Mean Meridional Motions Research Articles

Extension of Ekman (1905) wind-driven transport theory to the β plane

Abstract. The seminal Ekman (1905) f-plane theory of wind-driven transport at the ocean surface is extended to the β plane by substituting the pseudo-angular momentum for the zonal velocity in the Lagrangian equation. When the β term is added, the equations become nonlinear, which greatly complicates the analysis. Though rotation relates the momentum equations in the zonal and the meridional directions, the transformation to pseudo-angular momentum greatly simplifies the longitudinal dynamics, which yields a clear description of the meridional dynamics in terms of a slow drift compounded by fast oscillations; this can then be applied to describe the motion in the zonal direction. Both analytical expressions and numerical calculations highlight the critical role of the Equator in determining the trajectories of water columns forced by eastward-directed (in the Northern Hemisphere) wind stress even when the water columns are initiated far from the Equator. Our results demonstrate that the averaged motion in the zonal direction depends on the amplitude of the meridional oscillations and is independent of the direction of the wind stress. The zonal drift is determined by a balance between the initial conditions and the magnitude of the wind stress, so it can be as large as the mean meridional motion; i.e., the averaged flow direction is not necessarily perpendicular to the wind direction.

Long-term variations in meridional motion of sunspot groups: comparison of DPD and SOON data

AbstractWe have analyzed the Debrecen Photoheliographic Data (DPD) and the Solar Optical Observing Network (SOON) sunspot group data during the period 1977 – 2015 and find that during the maximum of solar cycle 23 there is a large difference in the mean meridional motion of sunspot groups determined from DPD and SOON data.

Analysis of the Equatorial Lower Stratosphere Quasi-Biennial Oscillation (QBO) Using ECMWF-Interim Reanalysis Data Set

The ERA-Interim data set from Europe Center for Medium Range Weather Forecasting (ECMWF) was used to quantitatively analyze the characteristic of equatorial quasi-biennial oscillation (QBO). Analysis of spatial and temporal of the data showed that the zonally symmetric easterly and westerly phase of QBO regimes alternate with period of ∼27.7 months. Based on Equivalent QBO Amplitude (EQA) method, the maximum amplitudes in zonal mean zonal wind (u), temperature (T), vertical shear (du/dz) and quadratic vertical shear (d2u/dz2) are ∼28.3 m/s, ∼3.4 K, ∼4.8 m/s/km, and ∼1.0 m/s/km2 respectively. The amplitudes decay exponentially with a Gaussian distribution in latitude. The twofold-structure of QBO descends downward at rate of ∼1 km/month. The temperature anomaly can be used to analyze the characteristic of QBO which satisfies the thermal wind balance relation in the lower- stratosphere due to very small contribution of the mean meridional and vertical motion. Moreover, the concentration of the total column ozone (TCO) in the tropics is significantly influenced by QBO. During the westerly phase of QBO, the TCO is relatively increased in the lower-stratosphere, but decreased during the opposite phase.

Erratum to: Predicting the Amplitude of a Solar Cycle using the North – South Asymmetry in the Previous Cycle: II. An Improved Prediction for Solar Cycle 24

