracked for several weeks, during which they can make up to two trips around the latitude circle. The energy life cycles of waves that make up several of these wave packets were analyzed, and apart from the incipient waves that started the wave packets, most of the subsequent wave development was found to be dominated by convergence and divergence of energy fluxes rather than due to baroclinic or barotropic conversions, demonstrating that these wave packets are indeed coherent entities that propagate due to downstream development (or dispersion). The energy life cycle of the most ...

A theoretical study of parcel stability and cloud distribution in a Jovian hot spot

The Galileo probe penetrated Jupiter’s atmosphere at a hot spot, a region of unusual clarity and dryness centered at 7°N. The measured abundances of condensable species were puzzling since strong depletion of volatiles was found from 0.4 to 12 bar, far below the condensation levels. In this paper we explore in detail thermally driven dry downdraft scenarios to simulate hot spot conditions. We compare the hot spot temperature profile with that of surrounding areas to evaluate their relative buoyancy and the parcel level of descent. For the hot spot we use the vertical thermal profile and composition measured in situ by the Galileo probe, as well as the distribution of condensable species inferred by the NIMS experiment on board of the Galileo orbiter. The environment is characterized by the thermal profile measured by the Voyager 1 radio probe experiment down to 1 bar. Downward, the profile is extended adiabatically using different values for the ortho–para distribution of H 2. Different concentrations of the condensables (relative humidity from 1 to 100%) were tested for the environment using a deep value ( P>8 bar) of 1 to 4 times the solar abundance of NH 3 and 1 to 2 times for H 2O. Under these conditions, dry downdrafts can sink in Jupiter’s atmosphere to the ∼5 to 15 bar level depending on the particular values used. Conversely, in a second numerical experiment, we simulate the hot spot’s composition and temperature profiles measured by the Galileo probe using a downward dry jet entraining model. Qualitative agreement is found for different parameterizations of the entrainment rates. Furthermore, a thermochemical model predicts at the hot spot, the existence of a NH 3 cloud with a base at ∼550 mbar and an NH 4SH cloud with a base at ∼1.6 bar. Finally, the number of hot spots presented in a latitude circle could be related to the depth of the hot spot motions through the Rossby deformation radius.

A VERIFICATION FOR TIDAL FLOW SIMULATION OF CLOSED BAY BY USING SATELLITE REMOTE SENSING

Omura Bay in Nagasaki Prefecture, which is called a double-closed bay, is connected with Sasebo Bay through very narrow channels. The authors had applied satellite remote sensing technique to confirm the exchange condition of seawater between these bays. This method is capable of investigating only the surface current. In this paper, the numerical simulation system was improved in order to simulate actual tides by using three-element control function. The results of satellite remote sensing were compared with the outcome of the simulation system. It follows that the results of satellite remote sensing and the outcome of the system corresponded well when the axis of coordinates which is rotated 34 degrees from circles of latitude and longitude and eddy viscosity in the 40m2/s to 60m2/s range were used for the numerical simulation.

1991年夏期の中国の強い降雨に関わる中国及南シナ海上の水蒸気収支

The moisture balance over the central part of China in May, June and July (MJJ) of 1991 is studied by utilizing the 24-hour prediction data of a global weather prediction model of JMA, in relation with the heavy rainfalls over China. The moisture balance over a domain bounded by 20-40°N and 110-125°E (Area-C, main part of China) indicates that the moisture flux convergence within this domain is mainly due to the southerly moisture inflow across the southern boundary. The precipitation in this domain increases in the early June with the abrupt increase of the southerly moisture inflow. The precipitation within this domain is highly correlated with the southerly moisture inflow crossing 20°N. To relate the southerly moisture flow into Area-C with the moisture balance in the tropical zone, the moisture balance in a domain bounded by 0-20°N and 110-125°E (Area-S, the South China Sea including the monsoon trough region) is further studied. The variation of the cross-equatorial moisture inflow in Area-S is significantly smaller than that of the moisture efflux across the northern boundary of Area-S. In Area-S, the strong correlation is found between the zonal moisture flux convergence and the southerly moisture flux across the 20°N latitude circle. This zonal moisture flux convergence is seen within the monsoon trough region between the monsoon westerly and the easterly flux in the southern rim of the Pacific subtropical anticyclone. The monsoon trough region plays the role of the channel to transport the moisture from 0-20°N zone toward north, although Area-S itself is not a moisture source region. The west-east intraseasonal oscillation of the monsoon trough, monsoon westerly and the North Pacific subtropical anticyclone have strong influence on the precipitation and westerly moisture transport over China. The features of precipitation over China in early July are characterized by a narrow intense rainfall zone over the Yangtze River Valley. The moisture flux field during this period indicates the strong moisture flux convergence within the precipitation zone.

