Meridional Potential Vorticity Gradient Research Articles

The Effects of Variations in Jet Width on the Growth of Baroclinic Waves: Implications for Midwinter Pacific Storm Track Variability

The effects of variations in jet width on the downstream growth of baroclinic waves are studied, using a simple quasigeostrophic model with a vertically varying basic state and variable channel width, as well as a simplified primitive equation model with a basic state that varies in latitude and height. This study is motivated by observations that in midwinter in the Pacific the storm track is weaker and the jet is narrower during years when the jet is strong. The linear models are able to reproduce the observed decrease of spatial growth rate with shear, if the narrowing of the jet is accounted for by assuming it decreases the meridional wavelength of the perturbations, which hampers their growth. A common suggestion has been that perturbations are weaker when the jet is strong because they move faster out of the unstable storm track region. The authors find that one needs to take into account that the jet narrows when it strengthens; otherwise, the increase of growth rate is strong enough to counteract the effect of increased advection speed. It is also found that, when the model basic state is Eady-like (small or zero meridional potential vorticity gradients in the troposphere), the short-wave cutoff for instability moves to large-scale waves as shear is increased, due to the accompanying increase in meridional wavenumber. This results in a transition from a regime where upper-level perturbations spin up a surface circulation very rapidly, and normal-mode growth ensues, to a regime where the initial perturbations take a very long time to excite growth. Since waves slow down when a surface perturbation develops, this can explain the observations that the storm track perturbations are more ‘‘upper level’’ during strong jet years and their group velocities increase faster than linearly with shear.

Dynamics of the 2‐day wave in a nonlinear model of the middle and upper atmosphere

The 2‐day wave is investigated in a three‐dimensional model of the nonlinear primitive equations. Extending upward from the tropopause into the thermosphere, the model is forced stochastically by fluctuations of tropospheric wave structure. The behavior recovered is broadly consistent with linear calculations of the Rossby‐gravity normal mode in the presence of instability. The mode's amplification depends sensitively upon details of the zonal‐mean flow, which determine instability through a reversal of potential vorticity (PV) gradient. On the other hand, the mode's period and structure are robust. They retain much the same form under a variety of conditions. The response is sharply discriminated to wave number 3 and westward periods of 2.0–2.3 days, even though forcing is broadband. Under January conditions, strong easterlies and the accompanying reversal of PV gradient enable the mode to amplify through sympathetic interaction with the mean flow. The amplifying signal eventually emerges from random variability, which is excited by broadband forcing. Amplification continues until eddy velocities approach 70 m/s, whereupon the signal saturates. Horizontal mixing then ensues, destroying the meridional gradient of PV, which in turn limits instability and further amplification. Under June conditions the response fluctuates randomly during the entire integration, with no evidence of sustained amplification. Nevertheless, it too is sharply discriminated to westward periods of 2.0–2.3 days, achieving eddy velocities of 10–20 m/s. The sharply discriminated response appears even when a reversal of PV gradient is absent, leaving the mean flow stable. Under all of these conditions the structure of eddy stream function Ψ′ is nearly barotropic, with antisymmetric character between the hemispheres. Eddy motion assumes the form of a wave‐number‐3 pattern of equatorial gyres, reducing in the tropics to v′ and strong cross‐equatorial motion. The pattern is very similar to the limiting structure of the 2‐day wave observed by UARS. Although magnified in the summer stratosphere and mesosphere, the structure of Ψ′ is global, with a phase reversal between the hemispheres. The modal structure extends upward into the thermosphere and, albeit weaker, as low as the tropopause.

