Baroclinic Growth Research Articles

Significance of a Midlatitude SST Frontal Zone in the Formation of a Storm Track and an Eddy-Driven Westerly Jet*

Abstract In a set of idealized “aquaplanet” experiments with an atmospheric general circulation model to which zonally uniform sea surface temperature (SST) is prescribed globally as the lower boundary condition, an assessment is made of the potential influence of the frontal SST gradient upon the formation of a storm track and an eddy-driven midlatitude polar front jet (PFJ), and on its robustness against changes in the intensity of a subtropical jet (STJ). In experiments with the frontal midlatitude SST gradient as that observed in the southwestern Indian Ocean, transient eddy activity in each of the winter and summer hemispheres is organized into a deep storm track along the SST front with an enhanced low-level baroclinic growth of eddies. In the winter hemisphere, another storm track forms just below the intense STJ core, but it is confined to the upper troposphere with no significant baroclinic eddy growth underneath. The near-surface westerlies are strongest near the midlatitude SST front as observed, consistent with westerly momentum transport associated with baroclinic eddy growth. The sharp poleward decline in the surface sensible heat flux across the SST frontal zone sustains strong near-surface baroclinicity against the relaxing effect by vigorous poleward eddy heat transport. Elimination of the midlatitude frontal SST gradient yields marked decreases in the activity of eddies and their transport of angular momentum into midlatitudes, in association with equatorward shifts of the PFJ-associated low-level westerlies and a subtropical high pressure belt, especially in the summer hemisphere. These impacts of the midlatitude frontal SST gradient are found to be robust against modest changes in the STJ intensity as observed in its interannual variability, suggesting the potential importance of midlatitude atmosphere–ocean interaction in shaping the tropospheric general circulation.

Environmental energetics of an exceptional high‐latitude storm

AbstractThis study presents the Lorenz energetics of an exceptional baroclinic storm propagating to Nome (Alaska) in October 1992, where it caused severe flooding. The storm track was greatly influenced by the interaction with a blocking high a week before the cyclone was formed. The energetics gives new insights into this dramatic storm, suggesting that the large‐scale environment was responsible for its long trajectory and intensification. The blocking high also provided the dynamic steering that facilitated the wave propagation and baroclinic growth. Our results show that the environmental energy transfers can be used as an informative metric for severe storms. Copyright © 2010 Royal Meteorological Society

A Wave Amplitude Transition in a Quasi-Linear Model with Radiative Forcing and Surface Drag

Abstract A quasi-linear two-layer quasigeostrophic β-plane model of the interaction between a baroclinic jet and a single zonal wavenumber perturbation is used to study the mechanics leading to a wave amplitude bifurcation—in particular, the role of the critical surfaces in the upper-tropospheric jet flanks. The jet is forced by Newtonian heating toward a radiative equilibrium state, and Ekman damping is applied at the surface. When the typical horizontal scale is approximately the Rossby radius of deformation, the waves equilibrate at a finite amplitude that is comparable to the mean flow. This state is obtained as a result of a wave-induced temporary destabilization of the mean flow, during which the waves grow to their finite-equilibrium amplitude. When the typical horizontal scale is wider, the model also supports a state in which the waves equilibrate at negligible amplitudes. The transition from small to finite-amplitude waves, which occurs at weak instabilities, is abrupt as the parameters of the system are gradually varied, and in a certain range of parameter values both equilibrated states are supported. The simple two-layer quasi-linear setting of the model allows a detailed examination of the temporary destabilization process inherent in the large-amplitude equilibration. As the waves grow they reduce the baroclinic growth by reducing the vertical shear of the mean flow, and reduce the barotropic decay by reducing the mean potential vorticity gradient at the inner sides of the upper-layer critical levels. Temporary destabilization occurs when the reduction in barotropic decay is larger than the reduction in baroclinic growth, leading to a larger total growth rate. Ekman friction and radiative damping are found to play a major role in sustaining the vertical shear of the mean flow and enabling the baroclinic growth to continue. By controlling the mean flow potential vorticity gradient near the critical level, the model evolution can be changed from one type of equilibration to the other.