The upper mesosphere/lower thermosphere wind field nonzonality as possible sign of the external forcing from above and from below

The geographical nonzonality of the global wind field was investigated through a long-term study of upper mesopause/lower thermosphere horizontal winds (average altitude 95 km) near Irkutsk, East Siberia, Russia (52° N, 104° E) in comparison with similar wind measurements at the stations, which are well spaced along the 52° N latitude circle. The longitudinal/regional variations in the dynamics of the upper middle atmosphere are illustrated. This effect may significantly manifest itself in the different amplitudes of the prevailing zonal wind, in the different times of the winter-spring transition of the zonal circulation, in the different response to the sudden stratosphere warmings, and in the different response to the geomagnetic storms and Solar Proton Events. The reasons of the nonzonality could be planetary scale mesopause oscillations structure; the external forcing from below due to different conditions for the generation and propagation upward of the internal atmospheric waves; the real difference of the geomagnetic latitudes. The nonzonality of the mesopause wind field may be important when empirical and physical upper atmosphere models are being formed.

A system of circumpolar waves in Jupiter's stratosphere

We report on the optical identification on Hubble Space Telescope (HST) and Voyager 1 archived images, of a circumpolar system of Jovian stratospheric quasi‐stationary waves, centered at planetographic latitudes −53°, −62°, and −67° with averaged zonal wavenumbers 23, 11, and 12 respectively. The most conspicuous wave is detected in the 890‐nm strong absorption band of methane at −67° latitude. Its amplitude changes in a matter of days, stretching and contracting but preserving the number of components in the latitude circle. This wave has been observed during several years and seems to be a persistent, long‐term phenomena in Jupiter's atmosphere.

Optimal Sites for Supplementary Weather Observations: Simulation with a Small Model

Anticipating the opportunity to make supplementary observations at locations that can depend upon the current weather situation, the question is posed as to what strategy should be adopted to select the locations, if the greatest improvement in analyses and forecasts is to be realized. To seek a preliminary answer, the authors introduce a model consisting of 40 ordinary differential equations, with the dependent variables representing values of some atmospheric quantity at 40 sites spaced equally about a latitude circle. The equations contain quadratic, linear, and constant terms representing advection, dissipation, and external forcing. Numerical integration indicates that small errors (differences between solutions) tend to double in about 2 days. Localized errors tend to spread eastward as they grow, encircling the globe after about 14 days. In the experiments presented, 20 consecutive sites lie over the ocean and 20 over land. A particular solution is chosen as the true weather. Every 6 h observations are made, consisting of the true weather plus small random errors, at every land site, and at one ocean site to be selected by the strategy being considered. An analysis is then made, consisting of observations where observations are made and previously made 6-h forecasts elsewhere. Forecasts are made for each site at ranges from 6 h to 10 days. In all forecasts, a slightly weakened external forcing is used to simulate the model error. This process continues for 5 years, and mean-square forecast errors at each site at each range are accumulated. Strategies that attempt to locate the site where the current analysis, as made without a supplementary observation, is most greatly in error are found to perform better than those that seek the oceanic site to which a chosen land site is most sensitive at a chosen range. Among the former are strategies based on the “breeding” method, a variant of singular vectors, and ensembles of “replicated” observations; the last of these outperforms the others. The authors speculate as to the applicability of these findings to models with more realistic dynamics or without extensive regions devoid of routine observations, and to the real world.