Developing Wave Packets in the North Pacific Storm Track

Developing wave packets in the western North Pacific storm track are diagnosed observationally. An abrupt upstream edge to baroclinic wave activity over the western North Pacific facilitates comparisons between the observational results and previous theoretical predictions on the spatiotemporal evolution of an impulse disturbance. Results show that surface cyclogenesis events are preceded by a sharply peaked wave packet that originates poleward of the Himalaya Plateau and develops rapidly across the North Pacific to North America. Composite wave-packet structure is broadly consistent with linear theory for idealized models such as Eady’s. The longitude‐height structure of the mature packet reveals deep growing waves with horizontal wavelengths of approximately 4000 km near the packet peak. Downstream from the peak, amplitude decays exponentially, and wavelength decreases approximately linearly to about 2500‐3000 km at the leading edge. Meridional potential vorticity gradients are concentrated near the tropopause. In contrast to linear theory, the packets show an abrupt upstream edge and no evidence of upstream development. As the packet travels through the along-stream variations of the Pacific jet stream, the packet-peak and leading-edge group velocity vary. These accelerations change the packet length and suggest that the Pacific jet may act to focus the packets. A sample of North Atlantic storm track events reveals similar results and suggests that the Atlantic storm track is often seeded by wave packets that originate over the western North Pacific Ocean. In contrast, Atlantic packets refract equatorward and become trapped on the subtropical jet to the south of Himalaya Plateau, suggesting perhaps less potential for seeding disturbances in the Pacific storm track.

Mechanisms of Hemispherically Symmetric Climate Variability*

Inspired by paleoclimate evidence that much past climate change has been symmetric about the equator, the causes of hemispherically symmetric variability in the recent observational record are examined using the National Centers for Environmental Prediction–National Center for Atmospheric Research reanalysis dataset and numerical models. It was found that the dominant cause of hemispherically symmetric variability is the El Niño–Southern Oscillation. During an El Niño event the Tropics warm at all longitudes and the subtropical jets in both hemispheres strengthen on their equatorward flanks. Poleward of the tropical warming there are latitude belts of marked cooling, extending from the surface to the tropopause in both hemispheres, at all longitudes and in all seasons. The midlatitude cooling is caused by changes in the eddy-driven mean meridional circulation. Changes in the transient eddy momentum fluxes during an El Niño event force upper-tropospheric ascent in midlatitudes through a balance between the eddy fluxes and the Coriolis torque. The eddy-driven ascent causes anomalous adiabatic cooling, which is primarily balanced by anomalous diabatic heating. Using a quasigeostrophic spherical model, forced by an imposed surface eddy disturbance of chosen wavenumber and frequency, it is shown that the anomalous eddy momentum fluxes are caused by the impact that the changes in the tropically forced subtropical jets have on the propagation in the latitude–height plane of transient eddies. Changes in zonal winds, and associated changes in the meridional gradient of potential vorticity, create an anomalous region of low meridional wavenumber in the midlatitudes that refracts waves away both poleward and equatorward. Tropical forcing of variability in the eddy-driven mean meridional circulation is another way, in addition to Rossby wave teleconnections, whereby the Tropics can influence extratropical climate. Unlike teleconnections this mechanism causes climate variability that has strong zonally and hemispherically symmetric components and operates throughout the seasonal cycle.

The Sensitivity of an Intermediate Model of the Midlatitude Troposphere's Equilibrium to Changes in Radiative Forcing*

The sensitivity of the equilibrated state of a dry high-resolution quasigeostrophic β-plane channel model, coupled to both a simplified model of the atmospheric boundary layer and an interactive static stability, to changes in forcings is investigated. An earlier study with the same model found that with standard parameter values, the potential vorticity in the center of the channel just above the atmospheric boundary layer was homogenized. The new experiments show that this result is robust does not vary strongly with variations in forcing over a wide range of forcing parameters. This is so even though the meridional temperature gradients and static stability are generally sensitive to the forcing; that is, the changes in these cooperate to keep the meridional potential vorticity gradient zero. The potential vorticity gradients at higher levels are also robust although nonzero. The homogenization in the lower troposphere does disappear if the differential diabatic heating is decreased sufficiently or if the tropopause level is lowered sufficiently. The model results are also used to assess proposed parameterizations of eddy effects. Stone's parameterization of the meridional eddy heat flux is most successful at reproducing the model's results for most of the experiments. However, no parameterizations of the eddy heat flux captured the results of the experiments in which the diabatic heating timescale was varied. In these experiments, changes in the eddy heat fluxes kept the tropospheric temperature structure essentially unchanged even though the timescale changed from 5 to 80 days.