Case Study of a Developing African Easterly Wave during NAMMA: An Energetic Point of View

Abstract The West African perturbation that subsequently evolved into Hurricane Helene (2006) during NASA’s African Monsoon Multidisciplinary Analysis (NAMMA), 15 August–14 September 2006, and AMMA’s third special observing period (SOP-3), 15–29 September 2006, has been simulated with the nonhydrostatic Méso-NH model using parameterized convection. The simulated disturbance evolved over West Africa and the adjacent eastern tropical Atlantic through interactions between different processes at the convective scale, mesoscale, and synoptic scale. The aim of this paper is to quantify the energetics of the simulated disturbance. A set of energy equations is first developed in the hydrostatic case to solve the limitations of Lorenz’s analysis when applied to a finite domain. It is shown that this approach is also valid in the compressible and in the anelastic case in order to apply it to the Méso-NH results. Application to the simulated pre-Helene disturbance allows one to determine the most important terms in these equations. These simplifications are taken into account to derive an energy cycle including barotropic and baroclinic conversions of eddy kinetic energy. The development of the simulated system was found to result from barotropic–baroclinic growth over West Africa and barotropic growth over the tropical eastern Atlantic. It is suggested that most of these energy conversions were the result of an adjustment of the wind field in response to the pressure decrease, presumably caused by convective activity.

Response of Idealized Baroclinic Wave Life Cycles to Stratospheric Flow Conditions

Abstract Dynamical stratosphere–troposphere coupling through a response of baroclinic waves to lower stratospheric flow conditions is investigated from an initial value approach. A series of adiabatic and frictionless nonlinear baroclinic wave life cycles in a midlatitude tropospheric jet with different initial zonal flow conditions in the stratosphere is simulated, using a dry primitive equation model with spherical geometry. When a stratospheric jet, located at various latitudes between 35° and 70°, is removed from the initial conditions, the wavenumber-6 life cycle behavior changes from the well-known LC1 to LC2 evolution, characterized by anticyclonic and cyclonic wave breaking, respectively. Linear theory, in terms of refractive index and the structure of the corresponding fastest-growing normal mode, is found to be unable to explain this stratosphere-induced LC1 to LC2 transition. This implies that altered nonlinear wave–mean flow interactions are important. The most significant stratosphere-induced change that extends into the nonlinear baroclinic growth stage is a region of downward wave propagation in the lower stratosphere associated with positive values of the squared refractive index near 20 km. Furthermore, it is demonstrated that the difference between the response of the tropospheric circulation to LC1 and LC2 life cycles closely resembles the meridional and vertical structure of the North Atlantic Oscillation (NAO), with positive (negative) NAO-like anomalies being driven by LC1 (LC2). Thus, a weakened stratospheric jet induces the generation of negative NAO-like anomalies in the troposphere, consistent with the observed stratosphere–NAO connection.

Two Types of Baroclinic Life Cycles during the Southern Hemisphere Summer

AbstractBaroclinic eddy life cycles of the Southern Hemisphere (SH) summer are investigated with NCEP–NCAR reanalysis data. A composite analysis is performed for the years 1980 through 2004. Individual life cycles are identified by local maxima in synoptic-scale eddy energy. Two types of baroclinic life cycles are examined, each defined by the strength of the barotropic energy conversion 2 days prior to the maximum baroclinic growth. For one life cycle, the barotropic conversion is anomalously weak before the maximum baroclinic growth; for the other, the barotropic conversion is anomalously strong. These two life cycles are referred to as the weak barotropic (WB) and strong barotropic (SB) life cycles.The analyses for the WB life cycle find that a poleward anomalous wave activity flux is observed within the SH tropics and subtropics just before the initial growth of the synoptic-scale eddies. In contrast, the SB life cycle exhibits an equatorward anomalous wave activity flux prior to the initial wave development. For the WB life cycle, these changes in the wave activity flux are shown to induce a mean meridional circulation that weakens and broadens the midlatitude zonal mean jet and reduces the baroclinicity in the midlatitude lower troposphere. Opposite characteristics are observed for the SB life cycle. Since the eddy growth rate is found to be greater in the WB life cycle, these results suggest that the influences of the barotropic governor mechanism (a reduction in horizontal shear coinciding with more rapidly growing baroclinic eddies) and the midlatitude baroclinicity oppose each other at the beginning of the life cycle, with the former being dominant.Both the WB and SB life cycles coincide with anomalous tropical convection. For the WB life cycle, there is a strengthening of the convection over the Maritime Continent, and for the SB life cycle there is a weakening in the convection over the same region. These results suggest that the two types of baroclinic life cycles are ultimately triggered by convection in the tropics.