Recently, using Greenwich and Solar Optical Observing Network sunspot group data during the period 1874 – 2006, Javaraiah (Mon. Not. Roy. Astron. Soc.377, L34, 2007: Paper I), has found that: (1) the sum of the areas of the sunspot groups in 0° – 10° latitude interval of the Sun’s northern hemisphere and in the time-interval of −1.35 year to +2.15 year from the time of the preceding minimum of a solar cycle n correlates well (corr. coeff. r=0.947) with the amplitude (maximum of the smoothed monthly sunspot number) of the next cycle n+1. (2) The sum of the areas of the spot groups in 0° – 10° latitude interval of the southern hemisphere and in the time-interval of 1.0 year to 1.75 year just after the time of the maximum of the cycle n correlates very well (r=0.966) with the amplitude of cycle n+1. Using these relations, (1) and (2), the values 112±13 and 74±10, respectively, were predicted in Paper I for the amplitude of the upcoming cycle 24. Here we found that the north – south asymmetries in the aforementioned area sums have a strong ≈44-year periodicity and from this we can infer that the upcoming cycle 24 will be weaker than cycle 23. In case of (1), the north – south asymmetry in the area sum of a cycle n also has a relationship, say (3), with the amplitude of cycle n+1, which is similar to (1) but more statistically significant (r=0.968) like (2). By using (3) it is possible to predict the amplitude of a cycle with a better accuracy by about 13 years in advance, and we get 103±10 for the amplitude of the upcoming cycle 24. However, we found a similar but a more statistically significant (r=0.983) relationship, say (4), by using the sum of the area sum used in (2) and the north – south difference used in (3). By using (4) it is possible to predict the amplitude of a cycle by about 9 years in advance with a high accuracy and we get 87±7 for the amplitude of cycle 24, which is about 28% less than the amplitude of cycle 23. Our results also indicate that cycle 25 will be stronger than cycle 24. The variations in the mean meridional motions of the spot groups during odd and even numbered cycles suggest that the solar meridional flows may transport magnetic flux across the solar equator and potentially responsible for all the above relationships.

Long-term variations in the mean meridional motion of the sunspot groups

We find that the mean meridional motion of the spot groups varies considerably on a time scale of about 5-20 years. The maximum amplitude of the variation is about 10-15 m/s in both the Sun's northern and the southern hemispheres. Variation in the mean motion is considerably different during different solar cycles. At the maximum epoch (year 2000) of the current cycle~23, the mean motion is relatively strong in the past 100 years and northbound in both the northern and the southern hemispheres. The power spectral analyses suggest the existence of approximate 3.2- and 4.3-year periodicities in the mean motion of the spot groups in the southern hemisphere, whereas a 13-16 year periodicity is found to exist in the mean motion of the northern hemisphere. There is strong evidence for a latitude-time dependency in the periodicities of the mean motion. The north-south difference in the mean motion also varies by about 10 m/s. During the recent cycles, the north-south difference is negligibly small. Approximate 12- and 22-year periodicities are found to exist in the north-south difference.

An equatorial oscillation in Saturn’s middle atmosphere

The middle atmospheres of planets are driven by a combination of radiative heating and cooling, mean meridional motions, and vertically propagating waves (which originate in the deep troposphere). It is very difficult to model these effects and, therefore, observations are essential to advancing our understanding of atmospheres. The equatorial stratospheres of Earth and Jupiter oscillate quasi-periodically on timescales of about two and four years, respectively, driven by wave-induced momentum transport. On Venus and Titan, waves originating from surface-atmosphere interaction and inertial instability are thought to drive the atmosphere to rotate more rapidly than the surface (superrotation). However, the relevant wave modes have not yet been precisely identified. Here we report infrared observations showing that Saturn has an equatorial oscillation like those found on Earth and Jupiter, as well as a mid-latitude subsidence that may be associated with the equatorial motion. The latitudinal extent of Saturn's oscillation shows that it obeys the same basic physics as do those on Earth and Jupiter. Future highly resolved observations of the temperature profile together with modelling of these three different atmospheres will allow us determine the wave mode, the wavelength and the wave amplitude that lead to middle atmosphere oscillation.

Morphology and dynamics of Venus oxygen airglow from Venus Express/Visible and Infrared Thermal Imaging Spectrometer observations

Images obtained by the Visible and Infrared Thermal Imaging Spectrometer (VIRTIS)‐M channel instrument onboard Venus Express have been used to retrieve maps and apparent motions of the O2 (1Δ) infrared nightglow on Venus at 1.27 μm. The nightglow distribution is highly inhomogeneous with the regions of brightest emission generally located at low latitudes near the midnight meridian. Unexpectedly some orbits show also intense airglow activity over the south polar region. The spatially resolved airglow is spectacularly variable not only in its morphology and intensity but also in the apparent motions of the airglow small‐ and large‐scale structures. Visual tracking of the bright features allowed to obtain mean zonal and meridional motions related to the subsolar to antisolar circulation. The zonal velocity is dominated by an intense prograde jet (contrary to the retrograde planetary rotation) from dawn to midnight extending up to 22 hours in local time with lower velocities and reversed sign from dusk. Typical zonal velocities range between +60 (prograde) to −50 (retrograde) m/s, whereas most meridional velocities range from −20 (poleward) to +100 m/s (equatorward) with an average meridional circulation of +20 m/s toward low latitudes. The brightest small‐scale (∼100 km) features appear correlated with locations of apparent convergence which may be a signature of compression and downwelling, whereas this is not evident for the large‐scale structures suggesting slow subsidence over large areas mixed with horizontal motions. We argue that part of the tracked motions are representative of real motions at the mesosphere over an altitude range of 95–107 km.