Climatology of Sign Reversals of the Meridional Potential Vorticity Gradient over Africa and Australia

Abstract A 10-yr climatology (1986–95) was performed using ECMWF gridded analyses on isentropic surfaces to identify regions where the lower-tropospheric meridional potential vorticity (PV) gradient changes sign across Africa and Australia during their respective summer seasons. While an African sign reversal has been documented, no similar study has been performed for the Australian region, which also has desert on the poleward side of open ocean. In each hemisphere, a northward decrease of PV is sufficient to produce a sign reversal. It was found that PV decreases northward in the lower troposphere across northern Australia, with the maximum reversal on the 315-K surface. It had comparable magnitude but smaller zonal extent (∼3000 km) than that on the 320-K surface in Africa (∼5000 km). In each region the sign reversal was associated with cyclonic PV anomalies on the equatorward side and anticyclonic anomalies on the poleward side. OLR was used as a proxy for deep convective heating in order to evaluate...

The Low-Level Structure of African Easterly Waves in 1995

Abstract The existence of African easterly waves (AEWs) north of the African easterly jet (AEJ) core with maximum amplitude at low levels has been confirmed and clarified using radiosonde data and the U.K. Meteorological Office global model analysis from the hurricane season of 1995. At Bamako (12.5°N, 8.0°W) the AEWs were characterized mainly by maximum amplitudes at the level of the AEJ (around 700 mb), whereas at Dakar (14.7°N, 17.5°W) the waves were characterized by maxima between 850 and 950 mb. The low-level waves to the north of the AEJ arise in association with baroclinic interactions between the negative meridional potential vorticity (PV) gradients in the jet core and the positive low-level gradient of potential temperature, θ, enhanced by the presence of low-static-stability air north of the AEJ. These waves follow the positive meridional θ gradients over northern Africa in contrast to the jet-level AEWs that follow the meridional PV gradients at the level of the AEJ. Cross-correlation analysis...

Three‐dimensional isoneutral potential vorticity structure in the Indian Ocean

The three‐dimensional isoneutral potential vorticity structure of the Indian Ocean is examined using World Ocean Circulation Experiment and National Oceanic and Atmospheric Administration conductivity‐temperature‐depth data and historical bottle data. The distribution of the potential vorticity is set by the Indian Ocean's source waters and their circulation inside the basin. The lower thermocline has a high potential vorticity signal extending westward from northwest of Australia and a low signal from the Subantarctic Mode Water in the south. The Antarctic Intermediate Water inflow creates patches of high potential vorticity at intermediate depths in the southern Indian Ocean, below which the field becomes dominated by planetary vorticity, indicating a weaker meridional circulation and weaker potential vorticity sources. Wind‐driven gyre depths have lower potential vorticity gradients primarily due to same‐source waters. Homogenization and western shadow zones are not observed. The β‐effect dominates the effect of the Somali Current and the Red Sea Water on the potential vorticity distribution. Isopleths tilt strongly away from latitude lines in the deep and abyssal waters as the Circumpolar Deep Water fills the basins in deep western boundary currents, indicating a strong meridional circulation north of the Antarctic Circumpolar Current. The lower‐gradient intermediate layer surrounded vertically by layers with higher meridional potential vorticity gradients in the subtropical Indian Ocean suggests that Rossby waves will travel ∼1.3 times faster than standard theory predicts. To the south, several pools of homogenized potential vorticity appear in the upper 2000 m of the Southern Ocean where gyres previously have been identified. South of Australia the abyssal potential vorticity structure is set by a combination of the Antarctic Circumpolar Current and the bathymetry.