Dynamical implications of the orientation of atmospheric eddies: a local energetics perspective

ABSTRACTA local quasi-geostrophic energetics analysis indicates that within the jet core, low-frequency (LF) eddies behave baroclinically essentially the same as high-frequency (HF) eddies. They both have a westward tilting vertical structure and both grow baroclinically by transporting heat poleward and by converting eddy potential energy to kinetic energy. However, the difference in the horizontal orientations of HF and LF eddies has several important implications to their amplitude and peak locations, as well as their interaction with stationary waves. The barotropic decay of meridionally elongated HF eddies tends to terminate the growth of HF eddies beyond the jet exit region. The barotropic growth of the zonally elongated LF eddies not only ensure a continuous growth of LF eddies in the jet exit region, but also results in a new baroclinic growth of LF eddies farther downstream due to the presence of the west—east temperature contrast associated with stationary waves. The continuous growth of LF eddies due to both barotropic and baroclinic processes in the jet exit region is consistent with the facts that LF eddies reach maximum variability farther downstream of the two major jet streams and that the LF variability is much stronger than HF eddies.The results of energetics analysis are confirmed by the feedback analysis, showing that HF eddies, being dominated by meridional orientations, mainly act to maintain (damp) stationary waves by locally enhancing (reducing) north—south gradient of the height (temperature) field near the jet core regions. The zonally elongated LF eddies, on the other hand, act to primarily reduce the zonal gradient associated with stationary waves both barotropically and baroclinically.

Diagnostic and dynamical analyses of two outstanding aspects of storm tracks

This paper reports diagnostic and dynamical analyses about two aspects of winter storm tracks in Northern Hemisphere. It is first established with NCEP/NCAR reanalysis data that the Atlantic storm track is distinctly more intense than the Pacific storm track in 30 out of 40 winters even though the Atlantic jet is considerably weaker and that the intensity of the two storm tracks are positively correlated. It is hypothesized that the continent–ocean configuration, in conjunction with the related differential friction, could be a significant contributing factor to such relative intensity. Supporting evidence for this hypothesis is presented in the context of a quasi-geostrophic two-level model analysis. We also establish that the intensity of the Pacific storm track is a minimum in mid-winter (MWMIN) in 21 of 40 winters, whereas in contrast the Atlantic storm track is most intense in mid-winter (MWMAX) in 25 of 40 winters. The structural characteristics of the composite mean flows as well as the energetics in MWMIN and MWMAX are compared. It is hypothesized that a significant enhancement of barotropic damping relative to the baroclinic growth in mid-winter is a major contributing factor to the occurrence of MWMIN. Supporting evidence for this hypothesis is presented from another idealized model analysis. Successful simulation of MWMIN can be made in a global spherical multi-level primitive equation model only if the proxy forcing is enhanced in early winter. However, the model still captures the difference between MWMIN and MWMAX even without using such enhanced forcing. The major caveat of this study is that moist dynamics has not been taken into consideration.

Nature of the Differences in the Intraseasonal Variability of the Pacific and Atlantic Storm Tracks: A Diagnostic Study