Dependence of the meridional motions of sunspot groups on life spans and age of the groups, and on the phase of the solar cycle

We have analyzed data on sunspot groups compiled during 1874–1981 and investigated the following: (i) dependence of the `initial' meridional motion (v ini(τ)) of sunspot groups on the life span (τ) of the groups in the range 2–12 days, (ii) dependence of the meridional motion (v(t)) of sunspot groups of life spans 10–12 days on the age (t) of the spot groups, and (iii) variations in the mean meridional motion of spot groups of life span 2–12 days during the solar cycle. In each of the latitude intervals 0°–10°, 10°–20° and 20°–30°, the values of both v ini(τ) and v(t) often differ significantly from zero. In the latitude interval 20°–30°, the forms of v ini(τ) and v(t) are largely systematic and mutually similar in both the north and south hemispheres. The form of v(t) suggests existence of periodic variation in the solar meridional motion with period of 4 days and amplitude 10–20 m s−1. Using the anchoring depths of magnetic structures for spot groups of different τ and testimated earlier, (Javaraiah and Gokhale, 1997), we suggest that the forms of v ini(τ) and v(t) may represent radial variation of meridional flow in the Sun's convection zone, rather than temporal variation of the flow. The meridional flows (v e(t)) determined from the data during the last few days (i.e., age t: 10–12 days) of spot groups of life spans of 10–12 days are found to have magnitudes (10–20 m s−1) and directions (poleward) similar to the those of the surface meridional plasma flows determined from the Dopplergrams and magnetograms. The mean meridional velocity of sunspot groups living 2–12 days seems to vary during the solar cycle. The velocity is not significantly different from zero during the rising phase of the cycle and there is a suggestion of equatorward motion (a few m s−1at lower latitudes and ∼10 m s−1at higher latitudes) during the declining phase (last few years) of the cycle. The variation during the odd numbered cycles seems to anticorrelate with the variation during the even numbered cycles, suggesting existence of ∼22-year periodicity in the solar meridional flow. The amplitude of the anticorrelation seems to be depending on latitude and the cycle phase. In the latitude interval 20°–30° the `surface plasma meridional motion', v e(t), is found to be poleward during maximum years (v e(t) ∼20 m s−1at 4th year) and equatorward during ending years of the cycle (v e(t) ∼−17 m s−1at 10th year).

Mean winds and tidal and planetary wave motions over Hawaii during airborne lidar and observations of Hawaiian Airglow ALOHA‐93

The mean and low‐frequency wave motions in the mesosphere and lower thermosphere from 70 to 98 km were observed with the MF radar on Kauai during the Airborne Lidar and Observations of Hawaiian Airglow ALOHA‐93 measurement campaign. Two‐day mean zonal winds exhibited a transition from a variable structure with altitude at the beginning of October having westward motions near 80 km and eastward motions at upper and lower levels to more uniform eastward winds with altitude at the end of October. Two‐day mean meridional motions were equally variable, with northward motions at all altitudes on October 1, northward motions above ∼80 km and southward motions below throughout much of the campaign, and largely northward motions at the end of October. Zonal and meridional winds exhibited significant oscillations at ∼5 to 30 day periods, indicative of planetary wave activity during this interval. Diurnal tidal amplitudes and phases were likewise variable, with maximum and minimum rms amplitudes of ∼ 35 and 10 ms−1, approximate phase quadrature with meridional leading zonal, amplitude and phase variability on time scales of ∼ 5 to 15 days, and an overall phase advance during October of ∼10 h. Other tidal components and the two‐day wave had rms amplitudes of ∼ 5 to 10 ms−1.