Spatially growing Rossby lee waves: Implications for a coupled ocean‐atmosphere global circulation model

Large‐amplitude Rossby lee waves are observed in the Southern Ocean of an ocean general circulation model. These waves result in a reduced ability of an associated coupled ocean‐atmosphere general circulation model to model the observed sea ice distribution around Antarctica. To understand this behavior, Rossby lee waves within a meridionally sheared eastward directed ocean current are investigated theoretically. It is found that shearing the current results in a significant asymmetry in the lee wave field, the amplitude of which increases linearly downstream. In the regions where the meridional gradient of the shear exceeds β but is of opposite sign, there are no Rossby lee waves. Waves are trapped to the region where β dominates. Investigation of the ocean general circulation model shows that the meridional shear does not exceed β. However, the meridional gradient of Ertel's potential vorticity, a more appropriate quantity in a continuously stratified ocean general circulation model, is negative as required.

The Modification of Long Planetary Waves by Homogeneous Potential Vorticity Layers

A mechanism by which long planetary waves in the ocean may propagate significantly faster than the classical long baroclinic Rossby waves is investigated. The mechanism depends on the poleward thickening of intermediate density layers and the concomitant thinning of near-surface and deep layers. These features of the mass distribution are associated with the well-known homogenization of potential vorticity in intermediate density layers and with significantly elevated meridional potential vorticity gradients near the surface and somewhat at depth. The mechanism is explored in a simple three-layer model, in which the middle layer has zero potential vorticity gradient and is sandwiched between a surface layer with large potential vorticity gradient and a bottom layer with modest potential vorticity gradient. The effective phase speed of the planetary waves is merely the sum of the phase speeds of virtual baroclinic Rossby waves propagating on the individual layer interfaces as though the other interface were not there and as though there were no mean vertical shear. The mechanism is also examined for a continuous model with zero potential vorticity gradient throughout the interior and large virtual potential vorticity gradients near the surface and bottom. Planetary waves in these models can propagate westward up to twice as fast as baroclinic Rossby waves would through an ocean with the same vertical stratification, but no mean vertical shear. This explanation of the Rossby wave speedup complements a recent detailed theoretical calculation of planetary-wave phase speeds based on geostrophic velocity profiles from archived hydrographic data.

Breaking Rossby waves in the barotropic atmosphere with parameterized baroclinic instability

In this study, we conducted a series of numerical experiments of breaking Rossby waves in thebarotropic atmosphere using a simple barotropic model which implements parametrization ofbaroclinic instability. Exponential growth of unstable modes must terminate eventually whenthe waves become finite amplitude. The non-linear evolution of amplified Rossby waves isexamined by analyzing the potential vorticity (PV) field in order to assess the criterion of theRossby wave breaking in a barotropic model atmosphere. For a control run of the wave-6experiment, growing unstable wavenumber n=6 is saturated when the wave energy attainsapproximately 20% of zonal energy of the basic flow. The energy supply at n=6 is balancedwith energy transfer to zonal flow and to its harmonics of n=12 and 18 by weak non-linearinteractions, maintaining a steady configuration of a surf zone structure. The existence of thenegative meridional gradient of PV is the necessary condition for the wave saturation. We thenattempted to break the waves intentionally by increasing the growth rate of the unstable mode.It is found that the regularity of Rossby wave progression is lost and the overturning of highand low PV centers occurs when the growth rate is increased by 30%. Associated with theRossby wave breaking, not only the harmonic waves but all zonal waves are amplified by thefully non-linear interactions among all waves. It is demonstrated that the transition from theweakly non-linear regime to fully non-linear regime in the energy transfer is the key factor forthe Rossby wave breaking so that the supplied energy is effectively dissipated by all waves.