Abstract On the basis of an intraseasonal variability index of storm track evaluated for 40 winters (1963–64 through 2003–04) of NCEP–NCAR reanalysis data, it is found that well-defined midwinter minimum [MWMIN; (midwinter maximum MWMAX)] occurs in 21 (8) winters over the North Pacific. In contrast, MWMIN (MWMAX) occurs in 4 (25) of the 40 winters over the North Atlantic. The power spectrum of such an index for the Pacific has a broad peak between 5 and 10 yr, whereas the spectrum of the index for the Atlantic has comparable power in two spectral bands: 2–2.8 and 3.5–8 yr. Over the North Pacific, the increase in the zonal asymmetry of the background baroclinicity as well as in the corresponding horizontal deformation of the time-mean jet from early/late winter to midwinter is distinctly larger in an MWMIN winter. Associated with these changes, there is a distinctly stronger barotropic damping rate in the January of an MWMIN winter. The increase in the net conversion rate of eddy kinetic energy from early/late winter to midwinter is much larger in an MWMAX winter than that in an MWMIN winter. Even though there is a modest increase in the barotropic damping from early/late winter to midwinter over the North Atlantic, it is overcompensated by a larger increase in the baroclinic conversion rate. That would result in MWMAX. These results are empirical evidences in support of a hypothesis that a significant enhancement of the barotropic damping relative to the baroclinic growth from early/late winter to midwinter is a major contributing factor to MWMIN of the Pacific storm track.

Case Study of an Intense African Easterly Wave

The life cycle of an intense African easterly wave (AEW) over the African continent is examined using European Centre for Medium-Range Weather Forecasts (ECMWF) operational analyses, Meteosat satellite images, and synoptic observations. This system, the strongest AEW of 2000, can be tracked from central North Africa into the eastern Atlantic Ocean, where it is associated with the genesis of Hurricane Alberto. Synoptic analysis of the kinematic and thermodynamic fields is supplemented by analysis of potential vorticity (PV), allowing exploration at the role of multiple scales in the evolution of this AEW. The authors’ analysis promotes the division of the AEW life cycle into three distinctive phases. (i) Initiation: The AEW development is preceded by a large convective event composed of several mesoscale convective systems over elevated terrain in Sudan. This convection provides a forcing on the baroclinically and barotropically unstable state that exists over tropical North Africa. (ii) Baroclinic growth: A low-level warm anomaly, generated close to the initial convection, interacts with a midtropospheric strip of high PV that exists on the cyclonic shear side of the African easterly jet, which is consistent with baroclinic growth. This interaction is reinforced by the generation of subsynoptic-scale PV anomalies by deep convection that is embedded within the baroclinic AEW structure. (iii) West coast development: Near the West African coast, the baroclinic structure weakens, but convection is maintained. The midtropospheric PV anomalies embedded within the AEW merge with one another and with PV anomalies that are generated by convection over topography ahead of the system. These mergers result in the production of a significant PV feature that leaves the West African coast and rapidly undergoes tropical cyclogenesis.

Combined Dynamics of Boundary and Interior Perturbations in the Eady Setting

A theoretical study is undertaken of the relative influence of intermediate-level, near-surface, and tropopause-level perturbations upon flow development and cyclogenesis. The study is based upon two complementary quasigeostrophic models with consideration given to different types of perturbations of Eady-like basic states. First, the standard linear Eady model is refined to include an interior, vertically localized potential vorticity (PV) wave perturbation, and the wave's influence upon the growth of the boundary waves is examined as a function of its vertical location and its initial phase and amplitude. It is shown that the pseudoresonance arising from the interaction of the interior PV wave with a surface-confined wave is highly transient and comparatively weak. In contrast, a PV wave located at midlevel can have supraexponential sustained impact upon the perturbation growth that results from a combination of its direct forcing effect and its ability to maintain the two Eady edge waves near quadrature for longer, rather than asymptoting to the normal mode setting. Second, an extended nonlinear configuration is adopted that comprised a baroclinic jetlike basic state of uniform PV perturbed by a localized upper-boundary thermal perturbation or an interior point PV vortex, and the vortex's influence upon the baroclinic growth and cyclogenesis is examined in terms of its vertical and horizontal vertical location. It is shown that the interior perturbation can exert an appreciable and sustained influence upon the growth when it is located on or near the so-called critical surface, and the attendant flow transits smoothly to modal behavior. The derived results point to the possible influence of low- and midtropospheric perturbations upon cyclone development, and are in accord with and can account for specific aspects of the structure and properties of singular vector (SV) perturbations.