Relationship of horizontal eddy displacements to mean meridional motions in the stratosphere

Equivalent‐barotropic integrations that have been carried out under high resolution reveal organized subsidence inside the polar‐night vortex. The potential temperature θ along a material surface, which is a prognostic variable in the equivalent‐barotropic system, drops to sharply lower values across the edge of the vortex. Resembling observed motion in the Brewer‐Dobson circulation, this behavior results from diabatic effects when the vortex is driven out of radiative equilibrium by wave advection. Lagrangian analyses carried out for ensembles inside and outside the vortex elucidate specific thermodynamic processes which act on individual bodies of air and are ultimately responsible for the Brewer‐Dobson circulation. When the vortex is displaced out of zonal symmetry, individual air parcels are driven out of thermodynamic equilibrium with their surroundings. As a result, they experience irreversible heat transfer that leads to a hysteresis and net change of θ with each complete orbit about the pole. Successive orbits then produce a drift of air to lower θ. Vertical motion estimated from ensemble‐mean properties corresponds to an average descent rate of about 1 mm/s, which is in qualitative accord with estimates derived elsewhere. Within the Lagrangian framework, the disturbed circulation functions as a radiative refrigerator by converting work performed at its lower boundary into heat that is eventually rejected to space through longwave cooling. The Lagrangian analyses suggest a similar analog for photochemical considerations. Driven out of photochemical equilibrium, the disturbed circulation can then function as a chemical engine by producing ozone and transferring it to the extratropical lower stratosphere.

Transport, Heat, and Freshwater Fluxes within a Diagnostic Numerical Model (FRAM)

Results are presented from the integration of a fine-resolution numerical model of the ocean operating in a diagnostic mode. The region covered lies south of 24°S, as depicted in the FRAM Atlas. Here transports, heat, salt, and freshwater fluxes are examined at 60°S and near 30°S in all three oceans. Results are found to be generally realistic. At midlatitude the meridional heat flux is largely determined by the structure of the mean meridional motions and to a lesser degree by the gyre-scale horizontal motions. These roles are reversed for freshwater fluxes. At a fixed high latitude the freshwater flux is determined by the mean meridional motions and the heat flux principally by the large-scale wandering of the circumpolar current across the latitude. Only in this latter case, namely for the heat flux at 60°S, do mesoscale motions contribute to a significant extent. The model underestimates the production and export of abyssal water; the climatological state with which it is initialized is identified as the likely cause. A suggestion is offered for assessing the accuracy of diagnostic integrations.

Mean winds, tides and gravity waves in the upper middle atmosphere during ALOHA‐90

Wind measurements made with a partial reflection radar located on Christmas Island (2°N, 157°W) are used to describe the dynamical state of the equatorial mesopause region during ALOHA‐90. Time mean westward (easterly) winds prevailed at most heights, and reached their maximum values of about −60 ms−1 near 85 km, but the mean meridional motions were weak. Strong oscillations due to the 24, 12, and 8 hr atmospheric tides were also observed in both wind components. The inferred vertical wavelengths were large, even for the diurnal tide. On the nights of 22 and 25 March, when airborne lidar observations were made in the vicinity of Christmas Island, the prevailing and tidal winds combined to produce especially strong westward winds ( −100 ms−1). Gravity wave activity was also high during March/April, especially for short period waves, although considerable day‐to‐day variability was noted.

Three-Dimensional Air Motions over the Baiu Front Observed by a VHF-Band Doppler Radar: A Case Study

Abstract Upper-tmpospheric three dimensional air motions have been observed for the first time during the Baiu period in 1984 by using a 46.5 MHz Doppler radar in Japan. This radar, called the MU radar, operates with an antenna aperture of 8330 m2 and peak and average radiation powers of 1000 and 50 kW, respectively. It can steer the antenna beam up to 30° from the zenith in each interpulse period. With the aid of this fast beam steerability the MU radar can measure the three dimensional air motion. Resolutions in time and altitude of, the present observations are 100 s and 150 m, respectively. Referring to the routine rawinsande observations the following results are obtained on the air motion over the Baiu front: 1) the observed mean meridional motion is upward and northward as expected but deviates upward from the frontal surface and pseudo-isentropes, 2) the upper-tropospheric mesoscale wind variations are not strongly correlated with the lower-tropospheric frontal activity such as precipitation and 3...