Transient eastward-propagating long-period waves observed over the South Pole

Abstract. Observations of the horizontal wind field over the South Pole were made during 1995 using a meteor radar. These data have revealed the presence of a rich spectrum of waves over the South Pole with a distinct annual occurrence. Included in this spectrum are long-period waves, whose periods are greater than one solar day, which are propagating eastward. These waves exhibit a distinct seasonal occurrence where the envelope of wave periods decreases from a period of 10 days near the fall equinox to a minimum of 2 days near the winter solstice and then progresses towards a period near 10 days at the spring equinox. Computation of the meridional gradient of quasi-geostrophic potential vorticity has revealed a region in the high-latitude upper mesosphere which could support an instability and serve as a source for these waves. Estimation of the wave periods which would be generated from an instability in this region closely resembles the observed seasonal variation in wave periods over the South Pole. These results are consistent with the hypothesis that the observed eastward propagating long-period waves over the South Pole are generated by an instability in the polar upper mesosphere. However, given our limited data set we cannot rule out a stratospheric source. Embedded in this spectrum of eastward propagating waves during the austral winter are a number of distinct wave events. Eight such wave events have been identified and localized using a constant-Q filter bank. The periods of these wave events ranges from 1.7 to 9.8 days and all exist for at least 3 wave periods. Least squares analysis has revealed that a number of these events are inconsistent with a wave propagating zonally around the geographic pole and could be related to waves propagating around a dynamical pole which is offset from the geographic pole. Additionally, one event which was observed appears to be a standing oscillation.Key words. Meteorology and atmospheric dynamics (Middle atmospheric dynamics; waves and tides).

On Divergent Barotropic and Inertial Instability in Zonal-Mean Flow Profiles

Abstract Impacts of atmospheric mean zonal flows on equatorial and subequatorial waves are examined using the generalized Laplace tidal equation, a horizontal structure equation that explicitly includes the effect of horizontal divergence on disturbances via the Lamb parameter e = (2Ωa)2/(ghe). Two important consequences of the mean zonal flow profile are the excitation of barotropically unstable waves associated with a reversed meridional gradient of potential vorticity and the excitation of inertially unstable waves related to anomalous potential vorticity values. Investigations show that both types of instabilities occur in realistic wind fields of the upper troposphere around 200 hPa. The barotropically unstable waves are divided into weakly divergent waves located in the middle latitudes and the more important class of strongly divergent waves located at the equatorward flank of the jet stream in the subtropics. The strongly divergent inertially unstable waves are identified by equatorially trapped w...

Climatological Ertel's potential-vorticity flux and mean meridional circulation in the extratropical troposphere − lower stratosphere

Abstract. We investigated to what extent the isentropic, non-geostrophic formulation of zonally averaged circulation derived for stratospheric conditions is applicable to climatological transport in the extratropical troposphere and lower stratosphere. The study is based on 10 years of daily data of ECMWF analysis and on the ECHAM3 climate model of the German Climate Computing Centre. The main result is a scalar isentropic mixing coefficient, Kyy, and a mean meridional transport circulation consistently derived from the same data base. For both data sources, isentropic mean meridional circulation is derived from horizontal mass flow rate for 4 representative months. Alternatively, a mean meridional circulation is calculated from total diabatic heating rates of the ECHAM3 model. It is shown that only the latter is in good agreement with the ECMWF mean meridional circulation. Isentropic analysis also comprises the seasonal cycle of the climatological meridional gradient and flux of Ertel's potential vorticity (PV). Application of Tung's flux-gradient relation yields that for all seasons Kyy is positive in height-latitude regions where statistical significance is reached. Large Kyy values, marking regions of more efficient mixing, have been found in the subtropical vertical band of weak westerly wind and in mid-latitudes in regions of upward-propagating baroclinic wave activity in the middle and upper troposphere. Based on the ECMWF data and results of baroclinic-wave behaviour, strong indications are presented that positive zonally averaged PV flux polewards of the jet core in the NH is strengthened by stationary waves and nonlinear effects. Reduced eddy transport is apparent in winter and spring slightly below the subtropical tropopause jet. The seasonal cycle of Kyy from ECHAM3 data is to a great extent in agreement with the result based on ECMWF analysis. In the model, reduced interannual variability enlarges the height-latitude range where sign of Kyy is significant.Key words. Meteorology and atmospheric dynamics · Climatology · General circulation · Middle atmosphere is significant.