The counter‐propagating Rossby‐wave perspective on baroclinic instability. II: Application to the Charney model

AbstractThe constant‐density Charney model describes the simplest unstable basic state with a planetary‐vorticity gradient, which is uniform and positive, and baroclinicity that is manifest as a negative contribution to the potential‐vorticity (PV) gradient at the ground and positive vertical wind shear. Together, these ingredients satisfy the necessary conditions for baroclinic instability. In Part I it was shown how baroclinic growth on a general zonal basic state can be viewed as the interaction of pairs of ‘counter‐propagating Rossby waves’ (CRWs) that can be constructed from a growing normal mode and its decaying complex conjugate. In this paper the normal‐mode solutions for the Charney model are studied from the CRW perspective.Clear parallels can be drawn between the most unstable modes of the Charney model and the Eady model, in which the CRWs can be derived independently of the normal modes. However, the dispersion curves for the two models are very different; the Eady model has a short‐wave cut‐off, while the Charney model is unstable at short wavelengths. Beyond its maximum growth rate the Charney model has a neutral point at finite wavelength (r=1). Thereafter follows a succession of unstable branches, each with weaker growth than the last, separated by neutral points at integer r—the so‐called ‘Green branches’. A separate branch of westward‐propagating neutral modes also originates from each neutral point. By approximating the lower CRW as a Rossby edge wave and the upper CRW structure as a single PV peak with a spread proportional to the Rossby scale height, the main features of the ‘Charney branch’ (0<r<1) can be deduced without prior calculation of the normal modes. Furthermore, CRWs from this branch are seen to make a smooth transition into the boundary and interior PV structure of the neutral modes appearing at r=1. The behaviour of the other branches and neutral points is essentially the same when viewed from the CRW perspective, but with cancelling interior PV structures reducing the self and mutual interaction of the CRWs. The underlying dynamics determining the nature of all the solutions is the difference in the scale‐dependence of PV inversion for boundary and interior PV anomalies, the Rossby‐wave propagation mechanism and the CRW interaction. The behaviour of the Charney modes and the first neutral branch, which rely on tropospheric PV gradients, are arguably more applicable to the atmosphere than modes of the Eady model where the positive PV gradient exists only at the tropopause. Copyright © 2004 Royal Meteorological Society

Piecewise Tendency Diagnosis of Weather Regime Transitions

Abstract Piecewise tendency diagnosis (PTD) is extended and employed to study the dynamics of weather regime transitions. Originally developed for adiabatic and inviscid quasigeostrophic flow on a beta plane, PTD partitions local geopotential tendencies into a linear combination of dynamically meaningful source terms within a potential vorticity (PV) framework. Here PTD is amended to account for spherical geometry, diabatic heating, and ageostrophic processes, and is then used to identify the primary mechanisms responsible for Northern Hemisphere weather regime transitions. Height tendency patterns obtained by summing the contributions of individual PTD forcing terms correspond very well to actual height tendencies. Composite PTD analyses reveal that linear PV advections provide the largest dynamical forcing for the weather regime development over the North Pacific. Specifically, linear baroclinic growth provides the primary forcing while barotropic deformation of PV anomalies provides a secondary contrib...

Nonmodal and unstable normal mode baroclinic growth as a function of horizontal scale

Linear, quasi-geostrophic, Cartesian, spectral models based on Grotjahn (1980) are solved as initial-value problems. The basic-state wind flow includes realistic vertical shear in the form of an upper-level jet but no horizontal shear. Two archetype initial vertical structures are selected. One structure, labeled “connected”, develops strong nonmodal growth (NG). The other structure, labeled “separated”, is intended to approximate better conditions prior to observed cyclogenesis. NG is deduced from growth rates of common growth measures: amplitude, total energy, potential enstrophy, and their components. Significant NG may occur, usually early on, before a solution asymptotes to the most unstable normal mode. This study focuses on how the relative amounts of NG and unstable normal mode growth vary for different scales in both horizontal dimensions. The peak NG in most growth measures is greatest for wavelengths much smaller than the most unstable normal mode wavelength. The peak NG occurs earlier as wavelength decreases consistent with relative phase speed and distance arguments applied to constituent eigenmodes coming into favorable superposition. The peak NG is much less at all wavelengths for a separated trough than a connected initial condition (IC), except for the boundary contribution to potential enstrophy. Also, the connected IC has peak NG at shorter wavelengths than the separated IC. The peak NG occurs at a shorter wavelength for amplitude than for total energy. The connected and separated ICs are shown with the horizontal structure of a square wave and for a wave having initially localized structure along the meridional axis but allowed to evolve in that dimension. The main differences are initially localized waves develop larger meridional scale and commensurately larger growth rates. When the meridional structure is allowed to evolve, transient horizontal tilts appear leading most commonly to zonal mean convergence of eddy momentum. Phase speed differences between the main eigenmodes comprising the total solution primarily explain this result.