Zonal mean circulation at the cloud level on Venus: Spring and fall 1979 OCPP observations

Cloud motions were obtained from a number of images acquired in reflected solar ultraviolet light during spring and fall of 1979 from the Pioneer Venus Orbiter Cloud Photopolarimeter (OCPP) to determine the zonal mean circulation of the atmosphere of Venus at the cloud top level. The meridional profile of the zonal component of motion is somewhat different from that previously obtained from Mariner 10 and preliminary Pioneer Venus observations, although the equatorial magnitude is about the same (−94 m/sec). The mean meridional motion is toward the south pole south of about 5° south latitude, and toward the north pole north of this latitude, with peak mean magnitudes of about 7 m/sec polewards of 20° north and 40° south latitudes in the respective hemispheres. From the few measurements obtained at higher latitudes the magnitude of the mean meridional component appears to decrease although it is still directed toward the respective poles. Due to the evolution of the cloud patterns over the duration of the images from which the cloud velocities are obtained, the uncertainties in the mean zonal and meridional components may be as large as 5–10 and 2–4 m/sec, respectively. Preliminary estimates of meridional momentum transport show that the mean circulation dominates the eddy circulation transport completely, in agreement with the estimates obtained from Mariner 10 data, although the uncertainties in both the mean and eddy circulation transports are large. The momentum transports are polewards and their peak magnitudes occur at latitudes between 20° and 40° in both the hemispheres.

An advective model for two‐dimensional transport of stratospheric trace species

A parameterization of two‐dimensional transport is developed for application to photochemical models of stratospheric trace species. It is shown that in a first approximation tracer transport in the meridional plane may be treated as an advective, rather than a diffusive, process. The mean meridional motion used in this advective model is a modified Eulerian mean which is a good approximation to the Lagrangian mean motion. The potential of the proposed parameterization is demonstrated by a simple numerical experiment in which the equilibrium distribution is computed for a tracer with a uniform tropospheric source and a stratospheric sink proportional to the local mixing ratio. The resulting distribution is qualitatively in accord with the oberved stratospheric distribution of trace species whose sources are in the troposphere, e.g., N2O.

Cloud Motions on Venus: Global Structure and Organization

Results on cloud motions on Venus obtained over a period of 3.5 days from Mariner 10 television images are presented. The implied atmosphere flow is almost zonal everywhere on the visible disk, and is in the same retrograde sense as the solid planet. Objective analysis of motions suggests the presence of jet cores (-130 m/s) and organized atmospheric waves. The longitudinal mean meridional profile of the zonal component of motion of the ultraviolet features shows presence of a midlatitude jet stream (-110 m/s). The mean zonal component is -97 m/s at the equator. The mean meridional motion at most latitudes is directed toward the pole in either hemisphere and is at least an order of magnitude smaller so that the flow is nearly zonal. A tentative conclusion from the limited coverage available from Mariner 10 is that at the level of ultraviolet features mean meridional circulation is the dominant mode of poleward angular momentum transfer as opposed to the eddy circulation.

Mean meridional circulations of the stratosphere and mesosphere

Observations from the Nimbus 6 pressure modulator radiometer (PMR) have been used to estimate monthly mean planetary wave fluxes of heat and momentum in the stratosphere and mesosphere. While the eddy heat fluxes play an important role in the mean meridional circulation of the winter stratosphere they are shown to be less important in the upper mesosphere. Incorporation of the observed momentum fluxes into the Oxford two-dimensional circulation model has shown that they are incapable of providing the momentum transport necessary to balance the zonal flow accelerations induced by the mean meridional motion. Other unspecified transfer processes represented by Rayleigh frictional damping of the zonal fow are shown to dominate. In contrast the observed fluxes in the stratosphere achieve the necessary redistribution of momentum. Moreover their interannual variability profoundly influences the stratospheric circulation, as demonstrated in the model by the use of two different annual sets of observed momentum fluxes. The desirability of calculating the planetary wave behaviour within the model is indicated.