The Effect of Basic-State Potential Vorticity Gradients on the Growth of Baroclinic Waves and the Height of the Tropopause

The stability characteristics of normal mode perturbations on idealized basic states that have meridional potential vorticity (PV) gradients that are zero in the troposphere, very large at the tropopause, and order b in the stratosphere are checked. The results are compared to the corresponding models that have a lid at the tropopause. The dispersion relations and the vertical structures of the modes are similar in the two models, thus confirming the relevance of the Eady problem to unbounded atmospheres. The effect of replacing the lid with a more realistic tropopause is to complicate the interaction of tropopause and surface waves, such as to inhibit phase locking for a range of wavenumbers. This causes the short-wave cutoff of the Eady model to move to longer waves. Also, there is a slight destabilization of the long waves, which have large amplitudes in the stratosphere. The effect of gradually changing the tropospheric PV gradients from zero (Eady-type profile) to b (Greentype profile) on the stability of normal modes is checked. The dispersion relations show a smooth transition from the Green profiles to the Eady profiles, and a short-wave cutoff is gradually formed. Finally, the possibility of neutralizing the atmosphere through the short-wave cutoff of the Eady model by lifting the tropopause while keeping PV gradients zero in the troposphere is examined. It is found that instability depends on some minimal amount of tunneling of waves between the surface and the tropopause. The amount of tunneling depends on the vertical integral of N in the troposphere. It is necessary for ∫ Nd zto increase for

Transient eastward-propagating long-period waves observed over the South Pole

Observations of the horizontal wind field over the South Pole were made during 1995 using a meteor radar. These data have revealed the presence of a rich spectrum of waves over the South Pole with a distinct annual occurrence. Included in this spectrum are long-period waves, whose periods are greater than one solar day, which are propagating eastward. These waves exhibit a distinct seasonal occurrence where the envelope of wave periods decreases from a period of 10 days near the fall equinox to a minimum of 2 days near the winter solstice and then progresses towards a period near 10 days at the spring equinox. Computation of the meridional gradient of quasi-geostrophic potential vorticity has revealed a region in the high-latitude upper mesosphere which could support an instability and serve as a source for these waves. Estimation of the wave periods which would be generated from an instability in this region closely resembles the observed seasonal variation in wave periods over the South Pole. These results are consistent with the hypothesis that the observed eastward propagating long-period waves over the South Pole are generated by an instability in the polar upper mesosphere. However, given our limited data set we cannot rule out a stratospheric source. Embedded in this spectrum of eastward propagating waves during the austral winter are a number of distinct wave events. Eight such wave events have been identified and localized using a constant-Q filter bank. The periods of these wave events ranges from 1.7 to 9.8 days and all exist for at least 3 wave periods. Least squares analysis has revealed that a number of these events are inconsistent with a wave propagating zonally around the geographic pole and could be related to waves propagating around a dynamical pole which is offset from the geographic pole. Additionally, one event which was observed appears to be a standing oscillation.Key words. Meteorology and atmospheric dynamics (Middle atmospheric dynamics; waves and tides).

Potential Vorticity, Easterly Waves, and Eastern Pacific Tropical Cyclogenesis

Abstract A significant sign reversal in the meridional potential vorticity gradient was found during the summer of 1991 on the 310-K isentropic surface (near 700 mb) over the Caribbean Sea. The Charney–Stern necessary condition for instability of the mean flow is met in this region. It is speculated that the sign reversal permits either invigoration of African waves or actual generation of easterly waves in the Caribbean. During the same season, a correlation existed between the strength of the negative potential vorticity gradient in the Caribbean and subsequent cyclogenesis in the eastern Pacific. The meridional PV gradient, convective heating measured by outgoing longwave radiation data, and eastern Pacific cyclogenesis all varied on the timescale of the Madden–Julian oscillation (MJO). It is hypothesized that upstream wave growth in the dynamically unstable region provides the connection between the MJO (or any other convective forcing) and the associated enhanced downstream tropical cyclogenesis.