Cold Season Synoptic-Scale Waves over Subtropical South America

The most active winter synoptic-scale wave patterns over South America are identified using an extended empirical orthogonal function (EEOF) technique and are physically diagnosed using composite methods. Results show that the leading modes of short timescale variability propagate along two main paths: over the subtropical jet latitudes (;308S) and over the subpolar jet latitudes (;608S). This research focuses on the subtropical mode and its evolution over South America. The observed structure of the systems associated with the subtropical mode resembles that of midlatitude baroclinic waves. Both cyclonic and anticyclonic perturbations display significant modifications in their threedimensional structure as they evolve over extratropical and subtropical South America. While the upper-level perturbations are mostly unaffected when moving eastward, the lower-level perturbations advance following the shape of the Andes Mountains and exhibit an abrupt equatorward migration at the lee side of the mountains. As a result of such detachment, smaller eddy heat fluxes are observed in the vicinity of the orography and consequently a weaker eddy baroclinic growth is observed. Once the upper-level system is on the lee side, the perturbations acquire a more typical baroclinic wave structure and low-level intensification of the system occurs. The latter is largest around 1000 km east of the orography, where enhanced moisture transports from tropical latitudes along the eastern portion of the low-level cyclone favor precipitation occurrence over southeastern South America. Those precipitation processes seem to provide a diabatic source of energy that further contributes to the strengthening of the low-level cyclone. In addition, an intensification of the cyclone once over the ocean was found in 60% of the situations considered, which is consistent with previous research suggesting an additional source of moisture and heat flux due to the warm waters of the Brazil Current.

The Evolution of a Stratospheric Wave Packet

This work examines the extent to which a few basic concepts that apply to plane waves, for example, the refraction of waves up the gradient of the index of refraction, apply to stratospheric planetary waves. This is done by studying the relation between group velocity (Cg) and the wave activity velocity, which is defined as the Eliassen–Palm flux divided by the wave activity density (Va = F/A). It is shown that although in the limit of plane waves Va equals Cg, the two velocities are not equal for stratospheric waves, because of reflection, tunneling, and superposition. The use of conservation of wave activity to understand the spatial variations of wave structure is explored. This is done by defining a wave activity packet as part of the wave that moves with Va. Integral lines of Va are then used to keep track of the wave packet location and volume. In the idealized case of an almost-plane wave, conservation of wave activity leads to variations in the amplitude of the wave when it is refracted by the slowly varying basic state. This effect is related to changes in wave packet volume. The wave activity packet framework is used to examine the importance of the “volume effect” for explaining the spatial variations of stratospheric waves. The wave packet formulation is also used to study the evolution of a wave propagating from the troposphere to the stratosphere. It is shown that the consequence of the polar night jet being a leaky waveguide is that perturbations initially concentrate up into the waveguide and only later leak out to the equatorial region. This can explain the observed stratospheric wave life cycle of baroclinic growth followed by a barotropic stage. Finally, integral lines of Va are used to estimate vertical propagation timescales of an observed wave, and it is shown that this estimate is consistent with linear wave dynamics.

Simple Initial Value Problems and Mechanisms for Baroclinic Growth

Abstract In this paper a simple 2D linear quasigeostrophic model is used to investigate how the development of local confined cyclonic perturbations is dependent on the perturbation scale, location, and tilt in Eady-type basic states. It is found that the initial growth of the perturbation can be maximized by reducing both the vertical and horizontal scale and using a “midtropospheric” vertical location. “Potential vorticity (PV) thinking” suggests the concept of “PV unshielding” to explain this result. Adding a meridional gradient of basic-state PV lowers the vertical location of perturbations that optimally excite sustained growth. This can be understood by considering the behavior of the upward and downward propagating parts of the initial perturbation. It is found that the importance of the initial perturbation tilt is diminished for confined perturbations. It is shown that diabatic heating in a vertically confined region can lead to a perturbation that exhibits rapid growth. The findings in this pape...