The Roles of Mean Meridional Motions and Large-Scale Eddies in Zonally Averaged Circulations

Abstract A hierarchy of zonally averaged atmospheric models is used to study the role of mean meridional motions and large-scale eddies in determining the zonal climate. Five models are developed: a radiative-convective equilibrium model (no large-scale motion), a zonally uniform model (no longitudinal asymmetries), an energy balance model (parameterized energy transport), a model that combines the physics of the two previous models, and a full statistical-dynamic model (with parameterizations of eddy momentum transport as well as eddy sensible heat transport). In the most complete model, the zonally averaged primitive equations are solved after parameterizing the eddies, friction and the diabatic heating rates. All the models have two layers in the vertical and a latitudinal grid resolution of 5°. For simplicity, we treat a “dry earth” case and calculate annual-average equilibrium states with each of the five models. We find that in the tropics a parameterized energy transport commonly used in energy bal...

The Observed Structure of Tropospheric Stationary Waves and the Local Balances of Vorticity and Heat

The physical structure and the associated transport properties of stationary waves in the troposphere are described using circulation statistics compiled from twice-daily hemispheric analyses covering 11 winters. The distributions of standing eddy meridional transports in middle latitudes are characterized by momentum flux convergence and equatorward geopotential energy transports in the upper troposphere, and by poleward heat fluxes at the lower levels. The contributions of steady and transient motions to the local, time-averaged budget of vorticity at 300 mb are evaluated. The dominant terms in the time-averaged vorticity equation are the local advection of relative vorticity by the stationary flow and the divergence term. The advection of planetary vorticity by the mean flow (the β-effect) and the convergence of vorticity fluxes by transient eddies appear to be of secondary importance. The hemispheric distributions of the principal terms in the vorticity balance are closely related to topographical features at the lower boundary. The hemispheric field of stationary flow divergence at various levels is determined as a residual in the time-averaged vorticity balance. This diagnosed divergence field is used to 1) demonstrate the feasibility of retrieving essential stationary flow features in the upper troposphere through solution of the linearized vorticity equation with a prescribed divergence forcing; 2) deduce the velocity potential field; and 3) compute the mean vertical motion field through vertical integration of the continuity equation. Standing eddy statistics involving vertical motions are described. Mean vertical motions in middle latitudes are found to be positively correlated with mean meridional motions and with mean temperature. The distributions of meridional and vertical transports of geopotential energy and westerly momentum in the meridional plane are presented in a vectorial format. The pattern depicting geopotential energy fluxes suggests that the enhanced standing wave kinetic energy over the subtropics is maintained by geopotential energy transports which originate from higher latitudes. A diagnosis of the local, time-averaged balance of heat at 1000 and 700 mb is performed. The heat transports by the transient eddies in the lower troposphere exhibit a strong tendency to destroy the zonally asymmetric component of the stationary temperature field. This dissipative mechanism acts on a time scale of several days. The hemispheric distributions of the diabatic heating deduced from the heat budget are indicative of the central role of geographically fixed influences such as ocean currents and sea-land contrast.

On the Mean Meridional Mass Motions of the Stratosphere and Mesosphere

Using a simplified, approximate 'Lagrangian-mean' dynamical formulation, the mean meridional mass circulation of the stratosphere and mesosphere is discussed. Under solstice conditions, it is shown that this Lagrangian-mean circulation may be inferred, as a first approximation, from the Eulerian-mean diabatic heating. Diabatic heating rates for the solstices, originally derived by Murgatroyd and Goody (1958), result in Lagrangian-mean rising motion at the tropical tropopause, subsidence across the extra-tropical tropopause, and a very strong summer-to-winter pole flow in the mesosphere. This circulation is exactly that obtained by Murgatroyd and Singleton (1961) for the solstices. Those authors, however, attempted to identify this circulation as the Eulerian-mean motion, and were later criticized for their neglect of the meridional eddy heat flux in the calculation, which proved to be extremely important in the winter hemisphere. The present study, nevertheless, indicates that Murgatroyd and Singleton's circulation may in fact be representative of actual air parcel motions in the stratosphere and mesosphere.