Adaptive Tuning of Numerical Weather Prediction Models: Simultaneous Estimation of Weighting, Smoothing, and Physical Parameters

In Wahba et al. it was shown how the randomized trace method could be used to adaptively tune numerical weather prediction models via generalized cross validation (GCV) and related methods. In this paper a “toy” four-dimensional data assimilation model is developed (actually one space and one time variable), consisting of an equivalent barotropic vorticity equation on a latitude circle, and used to demonstrate how this technique may be used to simultaneously tune weighting, smoothing, and physical parameters. Analyses both with the model as a strong constraint (corresponding to the usual 4D-Var approach) and as a weak constraint (corresponding theoretically to a fixed-interval Kalman smoother) are carried out. The conclusions are limited to the particular toy problem considered, but it can be seen how more elaborate experiments could be carried out, as well as how the method might be applied in practice. The authors have considered five adjustable parameters, two related to a distributed coefficient in the equivalent barotropic vorticity equation (“physical” parameters), one governing the relative weight given to observations versus forecast, one governing the relative weight given to observations versus goodness of fit to the model (in the weak constraint case), and one governing the damping of high-frequency oscillations in the analysis at the final time point (“smoothing” parameter). The weighting parameters and the smoothing parameter can, if desired, be interpreted as ratios of parameters in prior covariances. Analyses are made with a low-resolution model of the dynamics of the equivalent barotropic vorticity equation given noisy forecast (initial conditions) and noisy wind observations, and compared with nature evolved from exact initial conditions using a high-resolution forward integration. The authors found that these five (carefully chosen) parameters are simultaneously tunable on line, that is, simultaneously with the analysis, and 1) that the analysis is equally and strongly sensitive to both the choice of the observed versus forecast weighting parameter and the choice of the smoothing parameter; 2) that the analysis with the model as a weak constraint, based on the tuned estimate of the parameter governing how close the analysis satisfies the model, is somewhat better than the analysis using the model as a strong constraint, although estimation of this tuning parameter varies much more than the other parameters with replications of the experiment; and 3) good estimates of the physical parameters are obtained; however, these estimates are closer to those that make the model integrated forward with perfect initial conditions best fit nature, and these are not exactly the “true” parameters.

The Climatological Distribution of Potential Vorticity over the Abyssal Ocean

Climatological maps of the large-scale potential vorticity field Q along isopycnals are diagnosed for the abyssal waters over the global ocean. The inferred patterns of Q vary with density, the basin, and hemisphere. At middepths, the distribution of Q is controlled by the background planetary vorticity gradient. On deeper isopycnals, there are regions where Q contours deviate from latitude circles and even regions of nearly uniform Q. These different regimes appear to be robust features over the interior of ocean basins, as the standard error is found to be relatively small there. The nearly uniform Q occurs in the deep waters of the North Pacific and, possibly, in the bottom waters of the western North Atlantic and North Pacific. The nearly uniform Q has a low magnitude in each case, as well as a relatively low variability for the deep waters of the North Pacific. This nearly uniform Q signal appears to be formed when a single water mass enters the basin from a low latitude source.

An analysis of HALOE observations in summer high latitudes using airmass trajectory and photochemical model calculations

Stratospheric ozone, HCl, NO, and NO2 observed by the Halogen Occultation Experiment (HALOE) on the Upper Atmosphere Research Satellite (UARS) in late summer at high latitudes show nonuniform distributions in their volume mixing ratios along a latitude circle, while the mixing ratios of the long‐lived tracers, such as CH4 and HF, are observed to be quite uniform. These anomalous distributions of the reactive gases are confined in approximately 20–35 km. It is found that in the summer high latitudes, the isosurfaces of long‐lived tracers nearly coincide with isentropes. On the basis of meridional distributions of these species, it appears that the HALOE‐observed features are the results of quasi‐isentropic transport across latitude lines with a timescale shorter than the chemical relaxation times of those HALOE observed ozone and other gases. A spectral analysis of the United Kingdom Meteorological Office meridional wind data indicates the existence of episodically amplified wave 1 that is quasi‐stationary in July and then slowly westward moving after mid‐August 1992. A quasi‐stationary wave 2 is also found to be important in late August. The isentropic trajectory calculations show that these planetary scale waves seem to pull air masses out of the polar region as far as 10°–15° latitude equatorward in a week. Midlatitude air is also pushed into the high summer latitude region. A photochemical box model is used to simulate the chemical evolution of HALOE‐observed species along air parcel trajectories which are initialized at HALOE positions. Reasonable agreement is found in comparisons between coincident HALOE measurements and the model results in later days. The characteristic structures in HALOE‐observed ozone and other gases in the low and middle stratosphere indicate that the chemical relaxation times taken for these species to relax to their new local mixing ratios under new sunlit conditions are comparable with or longer than the timescale of the meridional transport induced by waves in late summer.