Dynamics of the North Brazil Current retroflection region from the Western Tropical Atlantic Experiment observations

Hydrographic and velocity observations of the North Brazil Current (NBC) retroflection region during the 1990–1991 Western Tropical Atlantic Experiment (WESTRAX) are examined with the intent of extracting dynamical information about the NBC eddy shedding. A comparison is performed between the depth structure of empirical orthogonal functions and the dynamical normal modes of the NBC retroflection region on a beta plane centered on 5°N. The barotropic and first two baroclinic modes account for about 75% of the vertical structure of the NBC flow. Thus, a three‐layer model of the NBC region is suitable for dynamical studies of the NBC retroflection. In terms of flow structure the upper layer represents the retroflection of the surface layers of the NBC to feed the North Equatorial Countercurrent. The middle layer represents the separating subthermocline waters of the NBC which feed the North Equatorial Undercurrent. The third layer represents a weak meandering flow that may be thought of as the Deep Western Boundary Current signature in the three‐layer ocean. In terms of PV, a well‐defined front separates the NBC waters from the North Equatorial Current in the upper layer. Both upper and middle layers present closed PV contours associated with the eddy in the process of pinching off from the retroflecting NBC. The NBC separation region, although equatorial, complies reasonably well with the basic quasigeostrophic (QG) assumptions. Therefore QG methods are applied to investigate the NBC meander growth. By isolating the effect of PV anomalies in each of the layers on each of the layers, baroclinic growth is verified to occur during the NBC eddy shedding.

A Model of Rossby Waves Linked to Submonthly Convection over the Eastern Tropical Pacific

Equatorward-propagating wave trains in the upper troposphere are observed to be associated with deep convection over the eastern tropical Pacific on the submonthly timescale during northern winter. The convection occurs in the regions of ascent and reduced static stability ahead of cyclonic anomalies in the wave train. In this study an atmospheric primitive equation model is used to examine the roles of the dry wave dynamics and the diabatic heating associated with the convection. Many features of a dry integration initialized with a localized wave train in the African‐Asian jet on a threedimensional climatological basic state quantitatively agree with the observations, including the zonal wavenumber 6‐7 scale of the waves, the time period of approximately 12 days, and the cross-equatorial Rossby wave propagation over the eastern Pacific. There is ascent and reduced static stability ahead of the cyclonic anomalies, consistent with the interpretation that the waves force the convection. The spatial scale of the waves appears to be set by the basic state; baroclinic growth upstream in the Asian jet favors waves with zonal wavenumber 6. On reaching the Pacific sector, lower-wavenumber components of the wave train are not refracted so strongly equatorward, while higher-wavenumber components are advected quickly along the Pacific jet before they can propagate equatorward. Once over the Pacific, the wave train approximately obeys barotropic Rossby wave dynamics. The observed lower-tropospheric anomalies include an equatorial Rossby wave that propagates westward from the region of cross-equatorial wave propagation and tropical convection. However, this equatorial Rossby wave is not forced directly by the dry equatorward-propagating wave train but appears in a separate integration as a forced response to the observed diabatic heating associated with the tropical convection.

Barotropic Effects on Atmospheric Storm Tracks

The effect of barotropic dynamics on atmospheric midlatitude storm tracks is investigated for the Northern Hemisphere winter season, using both observed data analyses and linear barotropic model experiments. It is shown that when the model flow is initialized with a realistic wave packet, barotropic processes alone can produce key features of the observed storm track structure. The author attributes this result both to the barotropic waveguide effect and to the fact that the geographical locations of barotropic growth coincide reasonably well with the baroclinic growth of the atmospheric storm track eddies. This study also shows that barotropic dynamics are more relevant for storm tracks in lower latitudes than for those in higher latitudes, consistent with the fact that the lower-latitude storm tracks are more closely associated with the subtropical jet, rather than with the polar front jet that is driven by baroclinic eddies.