The tropical stratopause in the UKMO stratospheric analyses: Evidence for a 2-day wave and inertial circulations

Large-scale atmospheric motions near the low-latitude stratopause in the UK Meteorological Office stratospheric analyses during the northern hemisphere winter are investigated, focusing in particular on the 2-day wave and inertial eddies. The 2-day wave is a westward-propagating planetary wave which appears recurrently in the summer subtropical upper stratosphere and mesosphere. Space-time spectral analysis of the assimilated fields of temperature and winds, and high-resolution transport of water vapour, reveal large-amplitude breaking planetary waves in the summer subtropics, principally a 2-day wave 3, and a wave 1 of a period near 7 days. A region of anomalous zonal-mean meridional gradient of potential vorticity also exists, pointing to a barotropic instability mechanism for the origin of the 2-day wave. By contrast, the winter subtropics are characterized by mixing on less-than-planetary scales during the period considered, occurring during the easterly phase of the stratospheric semi-annual oscillation. Water vapour is mixed turbulently into gyres, and drawn in narrow filaments out of the southern hemisphere. The potential-vorticity field displays vertically layered, zonally-asymmetric variations. The behaviour of the model assimilated fields is supported by along-track water vapour distribution observed by the Microwave Limb Sounder aboard the Upper Atmosphere Research Satellite which shows evidences for intermittent filamentary mixing in the northern subtropics, and a 2-day wave signature in the summer subtropics. Based on these results, a possible mechanism connecting the initiation of the austral 2-day wave events to strong planetary-wave activity in the winter is proposed. The winter planetary waves provide the background conditions for development of inertial instabilities at low latitudes of the winter hemisphere, which in turn create conditions favourable for barotropic instability of the summer subtropical easterly jet.

The tropical stratopause in the UKMO stratospheric analyses: Evidence for a 2‐day wave and inertial circulations

AbstractLarge‐scale atmospheric motions near the low‐latitude stratopause in the UK Meteorological Office stratospheric analyses during the northern hemisphere winter are investigated, focusing in particular on the 2‐day wave and inertial eddies.The 2‐day wave is a westward‐propagating planetary wave which appears recurrently in the summer subtropical upper stratosphere and mesosphere. Space‐time spectral analysis of the assimilated fields of temperature and winds, and high‐resolution transport of water vapour, reveal large‐amplitude breaking planetary waves in the summer subtropics, principally a 2‐day wave 3, and a wave 1 of a period near 7 days. A region of anomalous zonal‐mean meridional gradient of potential vorticity also exists, pointing to a barotropic instability mechanism for the origin of the 2‐day wave.By contrast, the winter subtropics are characterized by mixing on less‐than‐planetary scales during the period considered, occurring during the easterly phase of the stratospheric semi‐annual oscillation. Water vapour is mixed turbulently into gyres, and drawn in narrow filaments out of the southern hemisphere. The potential‐vorticity field displays vertically layered, zonally‐asymmetric variations.The behaviour of the model assimilated fields is supported by along‐track water vapour distribution observed by the Microwave Limb Sounder aboard the Upper Atmosphere Research Satellite which shows evidences for intermittent filamentary mixing in the northern subtropics, and a 2‐day wave signature in the summer subtropics.Based on these results, a possible mechanism connecting the initiation of the austral 2‐day wave events to strong planetary‐wave activity in the winter is proposed. The winter planetary waves provide the background conditions for development of inertial instabilities at low latitudes of the winter hemisphere, which in turn create conditions favourable for barotropic instability of the summer subtropical easterly jet.

Barotropic Aspects of ITCZ Breakdown

Abstract In satellite images the ITCZ (intertropical convergence zone) is sometimes observed to undulate and break down into a series of tropical disturbances. Tropical cyclones may later develop within these disturbances and move into higher latitudes allowing the ITCZ to reform. It has been proposed that ITCZ breakdown results from a convectively modified form of combined barotropic and baroclinic instability of the mean flow. An unstable mean flow can be produced by ITCZ convection in just a couple of days. In this sense, the ITCZ produces favorable conditions for its own instability and breakdown. In this study, a nonlinear shallow water model on the sphere is used to simulate barotropic aspects of ITCZ breakdown. In the shallow-water model, the ITCZ is simulated by a prescribed zonally elongated mass sink near the equator. The mass sink produces a cyclonic potential vorticity (PV) anomaly that has a reversed meridional PV gradient on its poleward side. According to the Ripa theorem, a flow that has a...