Atmospheric equilibrium, instability and energy transport at the last glacial maximum

A range of diagnostics from two GCM simulations, one of the present-day climate and one of the last glacial maximum (LGM) is used to gain insight into their different temperature structures and eddy dynamics. There are large local increases in baroclinicity at the LGM, especially in the Atlantic storm track, with large accompanying increases in the low level transient eddy heat flux. However, the differences in the zonal mean are much smaller, and the increases in both baroclinicity and heat flux are confined to low levels. Supplementary experiments with baroclinic wave lifecycles confirm the marked contrast between local and zonal mean behaviour, but do not adequately explain the differences between the zonal mean climates. The total flux of energy across latitude circles in the Northern Hemisphere does not change much during DJF, although its transient component is actually reduced at the LGM (during JJA the transient component is increased). Calculations of total linear eddy diffusivity reveal that changes in the time mean stationary waves are chiefly responsible for the seasonal range of this quantity at the LGM, while they only account for half the seasonal range at the present-day.

Dynamical structure functions in a four‐dimensional variational assimilation: A case study

AbstractThis paper contributes to the understanding of the structure functions used implicitly in the four‐dimensional variational assimilation (4D‐Var) developed at the European Centre for Medium‐Range Weather Forecasts in the last few years. The theoretical equivalence between 4D‐Var and the Kalman filter allows us to interpret (after normalization by the error standard deviations) the analysis increments produced by one single observation as the structure functions used implicitly in 4D‐Var. The shape of the analysis increments provides a three‐dimensional picture of the covariances of the background errors, modified by the dynamics.We study a baroclinic situation and observations have been regularly distributed along a latitude circle crossing the baroclinic wave. Eight standard pressure levels have been considered to sample the vertical.The forecast error standard deviations and the structure functions implied in 4D‐Var may differ considerably from those used in the 3D‐Var analysis. Unlike 3D‐Var, the structure functions are flow dependent: the effective background error standard deviation can be four times larger and the correlation length scale twice as short in the vicinity of a low. A meridional extension of the experimentation at the surface shows that the effective background error standard deviations at 1000 hPa are largest in the areas of strong pressure gradient.We quantify the link between the analysis increments produced by 4D‐Var and the fastest growing perturbations over the same time interval. In the depression, the explained variance of the analysis increments by the first 13 singular vectors reaches 30%.The impact of the temporal dimension is assessed. A period of 24 hours seems a minimum for the increments to develop fully baroclinic structures.

Synoptic Interpretation of Measurements from HALOE

The degree to which the Southern Hemisphere polar vortex is isolated against horizontal (isentropic) mixing is investigated using data from the Halogen Occultation Experiment (HALOE), U.K. Meteorological Office (UKMO) potential vorticity (PV), and contour advection diagnostics. Measurements of methane and water vapor taken by HALOE during a disturbed period in the Southern Hemisphere springtime (21 September - 15 October 1992) are interpreted in light of the prevailing synoptic meteorology. Daily fields of winds and PV are shown to be essential in the interpretation of the data. A climatological high pressure region is responsible for a distorted vortex, and a substantial 'vortex stripping' event is present, associated with the early stages of vortex breakdown. This leads to significant temporal, zonal, and altitudinal variations in the distribution of tracers. The authors point out the difficulties this presents for the interpretation of solar occultation data, especially with regard to the use of zonal average time series. Longitude-height methane distributions from two days during the period are examined. Both days show substantial variations in abundance around a latitude circle. In particular, the authors investigate HALOE measurements at 77 deg S on 15 October 1992, which indicate an abundance of methane in the height region 600-2000 K (approximately 30-1 mb) that is more typical of midlatitude air. Similar distributions, observed in the 1991 HALOE data, have previously been interpreted as evidence for the penetration of midlatitude air into the vortex. Gradients of potential vorticity and contour advection diagnositcs are employed to examine whether the UKMO winds are consistent with this hypothesis in 1992. Although midlatitude air is able to penetrate poleward of the main jet core by advection processes alone, an essentially intact inner core of vortex air remains, which does not mix to any great extent with air from lower latitudes. The authors show that the high-latitude HALOE abundances that are typical of midlatitude air were observed in a region of extensive filamentation and mixing, rather than within the inner, more isolated, core.